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Abstract Oxygen minimum zones (OMZs) play a critical role in global biogeochemical cycling and act as barriers to dispersal for marine organisms. OMZs are currently expanding and intensifying with climate change, however past distributions of OMZs are relatively unknown. Here we present evidence for widespread pelagic OMZs during the Pliocene (5.3-2.6 Ma), the most recent epoch with atmospheric CO₂analogous to modern (~400-450 ppm). The global distribution of OMZ-affiliated planktic foraminifer,Globorotaloides hexagonus, and Earth System and Species Distribution Models show that the Indian Ocean, Eastern Equatorial Pacific, eastern South Pacific, and eastern North Atlantic all supported OMZs in the Pliocene, as today. By contrast, low-oxygen waters were reduced in the North Pacific and expanded in the North Atlantic in the Pliocene. This spatially explicit perspective reveals that a warmer world can support both regionally expanded and contracted OMZs, with intermediate water circulation as a key driver. 

Abstract Oxygenation in the Eastern Equatorial Pacific is responsive to ongoing climate change in the modern ocean, although whether the region saw a deglacial change in extent or position of the Oxygen Minimum Zone remains poorly constrained. Here, stable isotopes from the shells of an Oxygen Minimum Zone-dwelling planktic foraminifer are used to reassess the position of the mid-water Oxygen Minimum Zone relative to both the thermocline and benthos. Oxygen isotopes record a rapid shoaling of the Oxygen Minimum Zone towards the thermocline associated with Heinrich Stadial 1 and persisting through the deglaciation. Meanwhile, carbon isotope similarities between Oxygen Minimum Zone-dwellingGloborotaloides hexagonusand benthicCibicidoides wuellerstorfisuggest a shared source water through the deglaciation. Results support a direct role for the Eastern Equatorial Pacific in venting carbon to the atmosphere through the deglaciation, a deglacial expansion of the Oxygen Minimum Zone, and a restructuring of mid-water oxygen and carbon dynamics from the glacial to Holocene intervals. 

Oxygen limited marine environments, such as oxygen minimum zones, are of profound importance for global nutrient cycling and vertical habitat availability. While it is understood that the extent and intensity of oxygen minimum zones are responsive to climate, the limited suite of viable proxies for low oxygen pelagic environments continues to pose a real barrier for paleoclimate interpretations. Here we investigate the proxy potential of an array of trace element (Mg, Mn, Zn, and Sr) to Ca ratios from the shells of Globorotaloides hexagonus , a planktic foraminifer endemic to tropical through temperate oxygen minimum zones. A species-specific relationship between Mg/Ca and temperature is proposed for quantitative reconstruction of oxygen minimum zone paleotemperatures. Both Mn/Ca and Zn/Ca ratios vary with oxygen concentration and could be useful for reconstructing G. hexagonus habitat where the primary signal can be d\istinguished from diagenetic overprinting. Finally, a robust correlation between Sr/Ca ratios and dissolved oxygen demonstrates a role for Sr as an indicator of oxygen minimum zone intensity, potentially via foraminiferal growth rate. The analysis of these relatively conventional trace element ratios in the shells of an oxygen minimum zone species has tremendous potential to facilitate multiproxy reconstructions from this enigmatic environment. 

Planktic foraminifera test iodine to calcium ratios represent an emerging proxy method to assess subsurface seawater oxygenation states. Several core-top studies show lower planktic foraminifera I/Ca in locations with oxygen depleted subsurface waters compared to well oxygenated environments. The reasoning behind this trend is that only the oxidized species of iodine, iodate, is incorporated in foraminiferal calcite. The I/Ca of foraminiferal calcite is thought to reflect iodate contents in seawater. To test this hypothesis, we compare planktic foraminifera I/Ca ratios, obtained from plankton tows, with published and new seawater iodate concentrations from 1) the Eastern North Pacific with extensive oxygen depletion, 2) the Benguela Current System with moderately depleted oxygen concentrations, and 3) the well oxygenated North and South Atlantic. We find the lowest I/Ca ratios (0.07 µmol/mol) in planktic foraminifera retrieved from the Eastern North Pacific, and higher values for samples (up to 0.72 µmol/mol) obtained from the Benguela Current System and North and South Atlantic. The I/Ca ratios of plankton tow foraminifera from environments with well oxygenated subsurface waters, however, are an order of magnitude lower compared to core-tops from similarly well-oxygenated regions. This would suggest that planktic foraminifera gain iodine post-mortem, either when sinking through the water column, or during burial. 

Abstract. Oxygen-depleted regions of the global ocean are rapidly expanding, withimportant implications for global biogeochemical cycles. However, ourability to make projections about the future of oxygen in the ocean islimited by a lack of empirical data with which to test and constrain thebehavior of global climatic and oceanographic models. We usedepth-stratified plankton tows to demonstrate that some species of plankticforaminifera are adapted to life in the heart of the pelagic oxygen minimumzone (OMZ). In particular, we identify two species, Globorotaloides hexagonus and Hastigerina parapelagica, living within theeastern tropical North Pacific OMZ. The tests of the former are preserved inmarine sediments and could be used to trace the extent and intensity oflow-oxygen pelagic habitats in the fossil record. Additional morphometricanalyses of G. hexagonus show that tests found in the lowest oxygen environments arelarger, more porous, less dense, and have more chambers in the final whorl.The association of this species with the OMZ and the apparent plasticity ofits test in response to ambient oxygenation invites the use of G. hexagonus tests insediment cores as potential proxies for both the presence and intensity ofoverlying OMZs.