Molecular Tracing of Riverine Soil Organic Matter From the Central Himalaya

Abstract

The isomer distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in soils has been shown to correlate to the local mean annual temperature. Here, we explore the use of brGDGT distributions as proxy for the elevation at which soil organic carbon is preferentially mobilized in the Central Himalaya. Soil brGDGT distributions collected along an altitudinal profile, spanning elevations from 200 to 4,450 m asl, are linearly correlated to elevation. We use this calibration to trace the provenance of soil organic matter in suspended sediments of rivers draining the Himalaya. BrGDGT distributions of fluvial sediments reflect the mean elevation of the soil cover in most catchments. Inverse modeling of the brGDGT data set suggests similar relative contribution to soil organic carbon mobilization from different land covers within a factor 2. We conclude that riverine soil organic carbon export in the Himalaya mostly occurs pervasively and is at the catchment scale insensitive to anthropogenic perturbations.

Reply on RC1 (David Naafs)

“…In contrast to in the dry season, brGDGT distributions do not change in the wet season, when the river plume extends beyond the Godavari mouth and the water is less saline (<3 psu) (Sridhar et al, 2008;Kirkels et al, 2020b). The seasonal difference in salinity indicates once more that the soil signal is only effectively exported to the Bay of Bengal in the wet season, and supports previous studies that have shown that substantial soil mobilisation is needed (e.g., by a distinct season with increased precipitation) to export soil-derived brGDGTs downstream (Guo et al, 2020;Kirkels et al, 2020a;Märki et al, 2020).…”

“…Although lipid biomarkers represent only a small part of the total OC, their source-or environmental-specific signature renders them as promising tracers in river systems (e.g., Eglinton et al, 2021). Over the past decade, bacterial membrane lipids, so-called branched glycerol dialkyl glycerol tetraethers (brGDGTs), have been used to trace soil organic matter inputs into rivers and continental margins (e.g., Hopmans et al, 2004;Weijers et al, 2009a;Kim et al, 2012;Zell et al, 2013a,b;Kirkels et al, 2020a;Märki et al, 2020). BrGDGTs occur globally in soils and peats, and can vary in the number (4-6) and position of methyl branches (5/5' or 6/6', defined as 5-methyl and 6-methyl) attached to linear C28 alkyl chains, as well as the presence of 0-2 cyclopentane moieties (Weijers et al, 2007a;De Jonge et al, 2013; see figures therein for the molecular structures).…”

“…These proxies are based on the assumption that brGDGTs preserved in continental margin sediments have a soil origin and represent a basinintegrated signal. Indeed, the composition of brGDGTs in rivers has been shown to resemble those in soils in upstream catchments (e.g., Märki et al, 2020). However, brGDGTs have also been found to be produced in rivers (e.g., Yang et al, 2013;Zell et al, 2013a,b;De Jonge et al, 2014b, 2015a, which complicates their use as soil-specific tracers.…”

An unshakable carbon budget for the Himalaya

“…The samples are further described in ref. 52 . The samples integrate the surface soil layer of a depth of up to 15 cm.…”

The Sources of Sedimentary Organic Matter Traced by Carbon and Nitrogen Isotopes and Environmental Effects during the Past 60 Years in a Shallow Steppe Lake in Northern China

“…The stoichiometric ratios and stable isotopic compositions of C (carbon) and N (nitrogen) in organic matter were developed as effective methods to trace the organic matter sources and identify predominant processes [7,8]. Moreover, the sediment profiles of stoichiometric ratios and stable isotopes of organic matter can reflect the historical changes in aquatic ecosystem productivity and terrigenous organic matter transportation processes, providing important information for the interpretation of paleoenvironmental conditions [4,5,9,10].…”