造山帶中岩石源有機碳氧化在碳循環的重要性：以卑南溪流域為例

Abstract

岩石源有機碳的氧化會釋放大量二氧化碳至大氣中，進而抵銷原本可藉由長期地質碳封存作用所去除的二氧化碳，造山運動可將岩石源有機碳快速抬升至地表，促進氧化作用的進行，然而其具體的氧化途徑與條件仍未被充分釐清，尤以造山帶變質岩中的高度石墨化岩石源有機碳為甚。在此框架下，台灣東部的卑南溪流域位處活躍造山帶，並廣泛分佈蘊含碳質的變質岩，可作為研究岩石源有機碳氧化的理想天然場域，卑南溪為台灣主要的沈積物輸出源之一，在此流域中，快速剝蝕雖得以促進氧化作用，但因高侵蝕速率，岩石源有機碳於短時間內經由河流系統輸送至海洋埋藏，氧化作用的相對重要性需進一步探討。本研究首先透過拉曼光譜分析，評估卑南溪流域及其相連的海底峽谷中，陸源及海洋材料的岩石源有機碳結構轉變，結果顯示，高度石墨化結構在土壤發育過程中轉變為相對無序的結構，後者又於海洋沈積物往深海搬運過程中耗損，揭示岩石源有機碳的二階段氧化歷程，並經由同位素質量平衡模型估算陸域近地表氧化所釋放的二氧化碳通量為20–35 tC km-2 yr-1。為探討地下龐大的岩石源有機碳儲庫所貢獻的氧化通量，本研究進一步設計適用於變質岩地層的地下二氧化碳現地連續測量系統，並於流域內一處坡頂井孔中，分別以被動模式或主動抽氣循環模式進行觀測，結果顯示，地下二氧化碳通量顯著受控於岩體結構，且主動抽氣可能導致通量的高估。此測量方法後續延伸應用至原井孔與流域內另一位於坡腳的井孔，並擴大觀測時間範圍，測量結果顯示坡腳的通量雖較高，但同位素質量平衡模型則指出岩石源有機碳氧化對坡頂二氧化碳有更高的貢獻比例；本研究以坡頂井孔層理清晰的地層特性作為代表參數設置，建立一維擴散反應模型，並結合實測數據，探討地層內非均質構造對於氣體遷移與通量估算的影響，此模型估算出地下岩石源有機碳氧化可釋放至大氣的二氧化碳通量為2–9 tC km-2 yr-1，此數值的估算並未考慮反應帶的分佈或於深度的延伸性，亦未納入不同反應物與環境條件的差異，此保守的估算量可達近地表通量的10%以上，更高於大河流域的地表氧化通量至少一個數量級，顯示造山帶變質岩中的石墨化岩石源有機碳具高度反應性，並扮演長期碳循環中的重要角色。

The oxidation of petrogenic organic carbon (OCpetro) has been reported to contribute a large CO2 flux to the atmosphere that can counteract geological sequestration processes. While mountain building processes rapidly expose OCpetro to surface environments conductive to aerobic oxidation, where and how OCpetro is susceptible to degradation in active orogens remain elusive. Particularly, the degradability of highly graphitic OCpetro in metamorphic rocks, which constitute a large portion of active orogens, is poorly constrained and requires further investigations. In this regard, the Beinan catchment, predominantly composed of carbonaceous metamorphic rocks and situated in the active Taiwan orogen, provides an ideal natural setting for studying OCpetro oxidation. The Beinan River represents one of the major sediment exports in Taiwan. While OCpetro oxidation is facilitated by rapid denudation in this catchment, OCpetro in rock detritus is efficiently transported by rivers to the ocean for burial, reducing the likelihood of further oxidation within such a short timeframe. Whether OCpetro oxidation predominates under these two competing pathways needs further investigations. In this thesis, we first demonstrated the structural transformation of OCpetro in both terrestrial and marine materials from the Beinan catchment and its connecting submarine canyon by using Raman spectroscopy. The increased disorder in graphitic structures for weathered materials and the depletion of disordered forms for marine sediments suggest a two-stage oxidation of highly graphitic OCpetro during soil development and submarine transit. The isotopic mixing model further yielded a CO2 flux of 20–35 tC km-2 yr-1 to the atmosphere during soil development. In order to account for the oxidation flux from the vast reservoir in the subsurface, we proposed a novel design for continuous in situ measurements and large-volume sampling of subsurface CO2 feasible for metamorphic rocks and applied it to a borehole at a hilltop in the studied catchment either by passive mode or active recirculation. Our measurements revealed that subsurface CO2 fluxes were highly dependent on geological context and suggest a potential overestimation by active recirculation. The same methodology was further extended to the measurements at the original site and the base of another hillslope over a broader observational timeframe. The CO2 measurements and isotopic analyses revealed a lower CO2 flux but higher contribution of OCpetro oxidation at the hilltop relative to the hill base. To transform the borehole observations to a more realistic and quantifiable framework for subsurface gas production and transport, a one-dimensional diffusion-reaction model was developed on the basis that the formation characteristics at the hilltop site was representative and clearly distinguishable to address the influence of heterogeneously distributed formation fabrics. Using field measurements as boundary constraints, the model yielded a CO2 emission from COpetro oxidation at 2–9 tC km-2 yr-1. This high oxidation flux represents a conservative estimate, as it does not account for the spatial and vertical extent of the reaction zone, nor variations in the graphitization degree of OCpetro and environmental conditions. Nevertheless, the flux amounts to over 10% of oxidation flux in the surface soil and is higher than the fluxes for large river systems, underscoring the reactivity of graphitic OCpetro in metamorphic rocks within active orogens and its important role in the geological carbon cycle.