台灣西南海域上部陸坡之底棲有孔蟲群落研究：以永安海槽為例

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Title:	台灣西南海域上部陸坡之底棲有孔蟲群落研究：以永安海槽為例
Authors:	葉仁筑;Yeh,Jen-Chu
Contributors:	地球科學學系
Keywords:	台灣西南外海;底棲性有孔蟲;沉積物;SW Taiwan;Benthic foraminifera;Sediment
Date:	2016-07-28
Issue Date:	2016-10-13 13:23:12 (UTC+8)
Publisher:	國立中央大學
Abstract:	本研究在台灣西南海域好景海脊與永安海脊之間的上部陸坡環境(水深介於1,100-1,600 m)建立底棲性有孔蟲的背景種屬組合，利用箱型沉積物採集器，於該區三處取得之海床表層沉積物進行有孔蟲的細胞質染色處理而後進行計量分析，並觀察該組合隨著水深以及約一百年來的時間變化。同時亦藉由岩心描述、X光攝影、粒徑分析、鉛-210活性分析以及有機碳、氮含量分析了解此研究區域的沉積環境狀況。由X光攝影影像顯示三處海床表層沉積物幾乎沒有擾動的痕跡，僅有一些有孔蟲軟泥和唯一一處肉眼可見的生物擾動痕跡。粒徑分析結果顯示，粒徑範圍約4-9ψ，中間粒徑範圍約6~8ψ，淘選度介於極良淘選(<0.35ψ)與良淘選(0.35~0.5ψ)之間，鉛210活性分析結果呈指數衰退亦證明此處環境穩定，同時亦藉此計算出三處的沉積速率範圍在每年2.6到0.13 mm之間。三處採樣點中，YT1的沉積速率很低，推測可能因此處位於斜坡而且接近高區，導致懸浮沉降速率較低，低沉積速率也導致該處有孔蟲殼體數量極高。種屬計量分析顯示此區域的優勢種為唇口蟲(Chilostomella) ，而次要優勢種屬有小泡蟲(Bulimina)、葡萄蟲(Uvigerina)、透鏡蟲(Lenticulina)和串房蟲(Reophax)等。本研究亦將三支沉積物樣本中的YT2和YT3以5公分的間隔進行殼體種屬與豐度計算以比較其隨時間以及有機碳含量(TOC)的變化，其結果顯示底棲有孔蟲的總豐度變化與有機碳含量的變化之間的關係並不顯著。此研究將幫助我們對好景海脊與永安海脊之間的海床環境有更多了解，亦讓台灣西南海域的現生底棲有孔蟲族群組合的背景資料延伸至更廣的水深，以利未來做更深入的研究。 ;This study attempts to establish the spatial distribution of benthic foraminifera and to discuss the physical and chemical properties of host marine sediments in the upper accretionary wedge off SW Taiwan. Three box cores ranging from 32 cm to 49 cm thick are retrieved onboard R/V Ocean Researcher I during 1092 cruise in 2014 at water depths ranging from 1,135 to 1,586 m lying in between the Good Weather Ridge and the Yuan-An Ridge. Sediments of uppermost 5 cm for each core are immersed in Rose Bengal for staining living benthic foraminifera, spatial species distribution with respect to water depths and temporal down-core variations over the past ~200 years are discussed. Additionally, visual core description, X-radiographs, grain size analyses and 210Pb dating are also carried out in this study. Analyses on grain size reveal that the sediment size ranges from clay to silt for all sites, fine to well-sorted with the exception of YT1 site where a small percentage of fine sands (< 20%) is found to distribute evenly in a 32 cm-thick box core. X-radiographs of the YT1 core show rare bioturbation with large white spots, probably caused by aggregates of foraminiferal tests. Trace fossil of Chondrites, is found at YT3 core, indicating anoxia condition at YT3 site. Age of sediment is obtained by using 210Pb dating method. The 210Pb concentration profile decays exponentially down core, indicating sedimentation from suspension. The measured sedimentation rate ranges from 0.013 to 0.26 g/cm2yr. Site YT1 has the lowest sedimentation rate (around 0.013 g/cm2yr), leading to high abundance of individual benthic foraminiferal species. YT1 site locates at a western slope (~17 degree in gradient) of the Good Weather Ridge, whereas the rest two sites (YT2 and YT3) situated in depositional trough. We suggest that the higher slope gradient at YT1 has resulted in lowered sedimentation rate comparing to that for sites at bathymetric trough. Living foraminiferal individuals were distinguished from dead assemblages by Rose Bengal staining method during the cruise. Our results show that the dominant living species of all studied cores is Chilostomella oolina, with subsidiary occurrences of Bulimina aculeata, Uvigerina auberiana, and Reophax spp. This foraminiferal occurrence indicates suboxic/dysoxic conditions of shallow sub-seafloor and stable hemipelagic sedimentation. Abundance of living foraminiferal assemblages from three cores is of similar, whereas core YT1 exhibits much higher abundance of dead assemblage, due probably to low sedimentation rate. Cores of YT2 and YT3 are selected for studying down-core variations of foraminiferal assemblages for every 5 cm interval. Results show a remarkable up-core decrease in the abundance of Bulimina and Uvigerina species, indicating suboxic environment, and increasing abundance of Chilostomella, a characteristic species of dysoxic environment. This indicates that the oxygen concentration of the water masses may have decreased since c. 50 year ago. This study documents living benthic foraminifera distribution in water depths down to c. 1,600 m off SW Taiwan, providing a basis for future benthic foraminiferal studies in the deep-sea around Taiwan.
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