Seasonal and Episodic Variability in Seawater Strontium-Calcium Ratios on the Coral Reefs of the Western Hemispheric Warm Pool: A Confounding Variable for Coral Paleoclimate Reconstructions

Coral skeletal geochemistry is widely used to reconstruct (sub)tropical climate variability, a key component of the climate system. Skeletal strontium-to-calcium ratios (Sr/Ca) are a commonly applied coral geochemical proxy for reconstructing sea surface temperature, although coral skeletal Sr/Ca is also influenced by ambient seawater Sr/Ca. The behavior of seawater Sr/Ca in oligotrophic nearshore environments where coral reefs proliferate is not well documented, and is hypothesized to be the cause of non-temperature related coral Sr/Ca variability in some regions. This study explores the temporal and spatial variability of seawater Sr/Ca in the Florida Keys and in the U.S. and British Virgin Islands, important locations for ongoing paleoclimate research. Continuous water monitoring stations deployed since September 2016 reveal significant seasonal and interannual variability in seawater Sr/Ca at some sites, while other sites appear relatively stable.

Seasonal variability dominates in the Florida Keys, with the largest variations in Florida Bay (average 8.67 ± 0.11 mmol/mol, 1σ, range 0.52 mmol/mol), and minima/maxima occurring in December/July. Variability decreases significantly moving seaward, presumably due to increased distance from Florida Bay and a transition to open-ocean seawater Sr/Ca ratio values. Surface seawater samples taken over a 10x40 km grid on the ocean side of the Florida Keys support this hypothesis.

Results from the water monitoring station on Anegada in the British Virgin Islands demonstrate relatively stable seawater Sr/Ca (average [1σ] 8.65 [0.03] mmol/mol, range 0.12 mmol/mol). Seawater Sr/Ca in St. John appeared to be mostly stable (average [1σ] 8.70 [0.04] mmol/mol, range 0.18 mmol/mol), but a particularly large negative excursion lasting multiple weeks (minimum 8.15 mmol/mol) was observed in late 2018. These results indicate that sites closer to shore are subject to greater seawater Sr/Ca variability due to (1) freshwater discharge, and (2) an increased potential for biogeochemical interaction due to benthic and surface fluxes in a relatively shallow water column. Importantly, even relatively stable environments exhibit small changes in seawater Sr/Ca ratios that could influence the highly sensitive coral Sr/Ca paleothermometer.