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

[摘要] Monsoonal rivers play an important role in theland-to-sea transport of soil-derived organic carbon (OC). However, spatialand temporal variation in the concentration, composition, and fate of thisOC in these rivers remains poorly understood. We investigate soil-to-seatransport of soil OC by the Godavari River in India using glycerol dialkylglycerol tetraether (GDGT) lipids in soils, river suspended particulatematter (SPM), and riverbed sediments, as well as in a marine sediment core from the Bayof Bengal. The abundance and composition of GDGTs in SPM and sediments inthe Godavari River differs between the dry and wet season. In the dryseason, SPM and riverbed sediments from the whole basin contain more6-methyl branched GDGTs (brGDGTs) than the soils. In the upper basin, wheremobilisation and transport of soils is limited due to deficient rainfall anddamming, contributions of 6-methyl brGDGTs in SPM and riverbed sediments arerelatively high year-round, suggesting that they have an aquatic source.Aquatic brGDGT production coincides with elevated values of the isoprenoidGDGT-0  /  crenarchaeol ratio in SPM and riverbed sediments from the upper basin,indicating low-oxygen conditions. In the wet season, brGDGT distributions inSPM from the lower basin closely resemble those in soils, mostly from thenorth and east tributaries, corresponding to precipitation patterns. ThebrGDGT composition in SPM and sediments from the delta suggests that soil OCis only effectively transported to the Bay of Bengal in the wet season, whenthe river plume extends beyond the river mouth. The sediment geochemistryindicates that also the mineral particles exported by the Godavari Riverprimarily originate from the lower basin, similar to the brGDGTs, suggestingthat they are transported together. However, river depth profiles in thedownstream Godavari reveal no hydrodynamic sorting effect on brGDGTs ineither season, indicating that brGDGTs are not closely associated withmineral particles. The similarity of brGDGT distributions in bulk andfine-grained sediments ( ≤  63  µ m) further confirms the absence ofselective transport mechanisms. Nevertheless, the composition of brGDGTs ina Holocene, marine sediment core near the river mouth appears substantiallydifferent from that in the modern Godavari basin, suggesting thatterrestrial-derived brGDGTs are rapidly lost upon discharge into the Bay ofBengal and/or overprinted by marine in situ production. The large change inbrGDGT distributions at the river–sea transition implies that this zone iskey in the transfer of soil OC, as well as that of the environmental signalcarried by brGDGTs from the river basin.

[学科分类] 大气科学