Proxy Potential of Trace Element/Calcium Ratios in Planktic Oxygen Minimum Zone Foraminifer Globorotaloides hexagonus

The conditions within the low-oxygen, low-pH regions known as Oxygen Minimum Zones (OMZ) play a key role in global climate and marine nutrient cycles. Understanding how these environments have changed in the past is required for elucidating the long-term drivers of OMZ migration, expansion, and contraction. However, the scarcity of proxies for conditions in deep pelagic OMZs is frequently limiting. Here we present multiple trace element to calcium ratios (TE/Ca) from shells of the foraminifer, Globorotaloides hexagonus, recovered live from discrete depth (MOCNESS) tows through the upper oxycline and OMZ in the Eastern Tropical North Pacific (21°N, 118°W). We compare TE/Ca in foraminifera shells with temperature (Mg/Ca), salinity (Na/Ca), and oxygenation and carbonate chemistry (Sr/Ca, Mn/Ca, Zn/Ca) at the time of capture using both solution (pooled) and laser ablation ICP-MS approaches. We present a species-specific Mg/Ca:temperature relationship suitable for reconstructing pelagic temperatures at OMZ depths. Low oxygen is correlated with low Sr/Ca, high Mn/Ca, and high Zn/Ca. We hypothesize that the trends in Mn/Ca and Zn/Ca result from decreasing oxygen and pH deeper within the OMZ. The strong correlation with Sr/Ca is likely mediated by the response of shell growth rate to low oxygen. Results suggest that all three elements, especially Sr/Ca, have potential for reconstructing the intensity of paleo-OMZs.