Apparently Seasonal Variations of the Seawater Sr/Ca Ratio Across the Florida Keys Reef Tract

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 similar to 5.5 degrees 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%).Plain Language Summary Tropical reef-building corals make their skeletons out of calcium (Ca) carbonate, acquiring the element Ca from the surrounding seawater. When the temperature of the seawater goes up, they incorporate less of the chemically similar element strontium (Sr) and vice versa. The coral skeleton grows in annual bands that can be exactly dated and from the Sr/Ca ratio of these bands one can thus derive seawater temperature in the corresponding years. These records can be then be applied to construct or verify models of climate in the recent past, which only works well if the Sr/Ca ratio of seawater itself is constant. On the Florida reef, problems with this technique were thought to be caused by significant seasonal variations of the seawater Sr/Ca ratio. With a new analytical method we have monitored the Sr/Ca ratio of local seawater for 4 years and found that it does indeed vary between winter and summer. These variations appear to be of the right timing and magnitude to explain the problems previously encountered. While this situation is probably unusual, it may well occur on coral reefs in comparable geological or hydrological settings elsewhere.