台灣西南海域極端事件引發海底地質災害對有機碳埋藏的影響

Abstract

台灣西南海域於2006及2009年分別經歷屏東地震與莫拉克颱風事件所引發的海底地質災害，並沿高屏海底峽谷造成一系列海底電纜斷裂。由於此區域具有陸棚短窄及多條海底峽谷等特徵，使得大量陸源沉積物得以快速傳輸至深海。本研究利用210Pb、粒徑大小與組成、總有機碳含量及穩定碳同位素值等分析方法，探討極端事件對有機碳埋藏的重要性。本研究選取2006~2013年間的所採集11支岩芯，範圍涵蓋高屏海底峽谷、枋寮海底峽谷及高屏陸坡，分別用以代表不同區域之特徵。整合岩芯之210Pb剖面及粒徑數據，並於適合深度取樣進行有機碳分析。結果顯示沉積物的TOC含量約為0.5%，C/N比值介於4∼8，且近岸較深海的變化幅度大。δ13C分析結果呈現隨離岸距離的增加趨於越大。本研究由210Pb剖面劃分出事件層與正常堆積時期，利用TOC含量及δ13C值進一步評估單次極端事件所埋藏之陸源有機碳量遠超過平日堆積，暗示大規模海底地質災害傳輸大量陸源沉積物至深海堆積與埋藏的潛能不容小覷，可能對碳循環的收支平衡意義重大。

The tectonically active setting and climatic conditions give Taiwan a high exposure to severe natural hazards. After the Pingtung Earthquake and Morakot Typhoon which occurred in 2006 and 2009, the turbidity currents caused a series of submarine cable breaks along the Gaoping and Fangliao Submarine Canyons off SW Taiwan. Large amounts of terrestrial sediments were fast transported bypass the narrow continental shelf and rapidly moved southward through submarine canyon to the deep sea. This study aims to evaluate the impact of extreme events on the transportation and burial of organic carbon. Two piston cores which were taken from the Maiden Ridge and its adjacent area (MT7 and MT6) might shed light on understanding the export of terrestrial organic carbon to the abyss by natural geohazards. The 210Pb profile of MT7 in conjunction with the grain size data indicates the existence of the Pingtung Earthquake and Morakot Typhoon related deposits. The sedimentation rate of these two cores which derived from 210Pb is approximately 0.05 cm/yr. The cores collected from the Gaoping Submarine Canyon, Gaoping Slope and Fangliao Submarine Canyon are used for analyzing TOC, organic C/N and δ13C ratios. The concentrations of total organic carbon are ~0.5%, and C/N rations almost remain between 4 and 8. The high TOC (~1%) and C/N ratio (>10) are observed in the samples with plant debris. The fluctuation of TOC and C/N ratios in near-shore samples is higher than deep sea. In terms of δ13C-values, it progressively decreases with distances from coastal zone to the deep sea. Due to the larger proportions of land-derived organic carbon, the δ13C-values in the surface sediment of upper Gaoping Submarine Canyon, Gaoping Slope, and the turbidite layers at the head of Fangliao Submarine Canyon are lighter. Furthermore, this study uses the TOC concentrations and δ13C-values to estimate the fractional contributions of terrestrial organic carbon by a simple two component mixing model, and integrate with the 210Pb-derived sediment accumulation rates to evaluate the organic carbon burial between event layers and normal accumulation period. Our results show the extreme natural geohazard events not only fast transported sediments into the deep sea but also delivered large amounts of organic carbon into abyss, and it may play an important role on the global carbon cycling system.