Contrasting terrestrial and marine sources of isoprenoid and branched glycerol monoalkyl glycerol tetraethers in marine sediments

Glycerol dialkyl glycerol tetraethers (GDGTs) are widely used as environmental proxies, yet the sources and environmental significance of the structurally related glycerol monoalkyl glycerol tetraethers (GMGTs) in marine sediments remain poorly constrained. We investigated isoprenoid and branched GMGTs in 16 coastal soils, 62 East China Sea surface sediments, five South China Sea sediment cores, and offshore suspended particles to distinguish terrestrial inputs from marine production. Isoprenoid GMGTs (isoGMGTs) occurred in coastal soils and declined across the shelf, with no detections in W07 particles or the offshore SCS cores. Elevated isoGMGTs/crenarchaeol ratios near the Yangtze River mouth and Hangzhou Bay resembled the published spatial pattern of the soil-derived bacteriohopanepolyol index Rsoil, supporting their use as an indicator of soil organic matter input. Soil brGMGT assemblages contained nine resolved peaks, including three H1048 components, whereas among offshore samples with quantifiable brGMGTs, H1020a and H1020b accounted for >90% of the assemblage, with H1020c detected in some samples. H1020a and H1020b also constituted the quantifiable brGMGT assemblage in suspended particles at station W07, where concentrations peaked at 75 m in oxygenated waters. This correspondence supports a marine water-column contribution to sedimentary brGMGTs and indicates that their occurrence extends to oxygenated environments. The ratio H1020b/(H1020a + H1020b), expressed as IRH1020, covaried inversely with TEX86 in all three offshore cores. These downcore variations may reflect glacial–interglacial environmental changes. Together, these findings support isoGMGTs/crenarchaeol as a tracer of relative soil organic matter input and identify a distinct marine brGMGT assemblage with potential for paleoceanographic applications.