Favia Corals: a New Paleoclimate Archive

Abstract:

Projections of future climate change contain large uncertainties stemming from our inability to confirm long-term trends in climate models with short instrumental records. Fossil corals are an important archive of past climate changes in the tropical oceans as oxygen isotopic ratios (δ¹⁸O) in their skeletons reflect ambient ocean temperature and salinity during the time they grew. In particular, El Niño/Southern Oscillation (ENSO) is a natural phenomenon with a complex array of sensitivities to climate change, rendering any future projections of its variability highly uncertain. The long-term behavior of ENSO and lower-frequency Pacific climate variability is recorded by annually banded fossil corals, providing insight into both natural and anthropogenic climate changes in this under-sampled region (Hughen et al. 1999; Tudhope et al. 2001; Cobb et al. 2003; Cobb et al. 2013). To date, most coral-based reconstructions have utilized cores from the genus Porites, owing its regular, concentric growth bands and fast growth-rates that allow for the generation of up to 20 samples per year of coral growth. However, this genus is neither evenly distributed across the tropics nor continuously available within the fossil record, so there is a pressing need to expand the types of corals available for reconstruction. Here, we test Favia species from Kiritimati Island (2°N, 157°W) as a paleoclimate recorder by comparing different δ¹⁸O timeseries from within a single coral as well as across multiple corals with instrumental sea-surface temperature (SST). We find significant and consistent differences between coral δ¹⁸O profiles sampled along thecal versus septa walls, and show that δ¹⁸O in the thecal wall is more reproducible, and more coherent with SST. Slow growth rates (8-10mm/yr), and small inter-colony δ¹⁸O offsets suggest that Favia may be an untapped climate archive that is capable of providing robust constraints on natural climate variability in the tropical Pacific. Applying our sampling protocols to two fossil Favia corals from the early 19^th century, we demonstrate that high-resolution, accurate reconstructions of ENSO and mean climate in the central tropical Pacific are achievable with Favia corals.