From soil to sea: sources and transport of organic carbon traced by tetraether lipids in the monsoonal Godavari River, India

Abstract

Abstract. Monsoonal rivers play an important role in the land-to-sea transport of soil-derived organic carbon (OC). However, spatial and temporal variation in the concentration, composition, and fate of this OC in these rivers remains poorly understood. We investigate soil-to-sea transport of soil OC by the Godavari River in India using glycerol dialkyl glycerol tetraether (GDGT) lipids in soils, river suspended particulate matter (SPM), and riverbed sediments, as well as in a marine sediment core from the Bay of Bengal. The abundance and composition of GDGTs in SPM and sediments in the Godavari River differs between the dry and wet season. In the dry season, SPM and riverbed sediments from the whole basin contain more 6-methyl branched GDGTs (brGDGTs) than the soils. In the upper basin, where mobilisation and transport of soils is limited due to deficient rainfall and damming, contributions of 6-methyl brGDGTs in SPM and riverbed sediments are relatively high year-round, suggesting that they have an aquatic source. Aquatic brGDGT production coincides with elevated values of the isoprenoid GDGT-0 / crenarchaeol ratio in SPM and riverbed sediments from the upper basin, indicating low-oxygen conditions. In the wet season, brGDGT distributions in SPM from the lower basin closely resemble those in soils, mostly from the north and east tributaries, corresponding to precipitation patterns. The brGDGT composition in SPM and sediments from the delta suggests that soil OC is only effectively transported to the Bay of Bengal in the wet season, when the river plume extends beyond the river mouth. The sediment geochemistry indicates that also the mineral particles exported by the Godavari River primarily originate from the lower basin, similar to the brGDGTs, suggesting that they are transported together. However, river depth profiles in the downstream Godavari reveal no hydrodynamic sorting effect on brGDGTs in either season, indicating that brGDGTs are not closely associated with mineral particles. The similarity of brGDGT distributions in bulk and fine-grained sediments (≤ 63 µm) further confirms the absence of selective transport mechanisms. Nevertheless, the composition of brGDGTs in a Holocene, marine sediment core near the river mouth appears substantially different from that in the modern Godavari basin, suggesting that terrestrial-derived brGDGTs are rapidly lost upon discharge into the Bay of Bengal and/or overprinted by marine in situ production. The large change in brGDGT distributions at the river–sea transition implies that this zone is key in the transfer of soil OC, as well as that of the environmental signal carried by brGDGTs from the river basin.

The dispersal of fluvially discharged and marine, shelf-produced particulate organic matter in the northern Gulf of Mexico

“…However, the brGDGT concentrations decrease more rapidly than other TerrOM sources. Possibly, due to the hydrophobic nature of brGDGTs, they could potentially form colloids rather than fully move to the dissolved phase (Kirkels et al, 2022). Studies on brGDGTs in rivers have shown that their composition is stable in suspended matter throughout the water column, i.e., there is no preferential loss or production of a certain brGDGT compound, whereas the grain size and mineral composition are clearly depth-segregated (Feng et al, 2016;Kirkels et al, 2020).…”

“…The spatial distribution of plant-derived OM in the GoM shows that, in particular, n-alkanes could be preferentially associated with clay particles and thereby transported further offand along shore, as these mineral associations not only facilitate their transport but also provide protection from degradation (Mayer, 1994a, b;Keil et al, 1997;Lalonde et al, 2012). The plant markers have a higher affinity to bind with mineral surfaces than C 32 1,15-diols or brGDGTs, which possibly do not form these associations (Kirkels et al, 2022). To support this, mineral-associated organic matter in the soil continuum generally consist of more plant-derived OM than microbial-derived OM, although this depends on many factors including substrate quality, land use, and climatic conditions (see review by Angst et al, 2021).…”

“…During river transport, partial degradation and/or (temporal) storage in sediments and inland wetlands can change the composition of TerrOM (Battin et al, 2009;Aufdenkampe et al, 2011). Therefore, only a fraction of the OM delivered to rivers (32 %-47 %) reaches the coastal zone (Cole et al, 2007;Battin et al, 2009;Tranvik et al, 2009;Aufdenkampe et al, 2011;Regnier et al, 2013;Li et al, 2017;Kirschbaum et al, 2019).…”

Carbon isotopic ratios of modern C3 and C4 vegetation on the Indian peninsula and changes along the plant–soil–river continuum – implications for vegetation reconstructions

“…In the Godavari basin, Usman et al (2018) reported similar 14 C values for soil OC in the upper and lower basin that have different C 3 and C 4 plant covers, suggesting that the net effect of preferential degradation (more young OC) and stabilisa-tion (more old OC) is minor. Indeed, extensive degradation of C 4 -plant-derived OC is unlikely, given that the upper basin with most C 4 plants contains clay-rich, fine particles from weathering of the Deccan basalts (Giosan et al, 2017;Usman et al, 2018;Kirkels et al, 2022b), which would contribute to stabilising the C 4 -derived OC. Fine soils (≤ 63 µm) sampled in the upper basin had on average slightly less negative δ 13 C org values than the bulk soils (−19.7 ± 1.0 ‰, n = 8 vs. −21.1 ± 0.7 ‰, n = 8; p ≤ 0.09), pointing towards preferential stabilisation of C 4 -derived OC in the fine fraction (Fig.…”

Provenance, transport, and the fate of organic matter and sediments drained through Himalayan Rivers in Nepal

“…Silicate weathering of the exhumed Himalayan sediments consumes CO 2, resulting in a worldwide cooling effect and changes in atmospheric circulation patterns (Fluteau, 2003;Li et al, 2014;Zhang et al, 2015). To constrain this mechanism, contemporary studies in the last few decades focused on weathering processes driven by fluvial systems using radiogenic isotopes, e.g., rubidium (Rb), strontium (Sr), and neodymium (Nd) and their ratios (Bayon et al, 2021;Bickle et al, 2005;Dalai et al, 2003;Derry and France-Lanord, 1996;Galy et al, 1999;Nakano, 2016;Saitoh et al, 2020), major element chemistry (Bickle et al, 2018;Blum et al, 1998;English et al, 2000;Liu et al, 2015;Singh et al, 1998), and organic compounds and their diagnostic proxies (Bhandari et al, 2022(Bhandari et al, , 2021Kirkels et al, 2022).…”