Reconciling drainage and receiving basin signatures of the Godavari River system

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

Global-scale evidence for the refractory nature of riverine black carbon

Author Posting. © The Author(s), 2018. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Nature Geoscience 11 (2018): 584-588, doi:10.1038/s41561-018-0159-8.Wildfires and incomplete combustion of fossil fuel produce large amounts of black carbon. Black carbon production and transport are essential components of the carbon cycle. Constraining estimates of black carbon exported from land to ocean is critical, given ongoing changes in land use and climate, which affect fire occurrence and black carbon dynamics. Here, we present an inventory of the concentration and radiocarbon content (∆14C) of particulate black carbon for 18 rivers around the globe. We find that particulate black carbon accounts for about 15.8 ± 0.9% of river particulate organic carbon, and that fluxes of particulate black carbon co-vary with river-suspended sediment, indicating that particulate black carbon export is primarily controlled by erosion. River particulate black carbon is not exclusively from modern sources but is also aged in intermediate terrestrial carbon pools in several high-latitude rivers, with ages of up to 17,000 14C years. The flux-weighted 14C average age of particulate black carbon exported to oceans is 3,700 ± 400 14C years. We estimate that the annual global flux of particulate black carbon to the ocean is 0.017 to 0.037 Pg, accounting for 4 to 32% of the annually produced black carbon. When buried in marine sediments, particulate black carbon is sequestered to form a long-term sink for CO2.A.C. acknowledges financial support from the University of Zurich Forschungskredit Fellowship and the University of Zurich (grant No. STWF-18-026). M.R., S.A. and M.S. acknowledge support from the University Research Priority Projection Global Change and Biodiversity (URPP-GCB). M.Z. acknowledges support from the National Natural Science Foundation of China (No. 41521064). T.E. acknowledges support from the Swiss National Science Foundation (“CAPS-LOCK” and “CAPS-LOCK2” #200021_140850). V.G. acknowledges financial support from an Independent Study Award from the Woods Hole Oceanographic Institution

Sedimentary hydrodynamic processes under low-oxygen conditions: implications for past, present, and future oceans

The Benguela Upwelling System is situated between 18°S and 26°S and is characterized by seasonally variable upwelling cells, elevated surface primary and secondary productivity, high sedimentary organic carbon concentrations (up to 20 %), and locally stable bottom water anoxia. Between 2014 and 2019, surface sediment cores were collected, subsampled, and analysed using geochemical and sedimentological tools. For the grain size fractionation, a combined sieve-centrifuge-filtering approach was developed to create seven fractions (> 250, 250-200, 200-125, 125-63, 63-10, 10-2, ≤ 2.0 µm) and the density fractionation followed a centrifugation sequence to yield four fractions using sodium polytungstate (NaW) heavy liquid (≤ 1.6, 1.6-2.0, 2.0-2.5, > 2.5 g cm-3) (cf., Wakeham et al., 2009, https://doi.org/10.1016/j.marchem.2009.08.005). The bulk surface sediment and each fraction were analysed for TOC (%), radiocarbon age (F14C), and surface area (m2 g-1). Together with literature data, these samples are used to test and illustrate a novel hypothesis on sediment hydrodynamic properties in oxygen-depleted environments