Biogeosciences, 15, 3975–4001, 2018 https://doi.org/10.5194/bg-15-3975-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. A 1500-year multiproxy record of coastal hypoxia from the northern Baltic Sea indicates unprecedented deoxygenation over the 20th century Sami A. Jokinen1, Joonas J. Virtasalo2, Tom Jilbert3, Jérôme Kaiser4, Olaf Dellwig4, Helge W. Arz4, Jari Hänninen5, Laura Arppe6, Miia Collander7, and Timo Saarinen1 1Department of Geography and Geology, University of Turku, 20014 Turku, Finland 2Marine Geology, Geological Survey of Finland (GTK), P.O. Box 96, 02151 Espoo, Finland 3Department of Environmental Sciences, University of Helsinki, P.O. Box 65, 00014 Helsinki, Finland 4Leibniz Institute for Baltic Sea Research Warnemünde (IOW), Seestrasse 15, 18119 Rostock, Germany 5Archipelago Research Institute, University of Turku, 20014 Turku, Finland 6Finnish Museum of Natural History, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland 7Department of Food and Environmental Sciences, University of Helsinki, P.O. Box 66, 00014 Helsinki, Finland Correspondence: Sami A. Jokinen (sami.jokinen@utu.fi) Received: 12 January 2018 – Discussion started: 16 January 2018 Revised: 1 June 2018 – Accepted: 6 June 2018 – Published: 5 July 2018 Abstract. The anthropogenically forced expansion of coastal hypoxia is a major environmental problem affecting coastal ecosystems and biogeochemical cycles throughout the world. The Baltic Sea is a semi-enclosed shelf sea whose central deep basins have been highly prone to deoxygenation during its Holocene history, as shown previously by numerous pa- leoenvironmental studies. However, long-term data on past fluctuations in the intensity of hypoxia in the coastal zone of the Baltic Sea are largely lacking, despite the significant role of these areas in retaining nutrients derived from the catchment. Here we present a 1500-year multiproxy record of near-bottom water redox changes from the coastal zone of the northern Baltic Sea, encompassing the climatic phases of the Medieval Climate Anomaly (MCA), the Little Ice Age (LIA), and the Modern Warm Period (MoWP). Our recon- struction shows that although multicentennial climate vari- ability has modulated the depositional conditions and deliv- ery of organic matter (OM) to the basin the modern aggrava- tion of coastal hypoxia is unprecedented and, in addition to gradual changes in the basin configuration, it must have been forced by excess human-induced nutrient loading. Alongside the anthropogenic nutrient input, the progressive deoxygena- tion since the beginning of the 1900s was fueled by the com- bined effects of gradual shoaling of the basin and warming climate, which amplified sediment focusing and increased the vulnerability to hypoxia. Importantly, the eutrophication of coastal waters in our study area began decades earlier than previously thought, leading to a marked aggravation of hy- poxia in the 1950s. We find no evidence of similar anthro- pogenic forcing during the MCA. These results have impli- cations for the assessment of reference conditions for coastal water quality. Furthermore, this study highlights the need for combined use of sedimentological, ichnological, and geo- chemical proxies in order to robustly reconstruct subtle re- dox shifts especially in dynamic, non-euxinic coastal settings with strong seasonal contrasts in the bottom water quality. 1 Introduction The expansion of hypoxic dead zones is an ongoing global problem both in the marine realm (Diaz and Rosenberg, 2008; Vaquer-Sunyer and Duarte, 2008; Gooday et al., 2009; Rabalais et al., 2010, 2014) and in lacustrine settings (Jenny et al., 2016a, b). Bottom water oxygen depletion (< 2 mg L−1 dissolved oxygen), caused by the combined effects of water column stratification and excess delivery of organic matter (OM) to the seafloor, deteriorates benthic ecosystems (Levin et al., 2009) and often triggers harmful algal blooms (Zhang Published by Copernicus Publications on behalf of the European Geosciences Union. 3976 S. A. Jokinen et al.: A 1500-year multiproxy record of coastal hypoxia from the northern Baltic Sea et al., 2010) due to the impact of hypoxia on biogeochemi- cal cycles at the sediment–water interface (Middelburg and Levin, 2009). Upon bottom water deoxygenation, phospho- rus (P) is released efficiently to the water column from sur- face sediments, fueling further primary productivity and dini- trogen (N2) fixation by diazotrophic cyanobacteria, thus trig- gering a self-sustaining positive feedback mechanism com- monly associated with eutrophication (Vahtera et al., 2007). In addition, the ability of benthic ecosystems to remove ni- trogen via denitrification and anaerobic ammonium oxida- tion may be reduced upon repeated or prolonged exposure to bottom water hypoxia (Conley et al., 2009a; Middelburg and Levin, 2009; Carstensen et al., 2014b). Due to these internal feedback mechanisms, recovery from hypoxia is often slow, hampering management of the problem through reductions in external nutrient loading (Vahtera et al., 2007). Further- more, global warming is likely to exacerbate the spreading of hypoxia in many regions through enhanced nutrient inputs (linked to increased precipitation and discharge), decreased solubility of oxygen due to increased temperature, and accel- eration of internal nutrient cycling (Meier et al., 2011; Meire et al., 2013). Over the past century, the Baltic Sea has seen a marked expansion of benthic hypoxia (Jonsson et al., 1990; Conley et al., 2011; Carstensen et al., 2014a), and the Baltic Sea dead zone is often referred to as the largest anthropogeni- cally induced hypoxic marine area in the world (Diaz and Rosenberg, 2008). Yet, although long-term trends in the ex- pansion of hypoxia in offshore areas of the Baltic Sea have been widely studied, little is known about the past evolution of hypoxia in the shallow coastal areas, where episodic or seasonal oxygen deficiency is forced by thermal rather than salinity stratification (Virtasalo et al., 2005; Conley et al., 2011). Importantly, these coastal areas act as a filter for nu- trients received from the catchment (Asmala et al., 2017). Thus, changes in biogeochemical cycles in coastal sediments may have an impact on nutrient transport to offshore areas of the Baltic Sea (Almroth-Rosell et al., 2016). Elucidating fluctuations in coastal hypoxia will aid in the understand- ing of the efficiency of the coastal filter and is therefore vi- tal for understanding the expansion of hypoxia in the entire Baltic Sea. Indeed, it is still debated whether the decisive fac- tor triggering the hypoxic event during the Medieval Climate Anomaly (MCA, 900–1350 AD) in the Baltic Proper was in- tensified land use in the catchment (Zillén and Conley, 2010) or anomalously warm climate (Kabel et al., 2012; Papado- manolaki et al., 2018). In this study, we present a multiproxy reconstruction of the development of hypoxia in an enclosed coastal setting in the Finnish Archipelago Sea (northern Baltic Sea) over the past 1500 years, covering the known climatic oscillations of the MCA, the Little Ice Age (LIA, 1350–1850 AD), and the Modern Warm Period (MoWP, after 1850 AD) in order to as- sess how the coastal zone responds to centennial–millennial climate variability and potential past inputs of nutrients from the catchment areas. We use diverse bulk sediment geochem- ical proxies in combination with integrated sedimentological and ichnological analyses to elucidate temporal changes in the intensity of near-bottom water oxygen deficiency. In or- der to constrain the drivers behind the observed oxygenation changes, we assess past fluctuations in hydrodynamic condi- tions at the study site, and in the delivery of OM, and com- pare these with the past climate variability and changes in the anthropogenic nutrient loading from the catchment. 2 Study location The Baltic Sea is a shallow (mean depth 54 m) semi-enclosed basin located on a continental shelf (Fig. 1a) between mar- itime temperate and continental sub-Arctic climate zones. Climatic conditions in the area are largely modulated by the North Atlantic Oscillation (NAO) as well as the summer low and winter high over Eurasia (e.g., Rutgersson et al., 2014). Winter mean air temperature ranges from−12 ◦C in the north to 0 ◦C in the south, whereas summer mean temperature has a narrower range of 14–17 ◦C. The sea is essentially non- tidal, but irregular variations in wind and atmospheric pres- sure modulate the water level with a maximum amplitude of 2 m. Surface salinity exhibits an increasing trend from north to south, from 3–5 in the Gulf of Finland and Gulf of Bothnia to 8–10 in the southern Baltic (Leppäranta and Myrberg, 2009). This horizontal salinity gradient results from combined ef- fects of high riverine freshwater input in the north and occa- sional inflows of saline water from the North Sea through the Danish straits in the south. The contrasting density of these two water masses leads to the formation of a strong 10–20 m thick pycnocline, which lies at a depth of 40–80 m depending on the sub-basin (Leppäranta and Myrberg, 2009). Irregular saline inflow events from the North Sea occasionally venti- late the deep stagnant bottom waters of the Baltic Proper, but this oxygen is readily exhausted with a net effect of stronger stratification and possibly even more severe oxygen deple- tion (Conley et al., 2002; Carstensen et al., 2014a). The Archipelago Sea, located in the southwestern coastal area of Finland in the northern Baltic Sea (Fig. 1a), is a mo- saic of thousands of islands and small bays within an area of ∼ 8000 km2. Salinity in the area ranges from 5 to 7, in- creasing towards the open sea. Mean water depth is only 23 m, although some depths reach over 100 m. The length of the ice season is 3–4.5 months (Seinä, 1994), but a current decreasing trend of 46 days per century has been reported (Ronkainen, 2013). The rate of the present glacio-isostatic uplift is 3–4 mm per year (Mäkinen and Saaranen, 1998), which exposes previously deposited sediments to wave ero- sion and modulates hydrographic conditions in the area. The complex topography of the Archipelago Sea results in restricted water exchange between the inner archipelago and the open-sea areas (Mälkki et al., 1979), and numerous small Biogeosciences, 15, 3975–4001, 2018 www.biogeosciences.net/15/3975/2018/ S. A. Jokinen et al.: A 1500-year multiproxy record of coastal hypoxia from the northern Baltic Sea 3977 30° E 20° E 10° E 60° N FINLAND SWEDEN NORWAY 200 km Basemap BALTIC SEA HYDROGRAPHIC COMISSION 2013 c B S GF B a lt ic P ro p e rK a tte g a t A S B B 200 40 100 400 300 70 10W a te r d e p th ( m ) 500 m N Turku Study site HAVERÖ ISLAND Arable land Forest Depth 0–10 m Depth 10–20 m Depth > 20 m Study site Fish farm (a) (b) N Lake Kassjön Lake Koltjärn Figure 1. Maps of the study area. (a) Bathymetric map of the Baltic Sea. The abbreviations for the indicated sub-basins are as follows: BB, Bothnian Bay; BS, Bothnian Sea; AS, Archipelago Sea; GF, Gulf of Finland. Locations of the varved lakes used for the correlation of the pollution Pb profiles are also shown. (b) Inset map of the study location. White dashed line indicates the 5 m contour line that roughly corresponds to the paleoshoreline at ∼ 760 AD. basins with contrasting bottom water conditions exist in close proximity (Virtasalo et al., 2005). Water exchange in the area mostly occurs through deep straits following the fault-lines of the crystalline bedrock, which are mostly aligned in a north–south direction. In enclosed basins, a strong thermo- cline impedes mixing of dissolved oxygen to the bottom wa- ters during summer, which together with high delivery of re- active OM to the seafloor commonly results in seasonal hy- poxia (Virtasalo et al., 2005; Jokinen et al., 2015). Mixing of the water column through thermal convection takes place in spring and autumn due to the lack of a permanent halocline (Leppäranta and Myrberg, 2009). The sediment fill of the Archipelago Sea since the deglacial to present comprises a succession of ice-proximal tills and outwash, glaciolacustrine rhythmites, patchily dis- tributed debrites, postglacial lacustrine clays, and brackish- water mud drifts (Virtasalo et al., 2007, 2014). The study area was deglaciated at ∼ 11 400 cal BP (Stroeven et al., 2016), which led to freshwater conditions in the area (Tuovinen et al., 2008). By∼ 7600 cal BP, the eustatic ocean-level rise sur- passed the glacio-isostatic rebound rate and the Danish straits became inundated, resulting in marine flooding of the Baltic Sea basin and consequent salinization of the Archipelago Sea (Tuovinen et al., 2008). Since then, sedimentation in the Archipelago Sea has been characterized by wave and cur- rent modulated deposition of brackish-water mud drifts with laminated intervals and local unconformities (Virtasalo et al., 2007). Haverö is a small, extremely enclosed basin in the mid- dle of the Archipelago Sea (Fig. 1). The Proterozoic bedrock surface in the area is dominated by microcline granites, with small patches of gneisses, amphibolite, and granodiorite (Bedrock of Finland, DigiKP). Due to the relatively high el- evation of the surrounding islands, Quaternary deposits have been largely removed by erosion after the islands were up- lifted above the wave base (Maankamara, DigiKP). The dis- tance to the mouths of the largest rivers in the area, Aurajoki River and Paimionjoki River, is 25 and 38 km, respectively. A small brook drains into the basin from a lake located on the Haverö island. Sedimentation in the Haverö basin is domi- nated by reworking of previously deposited late- and post- glacial clays and organic-rich brackish-water muds during autumn and winter, and by rapid settling of organic-rich ag- gregates during spring and summer (Jokinen et al., 2015). This seasonal contrast in the sedimentation, accompanied by severe seasonal hypoxia and consequent deterioration of macrobenthic fauna, has enabled the formation and preser- vation of annual laminations (varves) over the past decades (Jokinen et al., 2015). Population around Haverö is sparse, and the dominant direct anthropogenic nutrient loading is sourced from two local fish farming cages that were oper- ational from 1987 to 2008 AD (Fig. 1b). 3 Description of the proxies 3.1 Proxies for hydrodynamic conditions 3.1.1 Sediment grain size Sediment grain size depends on sediment inputs and hy- drodynamic conditions. Given that sediment provenance re- mains fixed, temporal variation in grain size distribution in the coastal zone of the Baltic Sea, excluding river mouths, is mainly governed by changes in wind stress and local seafloor morphometry that modulate the bottom water energy flux (Lehmann et al., 2002b; Jönsson et al., 2005a; Ning et al., 2016). In general, periods with enhanced near-bottom cur- www.biogeosciences.net/15/3975/2018/ Biogeosciences, 15, 3975–4001, 2018 3978 S. A. Jokinen et al.: A 1500-year multiproxy record of coastal hypoxia from the northern Baltic Sea rents become recorded in sediments through increased pro- portion of coarse grains (e.g., Hjulström, 1939). 3.1.2 Titanium to potassium ratio In fine-grained sediments, potassium (K) is mainly associated with illite ((K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2(H2O)]), whereas titanium (Ti) is mainly present in heavy min- erals such as rutile (TiO2) or ilmenite ((Fe, Mg, Mn, Ti)O3) and thus it concentrates in the coarse fraction of sediments due to sorting effects (Dellwig et al., 2000). Therefore, variations in Ti /K in locations with negligible changes in sediment provenance over time can be ascribed to changes in depositional energy and sediment transport processes (Piva et al., 2008; Spofforth et al., 2008), whereby an increase in the ratio denotes amplified bottom water currents. 3.2 Proxies for the source of organic matter 3.2.1 Carbon to nitrogen ratio Due to the contrasting composition of vascular plants in com- parison to phytoplankton, carbon (C) to nitrogen (N) ratios of sediment OM can be used to estimate relative contributions of OM originating from terrestrial and marine compartments (Meyers, 1994, 1997, and references therein). This difference arises from the high abundance of proteins in algae, while vascular plants are rich in cellulose instead. Accordingly, ma- rine and terrestrial OM are characterized by molar C /N ra- tios of 4–10 and > 20, respectively. However, C /N of OM produced in the euphotic zone is potentially elevated during sinking (Müller, 1977; Meyers, 2003) and diagenesis (Gäl- man et al., 2008) due to preferential degradation of N over C. Conversely, adsorption of ammonia (produced upon OM de- composition) onto clay mineral surfaces has the potential to decrease sediment C /N during diagenesis, especially in sed- iments with less than 0.3 % of Corg (Müller, 1977). Despite these potential constraints, the C /N ratio often accurately records past variations in the source of OM to the seafloor (Meyers, 1994; 1997), as suggested for the coastal Baltic Sea (Müller and Mathesius, 1999). 3.2.2 Stable isotope composition of carbon Stable isotope composition of sediment organic carbon (δ13Corg) depends on the isotopic ratio of the C source as well as on the fractionation between 12C and 13C during photosynthesis (e.g., Hayes, 1993). A vast majority of plants present in the study area fix C via the Calvin (C3) pathway, whereby 12C is preferentially incorporated, producing Corg with 20 ‰ lighter isotope composition than the inorganic C (Cinorg) source. Land plants use atmospheric carbon dioxide (CO2, δ13C∼−7 ‰) as their source of Cinorg, whereas ma- rine algae utilize dissolved bicarbonate (HCO−3 , δ13C∼ 0 ‰) in addition to CO2, resulting in higher δ13C values in marine phytoplankton. Although the preferential source of C for ma- rine algae is dissolved CO2, they incorporate a progressively higher proportion of HCO−3 with respect to CO2 under de- creasing pCO2 (Fogel et al., 1992; Rost et al., 2003), lead- ing to increasing δ13C values in their cells. Accordingly, or- ganic matter produced by land plants has an average δ13C value of ∼−27 ‰, whereas phytoplankton-derived marine organic matter is characterized by a δ13C signature from−20 to −22 ‰ (Meyers, 1994). Although early diagenetic pro- cesses potentially alter the original stable isotope composi- tion of OM (Lehmann et al., 2002a), a number of studies indicate that the δ13C signature of sediment OM often ro- bustly records past environmental changes in the water col- umn (Meyers, 1994, 1997; Freudenthal et al., 2001; Kohzu et al., 2011). 3.2.3 Stable isotope composition of nitrogen The stable isotope composition of nitrogen (δ15N) in marine OM produced in the euphotic zone is governed by the iso- topic ratio of nitrate (NO−3 ) and the extent to which this nu- trient reservoir is consumed by phytoplankton (Altabet and Francois, 1994; Voss et al., 1996). Due to the preferential assimilation of 14NO−3 during photosynthesis, the produced OM is depleted in 15N relative to the inorganic N source. If the available nutrient pool is not continuously replenished (e.g., due to thermal stratification), the δ15N signature of the remaining NO−3 pool becomes progressively enriched fol- lowing Rayleigh fractionation kinetics (Altabet and Francois, 1994). Consequently, upon complete exhaustion of the NO−3 pool due to a period of intensive primary productivity, such as a spring bloom, no net isotopic fractionation occurs and the “original” δ15N signature of NO−3 is communicated to the accumulated OM. On continental margin settings charac- terized by high sedimentation rates and intensive phytoplank- ton blooms that contribute the majority of OM delivery to the seafloor, as in the Baltic Sea, alteration of the primary δ15N signal attained by phytoplankton within the euphotic zone is minute both during sinking through the water column (Alta- bet et al., 1991; Altabet and Francois, 1994) and during early diagenesis (Kienast et al., 2002; Thunell et al., 2004). Hence, high δ15N values in coastal sediments often record eutrophi- cation through agricultural and urban contribution to riverine N loading (McClelland and Valiela, 1998; Voss et al., 2000, 2005; Struck et al., 2000; Cole et al., 2004), owing to the enrichment of 15N in fertilizer and manure due to N transfor- mations such as denitrification and ammonia volatilization in catchment soils (Heaton, 1986; Aravena et al., 1993). 3.2.4 Branched isoprenoid tetraether index Founded on the observation that branched glycerol dialkyl glycerol tetraethers (GDGTs I–III) are mainly sourced from terrestrial environment (soil OM), whereas crenarchaeol originates predominantly from the marine environment (a Biogeosciences, 15, 3975–4001, 2018 www.biogeosciences.net/15/3975/2018/ S. A. Jokinen et al.: A 1500-year multiproxy record of coastal hypoxia from the northern Baltic Sea 3979 characteristic lipid for aquatic Thaumarchaeota), Hopmans et al. (2004) defined the branched isoprenoid tetraether (BIT) index as a proxy for the relative abundance of terrestrial OM: BIT index= (1) [GDGT− I] + [GDGT− II] + [GDGT− III] [Crenarchaeol] + [GDGT− I] + [GDGT− II] + [GDGT− III] , where end-member values of 0 and 1 denote open marine and coastal environment, respectively. Diagenetic effects on the BIT index are regarded as minute due to the structural similarity of the compounds involved (Schouten et al., 2013). Possible constraints on the application of the BIT index in- clude potential in situ production of branched GDGTs in the water column (Sinninghe Damsté et al., 2009) and marine sediments (Peterse et al., 2009) as well as the potential pro- duction of crenarchaeol in soils (Weijers et al., 2006). 3.3 Redox proxies 3.3.1 Sedimentary fabric Sedimentary-fabric analysis is focused on the preservation of primary sedimentary structure, its mixing by macrofaunal bioturbation, and the characteristics of identifiable bioturba- tion structures (trace fossils). Trace fossil assemblages of the organic-rich brackish-water muds of the Baltic Sea are well applicable for reconstructing past bottom water redox shifts (Virtasalo et al., 2011a, b). Benthic faunal responses to bot- tom water hypoxia include avoidance or even mortality of large species, loss of diversity, and shoaling of penetration depth or emergence from sediment (Levin et al., 2009). Con- sequently, the vertical extent, diameter, and diversity of bur- rows constructed by macrobenthic fauna decrease with de- clining bottom water oxygenation, which is thought to be the decisive factor shaping biogenic sedimentary fabrics in the Baltic Sea (Savrda and Bottjer, 1986, 1991; Virtasalo et al., 2011a, b), although other factors such as salinity, sub- strate consistency, and food supply also affect trace fossil as- semblages in the area (Virtasalo et al., 2006, 2011a). Impor- tantly, the behavior of macrobenthic fauna responds rapidly to changes in the bottom water environmental conditions, and these responses can be readily recorded in the trace fossil assemblages (Savrda and Bottjer, 1986; Wetzel, 1991). The magnitude of this response is governed by the intensity and duration the deoxygenation as well as by the recovery time between consecutive hypoxic events (Levin et al., 2009). 3.3.2 Molybdenum content Sedimentary molybdenum (Mo) content is a well-established proxy for past redox fluctuations in bottom waters overly- ing marine sediments (e.g., Algeo and Lyons, 2006; Scott and Lyons, 2012; Helz and Adelson, 2013). It has been suc- cessfully applied to Baltic Sea sediments for bottom water redox reconstructions, especially in deep areas (Mort et al., 2010; Jilbert and Slomp, 2013; Jilbert et al., 2015; Dijk- stra et al., 2016; Hardisty et al., 2016; van Helmond et al., 2017). The sensitivity of sedimentary Mo content to redox fluctuations is due to the conversion of the relatively inert molybdate ion (MoO2−4 ) in seawater to a series of particle- reactive thiomolybdates (MoOxS4−x) under exposure to hy- drogen sulfide (H2S). Where the concentration of H2Saq ex- ceeds ∼ 11 µM, the so-called sulfide switch is activated and a quantitative conversion to tetrathiomolybdate MoS2−4 may occur (Helz et al., 1996; Erickson and Helz, 2000), trigger- ing effective fixation of Mo in association with Fe-S phases (Helz et al., 1996, 2011; O’Connor et al., 2015) and organic matter (Helz et al., 1996; Algeo and Lyons, 2006; Dahl et al., 2017). In addition, Mo is reduced from oxidation state (VI) to (IV) during burial in sediments (Dahl et al., 2013). Un- der non-euxinic conditions, where H2S is only found in pore waters but not in bottom waters, the initial sedimentation of water column Mo often occurs through adsorption to solid- phase manganese (Mn) oxides at the sediment–water inter- face (Scott and Lyons, 2012; Scholz et al., 2013; Noordmann et al., 2015). Upon reduction of Mn oxides, Mo is released into the pore waters, from where it may efflux back to the water column or become sequestered into the sediments in the presence of sufficiently high sulfide levels. In such set- tings, the depth and intensity of the pore water H2S maxi- mum (in Baltic Sea sediments typically associated with the sulfate–methane transition zone (SMTZ), Egger et al., 2015; Jilbert et al., 2018) is expected to regulate the amount of Mo sequestered in the sediment (Adelson et al., 2001; Scott and Lyons, 2012; Helz and Adelson, 2013; Sulu-Gambari et al., 2017). 3.3.3 Pristane to phytane ratio The application of the pristane to phytane ratio (Pr /Ph) as a redox proxy is based on the theory that both of these aliphatic hydrocarbons are mainly sourced from the phytol side chain of chlorophylls, with preferential diagenetic formation of Ph upon exposure to reducing conditions (Didyk et al., 1978). Didyk et al. (1978) postulated that Pr /Ph ratios of < 1 are indicative of deposition under anoxic water column, while ratios fluctuating about 1 record oscillations between anoxic and oxic bottom waters, and persistently oxic bottom waters result in ratios > 1. However, it has been shown that a de- cline in Pr /Ph may also be caused by excess production of Ph sourced from methanogenic microbes below the SMTZ (Brassel et al., 1981; Venkatesan and Kaplan, 1987; Duan, 2000). As a result, the ratio should be used cautiously and in association with other indicators of redox conditions. This redox proxy is widely used in petroleum geology to charac- terize source rocks (Peters et al., 2005) but is so far unutilized in studies of Baltic Sea sediments. www.biogeosciences.net/15/3975/2018/ Biogeosciences, 15, 3975–4001, 2018 3980 S. A. Jokinen et al.: A 1500-year multiproxy record of coastal hypoxia from the northern Baltic Sea 4 Materials and methods 4.1 Sediment coring, subsampling, and analysis of grain size The study site was selected based on previous studies by Jokinen et al. (2015), where it was found that the sediment in the basin comprises thick varves since the beginning of the 20th century, providing a high-resolution archive of envi- ronmental change for the corresponding period. Due to this apparent sensitivity of the basin to bottom water hypoxia, as manifested in the continuous laminations, together with the central location in the middle of the Archipelago Sea, we ex- pected the site to be representative of the past environmental changes in the area. Importantly, despite the contrasting bot- tom water oxygenation between adjacent sub-basins in the area (Virtasalo et al., 2005), the long-term trends in envi- ronmental conditions are largely congruent over the entire Archipelago Sea, excluding areas close to prominent nutri- ent point sources (Suomela, 2011). Two replicate sediment cores (HAV-KU-5 and HAV-KU- 6) were retrieved using a 5 m long piston corer onboard R/V Aurelia of the Archipelago Research Institute in June 2015 (Table 1). The coring device was adjusted to start the core retrieval with the piston positioned 1–2 decimeters above the sediment–water interface, in order to capture the sediment surface as intact as possible. In the laboratory, the cores were split lengthwise and trimmed for digital photography and description of lithology. All of the following analyses, ex- cept for X-radiography, were conducted for the HAV-KU-6 core only. The sediment was sub-sampled into cubic (7 cm3) polystyrene sample boxes at approximately every 3.0 cm for geochemical analyses. In addition, the HAV-KU-6 core was continuously subsampled at 1 cm resolution into reclosable polyethylene bags for 137Cs, 14C, and grain size analyses. Grain size distributions were analyzed at every 10 cm by a Coulter LS200 laser diffractometer after pretreatment with excess H2O2 and dispersing the particles in an ultrasonic bath. 4.2 Geochronological methods Age constraints for the age model were obtained based on visual varve counting (1900 AD onwards), 137Cs profiles, and recognizable features in the measured atmospheric lead (Pb) fallout profile. Three analysts independently counted the varves within the continuously laminated interval (0–76 cm core depth) from the freshly split sediment surface in order to constrain the reproducibility of the method. To fix this float- ing varve chronology, 137Cs activity was measured for the topmost 60 cm of the core by measuring the gamma spectra of the wet subsamples at the Geological Survey of Finland using an EG&E Ortec ACE™-2K spectrometer with a 4 in. (1 decimeter) NaI /Tl detector. Two or three consecutive 1 cm thick subsamples were combined for the 20–60 and 0– 20 cm depth intervals, respectively, to gain enough material for the measurement. No corrections were applied for the re- sults, because the target was only to detect relative 137Cs ac- tivity peaks (Jokinen et al., 2015). Due to the lack of datable macrofossils, we also attempted to constrain the age model with bulk sediment radiocarbon dating of the NaOH-soluble fraction as suggested by Rößler et al. (2011). The NaOH- extraction and accelerator mass spectrometry (AMS)-14C measurements for nine selected core depths were conducted at the Poznan´ Radiocarbon Laboratory, Poland (Goslar et al., 2004). In addition, we compared our bulk sediment Pb pro- files with the Pb fallout records from varved lakes in Sweden (Brännvall et al., 1999) to constrain the chronology. 4.3 X-radiography, trace fossils, and bioturbation index For X-radiography, plastic boxes (50×4×2 cm) were pressed into the sediment, cut out using a steel string, trimmed with a thin aluminum sheet, and sealed after inserting a lid (Vir- tasalo et al., 2006). Due to disturbance of the uppermost sediments in the reference half of HAV-KU-6 during split- ting of the core, the subsampling for X-radiography for the topmost 170 cm was done for the replicate core HAV-KU- 5. For the bottom part, the subsampling was conducted for HAV-KU-6 due to the longer core retrieval (Table 1). The cores were correlated visually based on the occurrence of the laminated intervals and the boundaries of the lithological units described below. The 2-D projections of the sedimen- tary structures were investigated from high-resolution digi- tal X-radiographs of the boxes, which were obtained by a custom-made tungsten-anode micro-computed-tomography Nanotom device (Phoenix|Xray Systems+ Services GmbH) at the University of Helsinki. Power settings of the X-ray source were 100 kV and 150 µA, and the detector was ad- justed to exposure time of 1 s and an averaging of 30 images per radiograph. The resulting pixel size in the X-radiographs was 38 µm. X-radiographs were used for further lithological description as well as for analysis of bioturbation index and ichnological structures to the ichnogenus level (for ichno- fossil descriptions, see Virtasalo et al., 2011a) in 6 cm thick sediment intervals. We assigned a bioturbation index of 1– 4, modified from Behl and Kennett (1996), to these intervals based on the preservation of the primary sedimentary fabric so that high values denote intense mixing (Table 2). 4.4 Geochemical analyses 4.4.1 Carbon and nitrogen contents and stable isotope ratios Lids of the polystyrene sample boxes were removed, the samples were freeze-dried, and the dry bulk density was cal- culated. Subsequently, the sediment samples were ground in an agate mortar and analyzed for total carbon (Ctot) and nitro- Biogeosciences, 15, 3975–4001, 2018 www.biogeosciences.net/15/3975/2018/ S. A. Jokinen et al.: A 1500-year multiproxy record of coastal hypoxia from the northern Baltic Sea 3981 Table 1. Retrieved sediment cores. Core ID Sampling date Latitude Longitude Water depth Gear Research Recovery (WGS 84) (WGS 84) (m) vessel (cm) HAV-KU-5 11 June 2015 60◦14.117 N 22◦02.642 E 23.3 Piston corer Aurelia 330 HAV-KU-6 11 June 2015 60◦14.116 N 22◦02.642 E 23.3 Piston corer Aurelia 390 Table 2. Classification scheme for assessing bioturbation index, modified from Behl and Kennett (1996). Bioturbation Description of the sedimentary fabric index 1 Unbioturbated sediment with distinct, continuous lamination 2 Diffuse, discontinuous, or irregular laminations 3 Slightly bioturbated sediment, with either faint, diffuse laminations/bedding or with few discrete patches of laminations surrounded by homogenized sediment 4 Completely bioturbated sediment wherein the primary sedimentary fabric is thoroughly obliterated gen (Ntot) by a CHNS analyzer (TruSpec Micro, LECO Cor- poration) in the Department of Food and Environmental Sci- ences at the University of Helsinki. The amount of inorganic C and N was assumed to be negligible in this setting, and thus Ctot and Ntot contents are assumed to equal to Corg and Norg, respectively (Jilbert et al., 2018). The relative contributions of terrestrial plant-derived (%OCterr) and phytoplankton- derived OM (%OCphyt) to the total Corg were estimated with a simple two-end-member mixing model for the molar N /C ratio, applying end-member values of N /Cterr = 0.04 (C /N= 25) and N /Cphyt = 0.13 (C /N= 7.7) for terrestrial and phytoplankton-derived OM, respectively (Goñi et al., 2003): %OCterr = (N/C)sample− (N/C)phyt (N/C)terr− (N/C)phyt × 100, (2) where %OCterr = 100−%OCphyt. The terrestrial end- member integrates various sources of terrestrial OM, from fresh vascular plant detritus to more degraded soil OM. Since virtually all of the OM transported by rivers to the coastal zone of the Archipelago Sea passes through the soil reser- voir and enters the marine environment as a mixture of vari- ably degraded material, further deconvolution of the terres- trial end-member by N /C ratios alone is impractical. The validity of these end-member values for determining terres- trial vs. phytoplankton material, however, was recently con- firmed by a study in the southern coast of Finland, where a strong gradient of N /C values of sediment OM was reported along an estuarine transect (Jilbert et al., 2018). Further as- sumptions in the mixing model are that the N /C ratios of the both end-members are temporally and spatially constant and that these values remain effectively unaltered during OM sedimentation and burial. To obtain the desired fraction of terrestrially derived Corg rather than Norg from this model, we use N /C instead of the C /N ratio for the calculation (Perdue and Koprivnjak, 2007). Elsewhere in the text and in the figures we refer to the more commonly used C /N ratio. In the Laboratory of Chronology at the University of Helsinki, selected freeze-dried and ground samples were measured for stable isotope compositions of carbon (δ13Corg) and nitrogen (δ15N) by an isotope ratio mass spectrome- ter (Delta V Plus, Thermo Fisher Scientific) coupled to an NC2500 elemental analyzer. The measured δ13Corg and δ15N values are reported relative to the Vienna Pee Dee Belemnite (V-PDB) and AIR scales for C and N, respectively. Precision of the measurements, as checked against in-house and ref- erence standards, yielded < 0.02 % (1σ ) for both elements. Based on analysis of replicate samples, precision of the en- tire procedure was < 0.2 and < 2.6 % for δ13Corg and δ15N, respectively. To account for the historic decline in δ13C of atmospheric CO2 due to fossil fuel burning and deforesta- tion, the measured values for samples postdating 1700 AD were corrected according to the equation suggested by Ver- burg (2007): %OCterrδ13C= 7.7738118× 10−16×Y 6 − 1.2222044× 10−11×Y 5 + 7.1612441× 10−8×Y 4 − 2.1017147× 10−4×Y 3 + 3.3316112× 10−1×Y 2 − 273.715025×Y + 91703.261, (3) where Y = year (AD) of the sediment accumulation. 4.4.2 Biomarker analyses Selected freeze-dried and homogenized samples were ana- lyzed for various biomarkers at the Department of Marine Geology in the Leibniz Institute for Baltic Sea Research (IOW) following Kaiser and Arz (2016). Briefly, 0.5–1.0 g of www.biogeosciences.net/15/3975/2018/ Biogeosciences, 15, 3975–4001, 2018 3982 S. A. Jokinen et al.: A 1500-year multiproxy record of coastal hypoxia from the northern Baltic Sea sediment was used for accelerated solvent extraction (Dionex ASE 350, Thermo Fisher Scientific) with a 9 : 1 volumet- ric mixture of dichloromethane and methanol using high pressure (100 bar) and temperature (100 ◦C). After the ad- dition of internal standards (squalane, nonadecan-2-one, 5α- androstan-3β-ol, and C46-GDGT) for quantification, the to- tal extracts were divided into four fractions by microscale silica gel column chromatography. For the determination of Pr and Ph contents, the apolar alkane fraction was measured on a multichannel Trace Ultra gas chromatograph (Thermo Fisher Scientific), utilizing a split/splitless inlet, a DB-5 MS capillary column, and a FID (flame ionization detector) de- tector. Peak identification from the obtained chromatograms was done based on the comparison of peak retention times with an external standard containing Pr, Ph, and n-C8 to n- C40 alkanes, complemented with GC-MS analyses (see for details Kaiser and Arz, 2016). To calculate the BIT index, the most polar fraction (including glycerol dialkyl glycerol tetraethers, GDGTs) was measured with HPLC APCI-MS (Dionex Ultimate 3000 UHPLC, Thermo Fisher Scientific system coupled to a MSQ Plus, Thermo Fisher Scientific) (see for details Kaiser and Arz, 2016). 4.4.3 Major and trace element contents Elemental contents of the freeze-dried and homogenized samples were estimated by a combination of ICP-OES (in- ductively coupled plasma optical emission spectrometry) and ICP-MS (inductively coupled plasma mass spectrometry) analysis. Initial ICP-OES analysis for K and Ti was per- formed at the Department of Food and Environmental Sci- ences at the University of Helsinki. A portion of 0.1–0.2 g of dry sediment was dissolved in 2.5 mL of HF (38 %) and 2.5 mL of a mixture of HClO4 (70 %) and HNO3 (65 %) (vol- umetric ratio 3 : 2) in closed Teflon bombs at 90 ◦C for 12 h. After evaporating the acids at 160 ◦C, the remaining gel was dissolved in Suprapur® 1 M HNO3 and analyzed for K and Ti by ICP-OES (Thermo Fisher Scientific, precision determined by replicate analyses < 5 %). Accuracy of the initial ICP-OES results was checked by digestion and analysis of a subset of 28 samples at IOW together with the international reference material SGR-1b (USGS). A total of 50 mg of dried and ground sediment was first treated in open Teflon vessels (PDS-6; Heinrichs et al., 1986) with 1 mL HNO3 (65 %) at 60 ◦C for 1 h to oxidize OM. After addition of 2 mL concentrated HF and 2 mL con- centrated HClO4, the closed vessels were heated at 180 ◦C for 12 h. After evaporation of the acids on a hot plate at 180 ◦C, the digestions were fumed off 3 times with 6 M HCl, re-dissolved in 25 ml 2 vol % HNO3, and finally measured by ICP-OES (iCAP 7300 Duo, Thermo Fisher Scientific) for K and Ti using Sc as internal standard. Precision and accuracy of the measurements of SGR-1b at IOW were respectively 0.7 and −5.9 % for K and 0.8 and −7 % for Ti. Constant off- sets of up to 20 % were observed between the datasets from Helsinki and IOW for the 28 samples. A correction factor was thus applied to the results from the Helsinki data using a linear regression between the IOW and Helsinki results. The K and Ti data reported in this paper are thus the corrected Helsinki data and may be considered to have precision and accuracy < 10 % (this value integrates both the quality of the IOW measurements and the goodness of fit of the linear re- gression). Internal reproducibility between the two sample sets was good (< 7 %), suggesting that the offsets observed between the respective ICP-OES datasets from Helsinki and IOW were related to analytical issues rather than the diges- tion protocols. ICP-MS was used to determine the contents of Mo, Pb, and the ratio of stable Pb isotopes (206Pb / 207Pb). ICP-MS analysis (iCAP Q, Thermo Fisher Scientific) was performed at IOW on the complete set of digests from Helsinki and the subset of 28 digested samples from IOW. The interna- tional reference material SGR-1b (USGS) served to deter- mine the precision and accuracy of Mo (0.7/− 1.6 %) and Pb (0.6/− 2.2 %) measurements using Rh and Ir as internal standards in KED (kinetic energy discrimination) mode us- ing He as collision gas. For determination of 206Pb / 207Pb in standard mode, the samples were diluted to a Pb concentra- tion of ∼ 1 µg L−1 and the instrument was tuned to provide best results for the NIST standard SRM-981, resulting in a precision < 0.07 %. All measured elemental contents were corrected for the weight of the salt in the pore water using the ambient salin- ity and porosity (Lenz et al., 2015). Mass accumulation rates (MARs) of the individual elements were calculated as fol- lows: MARx = Cx100 ×LSR×DBD, (4) where Cx is the element content (%), LSR is the linear sedimentation rate (cm a−1), DBD is the dry bulk density (g cm−3), and subscript x denotes the element in question. 5 Results 5.1 Age model The three independent varve counts for the continuously lam- inated recent sediments (3–76 cm) suggest that this section covers 113± 5 varve years of deposition. The 137Cs activ- ity increase at 33 cm core depth, derived from the Chernobyl nuclear power plant accident in 1986 AD, enabled us to fix the floating varve chronology, which indicated that the on- set of continuous lamination occurred at∼ 1900 AD (Fig. 2). A similar 137Cs activity increase at this site was found at 32 cm core depth in a core taken in 2013 AD (Jokinen et al., 2015), implying that no marked loss of surface sediment occurred during the piston coring. This is supported by the varve counting, which (after fixing with the 137Cs activity Biogeosciences, 15, 3975–4001, 2018 www.biogeosciences.net/15/3975/2018/ S. A. Jokinen et al.: A 1500-year multiproxy record of coastal hypoxia from the northern Baltic Sea 3983 profile) suggested that the sediment surface was indeed of modern age (Table 3). The apparent temporal fluctuations in the magnitude of the bulk sediment reservoir age as indicated by the downcore re- versals in 14C age (Fig. 2; Lougheed et al., 2017) impeded utilization of the bulk sediment AMS-14C dates for the con- struction of the age model. Because our data did not enable robust correction for such variations, we desisted from using bulk sediment 14C dates as age constraints. To estimate the content of atmospheric pollution Pb in the sediment, we applied a simple two-component mixing model following Brännvall et al. (1999): Pbpollution = (5) (206Pb/ 207Pb)sample− (206Pb/207Pb)background (206Pb/ 207Pb)pollution− (206Pb/ 207Pb)background ×Pbsample. The background isotope composition of Pb was calculated as the mean prior to the apparent onset of Pb pollution at 900 AD (Fig. 2), which yielded an estimate of 1.397. We used the age model based on 137Cs dating and varve count- ing for the sediments deposited after 1900 AD to assign a time-dependent 206Pb / 207Pb composition for the pollution Pb. We assumed a constant 206Pb / 207Pb ratio of 1.17 for the pollution Pb prior to 1900 AD, a linear decrease from 1.17 to 1.15 between 1900 and 1945 AD, and a constant ratio of 1.15 since 1945 AD (Brännvall et al., 1999, and references therein). We found a remarkable consistency between our Pb record and the pollution Pb profiles of varved lakes in eastern Swe- den, where the preindustrial Pb fallout was mainly sourced from mainland Europe and from the British Isles (Brännvall et al., 1999; Fig. 2). Hence, we used the main Pb pollution features found consistently in all of these lakes to constrain an age model for our sediment core (Fig. 2; Table 3). The onset of medieval Pb pollution (at 900 AD, Lougheed et al., 2012) and the medieval pollution maximum (at 1200 AD, Lougheed et al., 2012, 2017; Zillén et al., 2012) have pre- viously been used as age constraints in sediments from the Baltic Proper. However, our study is, to our best knowledge, the first to identify the 1530 AD pollution peak and the pol- lution minima of 1350 and 1600 AD (Brännvall et al., 1999; Renberg et al., 2002) as age–depth points in Baltic Sea sedi- ments. A conservative 1σ uncertainty of 50 years for all of the Pb constraints was assumed. In addition to this, we assigned an additional uncertainty for the Pb age constraints in cases where the position of the feature was ambiguous by first cal- culating the mean LSR for the 900–1900 AD period (from the sharp onset of Pb pollution to the onset of continuous lamination). Based on this LSR estimate (0.21 cm a−1) and the width of the Pb features, an additional uncertainty was added to these constraints by assuming that the width of each recognized feature in our Pb profile corresponds to the 2σ uncertainty in the correlation with the Swedish pollution Pb records. The obtained age constraints (Table 3) were used to con- struct a Bayesian age model in OxCal 4.2 software (Bronk Ramsey, 2009), using the P -sequence function (Bronk Ram- sey, 2008) with a k value of 20. The resulting age model in- dicates that the core HAV-KU-6 covers ∼ 1500 years of de- positional history in the area (Fig. 2). A substantial increase in LSR is observed coincident with the onset of laminated sediment accumulation at 76 cm depth (Fig. 2). 5.2 Lithology and trace fossils The sediment in the core HAV-KU-6 is characterized by organic-rich mud, which conforms to the brackish-water mud drift sensu Virtasalo et al. (2007), indicative of recent deposi- tion in the basin. Based on lithology, the core can be divided into four units as described below (Figs. 2 and 3). These units approximately correspond to the pre-MCA, MCA, LIA, and MoWP intervals (Fig. 3). The basal part (393–324 cm, ∼ 500–780 AD) of the core HAV-KU-6 roughly corresponds to the pre-MCA interval and comprises seemingly homogenous greenish-brown mud. The X-radiographs show that this mud is intensely burrow mot- tled (Planolites mottling) and is thus characterized by a bio- turbation index of 4 (Fig. 4), although discrete trace fossils are hardly discernible due to the high burrow density and low contrast enhancement by pyritization along the burrows (Fig. 3a), probably resulting in substantial underestimation of the number of traces. The upper contact to the thinly bed- ded mud is relatively sharp, but no clear signs of erosion are observed in the X-radiograph. The thinly bedded mud (324–201 cm, ∼ 780–1290 AD) roughly corresponds to the MCA interval and comprises 2–5 cm thick beds, which are burrow mottled and poorly graded, and locally inverse graded (Fig. 3b). The bioturba- tion index in this unit varies from 3 to 4 depending on the preservation of the beds (Fig. 4). In places where the bedding pattern is clearly visible, the basal parts of the beds are dark grey and appear dark in the X-radiographs, suggesting rela- tively low density. In contrast, the upper parts of the beds are greenish-grey and appear bright in the X-radiographs, point- ing to higher density and coarser grain size. The beds are mottled by abundant Planolites, rare to abundant Arenicol- ites, and rare large Planolites ichnofossils and complex bi- valve biodeformational structures (Figs. 3b and 4). The thinly bedded mud is gradationally overlain by greenish-grey mud roughly corresponding to the LIA inter- val (201–76 cm, ∼ 1290–1900 AD), with occasional 1–4 cm thick indistinctly laminated intervals. In general, this unit is characterized by a complete obliteration of the primary sed- imentary fabric (bioturbation index of 4) and the trace fos- sil assemblage comprises abundant Planolites and Arenico- lites, rare large Planolites, and complex bivalve biodefor- mational structures (Figs. 3c and 4). The bioturbation index www.biogeosciences.net/15/3975/2018/ Biogeosciences, 15, 3975–4001, 2018 3984 S. A. Jokinen et al.: A 1500-year multiproxy record of coastal hypoxia from the northern Baltic Sea 2000 1500 1000 500 0 400 300 200 100 0 2000 1500 1000 500 0 C o re d e p th ( c m ) M o W P L IA M C A Year (AD) Li th ol og y C lim at e 14 Bulk C dates (NaOH soluble) C o re d e p th ( c m ) V a rv e c o u n ti n g 137 Cs age constraints Pollution Pb age constraints 0 50 100 137 -1 Cs (Bq kg ) 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 -1 Pollution Pb (mg kg ) 206 207 Pb/ Pb Lake Koltjärnen -1 Pollution Pb (mg kg ) Lake Kassjön -1 Total Pb (mg kg ) 206 207 Pb/ Pb 20 30 40 1.4 1.35 1.30 1.25 0 10 20 30 40 206 207 Pb/ Pb 1.5 1.4 1.3 1.2 1.5 1.4 1.3 1.20 10 20 30 40 500 10 20 30 -1 Pollution Pb (mg kg ) Y e a r (A D ) Haverö Pb-5 Pb-4 Pb-3 Pb-2 Pb-1 Cs-1 0 0.4 0.8 1.2 1.6 -1 L S R ( c m a ) Year (AD) Figure 2. Simplified lithology, constructed age model, and 137Cs and Pb profiles for the sediment core HAV-KU-6. Correlation of Pb profiles with pollution Pb reconstructions from two varved lakes in Sweden (data from Brännvall et al., 1999) are also shown. For details of the indicated age constraints the reader is referred to Table 2. NaOH-extractable bulk sediment 14C dates are also indicated, although they were not used for constructing the age model due to the apparent temporal variation in the bulk sediment reservoir age. Note the reversed x axes for the 206Pb / 207Pb profiles. The error bars for the age model indicate 1σ uncertainty range. The linear sedimentation rate (LSR) profile calculated based on the age model is also shown. and the number of traces decline rapidly at 91 cm core depth (∼ 1820 AD) and remain low to the top of the unit (Fig. 4). The upper contact to the sharply laminated mud is grada- tional. The sharply laminated mud (76–0 cm) roughly corre- sponds to the MoWP and comprises rhythmically alternating light brown, black, and grey laminae. The thickness of the individual lamina successions varies from 2 to 13 mm, with a generally upward increasing trend, which partly results from decreasing compaction. These laminites correspond to the varves described by Jokinen et al. (2015). Trace fossils are virtually absent in this unit (bioturbation index of 1–2), al- though occasional blurring of the laminations is observed (Figs. 3d and 4). Black sulfide staining becomes predomi- nant from 53 cm upwards. 5.3 Geochemistry 5.3.1 Lithogenic components: trends over the entire study interval Median grain size (range 1.8–2.4 µm) and Ti /K (range 0.146–0.159) are closely coupled and show a generally de- creasing trend towards the present (Fig. 4). Superimposed on this trend the LIA stands out as an interval of de- creased values in both profiles (median grain size ∼ 1.9 µm, Ti /K∼ 0.149) in comparison to the pre-MCA, MCA, and MoWP. 5.3.2 Organic component: trends over the entire study interval The proxies for organic matter contents and composition cor- relate strongly with each other and show profiles very sim- ilar to those in Ti /K and grain size (e.g., Ti /K vs. Corg rs = 0.83, p < 0.001; Table 4). Namely, the organic prox- ies display a long-term trend (towards lower Corg content and δ13Corg, higher BIT and C /N) onto which changes dur- ing the MCA and MoWP (towards higher Corg content and δ13Corg, lower BIT and C /N) are superimposed (Fig. 5). Despite the relatively narrow ranges of values for Corg (2.9– 4.5 %), C /N (8.6–9.9), BIT index (0.18–0.29), and δ13Corg (−23.9 to−22.2 ‰), variability in these proxies is internally consistent over the last 1500 years (Fig. 5). The MAR of Corg remained between 20 and 50 g m−2 a−1 until the onset of the 20th century, after which it increased up to ∼ 100 g m−2 a−1 by the end of the century. Meanwhile, the bulk sedimentary δ15N remained fairly constant (3.1–3.5 ‰) throughout much of the study period, until a steady increase from 3.4 to 5.2 ‰ in the 1900s. Biogeosciences, 15, 3975–4001, 2018 www.biogeosciences.net/15/3975/2018/ S. A. Jokinen et al.: A 1500-year multiproxy record of coastal hypoxia from the northern Baltic Sea 3985 Table 3. Age constraints given as an input for the age model constructed using OxCal 4.2 software. Pb pollution features were obtained from Brännvall et al. (1999). The IDs are as in Fig. 2. Measured bulk sediment AMS-14C dates are given as 14C ages without any calibration or reservoir effect correction, although they were not used in the age model due to the large uncertainties in the bulk sediment reservoir effect. Dating Event ID Core depth Age Age uncertainty method (cm) (year AD) (±1σ ) Varves 3 2014 5 Varves 13 2006 5 Varves 23 1997 5 Cs-137 Chernobyl Cs-1 33 1986 2 Varves 43 1970 5 Varves 53 1951 5 Varves 63 1931 5 Varves 73 1908 5 Varves 76 1901 5 Pb pollution Pollution minimum Pb-1 140 1600 56 Pb pollution Pollution maximum Pb-2 155 1530 56 Pb pollution Pollution minimum Pb-3 179 1350 50 Pb pollution Medieval pollution peak Pb-4 219 1200 71 Pb pollution Onset of medieval pollution Pb-5 289 900 50 AMS-14C dates Radiocarbon Poz-76784 72.5 725 30 Radiocarbon Poz-76785 147.5 670 30 Radiocarbon Poz-81729 174.5 660 30 Radiocarbon Poz-81730 212.5 670 30 Radiocarbon Poz-79786 229.5 270 30 Radiocarbon Poz-81731 260.5 270 30 Radiocarbon Poz-76787 304.5 60 30 Radiocarbon Poz-81732 350.5 −15 30 Radiocarbon Poz-76788 389.5 15 30 5.3.3 Proxies for hypoxia: trends over the entire study interval Sedimentary Mo content and MAR (ranges 2–8 mg kg−1 and 0.09–1.21 µg cm−2 a−1) display negligible changes through- out the pre-MCA, MCA, and LIA intervals, but increase to maximum values during the MoWP (Fig. 4). When expressed as MAR, the recent increase in Mo continues to the present day (Fig. 6). The correlation between Mo and Corg is gen- erally weak (rs = 0.18, p= 0.04), although this improves to- wards the present (i.e., when considering the LIA and MoWP intervals only, Fig. S1 in the Supplement). Pr /Ph ratio also shows little systematic change through- out most of the record, recording values in the range of 0.4– 1.0 from the pre-MCA to the early MoWP. Coincident with the subrecent increase in Mo, Pr /Ph drops sharply to nearly zero at the depth interval corresponding to 1960 AD, be- fore recovering to 0.6 in the most recent sediments (Fig. 4). Throughout the entire record, Mo and Pr /Ph show a nega- tive correlation (rs =−0.48, p < 0.001; Fig. S1; Table 4). 5.3.4 Trends during the Modern Warm Period The recent changes observed in our proxies during the MoWP are complex and are described here in more detail. The LIA–MoWP transition (1800–1900 AD) was character- ized by low Mo and Corg contents and MARs (range during the MoWP 0.17–1.21 µg cm−2 a−1 and 22–122 g m−2 a−1, respectively) as well as low sediment MAR (range during the MoWP 0.7–2.9 kg m−2 a−1) and high Pr /Ph (Fig. 6). At 1900 AD, coinciding with the onset of continuous lamina- tions, MARs of Mo, Corg, and sediment increased contempo- raneously with a marked decrease in Pr /Ph. After this point, the strongly enhanced sediment MAR appears to partially di- lute the Mo content, particularly in the late 20th century (see also Fig. 4). Hence we focus on the MAR of Mo as a proxy for recent changes in hypoxia during the MoWP. The Mo MAR shows a steady increase throughout the MoWP, dis- playing several decadal-scale oscillations (Fig. 6). Despite declining rapidly after the onset of continuous laminations, Pr /Ph recovered to pre-MoWP values in the period 1990 AD–present (Fig. 6). The possible reasons for this observation are discussed below. Meanwhile, the proxies for organic matter composition during the MoWP are con- www.biogeosciences.net/15/3975/2018/ Biogeosciences, 15, 3975–4001, 2018 3986 S. A. Jokinen et al.: A 1500-year multiproxy record of coastal hypoxia from the northern Baltic Sea 280 275 380 375 120 125 60 55 Sharply laminated Partly laminated Thinly bedded Burrow mottling LIAMoWP MCA X-radiograph X-radiograph Interpretation X-radiograph Interpretation X-radiograph Ar Ar Pl Po Ar lPl Tr Pl Pl Pl Tr Lithology: Pl Pl Pl Pl Pl Pl Pl bbd Pl Pl Bed Bed Bed Bed Blurring of lamination Pl Tr Year (AD) 600800100012001400160018002000 (a) (b) (c) (d) Figure 3. X-radiographs of different lithological units together with their interpretations. (a) Intensely Planolites-mottled sedimentary fabric characterized by a low number of distinct traces. (b) Thinly bedded sedimentary fabric overprinted by abundant Planolites trace fossils. The lowermost bed is disturbed by bivalve biodeformation (bbd). (c) Intensely burrow-mottled sedimentary fabric with abundant Planolites (Pl) and Arenicolites (Ar) trace fossils. (d) Sharply laminated sedimentary fabric punctuated with gentle blurring of the lamination. The black holes in the X-radiographs are artefacts produced by holes in the sample boxes. Table 4. Spearman rank correlation (rs) matrix for selected variables. The number of stars denotes the degree of significance for each pair of variables: *** for very high significance (p < 0.001), ** for high significance (0.001

600 µM have been reported to prevail in the uppermost centimeters of the sediment column within the SMTZ at the study site (Karoliina Koho, University of Helsinki, personal communication, 2018). Assuming a salin- ity and temperature of 5.6 and 9 ◦C, respectively (Virtasalo et al., 2005), and a pH of 6.6 (our unpublished data) at the sediment–water interface, this total sulfide concentration cor- responds to a H2Saq concentration of > 400 µM (calculated with R package AquaEnv, Hoffmann et al., 2010), clearly exceeding the requirement of 11 µM H2Saq for the activation of the sulfide switch (Helz et al., 1996; Erickson and Helz, 2000). The annual accumulation rate of Mo is expected to be controlled by the duration of the late summer period when the SMTZ is located closest to the sediment surface (Adel- son et al., 2001; Helz and Adelson, 2013), which, in turn, is modulated by the balance between the delivery of labile OM and electron acceptors at the seafloor (Middelburg and Levin, 2009). By extension, when this period is longer in duration we expect seasonal hypoxia in the bottom water to be more intense and thus that the Mo accumulation rate provides a first-order proxy for bottom water hypoxia during the MoWP. We note that the amount of anthropogenically sourced Mo in our sediment record is likely negligible in comparison to the enrichment caused by authigenic processes. Indeed, it has been shown that modern sediment sequestration of Mo in the area shows no spatial trends but is largely controlled by bot- tom water oxygenation (Peltola et al., 2011). Furthermore, the similarity between raw Mo content and Mo /Al profiles (Fig. S3) implies that the sediment Mo content at the study site is determined by authigenic Mo sequestration. During the MoWP, the Haverö basin has undergone a pro- gressive aggravation of bottom water hypoxia, typified by two distinct regime shifts. First, a marked shoaling of the sed- iment redoxcline at 1900 AD is manifested in the contempo- raneous occurrence of continuous lamination (near-complete cessation of macrobenthic activity), a decrease in Pr /Ph, and an increase in the MAR of Mo (Fig. 6). Although the onset of the increased Mo MAR is hard to constrain due to the scarcity of age constraints prior to the preservation of continuous laminations, the appearance of subtle enrich- ments in Mo content evidences intermittent shoaling of the SMTZ and consequently intensified sequestration of Mo in the sediments since 1900 AD (Adelson et al., 2001; Helz and Adelson, 2013). Likewise, the preservation of laminated sed- imentary fabric suggests upward migration of the redoxcline towards the sediment–water interface, inhibiting burrowing by macroinfauna, whereas the occasional blurring of lami- nations (Fig. 3d) is ascribed to subtle mixing by meiofauna or transient colonization by opportunistic nectobenthos (Vir- tasalo et al., 2011b; Jokinen et al., 2015). Although annual recovery of macrofauna is common to systems prone to re- curring seasonal hypoxia (Diaz and Rosenberg, 1995; Levin et al., 2009), such rapid recolonization was effectively inhib- ited in the Haverö basin since 1900 AD, possibly owing to in- creased porewater H2S concentration (Diaz and Rosenberg, 1995). A critical threshold for defaunation in areas experi- encing seasonal hypoxia is often around 0.7 mg L−1 (Llansó, 1992; Diaz and Rosenberg, 1995), pointing to severe near- bottom water oxygen depletion at the study site already at 1900 AD. Considering the negligible variation in the proxies for the source of OM prior to 1930 AD (Fig. 6), the onset of seasonal hypoxia and the resulting preservation of contin- uous lamination since the beginning of 20th century was apparently not solely forced by potential changes in pri- mary productivity linked to human-induced eutrophication. Instead, we postulate that this deoxygenation was forced by the following complex interplay of factors: (1) increased source-to-sink ratio, combined with intensified lateral sed- iment transport especially during early winter due to the warming climate, leading to enhanced sediment focusing and higher MAR of Corg (Fig. 7; Sect. 6.2.2) – together with the prior accumulation of organic-rich brackish-water muds in the basin at least since ∼ 500 AD, this likely led to pro- gressive depletion of electron acceptors (“oxygen debt”) at the seafloor (Pamatmat, 1971); (2) decreased bottom wa- ter volume due to the gradual shoaling of the basin, result- ing in increased vulnerability to hypoxia (Caballero-Alfonso et al., 2015); (3) strengthened summer thermocline caused by global warming and gradual isolation of the basin ham- pered the replenishment of the bottom water oxygen reser- voir (Hordoir et al., 2017); (4) increased anthropogenic nu- trient loading from the catchment potentially stimulated pri- mary productivity and the delivery of OM to the seafloor (Sect. 6.2.2). Accordingly, the onset of recurring seasonal hy- poxia at around 1900 AD can be at least partly attributed to the naturally increased vulnerability to deoxygenation, which alongside global warming and direct anthropogenic forcing irreversibly tipped the ecosystem over a threshold, inducing a regime shift commonly associated with coastal oxygen de- ficiency (e.g., Conley et al., 2009b). Another marked redox shift is observed at 1950 AD, where a rapid increase in Mo MAR accompanied by a prominent decrease in Pr /Ph suggest unprecedentedly reducing con- ditions at the sediment–water interface (Fig. 6). This shift likely denotes shoaling of the redox zonation as a response to eutrophication in the area, which has been reported in pre- vious studies of the Baltic Sea (Slomp et al., 2013; Egger et al., 2015; Rooze et al., 2016; Jilbert et al., 2018) and re- flects intensified delivery of labile OM to the seafloor with respect to the supply of electron acceptors (Middelburg and Levin, 2009). Considering the decreasing trend in the local Biogeosciences, 15, 3975–4001, 2018 www.biogeosciences.net/15/3975/2018/ S. A. Jokinen et al.: A 1500-year multiproxy record of coastal hypoxia from the northern Baltic Sea 3993 summer temperatures over this interval, promoting less in- tense sediment focusing, weaker thermal stratification, and suppressed productivity, we infer that the upward migration of the SMTZ was more likely driven by the increased an- thropogenic nutrient loading from the catchment than by cli- matic factors (Fig. 6). Similar exacerbation of bottom water hypoxia around the 1950s has been reported in the coastal areas of Sweden (Persson and Jonsson, 2000; Savage et al., 2010) and in the Baltic Proper (Fig. 6; Jonsson et al., 1990; Carstensen et al., 2014a), reflecting synchronous increases in the anthropogenic nutrient loading around the Baltic Sea (Conley et al., 2009a; Savage et al., 2010; Gustafsson et al., 2012; Carstensen et al., 2014a). This deoxygenation around the 1950s conforms to the global spread of hypoxia in coastal areas (Vaquer-Sunyer and Duarte, 2008) following the “Great Acceleration” (Steffen et al., 2015). Accordingly, the steep- est gradient in the δ15N profile at our study site is reached between 1950 and 1995 AD, paralleled by intensified agri- cultural practices and fast population growth rate (Fig. 6). Although the decline in Mo content and concomitant in- crease in Pr /Ph suggest a slight improvement in the bot- tom water oxygenation since 1990 AD (Fig. 6), this is an unlikely scenario. Instead, the MAR of Mo has remained high since 1950 AD and reached the highest values of the record as late as 2005 AD, suggesting that aggravation of bottom water hypoxia has continued up to the present, as ev- idenced by the local monitoring data directly at the study site (Fig. S4). In line with this, monitoring data in other parts of the Archipelago Sea consistently demonstrate progressive deoxygenation in the area during last two decades (Fig. 8; Suomela, 2011; Caballero-Alfonso et al., 2015). Therefore, we ascribe the decrease in Mo content at 1990 AD to dilu- tion by the concurrently enhanced sediment MAR (Fig. 6). We postulate that, in addition to effective sediment focusing, the increased sediment MAR was likely fueled by ballast- ing effects, whereby eutrophication-induced increase in the OM production in the euphotic zone drives the sedimenta- tion of fine-grained lithogenic material through aggregation (Passow, 2004; Passow and De La Rocha, 2006; De La Rocha et al., 2008), as suggested by the close covariation between sediment and Corg MARs (Fig. 6). In line with this, it has been shown that rapid sedimentation events during vernal phytoplankton blooms in the study area are caused by the formation of organomineralic aggregates adhered together by sticky transparent exopolymers (TEP) excreted by phy- toplankton (Jokinen et al., 2015), coupling the sedimentation of lithogenic material to autochthonous primary production. Therefore, while Corg content remained fairly constant over the shift to enhanced sediment MAR at 1985 AD due to in- tensified sediment focusing and ballasting effects, the content of Mo was diluted as the depth and intensity of the H2S front remained relatively unaffected. The recovery in Pr /Ph at the core top (Fig. 6), which is observed despite the obvious aggravation of hypoxia until the present, suggests that the distinct minimum in Pr /Ph be- Figure 8. Contour plots for water column dissolved oxygen con- centration in August over the past decades at four intensive mon- itoring stations (data from HERTTA database) located around the study site (HAV-KU-6). The interpolations were produced with the Ocean Data View software (Schlitzer, 2017). White dots represent the original measurement data. tween 1950 (53.5 cm) and 1990 AD (29 cm) is caused by a post-depositional overprint. Although the mechanism caus- ing a diagenetic decline in Pr /Ph in this depth interval at the present day remains speculative, a likely candidate is the ex- cess Ph production by methanogenic microbes (Brassel et al., 1981; Venkatesan and Kaplan, 1987; Duan, 2000). Indeed, the current position of the SMTZ at our study site is likely lo- cated in the uppermost 10–20 cm below the sediment surface (see Sawicka and Brüchert, 2017 and Jilbert et al., 2018, for comparable systems), below which methane concentrations are expected to increase to the millimolar range. As such, Pr /Ph may not be a direct proxy for bottom water oxygena- tion at our study site. Instead, it likely records the rate of methanogenesis below the SMTZ, reflecting the amount of labile OM that escapes aerobic degradation. Importantly, the invariably high Pr /Ph prior to the 20th century (Fig. 4) could www.biogeosciences.net/15/3975/2018/ Biogeosciences, 15, 3975–4001, 2018 3994 S. A. Jokinen et al.: A 1500-year multiproxy record of coastal hypoxia from the northern Baltic Sea denote that intensive methane formation, due to increased productivity and subsequent shoaling of the redox zonation over the past few decades (Slomp et al., 2013; Egger et al., 2015; Rooze et al., 2016; Jilbert et al., 2018), is unprece- dented in our sediment record. However, the low Mo content of < 10 mg kg−1 (Fig. 6) suggests that the basin has remained non-euxinic until the present (Scott and Lyons, 2012). This is also evidenced by the slope of ∼ 2.7 for the linear regres- sion between Mo and Corg (Fig. S1), being slightly shal- lower than reported for continental margin upwelling sys- tems characterized by persistently hypoxic, but non-euxinic bottom water conditions (Algeo and Rowe, 2012, and refer- ences therein). Furthermore, the MAR of Mo has remained in less than half of the MAR reported for the seasonally mildly euxinic Chesapeake Bay (Helz and Adelson, 2013). 7 Implications Previous studies have suggested that the eutrophication of the Archipelago Sea began in the late 1960s (Bonsdorff et al., 1997a, b; Hänninen et al., 2000; Suomela, 2011). Our data show that environmental conditions around the study site in the Archipelago Sea likely deteriorated several decades prior to this and therefore also prior to the establishment of water quality monitoring campaigns in the 1960s. This highlights the use of sediment-core studies for the long-term recon- struction of environmental conditions in such settings. Our δ15N record demonstrates increased anthropogenic nutrient input already at the beginning of the 20th century (Fig. 6), although the onset of hypoxia and laminated sediment depo- sition at this time was additionally driven by physical factors (Sect. 6.3.2). However, the prominent aggravation of hypoxia in the 1950s was unequivocally anthropogenically induced. The timing of this shift predates the major establishment pe- riod of fish farms in the Archipelago Sea in 1970s (Hänninen et al., 2000), suggesting that, although aquaculture has ag- gravated hypoxia since the 1970s (Bonsdorff et al., 1997a), other human activities had significantly degraded the coastal water quality up to 20 years previously. Despite the decreased loading of sewage waters since the 1980s, following the establishment of a wastewater treat- ment plant for the city of Turku (Suomela, 2011; Fig. 6), the continued leakage of nutrients from agriculture to the Archipelago Sea (Ekholm et al., 2015) together with inten- sive P regeneration from surface sediments mainly upon the dissolution of Fe-bound P (Puttonen et al., 2014) has sus- tained the trend toward increasing eutrophication and shoal- ing of hypoxia until the present (Figs. 6 and 7). In addi- tion, the recent trajectory towards further aggravation of hy- poxia has likely been amplified by the progressively increas- ing summer temperatures (Fig. 6), which is also supported by the increased importance of climatic effects in the forcing of oxygen depletion in the Swedish coast of the Baltic Sea since the late 1970s (Savage et al., 2010). Hence, while reductions in nutrient loading appear to have improved bottom water oxygenation in the Stockholm Archipelago since the 1990s (Karlsson et al., 2010), we observe no signs of recovery in the Archipelago Sea so far. 8 Conclusions This study shows that multicentennial-scale climatic oscil- lations affect near-bottom water oxygenation of a shallow coastal basin in the northern Baltic Sea currently suffer- ing from severe seasonal hypoxia. During warm phases, in- creased export production of labile, phytoplankton-derived OM combined with effective sediment focusing to the deep- est part of the basin drives deoxygenation of the near-bottom waters in summer. Accordingly, decreased oxygen levels are observed during the MCA and MoWP, but the intensity of the MoWP hypoxia, typified by complete deterioration of the macrobenthic community, is unprecedentedly severe. The progressive deoxygenation during the 1900s was originally triggered by gradual shoaling of the basin due to glacio- isostatic uplift and basin infilling that, together with warm- ing climate and anthropogenic nutrient input, promoted the vulnerability of the basin to hypoxia and intensified OM ac- cumulation. By contrast, the marked aggravation of hypoxia in the 1950s was unequivocally attributed to the excessive an- thropogenic nutrient loading from the catchment, which sub- stantially stimulated autochthonous primary production. Our results demonstrate that the markedly more severe hypoxia during the MoWP in comparison to the MCA is not only at- tributed to the excess anthropogenic nutrient loading, but also to the gradual changes in the basin configuration that have in- creased the sensitivity to deoxygenation towards the present. Such natural changes should be considered when elucidating anthropogenic contribution to hypoxia. Furthermore, signs of eutrophication in the area are readily discernible in our sedi- ment record already in the beginning of 1900s, implying that the water quality diverged from natural conditions decades prior to the establishment of monitoring campaigns. This has important implications for the assessment of reference con- ditions for water quality in the area. Despite the recent mea- sures taken to reduce anthropogenic nutrient loading to the area, we find no evidence of recovery from hypoxia, sug- gesting that further measures are needed to alleviate oxygen depletion. Data availability. The data we have produced ourselves can now be found in Pangaea: https://doi.pangaea.de/10.1594/PANGAEA. 891284. In cases where data from a third party were used, this has been clearly indicated in the figure captions. The Supplement related to this article is available online at https://doi.org/10.5194/bg-15-3975-2018-supplement. Biogeosciences, 15, 3975–4001, 2018 www.biogeosciences.net/15/3975/2018/ S. A. Jokinen et al.: A 1500-year multiproxy record of coastal hypoxia from the northern Baltic Sea 3995 Author contributions. SJ devised the study, conducted field and laboratory work, interpreted the data, produced the figures, and drafted the paper. JV devised the study, interpreted the data, and assisted with trace fossil analysis and writing the paper. TJ assisted with laboratory work, interpreted the data, and assisted with writ- ing the paper. JK assisted with the biomarker analyses, interpreted the data, and assisted with writing the paper. OD carried out the ICP-OES and ICP-MS analyses at IOW, interpreted the data, and assisted with writing the paper. HA interpreted the data and assisted with writing paper. JH conducted fieldwork, interpreted the data, and assisted with writing the paper. LA carried out the IRMS anal- ysis and assisted with writing the paper. MC assisted with the ICP- OES analysis in Helsinki. TS conducted field and laboratory work and assisted with writing the paper. Competing interests. The authors declare that they have no conflict of interest. Acknowledgements. We acknowledge the crew on R/V Aurelia for their valuable help in sediment coring. Nadine Hollman, Anne Köhler, Arto Peltola, Jouko Saren, and Hannu Wenho are thanked for their assistance with the laboratory work. Ilppo Vuorinen is thanked for stimulating discussions on the eutrophication of the Archipelago Sea. This research was funded by the Finnish Cultural Foundation project 00150315, Maa- ja Vesitekniikan Tuki project 32719, and by the BaltRap project SAW-2017-IOW-2, which was funded by the Leibniz Association. SJ received funding from the Doctoral Programme in Biology, Geography and Geology (BGG) at the University of Turku. This study has utilized research infrastructure facilities provided by FINMARI (Finnish Marine Research Infrastructure network). 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