Planktic foraminifera from the Paleocene-Eocene Thermal Maximum at Shatsky Rise, Pacific Ocean@en

High-resolution biostratigraphic and quantitative studies of subtropical Pacific planktonic foraminiferal assemblages (Ocean Drilling Program, Leg 198 Shatsky Rise, Sites 1209 and 1210) are performed to analyse the faunal changes associated with the Paleocene–Eocene Thermal Maximum (PETM) at about 55.5 Ma. At Shatsky Rise, the onset of the PETM is marked by the abrupt onset of a negative carbon isotope excursion close to the contact between carbonate-rich ooze and overlying clay-rich ooze and corresponds to a level of poor foraminiferal preservation as a result of carbonate dissolution. Lithology, planktonic foraminiferal distribution and abundances, calcareous plankton and benthic events, and the negative carbon isotope excursion allow precise correlation of the two Shatsky Rise records. Results from quantitative analyses show that Morozovella dominates the assemblages and that its maximum relative abundance is coincident with the lowest delta 13C values, whereas subbotinids are absent in the interval of maximum abundance of Morozovella. The excursion taxa (Acarinina africana, Acarinina sibaiyaensis, and Morozovella allisonensis) first appear at the base of the event. Comparison between the absolute abundances of whole specimens and fragments of genera demonstrate that the increase in absolute abundance of Morozovella and the decrease of Subbotina are not an artifact of selective dissolution. Moreover, the shell fragmentation data reveal Subbotina to be the more dissolution-susceptible taxon. The upward decrease in abundance of Morozovella species and the concomitant increase in test size of Morozovella velascoensis are not controlled by dissolution. These changes could be attributed to the species' response to low nutrient supply in the surface waters and to concomitant changes in the physical and chemical properties of the seawater, including increased surface stratification and salinity. Comparison of the planktonic foraminiferal changes at Shatsky Rise to those from other PETM records (Sites 865 and 690) highlights significant similarities, such as the decline of Subbotina at the onset of the event, and discrepancies, including the difference in abundance of the excursion taxa. The observed planktonic foraminifera species response suggests a warm–oligotrophic scenario with a high degree of complexity in the ocean structure.针对亚热带太平洋浮游有孔虫组合（planktonic foraminiferal assemblages）开展的高分辨率生物地层学与定量研究（基于大洋钻探计划（Ocean Drilling Program）第198航次斯库姆脊（Shatsky Rise）1209与1210站位数据），旨在解析约5550万年前古新世-始新世极热事件（Paleocene–Eocene Thermal Maximum, PETM）伴随的动物群演化特征。在斯库姆脊区域，古新世-始新世极热事件的起始以碳同位素负偏移的突发性出现为标志，该偏移紧邻富碳酸盐软泥与上覆富黏土软泥的接触界面，同时对应因碳酸盐溶解作用导致有孔虫保存状况极差的地层段。岩性特征、浮游有孔虫分布与丰度、钙质浮游与底栖生物事件以及碳同位素负偏移，可实现斯库姆脊两处站位沉积记录的精准对比。定量分析结果显示，莫罗兹虫属（Morozovella）为组合中的优势类群，其相对丰度峰值与最低δ¹³C（delta 13C）值恰好重合，而在莫罗兹虫属丰度达到峰值的层段内，苏伯特虫类（subbotinids）完全消失。极热事件特征类群（非洲阿卡丁虫（Acarinina africana）、西巴亚阿卡丁虫（Acarinina sibaiyaensis）及艾利森莫罗兹虫（Morozovella allisonensis））首次出现于该事件的底界位置。通过对比完整壳体与属内碎片的绝对丰度，证实莫罗兹虫属绝对丰度的升高以及苏伯特虫属（Subbotina）丰度的降低并非选择性溶解作用导致的人为假象。此外，壳体碎片数据表明，苏伯特虫属是更易发生溶解的类群。莫罗兹虫属物种丰度随剖面向上逐渐降低，以及伴生的维拉帕斯莫罗兹虫（Morozovella velascoensis）壳体尺寸增大的现象，均不受溶解作用调控。上述演化特征可归因于物种对表层水体低营养条件的响应，以及海水物理化学性质的协同变化——包括表层水体层化作用增强与盐度升高。将斯库姆脊的浮游有孔虫演化特征与其他古新世-始新世极热事件记录（865及690站位）进行对比后，可发现显著的共性与差异：共性表现为事件起始阶段苏伯特虫属丰度下降，差异则包括事件特征类群的丰度差异。本次观测到的浮游有孔虫物种响应特征，表明当时海洋环境呈现暖水贫营养状态，且海洋结构具有高度复杂性。