K. Halimeda Kilbourne

A 4‐year time‐series of surface seawater Sr/Ca ratios was assembled across a section of the Florida Keys Reef Tract, in order to uncover any variability that might explain previously reported anomalies in regional calibrations of the coral aragonite Sr/Ca paleotemperature proxy. Samples were collected semiannually on a grid of 54 sites, from September of 2016 until January of 2020. The 325‐km 2 grid extended from the ocean shore to the forereef wall and from the east end of Long Key to the west end of Marathon. A novel ICP‐AES method was used to measure the Sr/Ca ratio, with ratio calibration and normalization against an in‐house seawater reference, yielding a long‐term precision of better than 0.2%. Significant variations (2%–3%) of the seawater Sr/Ca ratio were found. While it was relatively constant offshore, near the coast the ratio alternated seasonally between higher and lower values, generally resulting in seaward Sr/Ca gradients that were markedly negative in summer but reversed in winter. Inshore seawater Sr/Ca ratios ranged from a summer high of 8.83 mmol/mol to a winter low of 8.54 mmol/mol, the difference corresponding to a potential bias of ∼5.5°C in terms of the coral Sr/Ca paleotemperature proxy. This seasonal variation should diminish the slope of empirical Sr/Ca–SST calibration lines, as has indeed been observed in prior studies with local coral species. Open ocean samples obtained from the Atlantic, Indian, and Pacific enlarge the published Sr/Ca data set for surface seawater and show a much smaller variability of 8.646 ± 0.018 mmol/mol (0.2%).