Biogeosciences, 15, 3357–3375, 2018 https://doi.org/10.5194/bg-15-3357-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. Reconciling drainage and receiving basin signatures of the Godavari River system Muhammed Ojoshogu Usman1, Frédérique Marie Sophie Anne Kirkels2, Huub Michel Zwart2, Sayak Basu3, Camilo Ponton4, Thomas Michael Blattmann1, Michael Ploetze5, Negar Haghipour1,6, Cameron McIntyre1,6,7, Francien Peterse2, Maarten Lupker1, Liviu Giosan8, and Timothy Ian Eglinton1 1Geological Institute, ETH Zürich, Sonneggstrasse 5, 8092 Zürich, Switzerland 2Department of Earth Sciences, Utrecht University, Heidelberglaan 2, 3584 CS Utrecht, the Netherlands 3Department of Earth Sciences, Indian Institute of Science Education and Research Kolkata, 741246 Mohanpur, West Bengal, India 4Division of Geological and Planetary Science, California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, USA 5Institute for Geotechnical Engineering, ETH Zürich, Stefano-Franscini-Platz 3, 8093 Zürich, Switzerland 6Laboratory of Ion Beam Physics, ETH Zürich, Otto-Stern-Weg 5, 8093 Zürich, Switzerland 7Scottish Universities Environmental Research Centre AMS Laboratory, Rankine Avenue, East Kilbride, G75 0QF Glasgow, Scotland 8Geology and Geophysics Department, Woods Hole Oceanographic Institution, 86 Water Street, Woods Hole, Massachusetts 02543, USA Correspondence: Muhammed Ojoshogu Usman ([email protected]) Received: 12 January 2018 – Discussion started: 8 February 2018 Revised: 18 May 2018 – Accepted: 24 May 2018 – Published: 7 June 2018 Abstract. The modern-day Godavari River transports large sediment mineralogy, largely driven by provenance, plays an amounts of sediment (170 Tg per year) and terrestrial organic important role in the stabilization of OM during transport carbon (OCterr; 1.5 Tg per year) from peninsular India to the along the river axis, and in the preservation of OM exported Bay of Bengal. The flux and nature of OCterr is considered to by the Godavari to the Bay of Bengal. The stable carbon have varied in response to past climate and human forcing. In isotopic (δ13C) characteristics of river sediments and soils order to delineate the provenance and nature of organic mat- indicate that the upper mainstream and its tributaries drain ter (OM) exported by the fluvial system and establish links to catchments exhibiting more 13C enriched carbon than the sedimentary records accumulating on its adjacent continen- lower stream, resulting from the regional vegetation gradient tal margin, the stable and radiogenic isotopic composition and/or net balance between the upper (C4-dominated plants) of bulk OC, abundance and distribution of long-chain fatty and lower (C3-dominated plants) catchments. The radiocar- 14 acids (LCFAs), sedimentological properties (e.g. grain size, bon contents of organic carbon (1 COC) in deep soils and mineral surface area, etc.) of fluvial (riverbed and riverbank) eroding riverbanks suggests these are likely sources of “old” sediments and soils from the Godavari basin were analysed or pre-aged carbon to the Godavari River that increasingly and these characteristics were compared to those of a sed- dominates the late Holocene portion of the offshore sedimen- iment core retrieved from the continental slope depocenter. tary record. While changes in water flow and sediment trans- Results show that river sediments from the upper catchment port resulting from recent dam construction have drastically exhibit higher total organic carbon (TOC) contents than those impacted the flux, loci, and composition of OC exported from the lower part of the basin. The general relationship from the modern Godavari basin, complicating reconciliation between TOC and sedimentological parameters (i.e. mineral of modern-day river basin geochemistry with that recorded surface area and grain size) of the sediments suggests that in continental margin sediments, such investigations provide Published by Copernicus Publications on behalf of the European Geosciences Union. 3358 M. O. Usman et al.: Reconciling drainage and receiving basin signatures of the Godavari River system important insights into climatic and anthropogenic controls The Godavari River basin (Fig. 1) is an example of on OC cycling and burial. a monsoon-influenced low-latitude river basin and, as the largest non-Himalayan river in India, is of special interest due to its large catchment size and sediment flux to the ocean (Kale, 2002). Draining central peninsular India, the river in- 1 Introduction tegrates rainfall within the core monsoon zone of central In- Rivers form a key component of the global carbon cycle, dia, both reflecting the mean monsoon regime and captur- transporting about 200–400 Tg of particulate organic carbon ing fluctuations in monsoonal rains over the sub-continent. (POC) to the oceans annually (Degens et al., 1991; Ludwig With over 90 % of discharge from the Godavari deriving et al., 1996; Schlünz and Schneider, 2000), with the major- from summer monsoon precipitation (Rao et al., 2005), cor- ity of this POC deposited on the continental margins (Berner, responding offshore sedimentary sequences record past vari- 1989; Hedges, 1992). Much of this POC is mobilized from ations in continental climate as well as anthropogenic activ- soils (Meybeck, 1982; Tao et al., 2015) and augmented by ity within the drainage basin (Cui et al., 2017; Giosan et al., recently biosynthesized higher plant debris, recycled fossil 2017; Ponton et al., 2012; Zorzi et al., 2015). OC derived from erosion of sedimentary rocks, and in situ Prior studies, spanning the Holocene, of a Godavari River- aquatic productivity within the rivers (Hedges et al., 1986). proximal sediment core NHGP-16A from the Bay of Ben- Rivers act not only as conduits linking terrestrial and marine gal (BoB) have revealed marked geochemical and sedimen- tological variations that have been interpreted in the con- reservoirs but also as reactors where terrestrial OC (OCterr) is subject to a myriad of processes resulting in degradation text of both evolving regional hydroclimate and accompa- and modification of the suspended OC load (Aufdenkampe nying changes in land use within the Godavari catchment et al., 2011; Wu et al., 2007; Cole et al., 2007). Although (Giosan et al., 2017; Ponton et al., 2012). Specifically, a dis- 13 a general framework for describing the origin and evolution tinct change in the stable carbon isotopic (δ C) composition of molecular markers of terrestrial vegetation implies an in- of OCterr in different types of river basins is emerging (e.g. Blair and Aller, 2012), a detailed understanding of the im- crease in the proportion of aridity-adapted C4 vegetation be- pact of the diverse and complex array of processes occurring ginning around 4.5 kyr BP (Ponton et al., 2012). This shift in vegetation type is accompanied by increased variability in within river basins on the amount and composition of OCterr that is ultimately exported offshore is still developing. the oxygen isotopic composition of planktonic foraminiferal The flux and nature of OC discharged to the ocean is de- carbonate, suggesting enhanced hydrological variability, po- pendent on a number of factors, including the composition tentially reflecting less frequent (“break monsoon”) but in- of underlying bedrock, geomorphologic properties, and cli- tense rainfall activity within the drainage basin (Ponton et matic factors like temperature and precipitation (Hilton et al., al., 2012). Subsequent down-core geochemical and sedimen- 2008; Leithold et al., 2006). Climate variability on millen- tological measurements on the Godavari-proximal BoB sed- nial and longer timescales is considered to exert an impor- iments have served to paint a more comprehensive picture of tant influence on the export of OC from the terrestrial bio- past changes within the Godavari basin (Giosan et al., 2017). sphere and burial in ocean sediments, with important feed- Notably, sharp increases in sediment accumulation rate dur- ing the late Holocene imply a concomitant increase in flu- backs on atmospheric CO2. Variations in exhumation, oxi- dation, and burial of bedrock OC exported from river basins vial sediment discharge, despite the onset of increasingly arid is considered to exert fundamental controls on atmospheric conditions. Furthermore, detrital Neodymium (Nd) isotopic compositions indicate a shift in sediment provenance at ca. CO2 = O2 balance over longer (> million year) timescales (Berner, 2003). 4.5 kyr BP from a relatively unradiogenic signature consis- Tropical and subtropical rivers are estimated to account for tent with lower-basin bedrock as the primary detrital mineral sources prior to increased contributions from the more ra- more than 70 % of the global OCterr delivery to the oceans (Ludwig et al., 1996; Schlünz and Schneider, 2000) and thus diogenic rocks in the upper basin (Deccan Plateau). Finally, comprise major vectors in land–ocean carbon transport (Auf- these changes recorded in the sediment core are also asso- 14 denkampe et al., 2011; Galy and Eglinton, 2011; Hedges et ciated with increased C age offsets between bulk OC and al., 1986; Schefuss et al., 2016; Spencer et al., 2012). The coeval planktonic foraminifera, suggesting enhanced erosion discharge of such rivers is sensitive to variations in climate, and export of pre-aged OCterr exhumed from deeper soil lay- such as the location and intensity of monsoonal rains and ers (Giosan et al., 2017). Collectively,