BAYSPLINE: A New Calibration for the Alkenone Paleothermometer

The alkenone-based U U^K’₃₇ proxy is a cornerstone of paleoclimatology, providing insight into the temperature history of the Earth's surface ocean. Although the relationship between U U^K’₃₇ and sea surface temperatures (SSTs) is robust and well supported by experimental data, there remain outstanding issues regarding the seasonality of production of alkenones and the response of U U^K’₃₇ at very warm and cold SSTs. Using a data set of over 1,300 core-top U U^K’₃₇ measurements, we find compelling evidence of seasonal production in the North Atlantic, North Pacific, and Mediterranean Oceans. We also find significant attenuation of the U U^K’₃₇ response to SST at warm temperatures (>24°C), with the slope reduced by nearly 50% as U U^K’₃₇ approaches unity. To account for these observations in a calibration, we develop a new Bayesian B-spline regression model, BAYSPLINE, for the U U^K’₃₇ paleothermometer. BAYSPLINE produces similar estimates as previous calibrations below ∼24°, but above this point it predicts larger SST changes, in accordance with the attenuation of the U U^K’₃₇ response. Example applications of BAYSPLINE demonstrate that its treatment of seasonality and slope attenuation improves paleoclimatic interpretations, with important consequences for the inference of SSTs in the tropical oceans. BAYSPLINE facilitates a probabilistic approach to paleoclimate, building upon growing efforts to develop more formalized statistical frameworks for paleoceanographic reconstruction.