High‐Resolution Annual Cycles of the Sr/Ca Ratio in Florida Bay Seawater Sampled With Osmotic Pumps

Abstract Coral skeletal Sr/Ca ratios are a commonly used paleotemperature proxy that relies on stable seawater Sr/Ca ratios ( R sw ). Recent observations call this assumption into question at some locations, particularly in the Florida Keys, USA. We collected R sw time series data for 4 years from the Middle Keys with about weekly resolution, as well as several 24‐hr time series from coral reef sites throughout the Caribbean, to quantify the magnitude and mode of R sw variability and to assess its likely impact on coral Sr/Ca‐based paleoclimate reconstructions. Continuous seawater samples were collected with four bottom‐mounted osmotic pumps placed on a transect from inside Florida Bay to approximately 10 km offshore. In Florida Bay, R sw displays annual cycles with high values in the summer and an amplitude of 0.4–0.5 mmol/mol that attenuates and becomes less periodic with increasing distance from the coast. We propose that this seawater Sr/Ca cycle may be due to a seasonal biogeochemical process related to the precipitation and dissolution of CaCO 3 in Florida Bay, with the signal carried onto the shelf through mixing. The annual R sw cycle can explain the low Sr/Ca–SST slopes that have been reported in local corals. Pseudo‐coral Sr/Ca calculations indicate that R sw causes enhanced uncertainty in reconstructed SST, even at the site 10 km offshore. The 24‐hr time series show smaller diurnal variability in seawater Sr/Ca that appears to reflect local tides and hydrography, suggesting a similar potential for aberrant coral Sr/Ca–SST calibrations at other reef locations.