The carbon budget of the Mekong River Basin : a spatial and temporal study of chemical weathering

The chemical weathering of silicate rocks with carbonic acid is thought to play an important role in the consumption of atmospheric carbon dioxide (CO₂), which regulates global climate over million-year timescales. However, the climatic implications of chemical weathering of carbonate rocks with sulfuric acid, a process that can release geologically stored carbon to the atmosphere, are not thoroughly understood. Depending on the reaction environment the lithologically-sourced carbon released from the sulfuric acid weathering of carbonates can either degas as CO₂ instantaneously to the atmosphere, or can be transferred as bicarbonate (HCO₃⁻) to the oceans to be precipitated as carbonate, releasing CO₂ on million-year timescales. It is important to consider the timescale of CO₂ release to assess whether a river basin is a transient, or long-term source of carbon to the atmosphere. Few studies have highlighted the importance of this weathering reaction and less have quantified the impact of CO₂ release from sulfuric acid weathering of carbonates in large scale catchments. Quantifying carbonate weathering with sulfuric acid requires the source of riverine sulfate (SO₄²⁻) to be determined. This comes predominantly from two sources: sedimentary sulfate and sulfide. Sulfate released from the weathering of gypsum or anhydrite plays no role in the carbon cycle. Oxidative weathering of sedimentary sulfides, predominantly pyrite, produces sulfuric acid which can react with carbonates to release CO₂. Here, new coupled sulfur, δ³⁴S_SO₄, and oxygen, δ¹⁸O_SO₄, isotope data on dissolved riverine sulfate and river water isotopes, δ¹⁸O_H₂O, from one of the world's largest rivers, the Mekong in Southeast Asia, are presented. A new two end member mixing model is used to partition sources of dissolved sulfate. Importantly, sulfate sources cannot be distinguished using δ³⁴S_SO₄ alone, and hence δ¹⁸O_SO₄ must also be determined. The Mekong is the world's 10th largest river in terms of discharge, yet is disproportionately understudied compared to many other large rivers where chemical weathering rates have been investigated. This study sampled 36 tributaries and 10 locations along the main channel to calculate the carbon budget of the Mekong River. Samples were collected over three field seasons, as well as a bi-monthly time-series from 2014-2017. HCO₃⁻ and SO₄²⁻ concentrations in the Mekong are up to 2709μmol/L and 720.3μmol/L respectively, and generally decrease downstream from the headwaters on the Tibetan Plateau. Samples display up to 20.7‰ difference in Δ¹⁸O_SO₄⁻H₂O, and δ³⁴S_SO₄ ranges by ~ 13.5‰ over the basin. The proportion of sulfate derived from the oxidative weathering of pyrite, f_pyr, calculated by the two end member mixing model varies between 0.18 and 0.83 in Mekong tributaries with a mean of 0.60 in the main channel. Sources of dissolved inorganic carbon (DIC) are partitioned using a forward model which incorporates the source of acidity in weathering reactions (carbonic or sulfuric) and the use of individual Ca/Na and Mg/K ratios in silicate end members of each tributary. Charge balance calculations with partitioned cations suggest the most likely reaction pathway of CO₂ release from sulfuric acid weathering of carbonates is instantaneous degassing. The weathering of carbonates by carbonic acid accounts for on average 81% of the total DIC flux for all tributaries, whilst carbonic acid weathering of silicates contributes on average 19% (ranging 7-60%) of the total DIC flux in tributaries. Using a framework to track partitioned Ca²⁺ ions, CO₂ consumption or release in the Mekong basin is shown graphically. On timescales shorter than carbonate precipitation, all Mekong tributaries are a sink of atmospheric CO₂. Annually, the instantaneous release of CO₂ from oxidative pyrite--driven weathering of carbonates is ~ 16 times smaller than the drawdown of atmospheric carbon by carbonic acid weathering of both silicate and carbonate minerals. On million-year timescales, the headwater regions in China and one karst dominated tributary in the Middle Mekong release CO₂. The carbon budget of the Mekong varies throughout the year: during the monsoonal months the Mekong consumes atmospheric CO₂ but CO₂ is released during the dry season. Carbonic acid weathering of carbonates is carbon-neutral on million-year timescales, whereas the release of CO₂ from sulfuric acid weathering of carbonates is large enough to marginally offset the long-term sequestration of atmospheric CO₂ by carbonic acid weathering of silicates. Time-series samples and discharge measurements at Chroy Changvar, close to the mouth, are used to determine that the Mekong River basin is an annual net source of 0.01tC.km⁻².yr⁻¹ to the atmosphere. This differs to previous estimates, which indicate atmospheric CO₂ consumption by the Mekong. This thesis highlights the importance of determining the origin of sulfate in the world's largest rivers for the global carbon cycle, particularly in catchments with a high proportion of carbonate lithology.