Planktic foraminiferal assemblages in sediments of four ODP Leg 198 holes

A pulse of intense carbonate dissolution occurred during the early late Paleocene at 58.4 Ma. A prominent 5 to 25 cm-thick dark brown clay-rich calcareous nannofossil ooze was found on Shatsky Rise at Sites 1209, 1210, 1211, and 1212 during Ocean Drilling Program Leg 198. The layer corresponds to the lower part of planktonic foraminiferal Zone P4 and coincides with the evolutionary first occurrence of the nannolith Heliolithus kleinpellii, an important component of late Paleocene assemblages and a marker for the base of Zone CP5. The clay-rich layer contains common crystals of phillipsite, fish teeth, and phosphatic micronodules and corresponds to a prominent peak in magnetic susceptibility that probably reflects these high amounts of detrital and authigenic materials. Detailed quantitative analysis of planktonic foraminiferal assemblages across the clay-rich nannofossil ooze layer shows that fundamental changes in faunal composition occurred before, during, and after deposition of the clay-rich ooze. Planktonic foraminifers in the clay-rich layer are characterized by a low-diversity, largely dissolved assemblage dominated by representatives of the genus Igorina (mainly Igorina tadjikistanensis and Igorina pusilla). Conversely, Igorina albeari, morozovellids, acarininids, globanomalinids, subbotinids, and chiloguembelinids are common below the clay-rich layer, almost disappear within it, and reappear in low abundances above the clay-rich layer. These changes in faunal compositions are likely a response to a change in carbonate saturation that caused increased dissolution on the seafloor owing to the shoaling of the lysocline and the carbonate compensation depth.

在距今58.4 Ma的晚古新世早期，发生了一期强烈的碳酸盐溶解脉冲事件。大洋钻探计划（Ocean Drilling Program）第198航次在萨茨基海隆（Shatsky Rise）的1209、1210、1211和1212站位，发现了一层厚度5至25厘米的标志性深棕色富黏土钙质超微化石软泥（calcareous nannofossil ooze）。该层位对应浮游有孔虫带（planktonic foraminiferal Zone）P4的下部，同时与超微化石颗石（nannolith）*Heliolithus kleinpellii*的演化首次出现事件重合——该颗石是晚古新世生物组合的重要组分，也是CP5带底界的标志性分子。该富黏土层中常见钙十字沸石（phillipsite）晶体、鱼齿以及磷酸盐微结核，同时对应一个显著的磁化率（magnetic susceptibility）峰值，该峰值或反映了该层中较高含量的陆源（detrital）与自生（authigenic）物质。对该富黏土超微化石软泥层位上下的浮游有孔虫组合开展的详细定量分析显示，在该软泥沉积前、沉积期及沉积后，动物群组成均发生了根本性变化。富黏土层中的浮游有孔虫以低多样性、大部被溶解的组合为特征，优势类群为伊戈林虫属（Igorina）的代表种（主要为塔吉克伊戈林虫*Igorina tadjikistanensis*与普西利伊戈林虫*Igorina pusilla*）。与之相反，阿尔巴雷伊戈林虫*Igorina albeari*、莫罗佐夫虫类（morozovellids）、阿克瑞尼虫科（acarininids）、球拟抱球虫科（globanomalinids）、苏博特虫科（subbotinids）以及奇洛根比林虫科（chiloguembelinids）在富黏土层之下较为常见，在该层内几乎消失，并在富黏土层之上以极低的丰度重新出现。上述动物群组成的变化或为碳酸盐饱和状态改变的响应——由于溶跃面（lysocline）与碳酸盐补偿深度（carbonate compensation depth）变浅，海底的碳酸盐溶解作用增强。