Hypoxia in the Stockholm Archipelago during the last three millennia: natural factors versus human influence
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Over the last decades, the coastal areas of the Baltic Sea have been experiencing severe hypoxia, which is related to anthropogenic eutrophication and climate change. In the past three millennia, two other periods of hypoxia in the Baltic Sea have been recognized, coinciding with the Holocene Thermal Maximum and the Medieval Climate Anomaly (MCA). The causes for hypoxia during these periods are still debated, although higher temperatures and stratification seem to have played a role. In order to get a better understanding of the factors that played a role in the development and recovery of hypoxia in the last 3 millennia, the geochemical composition of sediment cores obtained from three different sites in the Stockholm Archipelago, Baggensfjärden, Ingaröfjärden and Erstaviken, was analyzed. Geochemical profiles of redox sensitive elements, total organic carbon (TOC), phosphorus and trace metals were generated and Glycerol Dialkyl Glycerol Tetraether (GDGT) lipids were extracted to reconstruct sea surface temperatures (SSTs) using the TEX86 proxy. Afterwards a combined 210Pb and 14C age model was used to translate depth into calendar years. The generated geochemical data were compared to historical data on population growth and mining activities in the area surrounding the Stockholm Archipelago. The geochemical profiles of redox sensitive elements and TOC confirm the occurrence of hypoxia before and during the MCA while a recovery of oxygen conditions is seen during the Little Ice Age (LIA), a period of cooler climate in northern Europe. TEX86-based SST reconstructions reveal a temperature trend that correlates with the expected temperature change during the MCA and LIA and confirm the change of climate in the transition from the MCA to LIA. This indicates the coupling of temperature and hypoxia. Profiles of copper, lead and zinc, indicate human activity in the form of mining from 1000 AD, which corresponds to the historical data on mining activity in the area surrounding the Stockholm Archipelago. As hypoxia was already present before the onset of the MCA and the estimated start of mining activities, the development of hypoxia before and during the MCA was most likely to a large extent driven by natural factors such as changes in temperature and nutrient availability in the water.