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

The TetraEther indeX of 86 carbon atoms (TEX86) 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 TEX86 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 TEX86 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 TEX86. 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.

Chemistry and Release of Gases from the Surface Ocean

The sea-surface layer is the very upper part of the sea surface where reduced mixing leads to strong gradients in physical, chemical and biological properties1. This surface layer is naturally reactive, containing a complex chemistry of inorganic components and dissolved organic matter (DOM), the latter including amino acids, proteins, fatty acids, carbohydrates, and humic-type components,2 with a high proportion of functional groups such as carbonyls, carboxylic acids and aromatic moieties.3 The different physical and chemical properties of the surface of the ocean compared with bulk seawater, and its function as a gateway for molecules to enter the atmosphere or ocean phase, make this an interesting and important region for study. A number of chemical reactions are believed to occur on and in the surface ocean; these may be important or even dominant sources or sinks of climatically-active marine trace gases. However the sea surface, especially the top 1um to 1mm known as the sea surface microlayer (ssm), is critically under-sampled, so to date much of the evidence for such chemistry comes from laboratory and/or modeling studies. This review discusses the chemical and physical structure of the sea surface, mechanisms for gas transfer across it, and explains the current understanding of trace gas formation at this critical interface between the ocean and atmosphere.