Targeted equatorial Pacific foraminifera-bound N isotope measurements: implications for a rare record of nutrient dynamics and the El Nino-Southern Oscillation

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The El Niño-Southern Oscillation (ENSO) is the largest source of global climate variability and has a profound influence on surface temperatures and precipitation around the world. This project seeks to test preliminary results that suggest that ENSO did not respond to past changes in greenhouse gas concentrations during the last geological period (the Pliocene) when carbon dioxide levels are thought to have been similar to today's. These findings have important implications for future ENSO behavior and, consequently, this project seeks to thoroughly scrutinize the preliminary results with additional high quality measurements. The research involves training and mentoring of University of California, Irvine undergraduate students in fundamental laboratory skills and scientific communication, and supports an early career scientist with no prior NSF support.This research seeks to understand how ENSO responds to changes in greenhouse gas forcing by examining ENSO variability during the Pliocene, the last time pCO2 levels are thought to have been similar to modern. Modern equatorial Pacific nitrate consumption variability has been linked to ENSO and, based on this relationship, a 5 million year long record of nitrate consumption was constructed using nitrogen isotope signature of bulk sediments. These results suggest that long-term equatorial Pacific nitrate consumption variability did not respond to changes in greenhouse gas concentrations over the past 5 million years, and imply that long-term ENSO variability is also insensitive to greenhouse gas radiative forcing. The funded research seeks to determine the fidelity of these bulk sediment results, which can be overprinted by diagenetic effects, by measuring the nitrogen isotope signature of planktic foraminifera from the same sediment samples. These "foraminifer-bound" signatures should be less subject to post-depositional alteration and thus a more robust proxy for past nitrate utilization. These results will address two questions: (1) what is the fidelity of equatorial Pacific bulk sediment nitrogen isotopes? (2) how did equatorial Pacific nitrate consumption evolve over the past 5 million years?