Holocene Refreshening and Reoxygenation of a Bothnian Sea Estuary Led to Enhanced Phosphorus Burial

Abstract

Salinity variations in restricted basins like the Baltic Sea can alter their vulnerability to hypoxia (i.e., bottom water oxygen concentrations <2 mg/l) and can affect the burial of phosphorus (P), a key nutrient for marine organisms. We combine porewater and solid-phase geochemistry, microanalysis of sieved sediments (including XRD and synchrotron-based XAS), and foraminiferal δ 18 O and δ 13 C analyses to reconstruct the bottom water salinity, redox conditions, and P burial in the Ångermanälven estuary, Bothnian Sea. Our sediment records were retrieved during the Integrated Ocean Drilling Program (IODP) Baltic Sea Paleoenvironment Expedition 347 in 2013. We demonstrate that bottom waters in the Ångermanälven estuary became anoxic upon the intrusion of seawater in the early Holocene, like in the central Bothnian Sea. The subsequent refreshening and reoxygenation, which was caused by gradual isostatic uplift, promoted P burial in the sediment in the form of Mn-rich vivianite. Vivianite authigenesis in the surface sediments of the more isolated part of the estuary ultimately ceased, likely due to continued refreshening and an associated decline in productivity and P supply to the sediment. The observed shifts in environmental conditions also created conditions for postdepositional formation of authigenic vivianite, and possibly apatite formation, at ∼8 m composite depth. These salinityrelated changes in redox conditions and P burial are highly relevant in light of current climate change. The results specifically highlight that increased freshwater input linked to global warming may enhance coastal P retention, thereby contributing to oligotrophication in both coastal and adjacent open waters.

Post-depositional formation of vivianite-type minerals alters sediment phosphorus records

“…Vivianite can also form post-depositionally at lake-marine transitions, where upward-diffusing porewater Fe 2+ from lake deposits meets PO 4 from the overlying organic-rich marine deposits, as recently confirmed by microspectroscopy in sediments from the Landsort Deep (Baltic Sea; Dijkstra et al, 2016). Postdepositional vivianite authigenesis is also assumed to have caused a distinct P maximum (∼ 60 µmol g −1 ) at the lakemarine transition in coastal Bothnian Sea sediments (Dijkstra et al, 2017a). The occurrence of vivianite in sediments can thus not always be ascribed to the environmental conditions in the surface sediments and overlying waters upon deposition.…”

“…Vivianite is also likely present in Ancylus Lake sediments in the coastal Bothnian Sea (Dijkstra et al, 2017a) and may explain the enrichments in CDB-extractable P (Fe-bound P) in lake sediments from the Archipelago Sea in the northern Baltic Sea (Virtasalo and Kotilainen, 2008) and the large centimeter-scale blue aggregates observed in Ancylus Lake sediments in the Little Figure 14. Simplified schematic of methane (CH 4 ), sulfur (S), iron (Fe) and phosphorus (P) diagenesis in Baltic Sea (Bornholm Basin) and Black Sea sediments (see Sect.…”

Holocene environmental history of the Ångermanälven Estuary, northern Baltic Sea

“…Pollen was used to augment the age model generated by Dijkstra et al . (). A particularly important stratigraphical signal in the northern Baltic region is the invasion of Picea (Seppä et al .…”

“…A radiocarbon‐based age model is available for the upper 6 m of Site M0061 (Dijkstra et al . ). Unfortunately insufficient material for dating was found beneath 6 m at Site M0061 and no datable material was recovered at Site M0062 (Dijkstra et al .…”

“…Unfortunately insufficient material for dating was found beneath 6 m at Site M0061 and no datable material was recovered at Site M0062 (Dijkstra et al . ).…”