By Jessica Oberlies

Faculty Mentor: Dr. Pamela Grothe

Abstract

The central tropical Pacific is a key region in modulating global climate changes, yet the lack of continuous long-term instrumental climate records from this area inhibits the robust evaluation of future climate impacts. In particular, the sensitivity of the El Niño Southern Oscillation (ENSO) to a warming climate is complex and remains difficult to predict. Current climate models here rely on records from Porites corals, which are an excellent climate recorder due to their fast growth rate, concentric banding, and dense skeletal features. However, the uneven distribution of Porites across location and time in the fossil record prevents a nuanced understanding of the impact that anthropogenic perturbations are having on the tropical Pacific. This creates an urgent need to expand the variety of records from this region for paleoclimate reconstructions. Here, we test two corals – Favia species and Hydnophora microconos (HM) – from Kiritimati Island (1.8°N, 157.4°W) as paleoclimate recorders of both sea surface temperature (SST) and sea surface salinity (SSS). These species grow slowly compared to Porites and have more open skeletal architecture. Oxygen isotope ratios (δ18O) and strontium calcium ratios (Sr/Ca) recorded in the coral skeleton reflect SST (Sr/Ca) or both SST and SSS (δ18O) at the time the coral was growing. Pairing both measures provides parallel SST and SSS records. We compare both δ18O and Sr/Ca timeseries within a single coral as well as across multiple corals with both instrumental SST records and published modern Porites records from Kiritimati Island. In Favia, we find consistent differences between δ 18O profiles sampled along the thecal versus septa walls and show that δ18O in the thecal wall is more reproducible, and more coherent with SST. Mirroring this approach, we sample along the densest skeletal features in additional modern Favia and HM corals and find consistent variability and small δ18O offsets, suggesting that they may be an untapped climate archive. Sr/Ca records for HM are less consistent, and further evaluation is needed to determine if Sr/Ca records are reliable for this species. Fossil samples of these new corals could be a way to expand the existing archive and reconstruct more robust climate records, increasing the accuracy of future climate predictions in the tropical Pacific.