Benthic foraminiferal accumulation rates of the late Pliocene-Pleistocene in the northern Indian Ocean

During the late Pliocene–middle Pleistocene, 63 species of elongate, bathyal–upper abyssal benthic foraminifera (Extinction Group = Stilostomellidae, Pleurostomellidae, some Nodosariidae) declined in abundance and finally disappeared in the northern Indian Ocean (ODP Sites 722, 758), as part of the global extinction of at least 88 related species at this time. The detailed record of withdrawal of these species differs by depth and geography in the Indian Ocean. In northwest Indian Ocean Site 722 (2045 m), the Extinction Group of 54 species comprised 2–15% of the benthic foraminiferal fauna in the earliest Pleistocene, but declined dramatically during the onset of the mid-Pleistocene Transition (MPT) at 1.2–1.1 Ma, with all but three species disappearing by the end of the MPT (~0.6 Ma). In northeast Indian Ocean Site 758 (2925 m), the Extinction Group of 44 species comprised 1–5% of the benthic foraminiferal fauna at ~3.3–2.6 Ma, but declined in abundance and diversity in three steps, at ~2.5, 1.7, and 1.2 Ma, with all but one species disappearing by the end of the MPT. At both sites there are strong positive correlations between the accumulation rate of the Extinction Group and proxies indicating low-oxygen conditions with a high organic carbon input. In both sites, there was a pulsed decline in Extinction Group abundance and species richness, especially in glacial periods, with some partial recoveries in interglacials. We infer that the glacial declines at the deeper Site 758 were a result of increased production of colder, well-ventilated Antarctic Bottom Water (AABW), particularly in the late Pliocene and during the MPT. The Extinction Group at shallower water depths (Site 722) were not impacted by the deeper water mass changes until the onset of the MPT, when cold, well-ventilated Glacial North Atlantic Intermediate Water (GNAIW) production increased and may have spread into the Indian Ocean. Increased chemical ventilation at various water depths since late Pliocene, particularly in glacial periods, possibly in association with decreased or more fluctuating organic carbon flux, might be responsible for the pulsed global decline and extinction of this rather specialised group of benthic foraminifera.

上新世晚期至中更新世时期，印度洋北部的大洋钻探计划（Ocean Drilling Program, ODP）722、758站位中，63种细长型半深海-超深渊底栖有孔虫（灭绝类群=旋塑虫科、肋管虫科及部分节房虫科）丰度逐渐下降并最终灭绝，该事件属于当时全球至少88种相关类群灭绝事件的组成部分。
该类群在印度洋内的消亡时序随水深与地理区域存在显著差异。在印度洋西北部722站位（水深2045米），54种灭绝类群在早更新世占底栖有孔虫动物群的2%~15%，但在1.2~1.1 Ma发生的中更新世转型（mid-Pleistocene Transition, MPT）初期出现急剧衰减，至中更新世转型结束（约0.6 Ma）时，仅余3个物种。
在印度洋东北部758站位（水深2925米），44种灭绝类群在约3.3~2.6 Ma时占底栖有孔虫动物群的1%~5%，随后在约2.5 Ma、1.7 Ma及1.2 Ma分三步出现丰度与多样性下降，至中更新世转型结束时仅存1个物种。
两处站位的灭绝类群堆积速率与反映低氧环境及高有机碳输入的代用指标均呈现显著正相关。两处站位的灭绝类群丰度与物种丰富度均呈现脉冲式下降，尤以冰期阶段最为显著，间冰期阶段存在部分恢复。
我们推测，水深更深的758站位的冰期衰减事件，是由南极底层水（Antarctic Bottom Water, AABW）生成量增加、水体更冷且通风性更强所导致，这一过程在上新世晚期及中更新世转型期尤为明显。
水深较浅的722站位灭绝类群并未受深层水团变化影响，直至中更新世转型初期，当冰川期北大西洋中层水（Glacial North Atlantic Intermediate Water, GNAIW）生成量增加并可能扩散至印度洋后，才受到冲击。
上新世晚期以来，各水深层的化学通风作用持续增强，尤其是在冰期阶段，这可能与有机碳通量下降或波动加剧相关，或是造成这类特化底栖有孔虫类群出现脉冲式全球衰减与灭绝的核心原因。