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 (CO$_{2}$), which regulates global climate over million--year timescales. However, the climatic implications of chemical weathering of carbonate rocks wi...
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| Format: | Doctoral or Postdoctoral Thesis |
| Language: | English |
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Queens' College
2020
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| Online Access: | https://doi.org/10.17863/CAM.49061 https://www.repository.cam.ac.uk/handle/1810/301985 |
| Summary: | The chemical weathering of silicate rocks with carbonic acid is thought to play an important role in the consumption of atmospheric carbon dioxide (CO$_{2}$), 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 CO$_{2}$ instantaneously to the atmosphere, or can be transferred as bicarbonate (HCO$_{3}^{-}$) to the oceans to be precipitated as carbonate, releasing CO$_{2}$ on million--year timescales. It is important to consider the timescale of CO$_{2}$ 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 CO$_{2}$ release from sulfuric acid weathering of carbonates in large scale catchments. Quantifying carbonate weathering with sulfuric acid requires the source of riverine sulfate (SO$_4^{2-}$) 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 CO$_{2}$. Here, new coupled sulfur, $\delta^{34}S_{SO_4}$, and oxygen, $\delta^{18}O_{SO_4}$, isotope data on dissolved riverine sulfate and river water isotopes, $\delta^{18}$$O_{H_2O}$, 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 $\delta^{34}S_{SO_4}$ alone, and hence $\delta^{18}O_{SO_4}$ must also be ... |
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