Validation of the U37K′ paleotemperature proxy in the South Brazilian Bight from core-top sediments

Abstract

The paleothermometer based on the alkenone unsaturation index (${\text{U}}_{\text{37}}^{{\text{K}}^{\prime }}$) is often used to reconstruct past sea surface temperatures (SST). In the SW Atlantic Ocean, however, a limited understanding of the seasonal and depth distribution of coccolithophores, which generates the ${\text{U}}_{\text{37}}^{{\text{K}}^{\prime }}$ signal preserved in ocean sediments, hinders accurate estimates of past regional SSTs. We analyzed 45 core-top sediment samples from the continental shelf of the South Brazilian Bight (SBB, 23 °S to 28 °S) to assess regional spatial ${\text{U}}_{\text{37}}^{{\text{K}}^{\prime }}$-SST gradients and improve SST estimates. The ${\text{U}}_{\text{37}}^{{\text{K}}^{\prime }}$ data were converted to SST using six published paleotemperature equations and compared to modern observational SST data from the World Ocean Atlas (2018) data set. Data indicate that the ${\text{U}}_{\text{37}}^{{\text{K}}^{\prime }}$ signal is produced during the austral summer and autumn when regional episodic upwelling events occur over the inner-shelf and the nutricline shoals at the slope. Our analysis of core-top data shows that SST estimates derived from most calibration equations closely align with modern observed temperatures but are skewed toward warmer months associated with upwelling-derived nutrients. Our findings underscore the importance of considering regional and seasonal biases to improve the accuracy of paleotemperature reconstructions. Understanding the factors influencing the SBB ${\text{U}}_{\text{37}}^{{\text{K}}^{\prime }}$ signal enables a more meaningful comparison between regional paleoceanographic studies, improving our understanding of past changes in the SW Atlantic Ocean and our ability to predict regional SST response to ongoing and future warming.