Coastal oxygen changes and element cycling: foraminifera as benthic monitors

The decline of ocean oxygen concentrations is an environmental issue of increasing concern. Detrimental changes have been observed in the last century, most prevalently in coastal marine environments. Drivers include warming, increased water stratification, higher biological oxygen demands, and other anthropogenic influences such as eutrophication. Current trends are predicted to continue in coming decades to centuries, but model uncertainties in future outcomes and severity persist. Long-term records of marine oxygen dynamics are crucial for improving climate models, contextualizing modern deoxygenation trends and understanding underlying mechanisms. Efforts to expand the toolbox of proxies capable of resolving low-oxygen states in the geological past are ongoing. Redox sensitive trace elements (e.g., manganese, iodine or uranium) in biogenic calcium-carbonates are among the emerging proxies. I will explore aspects of coastal oxygen dynamics and elemental cycling relevant for the calibration of trace element proxies on the example of manganese-to-calcium ratios (Mn/Ca) in benthic foraminifera. I showcase studies demonstrating (1) the potential of foraminiferal Mn/Ca as high-resolution archives of oxygen changes, and its current limitations, (2) the use of micro-analytical techniques such as laser-ablation ICP MS analyses and µXRF imaging in calibration approaches, and (3) the influence of biological factors on oxygen-proxy relationships. In conclusion, understanding the complexity of geochemical cycling in the light of oxygen thresholds, environmental settings and biological controls is critical for developing robust trace element proxies. Continued efforts of refining the manganese and other trace element proxies offer promising avenues towards quantitative reconstructions of bottom-water oxygen concentrations in the low oxygen-range, and a better understanding of past and present states of the marine environment.