Benthic foraminifera abundance in ODP Leg 155 holes

The general Pleistocene architecture of the Amazon Fan has been reconstructed using sediment recovered by Ocean Drilling Program Leg 155. Huge regional mass-transport deposits (MTDs) make up a significant component of the Amazon Fan. These deposits each cover an area over 15,000 km**2 (approximately the size of Jamaica), reach a maximum thickness of 200 m, and consist of ~5000 Gt of sediment. Benthic foraminiferal fauna analysis and sedimentology indicate that the MTDs originated on the continental slope, which is at least 200 km laterally and 1500 m above their present position. Each mass-failure event was formed by the catastrophic failure of the continental slope and has been dated and correlated with climate-induced changes in sea level. Studies of the benthic foraminiferal assemblages in the Amazon Fan has been essential to our reconstruction of the origin and cause of these failures. The MTDs contain rare shelf (Quinqueloculina cf. stalkeri, Brizalina aenariensis, Q. lamarckiana, and Pseudononion atlanticum) and dominant upper-middle bathyal species (cassidulinids and buliminids). We conclude that the MTD originated between 200 and 600 m water depth, approximately the same zone in which gas hydrates occur. We suggest that the glacial MTDs referred to as Deep Eastern MTD (35–37 ka) and Unit R MTD (41–45 ka) correlate with rapid drops in sea level which destabilized continental slope gas-hydrate reservoirs causing catastrophic slope failure. An alternative explanation is required for the deglacial MTDs referred to as Western and Eastern Debris Flows (13–14 ka) which occurred as sea level rose rapidly during the Bølling-Allerød period. We suggest that the deglaciation of the Andes and the consequent enhanced sediment supply coupled with a shift of the depo-centre to the continental shelf, caused over-burdening and thus slope failure. Evidence for a 2 per mil negative d13C shift in both planktonic foraminifera and organic matter coeval with these failures suggest that whatever the cause, there was a large release of methane hydrate associated with each failure.

本研究借助大洋钻探计划（Ocean Drilling Program）第155航次所获取的沉积物，重建了亚马逊扇的更新世整体沉积架构。巨型区域性块体搬运沉积（mass-transport deposits, MTDs）构成了亚马逊扇的重要组成部分：单块此类沉积的覆盖面积均超过15000平方千米（约等于牙买加的国土面积），最大厚度可达200米，沉积物总量约达5000吉吨。底栖有孔虫动物群分析与沉积学研究表明，这些块体搬运沉积源自大陆坡，其形成位置相较于现今沉积位置，横向偏移至少200千米，垂向高度高出至少1500米。每一次块体崩塌事件均由大陆坡的灾难性崩塌所引发，经年代测定并与气候驱动的海平面变化建立了关联。对亚马逊扇底栖有孔虫组合的研究，是重建此类崩塌事件起源与成因的核心依据。这些块体搬运沉积中包含稀有的陆架属种（Quinqueloculina cf. stalkeri、Brizalina aenariensis、Q. lamarckiana及Pseudononion atlanticum），以及占优势的中上部半深海物种（盔形虫类（cassidulinids）和泡虫类（buliminids））。我们据此推断，该块体搬运沉积形成于水深200至600米的区域，这一水深区间恰好与天然气水合物（gas hydrates）的赋存范围一致。我们提出，被称为"东部深部块体搬运沉积（3.5~3.7万年前）"和"R单元块体搬运沉积（4.1~4.5万年前）"的冰期块体搬运沉积，与海平面快速下降存在关联——海平面下降导致大陆坡天然气水合物储层失稳，进而引发了坡体的灾难性崩塌。而针对被称为"西部碎屑流"和"东部碎屑流"的冰消期块体搬运沉积（1.3~1.4万年前），其发生于波令-阿勒罗德期（Bølling-Allerød period）海平面快速上升阶段，则需要另一种成因解释。我们认为，安第斯山脉（Andes）的冰消作用伴随而来的沉积物供给增强，加之沉积中心向大陆架的迁移，共同造成了上覆载荷过载，最终引发了坡体崩塌。与这些崩塌事件同期的浮游有孔虫与有机质中均出现了2‰的碳同位素δ13C负偏移，这一证据表明，无论具体成因如何，每一次崩塌事件均伴随有大量甲烷水合物（methane hydrate）的释放。