Multi-species Coral Sr/Ca-Based Sea-Surface Temperature Reconstruction Using Orbicella faveolata and Siderastrea siderea from Dry Tortugas National Park, FL

Abstract:

Massive coral skeletons provide an important geologic archive of climate information over the past several centuries, due to their annual banding patterns and longevity. The ratio of strontium to calcium (Sr/Ca) in coral skeletons is dependent on the temperature of the seawater at the time of growth and, therefore, provides a powerful proxy of historic sea-surface temperature (SST). This study used cores from two modern coral colonies—Orbicella faveolata (formerly Montastraea faveolata) and Siderastrea siderea—collected from the Dry Tortugas National Park, Florida (DRTO), located in the southern Gulf of Mexico. The corals were subsampled at monthly resolution and Sr/Ca determinations were made using ICP-OES. Sr/Ca was calibrated to SST using previously-published calibration equations specific to each species. Both the O. faveolata (1893–2008) and S. siderea (1837–2012) records have well-defined annual cycles of Sr/Ca, which suggest a seasonal SST range of ~10–15°C: a range consistent with in situ instrumental SST records from DRTO. Our reconstructions of mean annual Sr/Ca-based SST and Sr/Ca variability from the two coral species show similar multi-decadal trends and strong coherency through overlapping time intervals. We investigated the intervals of divergence among the three independent DRTO records (two different species), and found that sudden drops in linear extension in S. siderea likely skewed the mean annual temperature record. This result underscores the importance of using multiple-coral geochemical records when reconstructing regional climate. A stacked record of SST derived from the three different coral colonies growing at DRTO from 1837–2008 showed a distinct trend toward cooler mean annual Sr/Ca-based SST from the late 1950s to the late 1960s, which coincides with a cooling trend found in several other Northern Hemisphere temperature reconstructions, and is likely linked to variability of the Atlantic Multi-decadal Oscillation.