The reconstruction of accurate sea‐surface temperatures (SST) is of utmost importance due to the ocean's central role in the global climate system. Yet, a number of environmental processes might bias reliable SST estimations. Here, we investigate the fidelity of SST reconstructions for the western tropical South Atlantic (WTSA) for the interval covered by Marine Isotope Stages (MIS) 6–5, utilizing a core collected off eastern Brazil at ∼20°S. This interval was selected as previous SST estimates based on Mg/Ca ratios of planktic foraminifera suggested a peculiar pooling of warm surface waters in the WTSA during MIS 6 despite glacial boundary conditions. To ground‐truth the Mg/Ca‐based SST data we generated SST reconstructions on the same core material using the alkenone and TEX86 paleothermometers. Comparison with alkenone‐based temperature estimates corroborate the previous Mg/Ca‐based SST reconstructions, supporting the suggestion of a warm‐water anomaly during MIS 6. In contrast, TEX86‐derived temperatures, albeit representing annual mean SST in recent core top samples, are up to 6°C colder than Mg/Ca‐ and alkenone‐based SST reconstructions. We interpret the periods of anomalously cold TEX86‐temperatures as a result of a vertical migration of the TEX86 producers (heterotrophic marine Thaumarchaeota) toward greater water depths, following food availability during phases of enhanced fluvial suspension input. Likewise, the data suggest that alkenone‐based SST are, albeit to a minor degree when compared to TEX86, affected by river run‐off and/or a seasonal bias in the growth season of haptophyte algae.
N2 - Plain Language Summary: In this study, we investigate the accuracy of sea‐surface temperature (SST) reconstructions for the western tropical South Atlantic (WTSA) for the interval covered by Marine Isotope Stages (MIS) 6–5—the penultimate glacial‐interglacial cycle (ca. 190,000 to 70,00 years before present). This time interval was selected because previous SST estimates based on Mg/Ca ratios of planktic foraminifera suggested a pooling of warm surface waters in the WTSA during late MIS 6 despite the cold glacial conditions. To verify the Mg/Ca‐based SST data, we generated temperature reconstructions from a core located off Eastern Brazil using two common paleothermometers that based on lipid biomarkers: alkenone and TEX86. The alkenone‐based temperature estimates agree with previous Mg/Ca‐based SST reconstructions, supporting the existence of a warm‐water anomaly in the WTSA during MIS 6. On the other hand, TEX86‐derived temperatures were up to 6°C colder than Mg/Ca‐ and alkenone‐based SST reconstructions. This discrepancy might be a result of a vertical migration of the TEX86 producers toward greater water depths where they feed on particles of organic matter. These migrations into deeper waters occurred during phases of increased river run‐off fluvial suspension input which enhanced surface primary productivity and facilitated vertical particle flux through the water column. N2 - Key Points:Parallel Mg/Ca (
Mg/Ca and alkenones represent annual mean sea surface temperatures, but most TEX86‐based temperatures deviate to colder values
Anomalous sea surface warming in the WTSA during late MIS 6 appears as a robust signal