Abstract
•Maximum GDGT concentration occurred at the base of the NO2− maximum.•Between 0 and 1000 m, > 98% of GDGT inventory present below the NO2− maximum.•GDGT export from ca. 80–250 m consistent with TEX86 and [2/3] ratio in sediments.•TEX86 remarkably insensitive to change in modeled export depth.•Local productivity an important control on exported GDGT ratio values.
The TEX86 paleotemperature proxy is based on the distribution of archaeal glycerol dibiphytanyl glycerol tetraether (GDGT) lipids preserved in marine sediments, yet both the influence of different physiological factors on the structural distribution of GDGTs and the mechanism(s) by which GDGTs is(are) exported to marine sediments remain(s) unresolved. We investigated the abundance and structural distribution of GDGTs in the South-west and Equatorial Atlantic Ocean in four water column profiles spanning 48 degrees of latitude. The depth distribution was consistent with production by ammonia-oxidizing Thaumarchaeota; maximum GDGT concentration occurred at the base of the NO2− maximum, core GDGTs dominated the structural distribution in surface waters above the NO2− maximum, and intact polar GDGTs – potentially indicating live cells – were more abundant below the NO2− maximum. Between 0 and1000 m, > 98% of the integrated GDGT inventory was present in waters at and below the NO2− maximum. Depth profiles of TEX86 temperature values displayed local minima at the NO2− maximum, while the ratio of GDGT-2:GDGT-3 increased with depth. A model based on the results predicts an average depth of origin for GDGTs exported to sediments between ca. 80–250 m. In the model, exported TEX86 values are remarkably insensitive to change in the average depth of origin of GDGTs. However, TEX86 values exported from the water column appear to reflect euphotic zone productivity, possibly due to the correlative intensity of organic matter remineralization providing substrates for ammonia oxidation. Predicting the influence of these regional controls on sedimentary TEX86 records requires a better understanding of the interaction between GDGT production, particle dynamics, and the depth of origin for exported organic matter.
