Evolving coastal character of a Baltic Sea inlet during the Holocene shoreline regression: impact on coastal zone hypoxia

Although bottom water hypoxia (O2 < 2 mg L−1) is presently widespread in the Baltic Sea coastal zone, there is a lack of insight into past changes in bottom water oxygen in these areas on timescales of millennia, and the possible driving factors. Here, we present a sediment-based environmental reconstruction of surface water productivity, salinity and bottom water oxygen for the past 5400 years at Gåsfjärden, a coastal site in SE Sweden. As proxies, we use dinoflagellate cysts, benthic foraminifera, organic carbon (Corg), biogenic silica (BSi), Corg/Ntot, Corg/Ptot, Ti/Al, K/Al and grain size distribution. The chronology of the sediment sequence is well constrained, based on 210Pb, 137Cs and AMS 14C dates. Between 3400 and 2100 BCE, isostatic conditions favored enhanced deep water exchange between Gåsfjärden and the open Baltic Sea. At that time, Gåsfjärden was characterized by relatively high productivity and salinity, as well as frequently occurring hypoxic-anoxic bottom water, despite the relatively large connection with the Baltic Sea. The most severe interval of oxygen depletion is recorded between 2400 and 2100 BCE, and appears to coincide with a similar hypoxic event in the Gotland Basin in the open Baltic Sea. As regional climate became wetter and colder between 2100 BCE and 700 BCE, salinity declined and bottom water oxygen conditions improved. Throughout the record, grain size, Ti/Al and K/Al data indicate an evolution towards a more enclosed coastal system, as suggested by reconstructions of the post-glacial shoreline regression. Gåsfjärden shifted to close to modern conditions after 700 BCE, and was characterized by less hypoxia and lower salinity compared with 3400–700 BCE. The timing of the shift corresponds with the Sub-Boreal/Sub-Atlantic transition in Europe. Human-induced erosion in the catchment is observed as early as 600 CE, and is particularly prominent since regional copper mining activity increased around 1700 CE. A sharp increase in sediment Corg concentration is recorded since the 1950s, indicating significant anthropogenic impact on biogeochemical cycles in the coastal zone, as observed elsewhere in the Baltic Sea.