Characteristics of isoprenoid and branched tetraether lipids in the deep ocean: implications from surface sediments in the seamount area of the Western Pacific Ocean

Abstract

The isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) and branched glycerol dialkyl glycerol tetraethers (brGDGTs) in marine sediments have been widely used for paleoceanographic reconstructions. However, their distributions in the deep ocean (> 4000 m) have not been well characterized. This study analyzed GDGTs in surface sediments from the seamount area of Marcus-Wake and Magellan, western Pacific Ocean (4431-5756 m). The results were further compared with published data of GDGTs in the South China Sea (SCS), equatorial Pacific Ocean, and Mariana Trench to reveal the provenance of GDGTs in the deep ocean and their suitability for paleoceanographic reconstruction. The ratios of GDGT-0/crenarchaeol (0.59–1.38) and GDGT-2/3 (10.65–14.78) indicate the contribution from benthic Thaumarchaeota in the seamount area. The isoGDGTs of the surface sediments from the seamount and the Mariana Trench regions were clustered for a redundancy analysis. They are characterized by the similar relative abundance of each isoGDGT, i.e. higher fractional abundance of GDGT-0 (26–49%) than those in the SCS (12–23%) and the equatorial Pacific Ocean (22–28%). This suggests isoGDGTs in the deep ocean (> 4000 m) are for both planktonic and benthic Thaumarchaeota and the benthic Thaumarchaeota likely produce excessive GDGT-0 in the seamount area. For brGDGTs, the higher IIIa/IIa ratios in the seamount area (1.67–4.83) than the marine-derived values (0.92), indicating in-situ production of brGDGTs. The discrepancy (0.7 to 5.0 °C) between sea surface temperature, deduced by TetraEther indeX (TEX86), and the observed sea surface temperature is likely due to less input of isoGDGTs from the pelagic Thaumarchaeota but more from the benthic Thaumarchaeota.