ENSO Variability and Mean State during the Late Holocene and Last Glacial Maximum Based on Mg/Ca of Individual Foraminifera

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Over large parts of the globe including the United States, El Niño-Southern Oscillation (ENSO) is a leading cause of climate variations from one year to the next. Regional impacts can be catastrophic, including drought, wildfires, flooding, mudslides, and fisheries collapse. Furthermore, ENSO has a significant influence on global mean air temperature, and a strong El Niño event contributed to the record warmth experienced during 2015-2016. Despite the clear importance of ENSO to modern climate variability, the computer models used to make climate projections offer no clear consensus on ENSO's future behavior. The various computer models can be improved, with broad benefit to society, by evaluating their ability to simulate past time periods when ENSO behaved differently. However, that approach requires more and better paleo-ENSO observations from geologic archives like ocean sediment cores. This project aims to ground-truth a new method to reconstruct ENSO variability during the geologic past. It relies on the fact that the chemistry of microscopic seashells (foraminifera) reflects the temperature at which the shell formed. By measuring the chemistry of numerous individual shells, researchers will be able to reconstruct the variability of sea surface temperature in regions where temperature is dominated by the ENSO phenomenon. This project will also provide extensive learning and training experiences for a female PhD student and two undergraduates.More specifically, the work proposed here will develop a new proxy for sea surface temperature variability, "wet chemistry" Mg/Ca ratios in individual planktic foraminifera, for probing ENSO variance during the past. Preliminary measurements and Monte Carlo simulations show that individual foraminifera from well-preserved sediments are reliable recorders of calcification temperature. Past changes in ENSO variance should therefore leave imprints on the distributions of individuals' Mg/Ca at carefully chosen core sites. In the tropical Pacific however, seafloor dissolution of CaCO3 and low sedimentation rates present dual challenges to this method, so it will be important to ground-truth the approach using late Holocene sediments. This work will use single-specimen Mg/Ca in two species of planktic foraminifera to test two main hypotheses: (1) The observed lowering of planktic foraminiferal Mg/Ca in partially dissolved samples is caused by a percentage Mg loss from each shell, and not by a molar amount lost, nor by breakup and loss of higher-Mg individuals formed in warmer waters; (2) Regardless of the exact nature of the dissolution artifact, information about temperature variability (including ENSO) is retained in single-specimen Mg/Ca distributions from tropical Pacific core top sediments above the lysocline. Hypothesis 1 will be tested using four core tops from a depth transect on Ontong Java Plateau in the western tropical Pacific, and Hypothesis 2 will be tested using 10 additional core tops spanning a range of annual and interannual temperature variability regimes across the tropical Pacific.