Validation of the U K' 37 and TEX 86 Paleothermometers across Bathymetric Gradients offshore Eastern Brazil

The U K' 37 and TEX 86 lipid paleothermometers, based on the relative distributions of alkenones and isoprenoid glycerol dibiphytanyl glycerol tetraethers (isoGDGTs), respectively, are the most widely used organic temperature proxies. Their applicability has previously been investigated along the SE Brazilian margin, where upper-ocean temperatures are influenced by the Brazil Current. However, these studies focused mainly on the Cabo Frio upwelling area, but information across larger gradients is as yet missing. Here, we evaluate U K' 37 and TEX 86 in surface sediments from the eastern Brazilian margin (10-23°S) by comparing temperatures derived from several calibration functions with modern observational data from the World Ocean Atlas 2023. Potential biases were assessed using organic and inorganic geochemical indices and grain-size distributions. Consistent with previous findings, U K' 37 largely reflects annual mean sea surface temperatures, although warm biases occur in samples with high sand contents. These biases are consistent with selective alkenone degradation under reduced mineral protection in coarse-grained sediments. At slope stations, most TEX 86 calibrations reproduce surface and subsurface temperatures within their calibration uncertainties, with the weighted 0-1,000 m calibration yielding the mean residual closest to zero. Elevated isoGDGT[2/3] ratios are consistent with substantial contributions from deep-dwelling archaea, which may partly overprint the temperature-sensitive TEX 86 signal. Shelf sediments yield anomalously low TEX 86 -temperatures. Their distinct isoGDGT distributions indicate non-thermal overprinting, potentially linked to changes in archaeal community composition and metabolic activity. A similar shelf-to-slope contrast in isoGDGT distributions is evident in the global surface sediment database, highlighting the need for regional evaluation of the TEX 86 at continental margins.