Award: OCE-2154081

The open ocean contains regions of very low oxygen below the surface known as Oxygen Minimum Zones or OMZs. Over the past decades of ocean observation, oxygen in the ocean has decreased and OMZs have expanded. Understanding whether these trends are likely to continue and on what timescales is a pressing question. One limitation to predicting how OMZs may change in the future is a lack of understanding of how these regions have behaved under past climate states. The goal of this project was to develop and apply new approaches to reconstructing open ocean OMZs based on the shells of planktic foraminifera, single celled marine organisms with an extensive fossil record. Establishing planktic foraminifera shells as recorders of the OMZ This project targeted a species of fossilizing plankton, Globigerinoides hexagonus, as a useful tool for reconstructing past OMZs. The affinity of this species for an OMZ habitat was demonstrated as were ways in which the shape of their shell reflected oxygen in their environment (Davis et al., 2021, Biogeosciences). Following this, relationships have been found between multiple trace element to calcium ratios in the shells and oxygen and temperature at collection (Davis et al., 2023, Frontiers in Marine Science). Together, this presents multiple ways in which the presence, and potentially morphology and chemistry, of G. hexagonus shells may be used to reconstruct oxygen in past oceans. Reconstructing past OMZs The tools developed in modern organisms were then applied to produce new insights into past OMZs. The fossil record of G. hexagonus was used to reconstruct OMZ extent during the last period with CO2 comparable to modern, the Pliocene. The distribution of G. hexagonus, supported by Earth System modeling, showed that many regions that host an OMZ today were also found to have little oxygen in the Pliocene. However, we found a smaller OMZ in the North Pacific and expanded OMZs in the North Atlantic compared to modern day. This demonstrates that a warmer climate can have alternate arrangements of low OMZs (Davis et al., 2023, Nature Communications). The vertical extent of the Equatorial Pacific OMZ was then reconstructed during the last period of rapid warming (the last deglaciation) using the oxygen and carbon isotopes of G. hexagonus. A rapid shoaling of the OMZ was found coincident with the last deglaciation as was a vertical restructuring of open ocean low-oxygen from the last glacial period into the Holocene (Davis, 2022, Communications Earth and Environment). Support for Early Career Researchers An additional outcome of this project was the initiation of the Pal(a)eoPERCS webinar series, a weekly seminar highlighting the research of Early Career Researchers across all subdisciplines of "paleo" sciences. As of this project’s conclusion, 132 speakers have shared their research through this venue with ~1750 people currently opting in to receive email updates and links to upcoming seminars. This project has also supported 2 postdoctoral researchers, 1 MS thesis, and 2 undergraduate researchers. Last Modified: 07/31/2023 Submitted by: Catherine V Davis