By Avery Duncan

Faculty Mentor: Pamela Grothe

Abstract

The shift from the mid-to-late Holocene 4 thousand years ago (4 kyBP) is marked by major hydroclimate anomalies and megadroughts leading to ancient civilization collapses. Currently, the cause of this major climate disruption is unclear, although more evidence supports an underlying role of the tropical Pacific, some suggesting it is due to an increase in the El Nino Southern Oscillation (ENSO) variability around 3.8 kyBP. I propose to utilize the geochemistry from a network of well-dated fossil corals from the central tropical Pacific that fall in the 3.8-4.2 cal yr BP window to better understand changes in ENSO, sea surface temperature (SST) and sea surface salinity (SSS) across the 4.0 kyBP abrupt climate transition. Specifically, I will analyze the oxygen isotopes (d18O) and trace elements in the coral skeleton to measure ENSO variability and to create paired SST/SSS records. To detect a statistical change in ENSO during this window, we need ~20 coral records of 8-20 years long in each 200-yr bin (3.8-4.0 and 4.0-4.2 kyBP). We currently have ~75 samples radiometrically dated around this time interval, increasing our chances for detecting a measurable change in ENSO statistics. Each sample will slabbed along the main growth axis of the coral, X-rayed for growth banding, and screened for secondary alteration using the scanning electron microscope (SEM) and X-ray diffraction (XRD). Then, the corals will be analyzed for d18O (SST/SSS signal) and a suite trace elements (Li, Mg, Sr, Ca, U) (SST-independent signal) every 1-mm down the major growth axis to create a monthly resolution record of SST/SSS and ENSO. I will compare these results with published records of ENSO and climate at this time to better understand how the central tropical Pacific and ENSO variability underlines the major hydroclimate anomalies at 4.0 kyBP. This work is critical for better understanding the global climate impacts driven by the tropical Pacific, especially in response to ENSO variability.