Coral Sr-U Thermometry Tracks Ocean Temperature and Reconciles Sr/Ca Discrepancies Caused by Rayleigh Fractionation

Abstract

Understanding climate change at the spatiotemporal scales necessary to improve climate projections requires proxy records that complement sparse and often contradictory observational temperature data sets. Massive long-lived corals have tremendous potential in this regard, continuously recording information about ocean conditions as they grow. Nevertheless, extracting accurate ocean temperatures from corals is challenging because factors other than temperature influence skeletal chemistry. Here, we tested the ability of the coral Sr-U thermometer to accurately capture annual sea surface temperatures (SSTs) in the subtropical Atlantic, where year-to-year temperatures vary by ∼1°C. Using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), we generated sufficient U/Ca – Sr/Ca pairs from a slow-growing (1−2 mm/yr) Siderastrea siderea coral to calculate annual Sr-U values. With the fine-scale spatial resolution attained using the laser, skeleton accreted during both fast and slow growing times of the year was represented in our sampling. The resulting 30-year-long Sr-U record tracked the amplitude and timing of annual SST to within ±0.2°C of observations (r = −0.71), whereas the Sr/Ca record did not (r = 0.23). Furthermore, Sr-U corrected for Sr/Ca offsets among adjacent skeletal elements approximately 1 mm apart. These offsets are equivalent to differences of 2–3°C if typical Sr/Ca–SST calibrations are applied. Our observations indicate that Sr-U can accurately constrain decadal-to-multidecadal variability and secular SST trends in regions where this information is urgently needed.