By Jessica Oberlies

Faculty Mentor: Pamela Grothe

Abstract

The central tropical Pacific is a key region for studying climate change because it modulates global climate variability, such as through the El Niño Southern Oscillation (ENSO). However, there is a lack of climate records prior to the mid-1950’s from this region, making it difficult to quantify the amount of warming and determine the human impact on this region compared to the changes arising due to natural variability from ENSO, a phenomenon that is complex and complicates reliable projections of future climate impacts. The geochemistry from fossil coral skeletal material can be used as a proxy to extend sea surface temperature records into the pre-industrial era, and this research is part of a larger project between multiple universities focused on reconstructing a continuous record of the sea surface temperature (SST) over the last 200 years in the tropical Pacific. Currently, corals from a single genus, known as Porites, are used for these climate reconstructions. Porites corals have been a critical archive of past climate change owing to their fast growth rate, regular concentric banding, and dense skeletal features, but this genus is not regularly distributed through the fossil record nor across the tropical Pacific, creating a pressing need to expand the types of corals used in this region for paleoclimate reconstructions. Here, we test two modern larger skeletal architectural corals – Favia species and Hydnophora microconos from Kiritimati Island (1.8°N, 157.4°W) as reliable recorders of sea surface temperature and salinity. Oxygen isotope ratios (δ18O) reflect SST and SSS at the time the coral was growing. We compare δ18O timeseries for Favia and Hydnophora microconos individually, as well as together with instrumental SST records and published modern Porites records from Kiritimati Island. In Favia, we find that δ18O sampled from the thecal wall versus the septa reproduces SST more accurately. Applying this sampling method to other modern Favia and Hydnophora microconos, we find consistent variability and small δ18O offsets, suggesting they may be an untapped climate archive that could be supplemented with Porites records to create more robust climate reconstructions to better constrain tropical Pacific climate variability and warming through the pre-industrial era.