Sulphuric acid-mediated weathering on Taiwan buffers geological atmospheric carbon sinks

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.

Figure 1

Gaoping River catchment. Geological and hydrological overview with sampling locations. Redrawn after geologic map of Taiwan, which is openly accessible (https://data.gov.tw/dataset/11004). Topographic features were added from open access digital elevation data (https://data.gov.tw/dataset/35430) and shaded with QGIS (QGIS Development Team (2018). QGIS Geographic Information System. Open Source Geospatial Foundation Project. www.qgis.org). The Gaoping River catchment was outlined from open access data from the Taiwanese government (https://www.wra07.gov.tw/12594/12595/12602/12605/70918/) and the final figure was generated with CorelDRAW (www.coreldraw.com).

Figure 2

Mineral weathering end-member mixing. (a) Mineral unit-normalised sulphate versus bicarbonate abundance. The Gaoping River data collected over multiple years in relation to the quaternary end-member mixing line. Literature data points where ionic compositions reflect incursion of seawater plot away from the quaternary mixing line. One data point from this study (datapoint with highest mineral unit normalised sulphate) deviated from the mixing line due to high ammonium amounts that are attributed to anthropogenic inputs and therefore wasn’t considered further in this study. Removing these anomalies, the data from this study correlates with R² = 0.99, and for all studies with R² = 0.99. (b) Mineral unit-normalised sulphate versus pMC of DIC. The three mineral weathering reactions producing bicarbonate span a ternary mixing triangle. All of the data lie outside of this mixing ternary due to the addition of sulphate from sulphuric acid weathering of silicates. The latter process, which leaves DI¹⁴C on the x-axis unaffected, adds sulphate until a theoretical maximum of one mole per weathered mineral unit, representing 100% silicate weathering by sulphuric acid.

Figure 3

Fluxes of atmospheric CO₂. Net total CO₂ balance of Taiwan in MtC/yr with positive and negative values representing an atmospheric source and sink, respectively. The effects of CO₂ drawdown and release by mineral weathering (this study) and organic geochemical cycles^,, are compared.