亞熱帶造山帶之化學風化及其控制因子

Abstract

化學風化作用為岩石為適應地表溫度、壓力所進行的一系列變化過程，會讓岩石中的元素釋放到生態系中，關係到地球系統內的元素循環，其中的碳循環，更因可能牽涉到氣候變化，為重要的研究熱點之一。然而，無論是化學風化的種類（矽酸鹽風化、碳酸鹽風化）、及其與控制因子的關係在台灣皆少有研究。本研究蒐集全島的河水化學資料，配合水利署流量資料進行各個集水區風化速率的計算。結果顯示：北部地區風化速率為102 t km-2 yr-1，其中矽酸鹽風化佔有較高的比例，為68 t km-2 yr-1；西部與東部地區的化學風化速率可高達350 t km-2 yr-1，大多由碳酸鹽風化所貢獻(~280 t km-2 yr-1)。在控制因子中，逕流與化學風化速率高度相關（R2 > 0.60）。另外，化學風化速率與物理侵蝕速率也呈現高度正相關 (R2可達0.71)。但若同時考慮抬升速率，則發現化學風化占總剝蝕速率的比例，會隨著抬升速率上升而降低，說明快速的抬升與構造活動同時促進岩石破碎與物理侵蝕，所造成的大量新鮮岩石碎屑，雖然能加速化學風化速率，但其增加率卻減少，顯現風化能力可能無法與物理侵蝕同步增揚。總結來說，全台灣的化學風化速率約為320 t km-2 yr-1，其二氧化碳消耗速率為32.5×105 mole km-2 yr-1，大約是世界平均的13倍，即使是由沉積岩與變質岩所構成的台灣島，風化速率也可與其他玄武岩以及安山岩島嶼相比擬。突顯台灣不只物理侵蝕，在化學風化的輸出仍然扮演相當重要的角色。

Chemical weathering with CO2 consumption attracts much attention, but the investigation of chemical weathering in Taiwan is very limited and thus the controlling factors on chemical weathering are still unclear. During the past decade, we collected the riverine water chemistry with 29 catchments in Taiwan to clarify the chemical weathering rate (CWR) in this high standing islands. The results show that northern Taiwan has lower CWR of 102 t km-2 yr-1 in which silicate weathering contributes 68 t km-2 yr-1 while western and eastern Taiwan have extremely high CWR of ~350 t km-2 yr-1 where carbonate weathering is the main source of ~280 t km-2 yr-1. Runoff and physical erosion rate (PER) are in proportion to CWR with R2= 0.62 and 0.71, respectively. Besides, comparing CWR with uplift rate and PER, we find that the ratio of CWR to total denudation (CWR/CWR+PER) decreases with increasing uplift rate. It indicates that although uplift and tectonic activities may promote physical erosion and consequently chemical weathering, their rates are not elevated at the same pace. In sum, CWR in Taiwan is 320 t km-2 yr-1 and the CO2 consumption rate is 32.5×105 mole km-2 yr-1, which are both 13 times higher than the global average. It reveals that this kind of the high-standing island may respond for not only sediment exports but the considerable amount of the global CO2 consumption by chemical weathering.