Holocene Spatiotemporal Redox Variations in the Southern Baltic Sea

Abstract

Low oxygen conditions in the modern Baltic Sea are exacerbated by human activities; however, anoxic conditions also prevailed naturally over the Holocene. Few studies have characterized the specific paleoredox conditions (manganous, ferruginous, euxinic) and their frequency in southern Baltic sub-basins during these ancient events. Here, we apply a suite of isotope systems (Fe, Mo, S) and associated elemental proxies (e.g., Fe speciation, Mn) to specifically define water column redox regimes through the Baltic Holocene in a sill-proximal to sill-distal transect (Lille Belt, Bornholm Basin, Landsort Deep) using samples collected during the Integrated Ocean Drilling Program Expedition 347. At the sill-proximal Lille Belt, there is evidence for anoxic manganous/ferruginous conditions for most of the cored interval following the transition from the Ancylus Lake to Littorina Sea but with no clear excursion to more reducing or euxinic conditions associated with the Holocene Thermal Maximum (HTM) or Medieval Climate Anomaly (MCA) events. At the sill-distal southern sub-basin, Bornholm Basin, a combination of Fe speciation, pore water Fe, and solid phase Mo concentration and isotope data point to manganous/ferruginous conditions during the Ancylus Lake-to-Littorina Sea transition and HTM but with only brief excursions to intermittently or weakly euxinic conditions during this interval. At the western Baltic Proper sub-basin, Landsort Deep, new Fe and S isotope data bolster previous Mo isotope records and Fe speciation evidence for two distinct anoxic periods but also suggest that sulfide accumulation beyond transient levels was largely restricted to the sediment-water interface. Ultimately, the combined data from all three locations indicate that Fe enrichments typically indicative of euxinia may be best explained by Fe deposition as oxides following events likely analogous to the periodic incursions of oxygenated North Sea waters observed today, with subsequent pyrite formation in sulfidic pore waters. Additionally, the Mo isotope data from multiple Baltic Sea southern basins argue against restricted and widespread euxinic conditions, as has been demonstrated in the Baltic Proper and Bothnian Sea during the HTM or MCA. Instead, similar to today, each past Baltic anoxic event is characterized by redox conditions that become progressively more reducing with increasing distance from the sill.

Mesoproterozoic surface oxygenation accompanied major sedimentary manganese deposition at 1.4 and 1.1 Ga

“…This cycle of Mn deposition and basin oxygenation was then repeated, forming the three observed Mn Zones (plus those zones present in the overlying weathered section). This redox cyclicity has been observed in the Baltic sea from the Holocene through to the present day where spatiotemporal variations in redox conditions in different positions and depths of the basin have led to enrichments in Mn and Fe (Hardisty et al, 2016 , 2021 ; Scholz et al, 2013 , 2018 ).…”

The depositional redox conditions of Fe‐speciation reference materials (BHW and WHIT) using redox‐sensitive trace metal enrichment

“…Trace metal enrichments [(X/Al) sample , where X = V, Mo and U] versus Fe py /Fe HR for BHW and WHIT samples (Alcott et al, 2020) and Baltic Sea (Hardisty et al, 2021), Black Sea (Canfield et al, 1996) and Cariaco Basin (Lyons et al, 2003; Piper & Dean, 2002; data in supplementary material Table S1). …”

“…It is also known that DOP can be used as a redox indicator if there is clear evidence of an anoxic water column (Poulton & Canfield, 2011). For this reason, when DOP is used alone to characterize the depositional redox condition of BHW, it gets misclassified F I G U R E 1 Trace metal enrichments [(X/Al) sample , where X = V, Mo and U] versus Fe HR /Fe T for BHW and WHIT samples (Alcott et al, 2020) and Baltic Sea (Hardisty et al, 2021), Black Sea (Canfield et al, 1996) and Cariaco Basin (Lyons et al, 2003;Piper & Dean, 2002; data in supplementary material Table S1).…”

“…Iron geochemistry and bio‐essential trace metals are widely used for studying the redox conditions in ancient marine environments (Anderson & Raiswell, 2004; Hardisty et al, 2021; Poulton & Canfield, 2005; Raiswell & Canfield, 1998; Tessier et al, 1979). This is because the trace metal availability exerts a significant control over phytoplankton productivity (Martin & Fitzwater, 1988), and the cell's trace element inventory is directly related to the conditions under which the host organism thrives (Da Silva & Williams, 2001).…”

Reviews and syntheses: Review of proxies for low-oxygen paleoceanographic reconstructions

“…In conclusion, sedimentary Mo isotopes can be used to trace the local euxinic water-column conditions when the global seawater Mo isotopic composition is known/close to modern (e.g., Holocene/Pleistocene to Paleocene sediments, Azrieli-Tal et al, 2014Matthews et al, 2017, Sweere et al, 2021, Riedinger et al, 2021.…”