Benthic Foraminiferal Mn/Ca as Low-Oxygen Proxy in Fjord Sediments

Fjord systems are typically affected by low-oxygen conditions, which are increasing in extent and severity, forced by ongoing global changes. Fjord sedimentary records can provide high temporal resolution archives to aid our understanding of the underlying mechanisms and impacts of current deoxygenation. However, such archives can only be interpreted with well-calibrated proxies. Bottom-water oxygen conditions determine redox regime and availability of redox-sensitive trace elements such as manganese, which in turn may be recorded by manganese-to-calcium ratios (Mn/Ca) in biogenic calcium carbonates (e.g., benthic foraminifera tests). However, biological influences on Mn incorporation (e.g., species-specific Mn fractionation, ontogeny, living and calcification depths) are still poorly constrained. We analyzed Mn/Ca of living benthic foraminifera (Bulimina marginata, Nonionellina labradorica), sampled at low- to well-oxygenated conditions over a seasonal gradient in Gullmar Fjord, Swedish West coast (71-217 mu mol/L oxygen (O-2)), by laser-ablation ICP-MS. High pore-water Mn availability in the fjord supported Mn incorporation by foraminifera. B. marginata recorded contrasting Mn redox regimes sensitively and demonstrated potential as proxy for low-oxygen conditions. Synchrotron-based scanning X-ray fluorescence nanoimaging of Mn distributions across B. marginata tests displayed Mn/Ca shifts by chambers, reflecting bottom-water oxygenation history and/or ontogeny-driven life strategy preferences. In contrast, Mn/Ca signals of N. labradorica were extremely high and insensitive to environmental variability. We explore potential biologically controlled mechanisms that could potentially explain this species-specific response. Our data suggest that with the selection of sensitive candidate species, the Mn/Ca proxy has potential to be further developed for quantitative oxygen reconstructions in the low-oxygen range. Plain Language Summary One of the impacts of global change on fjords and coastal zones is the decline of oxygen levels in seawater ("deoxygenation"), with serious environmental consequences. Important insights into the processes and consequences of deoxygenation can be gained through indicator records of past oxygen variability in seawater. An emerging indicator is the trace element manganese (Mn) due to its reduction and oxidation chemistry. We investigated the effects of low- to well-oxygenated seawater on Mn concentrations, measured as manganese-to-calcium ratio (Mn/Ca) in shells of marine micro-organisms (i.e., foraminifera), in a seasonal field-study of two sites in GF, Swedish west coast. We found that foraminiferal Mn/Ca varied with oxygen and Mn levels in the seawater. However, also dependent on biological factors. Of the two investigated candidate species (Bulimina marginata and Nonionellina labradorica), one showed promise as indicator of low-oxygen conditions. Our research shows that the foraminiferal Mn/Ca indicator can be used to better understand changes in oxygen levels in coastal environments.