Zhe-Xuan Zhang, Edith Parlanti, Christelle Anquetil, Jérôme Morelle, Anniet M Laverman, Alexandre Thibault, Elisa Bou, Arnaud Huguet

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial lipids that have been largely used as environmental proxies in continental paleorecords. Another group of related lipids, branched glycerol monoalkyl glycerol tetraethers (brGMGTs), has recently been proposed as a potential paleotemperature proxy. Nevertheless, the sources and environmental dependencies of both brGDGTs and brGMGTs along the river-sea continuum are still poorly understood, complicating their application as paleoenvironmental proxies in aquatic settings. In this study, the sources of brGDGTs and brGMGTs and the potential factors controlling their distributions are explored across the Seine River basin (NW France), which encompasses the freshwater to seawater continuum. To this aim, brGDGTs and brGMGTs were analyzed in soils, Suspended Particulate Matter (SPM) and sediments (n=237) collected all along this basin, from land to sea. Both types of compounds are shown to be produced in situ, in freshwater as well as saltwater. Redundancy analysis further shows that both salinity and nitrogen loadings dominantly control the brGDGT distributions. Furthermore, the relative abundance of 6-methyl vs. 5-methyl brGDGTs (IR6Me ratio), Total Nitrogen (TN), δ15N and chlorophyll a concentration co-vary in a specific zone with low salinity, suggesting that 6-methyl brGDGTs are preferentially produced under low-salinity and high-productivity conditions. In contrast with brGDGTs, brGMGT distribution appears to be primarily regulated by salinity, with a distinct influence on the individual homologues. Salinity is positively correlated with homologues H1020a and H1020b, and negatively correlated with compounds H1020c and H1034b. This suggests that bacteria thriving in freshwater preferentially produce compounds H1020c and H1034b, whereas bacteria primarily growing in saltwater appear to be predominantly responsible for the production of homologues H1020a and H1020b. Based on the abundance ratio of the freshwater-derived compounds (H1020c and H1034b) vs. saltwater-derived homologues (H1020a and H1020b), a novel proxy, Riverine IndeX (RIX) is proposed to trace riverine organic matter inputs, with high values (>0.5) indicating higher riverine contribution. RIX was then applied to the Godavari River basin (India) and a paleorecord across the upper Paleocene and lower Eocene, showing its potential applicability in both modern samples and in paleorecords.