Hypoxia in the Stockholm Archipelago over the Past 3 ka: A Natural Phenomenon or Human-Induced?

Excessive anthropogenic nutrient input and global warming
have led to a rapid expansion of hypoxia in the Baltic Sea
over the past century. Two earlier phases of widespread
hypoxia, coinciding with the Holocene Thermal Maximum
(HTM, ~8-4 ka before present; BP) and the Medieval Climate
Anomaly (MCA, ~1.2-0.8 ka BP), have previously been
identified in deeper parts of the Baltic Sea. While the warm
climate is thought to be a key driver of the hypoxia during the
HTM and MCA, anthropogenic nutrient input also may have
played a role. Relatively little is known about the bottom
water redox conditions in the coastal zone of the Baltic Sea
during the Holocene.
Here, we reconstruct Holocene bottom water redox
conditions and identify the key drivers of deoxygenation at
three sites in the coastal zone near Stockholm (Stockholm
Archipelago) based on sediment records. Sediment
concentrations of organic carbon and redox sensitive
elements (e.g. molybdenum) show that bottom waters in our
study area were hypoxic until ~1 ka BP, followed by a period
of oxygenation which lasted until the development of
modern-day hypoxia at the beginning of the 20th century.
TEX86-based sea surface temperature reconstructions
highlight that periods of deoxygenation and warming
coincide, suggesting a mechanistic link. Increased
anthropogenic input of Pb and Zn to the sediment is first
observed at ~1 ka during the period of oxygenation. We
conclude that, before the modern period, hypoxia was not
human-driven and that changes in temperature, combined
with isostatic rebound, were critical in determining trends in
bottom water oxygen.