Assessing Lateral Transport as a Source of Scatter in the Global TEX86-SST Calibration: a Case Study for the South Atlantic Ocean

Abstract

TEX86 (Tetraether index of tetraethers, which contains 86 carbon atoms) is a valuable proxy for absolute Sea Surface Temperature (SST) reconstructions. However, scatter in the relationship between sedimentary TEX86 ratios and the SST of the overlying water cause large calibration error bars in reconstructions. Hence, understanding the exact relationship between SST and sedimentary TEX86 index values might help reduce such calibration error bars. One of the questions is to what extent ocean currents laterally displace descending GDGTs-bearing particles through the water column. The GDGTs transport during sinking was simulated in a high-resolution model of the Subtropical South Atlantic of the modern ocean. The simulations allowed quantification of an offset between the region of origin of these particles and the directly overlying water. At several sinking speeds, the particles were tracked back from the position of the surface sediment sample to a water depth of 30 meter. Crucially, this has been concluded that water depth and the ratio between GDGTs bearing two vs. three cyclopentyl moieties ([2]/[3] ratio) reflecting a contribution from Archaea living in deeper waters are better discriminators of ΔSST bias values (an offset between the TEX86-derived SST and modeled average SST of transported particles) than lateral transport itself in the South Atlantic Ocean sub-dataset. Since lateral transport-related SST changes with water depth, it is no longer representative of directly overlying SST; the deeper the site, the larger ΔSST bias with the signal of the directly overlying water. It was also found that the shallow sites (<1000 m) have been consistent cold offset while the deeper sites (>1000 m) consistently have a warmer bias with a high [2]/[3] ratio. However, the lateral transport effect can be best seen if looked at a specific region, as ΔSST (the TEX86-derived SST (SSTTEX86) – modeled-overlying SST) is also a function of travel distance of particles, influencing the signal. The prevailing ocean current in the Southeast Atlantic is Agulhas Current while the Southwest Atlantic is induced by two currents coming from the Southern Ocean and dominantly from Brazilian Current (from the Equatorial Current), causing a cold ΔSST bias. This is a surprising observation due to the transport effect, yielding markedly elevated temperatures difference in some sites, ultimately affecting the calibration. A few aspects such as possible local events in the ocean which might cause an anomaly, the depth productivity, sub-sea surface temperature, the horizontal travel distance bias of the particles before sinking, and a global scale simulation need to be taken into account for future efforts.