The Present-Day Relation Between Observed Bottom Water Oxygenation and Marine Oxygen Proxies

Abstract

This study investigates how marine sediment proxies reflect dissolved oxygen levels in fjord basin water to improve our understanding of coastal marine deoxygenation that has been associated with ongoing climate change. We use western Norwegian fjords to test how marine sediment proxies for dissolved oxygen perform. Such knowledge is needed to evaluate if the recent reduced renewal of fjord basin waters is driven by anthropogenic impact or is within the range of natural variability. We have explored the potential and limitations of Mn/Ca, δ¹³C, benthic foraminiferal diversity, the calcareous and agglutinated foraminifera ratio, and the benthic foraminifera indicator species Stainforthia fusiformis as potential proxies for bottom water oxygen concentrations. Within the fjords investigated in western Norway, seven stations are well-oxygenated, one is close to hypoxic oxygen levels, and one shows nearly anoxic conditions. The relative abundance of S. fusiformis shows the strongest potential as a tool for reconstructing past oxygen concentrations. The Mn/Ca and δ¹³C measured in the seawater also strongly correlate with dissolved oxygen. The foraminiferal diversity indices follow the oxygenation gradient in the fjords. The calcareous/agglutinated species ratio has limitations under anoxic conditions, where an almost monospecific assemblage dominated by calcareous S. fusiformis was found. The signal of Mn/Ca and δ¹³C measured in shells of species Hyalinea balthica was difficult to assess due to low species abundance at some fjord sites. Hence, further analyses are necessary to employ the Mn/Ca and δ¹³C in foraminiferal shells as proxies for reconstructing past oxygen concentrations in fjords.