Net effect of chemical erosion in a tropical basin on carbon cycle: Constraints from elemental and sulfur isotopic composition of the Mahanadi river water

Abstract

Chemical erosion in river basins influence the climate by both consuming (via silicate erosion and carbon burial) and releasing (via H2SO4-mediated carbonate weathering and organic oxidation) atmospheric CO2. Net erosional effect of a basin on carbon cycle, therefore, requires precise rate assessment of all these pathways; combined data for the four processes for a given basin are sparse. In this contribution, we have quantified these process intensities for a large tropical basin (Mahanadi river) from the eastern part of India using new time-series (weekly) data for dissolved major ions and sulfur isotopes, and compiled data for rhenium concentrations and particulate carbon isotopes for the basin. The discharge-weighted silica concentrations (∼215 μM) of these samples is higher than that reported as global average for rivers (∼145 μM), indicating intense silicate erosion in the basin. The δ34S of the riverine sulfates vary between 8.8 ‰ and 17 ‰, with a discharge-weighted average of 12 ± 4 ‰ (global riverine value ∼4.4 ± 4.5‰). Temporal changes in concentrations for most of the elements (and δ34S) show relatively higher (enriched) values during lean-flow stages. Mass balance calculations involving Ca, Mg and Si values confirm that these seasonal changes are mainly due to increase (by 18 ± 9%) of groundwater supply to the stream during non-monsoon seasons. The silicate-derived cations (Catsilicate) for the Mahanadi show minimal changes during monsoon (36 ± 5%) and non-monsoon (33 ± 8%) seasons. These data correspond to a CO2 consumption rate of 2.4 ± 1.6 tC/km2/yr, which is about two times higher than that reported for global rivers. Comparison of δ34S values of riverine sulfate with its possible sources computes the relative supply of sulfates from atmospheric deposition (38%), gypsum (46%), and sulfides (16% (a lower limit)) to the river. The minimal contribution via sulfide oxidation confirms low CO2 release (0.1 ± 0.1 tC/km2/yr) in this basin through sulfuric-acid mediated carbonate weathering. Average rhenium for this river is 5 ± 3 pmol/kg. The source apportionment using S/Re and Na/Re ratios indicates that 85 ± 10% of the Re is derived through organic oxidation. The rhenium data are used to estimate the CO2 release rate (0.50 ± 0.57 tC/km2/yr) by oxidation of petrogenic carbon, which is higher than the estimated organic burial rate (0.27 ± 0.12) for the basin. The estimated four erosion rates confirm a net negative (∼ − 2.1 ± 1.7 tC/km2/yr) effect of chemical erosion in the Mahanadi basin on the CO2 budget. Low sulfide-oxidation in this supply-limited regime is possibly linked to limited exposure of fresher minerals, whereas high silicate erosion in the basin is consistent with its silicate-dominated lithology and tropical climate.