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

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 similar to 1 degrees 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 degrees 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 degrees 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. Plain Language Summary The chemistry of coral skeletons can be used to estimate past ocean temperatures and fill gaps in the instrumental record where observations are sparse. However, the accuracy of single element-to-calcium ratio thermometers is limited by biological factors that influence coral composition. A new coral thermometer, Sr-U, corrects for biological effects on Sr/Ca, a widely used coral paleothermometer. Here, we evaluate whether Sr-U can reconstruct temperatures in a slow- and seasonal-growing coral from the Bahamas. Unlike Sr/Ca, Sr-U tracks the observed average annual sea surface temperatures recorded at this site over a 30-year period. Our results reveal consistent offsets in Sr/Ca values among neighboring parts of the coral skeleton that grew at the same time. These offsets translate to 2-3 degrees C differences among parts of the coral that grew at the same temperature. However, we find no such difference in Sr-U values between these skeletal elements. We conclude that Sr-U can accurately reconstruct past ocean temperatures with a greater degree of confidence than those derived from Sr/Ca.