台灣西南海域海底泥火山之氣體通量時序變化

Abstract

台灣西南海域上部斜坡區域分布著13座海底泥火山，間歇性的噴發以甲烷為主的氣體、流體至海水中，並有機會逸出海表到大氣中，成為重要的溫室氣體來源，不論是水中氣柱的大小、底水溶解氣的濃度、或是海洋輸入到大氣中的通量，都有隨著時間而變化的現象，本研究利用中山大學海研三號研究船於2013–2014年之間的航次，採集了CTD海水剖面 ( water column ) 以及泥火山區的岩心沈積物，並佐以魚探深儀 ( echo sounder )的噴氣資料，以探討甲烷在濃度與通量上的變化。 岩心中間隙甲烷氣體在相當淺的位置 ( 70 cm ) 濃度便可以高達 38,522 μl/l 並且伴隨這硫酸鹽的濃度從背景值的 30 mM降到 5 mM左右 (SMI) ，且孔隙水中溶解無機碳 ( DIC ) 碳同位素從表層的0 ‰ 降到 –30 ‰左右。甲烷之碳同位素分析結果約為 –12 ~ –21 ‰，且辦隨著少量的乙烷，表示此區域甲烷絕大部分為熱分解性來源。 本研究在MV1、MV10、MV12三座海底泥火山採集海水剖面並記錄逐時的噴氣大小變化，利用噴氣大小、底層海水的甲烷濃度、表層海水的甲烷濃度，透過氣體擴散方程式 ( Gas diffusion equation )與擴散速率 ( Diffusion rate ) 的計算，結果顯示淺水、噴氣柱較大的泥火山有更多的機會將沈積物或是深部的甲烷帶到大氣中，三座泥火山估算出的最大通量分別是：MV10：4.3 μmol/ m2/d, MV1：47.1 μmol/ m2/d, MV12：4.9μmol/ m2/d。

Submarine mud volcanoes are features that episodically emit gases, fluids, and mud onto the seafloor. Methane is the representative gas transport by mud volcanoes efficiently from deep buried sediment to the water column, and potentially to the atmosphere as a greenhouse gas. Gas plume, dissolve methane concentration, methane flux to the atmosphere changes from time to time. This study was conducted during cruise OR3-1693, OR3-1755, OR3-1756, OR3-1776. To understand the activity of gas emissions of mud volcano, we utilized the 38kz echo sounder to scan back and forth over mud volcanoes and obtained acoustic images of plumes. Water column samples were collected above the venting at the tidal maximum and minimum. Three gravity cores taken at the mudflow site of G96 show high methane concentration (38,522 μl/l ) at shallow depth of 70 cm, while sulfate concentration drop from background value 30mM to 5mM. Dissolved inorganic carbon( DIC) decrease from 0 ‰ to -30 ‰. δ13C of methane located between -12 and -20‰, indicated that methane are mainly thermalgenic in origin. By calculating top water methane concentration, bottom water methane concentration with gas diffusion equation and diffusion rate, the results show that at shallow water mud volcanoes and mod volcanoes with larger plumes, methane can be transfer to the atmosphere more sufficiently. Methane flux to the atmosphere of three mud volcanoes: MV10：4.3 μmol/ m2/d, MV1：47.1 μmol/ m2/d, MV12：4.9μmol/ m2/d。