Abstract
The chemical composition of the Gaoping River in Taiwan reflects the weathering of both silicate and carbonate rocks found in its metasedimentary catchment. Major dissolved ion chemistry and radiocarbon signatures of dissolved inorganic carbon (DIC) reveal the importance of pyrite-derived sulphuric acid weathering on silicates and carbonates. Two-thirds of the dissolved load of the Gaoping River derives from sulphuric acid-mediated weathering of rocks within its catchment. This is reflected in the lowest reported signatures DI14C for a small mountainous river (43 to 71 percent modern carbon), with rock-derived carbonate constituting a 14C-free DIC source. Using an inverse modelling approach integrating riverine major dissolved ion chemistry and DI14C, we provide quantitative constraints of mineral weathering pathways and calculate atmospheric CO2 fluxes resulting from the erosion of the Taiwan orogeny over geological timescales. The results reveal that weathering on Taiwan releases 0.31 ± 0.12 MtC/yr, which is offset by burial of terrestrial biospheric organic carbon in offshore sediments. The latter tips the balance with respect to the total CO2 budget of Taiwan such that the overall system acts as a net sink, with 0.24 ± 0.13 MtC/yr of atmospheric CO2 consumed over geological timescales.
Highlights
Taiwan is one of the most rapidly uplifting orogens, with erosion rates in the order of 3–6 mm/yr continuously exposing fresh minerals for chemical weathering[1,2]
Carbon isotopic compositions were centred at −5.3 ± 1.3‰ for DI13C (n = 13), while DI14C values ranged from 43 to 68 pMC
Following the theoretical outline provided in the introduction, rainwater-corrected river data from the Gaoping River are plotted in Fig. 2a showing the mixing of the four end members, which adhere to theoretical expectations in the case of quaternary mixing of solute derived from silicate and carbonate weathering via sulphuric and carbonic acid in the absence of evaporite contributions (Fig. 2a)
Summary
The trails of evidence of these weathering reaction pathways (2–5) lead to unique signatures in the dissolved ion load (see Table 1). In reaction (2), carbon is sourced primarily from the atmosphere, which exhibits a modern signature, expressed as 100% modern carbon (pMC). The reaction pathway of silicates and carbonates dictates the net effect of weathering on atmospheric CO23,4, with the dissolution of carbonates by sulphuric acid acting as a CO2 source over geological time scales[10]. We quantitatively disentangle the inputs of silicate and carbonate weathering via carbonic and sulphuric acid dissolution by determining DI14C and dissolved ion compositions within the Gaoping River catchment of Taiwan, leading to new quantitative estimates on the effect of the Taiwan orogeny on atmospheric chemistry
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