台灣西南海域濁流沈積對沈積型黃鐵礦化作用影響之初步研究

Abstract

海洋沈積物中進行的黃鐵礦化作用是全球碳、硫、鐵循環的重要步驟之一，而本文的研究目的在於瞭解濁流沈積物中黃鐵礦化的特徵，以及影響沈積物黃鐵礦化程度的因素。本研究選擇位於台灣西南海域高屏海底峽谷及枋寮海底峽谷中含有濁流沈積物的岩芯進行分析，其中高屏海底峽谷有來自陸上高屏溪輸入的濁流沈積，枋寮海底峽谷則有來自海底邊坡崩塌的濁流沈積物，分析項目包含沈積物有機碳含量、氮含量、黃鐵礦-硫含量與鐵物種含量。 高屏海底峽谷同一岩芯中濁流沈積物較非濁流沈積物中產生較多黃鐵礦，水深較深的濁流沈積物相較於水深較淺的的濁流沈積物中也有較多黃鐵礦，但估算的原始有機碳含量及原始鐵含量在濁流沈積物中均未明顯較多，因此有機碳與鐵物種含量並未控制黃鐵礦的產量。濁流沈積物的黃鐵礦化程度均大於非濁流沈積物，且峽谷深處的濁流沈積物的黃鐵礦化程度大於峽谷淺處的濁流沈積物，因此推測濁流的快速沈積事件以及水深的增加是促進黃鐵礦產生的主要因素。 枋寮海底峽谷淺處濁流沈積物中的黃鐵礦含量與黃鐵礦化程度也高於非濁流沈積物，但估算的原始有機碳及原始鐵含量在濁流沈積物中也未明顯增加，碳氮比值亦顯示有機物的來源相近，說明有機碳的供應量、來源以及鐵物種含量均非造成濁流沈積物中黃鐵礦含量增加的因素。此外峽谷深處穩定沈積的沈積物原始有機碳含量、原始鐵物種含量、黃鐵礦化程度的結果均低於峽谷淺處沈積物，顯示枋寮海底峽谷深處沈積物黃鐵礦的產量與有機碳含量及鐵物種含量有關，而水深的增加可能是造成上述因子變化的因素。 綜合來看，本研究顯示高屏海底峽谷與枋寮海底峽谷濁流沈積物相較於非濁流沈積物可產生相對較多的黃鐵礦，可能是快速的濁流堆積產生了局部的還原環境，有利於有機碳、含鐵礦物及硫化氫的保存，而促進微生物硫酸鹽還原作用進行，並使黃鐵礦化程度增加而埋藏了較多的黃鐵礦。

Sedimentary pyrite formation is one of the most important processes controlling the global C-S-Fe cycle. The aim of this study is to evaluate the turbidite depositional influences on the sedimentary pyrite formation offshore southwestern Taiwan. Previous studies suggested that the turbidite deposits were derived from the Gaoping river in the Gaoping canyon and were transported from the sliding of adjacent slope in the Fangliau canyon. This study performs several geochemical analyses, including organic carbon, nitrogen, pyrite and reactive iron content in sediments to investigate the difference between turbidite and non-turbidite deposits. The pyrite content is higher in turbidite deposits than non-turbidite deposits in the same cores and the deeper turbidite deposits contain more pyrite than that in the shallower turbidite deposits in the Gaoping canyon. However, the calculated initial total organic carbon and reactive iron content among these cores are not significantly different. Thus, the pyrite formation may be not controlled by the supply of organic carbon and reactive iron. The rapid deposition and deeper water depth may enhance the degree of pyritization in the turbidite deposits. The turbidite deposits in the Fangliau canyon show similar behavior with that in the Gaoping canyon. The pyrite content increases in turbidite deposits but is not related to the calculated initial organic carbon content, Corg/Ntotal ratio and reactive iron content. Besides, the organic carbon, reactive iron and pyrite contents are lower in deeper non-turbidite deposits in the Fangliau Canyon, which infers that the organic carbon and reactive iron may control the degree of pyritization in different water depth. In summary, the result indicates that the pyrite content is higher in turbidite deposits both in the Gaoping canyon and the Fangliau canyon. It may infer that a relatively reduced environment could provide by turbidite deposits, which enhances the preservation of organic carbon, iron minerals and sulfide and then increases the degree of pyritization in the turbidite deposits.