台灣主要河域化學風化作用的特徵及其控制因子

Abstract

化學風化作用改變岩石中的礦物或化學成分，並釋放其組成成分至土壤、地下水，透過水的循環匯集到河流，最終排放至大海。其中溶解反應的進行，涉及二氧化碳的匯入或排出至大氣，對於地球系統的元素與碳循環，扮演著重要的角色。台灣位於活動構造造山帶，伴隨快速的抬升與侵蝕作用，亦呈現高於大河系統數個數量級的化學風化速率，因而經常作為研究小河集水區化學風化作用的模式區域。然而不同集水區的分佈與發育，受不同的構造活動與地質特徵的影響，對於化學風化機制與速率在時空的變化與可能的控制因子仍僅有有限的認識。 本研究分析台灣主要18條河流於乾濕季的水質，透過端成分混合模型計算，進而探討流經不同地層或岩性之河域化學風化的反應特徵與速率。研究結果顯示，除了海岸山脈集水區陽離子貢獻主要由矽酸岩風化產生外 (61-88 %)，全台灣河域風化作用是以碳酸岩風化為主，大多數河域碳酸岩化貢獻陽離子當量濃度達總量80 %以上，且其產生的陽離子當量濃度與硫酸鹽濃度呈高度正相關 (R2=0.72)。而根據穩定同位素資料，硫酸根主要為黃鐵礦氧化所產生，並且硫酸根中的氧主要源自於河水中的氧而非氧氣。若以區域區分，西部河域化學風化受岩性、崩塌率及乾濕季影響，碳酸岩風化所產生的陽離子當量濃度，隨集水區變質岩的比例增加而上升，並且乾、濕季都與崩塌率呈現高度正相關 (R2=0.89及0.76)；矽酸岩風化離子當量變化相對較小，僅在岩性涵蓋了西南部麓山帶的集水區，在乾季時發現較高矽酸岩風化產生的陽離子當量濃度。相較之下，東部河域化學風化則與崩塌率呈現正相關，而在海岸山脈隨著集水區內沈積岩的比例增加，矽酸岩風化的程度也會上升。就地質時間尺度的碳循環，台灣大部分的集水區的化學風化作用，是造成大氣二氧化碳濃度的增加，僅有在部分西南部及海岸山脈的集水區為消耗大氣二氧化碳。

Chemical weathering alters mineral or chemical compositions of rocks and releases their constitute elements into soil, groundwater, and through water cycle to rivers prior to eventually reaching the ocean. The dissolution reactions, which either consume or export CO2, play an important role in regulating carbon cycles embedded within the Earth system. Taiwan is located in an active orogenic belt with rapid uplift and erosion. Chemical weathering rates exceed those of large river systems by several orders of magnitude, thereby often being considered as a model for studying chemical weathering in small river basins. While the distribution and development of different catchments are influenced by different tectonic activities and geological features, the exact mechanisms and rates of chemical weathering and possible controlling factors are still unclear. This study analyzed aqueous chemistry of 18 major rivers in Taiwan, and further investigated the characteristics and rates of chemical weathering processes for rivers flowing through different strata or lithologies. The results show that chemical weathering in most catchments is dominated by carbonate dissolution, contributing more than 80% of the total cation equivalents. Exceptions occurred for the Coastal Range where 61-88% of the total cation equivalents were produced from silicate weathering. The cation equivalents derived from carbonate weathering were also highly positively correlated with sulfate concentration (R2=0.72). Analyses of stable isotopes of sulfate yielded a data pattern that suggests the production of sulfate from the oxidation of pyrite, and the oxygen in sulfate mainly derived from the oxygen in the river water rather than from atmospheric dioxygen. Furthermore, weathering characteristics varied with lithology, landslide ratio and wet/dry seasons at different degrees. For western catchments, the cation equivalents from carbonate weathering increased with the proportions of metamorphic rocks and with the landslide ratios (R2 of 0.89 and 0.76 for dry and wet seasons). In contrast, the charge equivalent variation associated with silicate weathering was relatively small with higher charge equivalents only found in the dry season in the catchment draining the southern Western Foothill. For eastern catchments, the cation equivalents from carbonate weathering increased with the landslide ratios, while the proportions of cations contributed by silicate weathering increased with the proportions of sedimentary formations in the Coastal Range . Over a geological time scale, chemical weathering renders the net export of CO2 into the atmosphere from the majority of catchments but vice versa only from some southwestern catchments and the Coastal Range.