Environmental Controls on the Riverine Export of Dissolved Black Carbon

Matthew W Jones,Thiago P Rangel,Timothy A Quine,Carlos E De Rezende,Marcelo G Almeida,Luiz E O C De Aragão,Jutta Niggemann,Ben T Johnson,Thorsten Dittmar,Jomar S J Marques

doi:10.1029/2018gb006140

Abstract

AbstractEach year, tropical rivers export a dissolved organic carbon (DOC) flux to the global oceans that is equivalent to ~4% of the global land sink for atmospheric CO2. Among the most refractory fractions of terrigenous DOC is dissolved black carbon (DBC), which constitutes ~10% of the total DOC flux and derives from the charcoal and soot (aerosol) produced during biomass burning and fossil fuel combustion. Black carbon (BC) has disproportionate storage potential in oceanic pools and so its export has implications for the fate and residence time of terrigenous organic carbon (OC). In contrast to bulk DOC, there is limited knowledge of the environmental factors that control riverine fluxes of DBC. We thus completed a comprehensive assessment of the factors controlling DBC export in tropical rivers with catchments distributed across environmental gradients of hydrology, topography, climate, and soil properties. Generalized linear models explained 70 and 64% of the observed variance in DOC and DBC concentrations, respectively. DOC and DBC concentrations displayed coupled responses to the dominant factors controlling their riverine export (soil moisture, catchment slope, and catchment stocks of OC or BC, respectively) but varied divergently across gradients of temperature and soil properties. DBC concentrations also varied strongly with aerosol BC deposition rate, indicating further potential for deviation of DBC fluxes from those of DOC due to secondary inputs of DBC from this unmatched source. Overall, this study identifies the specific drivers of BC dynamics in river catchments and fundamentally enhances our understanding of refractory DOC export to the global oceans.

Highlights

The riverine export of dissolved organic carbon (DOC) is one of the major fluxes of carbon across the land‐to‐ ocean aquatic continuum (Regnier et al, 2013)
It is estimated that 208 ± 28 Tg of DOC per year is exported by rivers to the global oceans, predominantly from stocks of soil organic carbon (SOC) in their drainage catchments, with 62% of this export occurring in tropical rivers (Dai et al, 2012)
Our results show that the majority of the observed variance in dissolved black carbon (DBC) and DOC concentration is explained by catchment properties, especially hydrological factors; the unexplained variance of the fitted models might relate in part to active pipe processes that are not represented by our study design

Summary

Introduction

The riverine export of dissolved organic carbon (DOC) is one of the major fluxes of carbon across the land‐to‐ ocean aquatic continuum (Regnier et al, 2013). The recent realization that ignoring lateral fluxes of carbon from terrestrial to marine environments results in nontrivial errors in terrestrial carbon accounting, combined with evidence for the anthropogenic perturbation of the DOC export, has triggered a renewed focus on identifying the environmental factors that control its export, character, and fate (Battin et al, 2009; Cole et al, 2007; Raymond et al, 2016; Regnier et al, 2013) As part of this agenda it will be pivotal to develop a mechanistic understanding of the environmental factors that control the export of DBC across the land‐to‐ocean aquatic continuum (Coppola et al, 2018; Dittmar & Stubbins, 2014; Wagner et al, 2018)

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