Dynamics in riverine inorganic and organic carbon based on carbonate weathering coupled with aquatic photosynthesis in a karst catchment, Southwest China

Highlights

• [HCO₃⁻] depended on geological condition and varied with discharge in a karst river.

• In-river primary production contributed great to organic carbon in karst area.

• Carbon sink in karst varied with discharge and inputs of sulfuric and nitric acids.

• Carbon sink produced by “biological carbon pump” in a karst river was considerable.

Abstract

Carbonate mineral weathering coupled with aquatic photosynthesis, herein termed ‘coupled carbonate weathering’ (CCW), represents a significant carbon sink which is determined by riverine hydrochemical variations. The magnitudes, variations and mechanisms responsible for the carbon sink produced by CCW are still unclear. In this study, major ions, TOC and discharge data at the Darongjiang, Lingqu, Guilin and Yangshuo hydrologic stations in Li River basin, a karst catchment typical of this geographic region, were analysed from January 2012 to December 2015 to elucidate the temporal variations in riverine inorganic and organic carbon and their controlling mechanisms. The results show that (1) HCO₃⁻ was sourced from carbonate weathering and silicate weathering, carbonate weathering by carbonic acid being predominant; (2) TOC was created chiefly by the transformation of bicarbonate to organic carbon by aquatic phototrophs during the non-flood period; (3) The carbon sink produced by coupled carbonate weathering in the Li River basin was calculated to be 14.41 tC·km⁻²·yr⁻¹, comprised of the sink attributable to carbonate weathering (12.17 tC·km⁻²·yr⁻¹) and sink due to the “biological carbon pump” (S_BCP) (2.24 tC·km⁻²·yr⁻¹). The S_BCP thus accounted for approximately 15.54% of the total carbon sink, indicating that the proportion of riverine TOC sourced by the transformation from bicarbonate to organic carbon by aquatic phototrophs may be high and must be considered in the estimation of carbonate weathering-related carbon sinks elsewhere.

Introduction

The chemical composition of river waters provides indications of the weathering of rock and associated CO₂ consumption, biological functions, and human activities in their basins (Gaillardet et al., 1999; Hindshaw et al., 2011; Li et al., 2011). The ion content is remarkably higher in carbonate terrains than in silicate terrains, and dominated by bicarbonate and calcium because of the rapid and sensitive process of carbonate weathering (Liu and Dreybrodt, 1997; Liu et al., 2007; Raymond et al., 2008). Therefore, the reaction between carbonate minerals and CO₂, which increase dissolved inorganic carbon (DIC) concentrations, may impact the carbon cycle and represent a net sink of atmospheric CO₂ at short time scales (Martin, 2017). In contrast, carbonate minerals dissolved by sulfuric and nitric acids may represent an atmospheric sources of CO₂ with no rapid sink to balance them (Perrin et al., 2008; Torres et al., 2014; Martin, 2017). Due to the inputs of sulfuric and nitric acids, the participation of sulfuric and nitric acids in carbonate dissolution was common in karst area (Li et al., 2008; Perrin et al., 2008; Yu et al., 2016; Zhang et al., 2020; Li et al., 2020; Li et al., 2021). Which are the dominant acids in carbonate weathering should thus be established first in karst basin studies. Bicarbonate was mainly sourced from carbonate mineral weathering in carbonate terrains and mixed silicate-carbonate terrains, and can be consumed by aquatic phototrophs and transformed into organic carbon in inland waters (Liu et al., 2011). During the bicarbonate uptake by aquatic phototrophs, the hydrochemical characteristics such as HCO₃⁻ concentration, pH, dissolved oxygen and carbon isotope of DIC changed significantly, which has been demonstrated both in laboratory studies (Liu et al., 2010; Wu et al., 2012; Wang et al., 2013), in the field in the outflow of springs and subterranean rivers, for example, Zhang et al. (2012); Chen et al. (2014); and Pu et al. (2017), and in Li River basin (Zhang et al., 2017; Wang et al., 2019; Sun et al., 2019). Such atmospheric CO₂ uptake by the reaction between carbonate rock and carbonic acid and the resultant bicarbonate transformation into organic carbon by aquatic photosynthesis is termed ‘coupled carbonate weathering’ (CCW - Liu et al., 2018).

The balancing of the atmospheric CO₂ budget is a critical question in research on global climate change. At present, it is accepted that there is a large residual land sink of 1.8~3.4 PgC·a ^− 1 in the global carbon budget (Melnikov and O'Neill, 2006; Lal, 2008; Ciais et al., 2013; Le Quéré et al., 2013, 2014; Friedlingstein et al., 2019). The locations, magnitudes, variations and controlling mechanisms of this sink need to be more fully understood. Previous studies show that rock chemical weathering contributes to the atmospheric carbon sink (Berner et al., 1983; Liu et al., 2010), the contributions of carbonate weathering being greater than 90% (Liu et al., 2018) due to its abundance and comparatively rapid solubility (Liu et al., 2011). The bicarbonate concentration in global rivers ranged from 0.1 to 4.7 mmol·L⁻¹ (Li et al., 2019; Zhong et al., 2020) and global fluxes from rivers to the ocean was 0.211 PgC·yr⁻¹ (Liu et al., 2018). Based on the CCW model, atmospheric CO₂ uptake by interactions between water, carbonate minerals, CO₂, and aquatic phototrophs on land (the CCW-related carbon sink) has been estimated to be as large as 0.5 PgC·a ^− 1 (Liu et al., 2018), and thus should be included in global carbon budget calculations. CCW-related carbon sink estimations should be based on hydrochemical data, e.g., discharge, concentrations and sources of the inorganic and organic solutes and suspensions, all of which can be strongly impacted by climate, by land-use changes and other anthropogenic activities (Liu et al., 2010; Cao et al., 2011; Zhang et al., 2015; Zeng et al., 2016; Chen et al., 2017; Zeng et al., 2017; Zhang et al., 2020). Further investigations of the variations, mechanisms, and accurate estimates of CCW-related carbon sinks are needed for river basins with substantial karst rock content.

This study determines the amounts of bicarbonate-transformed organic carbon in the riverine organic carbon and assesses the CCW-related carbon sinks based on the sources of inorganic and organic carbon, and the discharge through revealing the dynamics of bicarbonate sourced from carbonate weathering by carbonic acid, by sulfuric or nitric acid, and from silicate weathering, over the period from January 2012 to December 2015 at four gauging and sampling stations, upstream to downstream, along a karst river in southern China. The amounts of bicarbonate-transformed organic carbon in the riverine organic carbon were determined. The CCW-related carbon sinks were estimated based on the sources of inorganic and organic carbon, and the discharge. The analytical results combined with the geological setting will help to improve evaluations of the sources of riverine inorganic and organic carbon and improve our understanding of CCW-related carbon sinks.