Validating the UK'37-SST relationship in the SW Atlantic Ocean From Core-Top Samples - UK'37; Annual and Seasonal SSTs from Aqua MODIS and Water Column from WOA13 and Coccolithophores Abundance

Few studies have evaluated the seasonal and depth distribution of the alkenone unsaturation index (UK'37) signal in sediments in the SW Atlantic Ocean, limiting the understanding of past regional SST estimates from this biomarker proxy. We analyzed 45 core-top sediment samples collected on the continental shelf of the South Brazilian Bight (SBB, 23 °S to 28 °S) to assess the spatial gradient in the UK'37-SST signal and improve future paleoceanographic studies in the region. The proxies were converted to SST using four published paleotemperature equations and then compared to modern observed SST from the World Ocean Atlas (2013) and Aqua MODIS mission data sets. The Bayspline calibration (Tierney and Tingley, 2018, doi:10.1002/2017PA003201) results in the most accurate estimates of modern mean annual SST (p-value 0.99, n = 45, ΔSST of -0.0006°C ± 0.99). We regressed our UK'37 determinations against modern observational SST and found that core-top UK'37 was significantly correlated to mean annual SST, with marginally higher sensitivity to autumn and winter SST patterns. For downcore SST reconstruction in the SBB region, we recommend applying the Bayspline equation for mean annual SST estimates. Standardizing the UK'37-SST equation used in the SBB region will allow for a more meaningful comparison between regional paleoceanographic studies, improving our understanding of past changes in the SW Atlantic.