卑南溪流域化學風化作用對二氧化碳收支的影響

岩石的化學風化在地質時間尺度的全球碳循環中扮演著至關重要的角色。研究重點大多關注於風化酸源的產生路徑、岩石抵抗風化的程度以及風化速率與氣候、構造運動的相關性。前人研究集中在測量大河系統的溶質及沉積物傳輸總量，然而位於亞洲以及大洋洲之快速造山帶的河系其單位流域面積所傳輸的溶質及沉積物速率明顯高於大河系統。但上述流域確切的化學風化速率及途徑仍難以量化。本研究旨在研究台灣東南部卑南溪流域的化學風化模式，及其隨著時空分布上的變化。卑南溪流域位於全台灣抬升速率最快的地區之一，並且有著全台灣河川中最高的沉積物傳輸通量。本研究在 2016 至 2018 年間對不同支流的河水進行採樣與溶質分析，結果顯示Ca2+、SO42-以及溶解態無機碳是河水中主要成分，尤其是主要來自黃鐵礦所氧化產生之 SO42- 的濃度可以高於世界河川平均值的2 至 16 倍。本研究使用新的計算方法來評估河水中的溶解物質的來源與成因，結果顯示由硫酸驅動碳酸岩風化產生的陽離子電荷量比例在全流域中占最高比例，證明此途徑在整個集水區內的岩石化學風化佔有重要地位，而全流域產出的化學風化通量至少高出全球大河平均值 3 個數量級以上。除此之外，計算結果指示化學風化過程可造成 CO2 釋放至大氣圈中，其總量將與硫酸驅動碳酸岩風化的總量有緊密關係。卑南溪上游全年岩石風化皆造成淨釋放 CO2 至大氣圈中，其下游平原與河口區在乾季比濕季有更多的大氣二氧化碳淨消耗總量。總結來說，本研究證明在快速造山帶流域中夾藏在變質岩層中的黃鐵礦氧化所產生的硫酸，調節著集水區內的化學風化路徑與二氧化碳淨釋放量。

關鍵字

卑南溪；化學風化；二氧化碳；快速造山帶河川系統

並列摘要

Chemical weathering plays a vital role in the global carbon cycle over geological time scales. The central themes focus on the pathways of acid production, rock availability for dissolution, and correlations between rates and climatic or tectonic forcing. Most previous efforts are diverted to measure great exports of solutes and sediments from large rivers systems. As small rivers in active orogens across Asia and Oceania could deliver solutes and sediments at a rate surpassing large rivers on the per area basis, the exact flux and reaction pathway of chemical weathering in these catchments remain poorly quantified. This study aims to investigate the spatial and temporal variations in patterns of chemical weathering along the Beinan-Sinwulyu river in southeastern Taiwan where uplifting rates range up to 3 cm/yr and fluxes of sediment export are among the largest in Taiwan. Analysis of river water collected from different tributaries between 2016 and 2018 yields Ca2+, SO42- and dissolved inorganic carbon as the major constitutes. In particular, SO42- concentrations are two to sixteen times higher than the world average value of ~300 μM, which is primarily attributed to be produced from pyrite oxidation. Using a new computational approach to evaluate these solute data, we demonstrate that the major proportion of solutes was contributed from carbonate weathering driven by sulfuric acid at all sites. The catchment-wide flux derived from chemical weathering is at least three orders of magnitude greater than the average of global large rivers. The computation also indicates that the CO2 efflux was correlated with the flux of pyrite-derived carbonate weathering with the net emission occurring in upstream, mountainous regions in all seasons and the net consumption in the downstream plain to estuarine regions in dry seasons. Overall, these results demonstrate a strong modulation of the oxidation of pyrite inherited with metamorphic rocks in chemical weathering and CO2 emission in a rapidly uplifting catchment.