Identifying marine and freshwater overprints on soil-derived branched GDGT temperature signals in Pliocene Mississippi and Amazon River fan sediments

The fractional abundance of branched glycerol dialkyl glycerol tetraether (brGDGT) membrane lipids in coastal marine sediments has been posited as a proxy for the reconstruction of terrestrial temperatures on the nearby land, based on the assumption that they are produced in soils and delivered to the marine realm by rivers following erosion. Here, we test the suitability of brGDGTs as a continental paleothermometer in Pliocene age sediments from the northern Gulf of Mexico (GoM; speculated Mississippi River input) and the Ceará Rise (speculated Amazon River input). Low branched to isoprenoid tetraether (BIT) index values of 0.00–0.13 and the near absence of pollen and long-chain plant waxes in the GoM sediments suggest that the Mississippi River did not have a strong influence on the delivery of terrestrial organic matter to the site during the Pliocene and soil input was limited. Indeed, the high weighted average of cyclopentane-containing tetramethylated brGDGTs (#rings_tetra) in the GoM sediments (0.50 ± 0.09) relative to that of modern soils from the Mississippi catchment (0.25 ± 0.16) indicates that the brGDGTs in the GoM sediments were mostly produced in situ in the marine realm, hampering reliable land temperature reconstruction using the global soil transfer function. In contrast, high BIT index values (0.46 ± 0.21) and low #rings_tetra (0.25 ± 0.15) in sediments from the Ceará Rise suggest that these brGDGTs are primarily derived from soils. However, reconstructed temperatures were 11–18 °C lower than modern Amazon catchment temperatures. The relative abundance of 6-methylated brGDGTs (Isomerisation Ratio; IR) in the sediments is 0.82 ± 0.10, which resembles that of suspended particulate matter (SPM) in the modern Amazon River more than that of catchment soils (IR = 0.18 ± 0.18). This reveals that brGDGTs in the Ceará Rise sediments likely have a freshwater, riverine origin. Thus, the majority of the brGDGTs in both the GoM and Ceará Rise sediments are produced in situ, in the marine or river realms, which precludes application of the brGDGT paleothermometer. Our study shows that the sources of brGDGTs in coastal marine sediment archives must be critically evaluated prior to using the proxy for paleoclimate reconstruction.