Reconciling drainage and receiving basin signatures of the Godavari River system

Abstract

The modern-day Godavari River transports large amounts of sediment (170 Tg per year) and terrestrial organic carbon (OC_(terr); 1.5 Tg per year) from peninsular India to the Bay of Bengal. The flux and nature of OC_(terr) is considered to have varied in response to past climate and human forcing. In order to delineate the provenance and nature of organic matter (OM) exported by the fluvial system and establish links to sedimentary records accumulating on its adjacent continental margin, the stable and radiogenic isotopic composition of bulk OC, abundance and distribution of long-chain fatty acids (LCFAs), sedimentological properties (e.g. grain size, mineral surface area, etc.) of fluvial (riverbed and riverbank) sediments and soils from the Godavari basin were analysed and these characteristics were compared to those of a sediment core retrieved from the continental slope depocenter. Results show that river sediments from the upper catchment exhibit higher total organic carbon (TOC) contents than those from the lower part of the basin. The general relationship between TOC and sedimentological parameters (i.e. mineral surface area and grain size) of the sediments suggests that sediment mineralogy, largely driven by provenance, plays an important role in the stabilization of OM during transport along the river axis, and in the preservation of OM exported by the Godavari to the Bay of Bengal. The stable carbon isotopic (δ^(13)C) characteristics of river sediments and soils indicate that the upper mainstream and its tributaries drain catchments exhibiting more ^(13)C enriched carbon than the lower stream, resulting from the regional vegetation gradient and/or net balance between the upper (C_4-dominated plants) and lower (C3-dominated plants) catchments. The radiocarbon contents of organic carbon (Δ^(14)C_(OC)) in deep soils and eroding riverbanks suggests these are likely sources of old or pre-aged carbon to the Godavari River that increasingly dominates the late Holocene portion of the offshore sedimentary record. While changes in water flow and sediment transport resulting from recent dam construction have drastically impacted the flux, loci, and composition of OC exported from the modern Godavari basin, complicating reconciliation of modern-day river basin geochemistry with that recorded in continental margin sediments, such investigations provide important insights into climatic and anthropogenic controls on OC cycling and burial.

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