Variations in GDGT distributions through the water column in the South East Atlantic Ocean

Abstract The TetraEther indeX of 86 carbon atoms (TEX 86 ) temperature proxy is widely used in reconstructions of past sea surface temperature. Most current calibrations are based on surface sediment distributions of the glycerol dialkyl glycerol tetraether lipids (GDGTs) that comprise TEX 86 and assume that these GDGTs are exported from the upper mixed layer. However, GDGT export from deeper waters could impact sedimentary GDGT distributions and therefore TEX 86 paleothermometry. Here we examine GDGT distributions in suspended particulate matter (SPM) and underlying sediments collected from the Southeast Atlantic Ocean. Our results reveal different GDGT distributions – specifically the ratio between GDGTs bearing 2 vs. 3 cyclopentyl moieties, [2/3] ratios – between surface, subsurface (>50–200 m) and deep water (>200 m) SPM, which suggests the occurrence of in situ (deep) production that is not apparent when considering TEX 86 . The GDGT distributions in sediments match those of subsurface waters rather than surface waters, suggesting that they have not been preferentially derived from the upper mixed layer; this is consistent with GDGT abundances being highest in shallow subsurface SPM (∼100 to 200 m). It remains unclear what governs the different [2/3] ratios throughout the water column, but it is likely related to a combination of temperature and thaumarchaeotal community structure.