Stable isotope record and planktonic foraminiferal fragmentation of the late Pleistocene section of ODP Hole 134-828A

A high-resolution record of foraminiferal fragmentation (a dissolution indicator) for the last 250 k.y. (isotopic Stages 1 to 7) is identified in the upper 61.9 m of Ocean Drilling Program (ODP) Hole 828A, west Vanuatu. This record is comparable in detail to the atmospheric CO2 record and the d18O stack. Phase shifts between preservation spikes and maximum ice volumes (d18O of Globigerinoides sacculifer) are analogous to those on Ontong Java Plateau. Mass spectrometer (AMS14C) dating of a sample taken at the base of dissolution cycle B1 and the position of the last glacial maximum indicates a lag in time of ~8 k.y. in the Vanuatu region for the last glacial termination. When dissolution spikes are compared with minimum ice volumes there is no phase shift for the last two glacial terminations. The difference between Vanuatu and Ontong Java Plateau may be explained by local CO2 sinks and the interplay between intermediate and deep water masses. Terrigenous input increasingly affected sediment of Hole 828A on the North d'Entrecasteaux Ridge (NDR) as it approached Espiritu Santo Island. Mud and silt suspended in mid-water flows become important after 125 ka, while turbidites bypass the New Hebrides Trench only towards the last glacial maximum (LGM). Terrigenous supply seems to affect the lysocline profile that changed from an "open ocean" to a "near continent" type, thus favoring dissolution. Fragmentation of planktonic foraminifers is a more sensitive indicator of lysocline variations than is foraminiferal susceptibility to dissolution, the foraminiferal dissolution index, the abundance of benthic foraminifers, or CaCO3 content. A modern foraminiferal lysocline for the neighboring area (between 10°S and 30°S, and 160°E and 180°E) is found at 3.1 km below sea level, compared to west Vanuatu where it is shallower. The past lysocline level was deeper than 3086 m during intervals of dissolution minima, and ranged from ~2550 to 3000 m during intervals of dissolution maxima. The high sedimentation rates (in the order of 10 to 50 cm/k.y.) found in Hole 828A offer a great potential for future high-resolution studies either in this hole or other western localities along the NDR. Areas of high sedimentation near continental regions have been discarded for paleoceanographic and/or paleoclimatic studies. Nonetheless, conditions analogous to those found in Hole 828A are expected to occur in many trench areas around the world where mid-water flows have preserved as yet undiscovered fine high-resolution sedimentary records.

针对过去25万年（同位素阶段1至7）的有孔虫破碎率（溶解指标）高分辨率记录，在瓦努阿图以西的大洋钻探计划（Ocean Drilling Program, ODP）828A孔上部61.9米地层中被识别得出。该记录的细节可与大气CO₂记录及δ¹⁸O堆叠曲线相媲美。保存峰值与最大冰量（囊状球室虫（Globigerinoides sacculifer）的δ¹⁸O值）之间的相位偏移，与翁通爪哇高原（Ontong Java Plateau）的观测结果相仿。对溶解旋回B1底部采集的样品以及末次冰盛期（last glacial maximum, LGM）位置开展的加速器质谱（AMS¹⁴C）测年结果显示，瓦努阿图区域的末次冰消期存在约8千年的时间滞后。当将溶解峰值与最小冰量进行对比时，末次两次冰消期均未出现相位偏移。瓦努阿图与翁通爪哇高原之间的差异，可通过区域碳汇以及中层水与深层水团的相互作用来解释。随着德恩特雷卡斯托北脊（North d'Entrecasteaux Ridge, NDR）上的828A孔邻近圣灵岛（Espiritu Santo Island），陆源输入对其沉积物的影响逐渐增强。125千年以来，中层水流中悬浮的泥质与粉砂质组分逐渐成为沉积物的重要组成部分；而浊积岩仅在临近末次冰盛期（LGM）时才会绕过新赫布里底海沟。陆源供给似乎改变了溶跃面（lysocline）的剖面特征，使其从“远洋型”转变为“近陆型”，进而加剧了碳酸盐溶解作用。相较于有孔虫溶解敏感性、有孔虫溶解指数、底栖有孔虫丰度以及碳酸钙（CaCO₃）含量，浮游有孔虫破碎率是反映溶跃面变化更为灵敏的替代指标。邻近区域（10°S至30°S、160°E至180°E之间）的现代有孔虫溶跃面位于海平面以下3.1公里处，而瓦努阿图以西区域的溶跃面则更浅。在溶解作用最弱的时段，古溶跃面深度超过3086米；而在溶解作用最强的时段，其深度范围约为2550米至3000米。828A孔的沉积速率较高（约10至50厘米/千年），为该孔以及德恩特雷卡斯托北脊沿线其他西部点位未来开展高分辨率古海洋研究提供了极佳潜力。大陆周边的高沉积速率区域此前已被排除在古海洋学与古气候学研究之外。尽管如此，全球诸多中层水流保存了尚未被发现的精细高分辨率沉积记录的海沟区域，预计存在与828A孔类似的沉积环境。