Middle Eocene-Early Pliocene Subantarctic planktic foraminiferal biostratigraphy of ODP Site 1090 on the Agulhas Ridge, South Atlantic

The analysis of planktic foraminiferal assemblages from Site 1090 (ODP Leg 177), located in the central part of the Subantarctic Zone south of South Africa, provided a geochronology of a 330-m-thick sequence spanning the Middle Eocene to Early Pliocene. A sequence of discrete bioevents enables the calibration of the Antarctic Paleogene (AP) Zonation with lower latitude biozonal schemes for the Middle-Late Eocene interval. In spite of the poor recovery of planktic foraminiferal assemblages, a correlation with the lower latitude standard planktic foraminiferal zonations has been attempted for the whole surveyed interval. Identified bioevents have been tentatively calibrated to the geomagnetic polarity time scale following the biochronology of Berggren et al. (1995). Besides planktic foraminiferal bioevents, the disappearance of the benthic foraminifera Nuttallides truempyi has been used to approximate the Middle/Late Eocene boundary. A hiatus of at least 11.7 Myr occurs between V78 and V71 m composite depth extending from the Early Miocene to the latest Miocene-Early Pliocene. Middle Eocene assemblages exhibit a temperate affinity, while the loss of several planktic foraminiferal species by late Middle to early Late Eocene time reflects cooling. During the Late Eocene-Oligocene intense dissolution caused impoverishment of planktic foraminiferal assemblages possibly following the emplacement of cold, corrosive bottom waters. Two warming peaks are, however, observed: the late Middle Eocene is marked by the invasion of the warmer water Acarinina spinuloinflata and Hantkenina alabamensis at 40.5 Ma, while the middle Late Eocene experienced the immigration of some globigerinathekids including Globigerinatheka luterbacheri and Globigerinatheka cf. semiinvoluta at 34.3 Ma. A more continuous record is observed for the Early Miocene and the Late Miocene-Early Pliocene where planktic foraminiferal assemblages show a distinct affinity with southern mid- to high-latitude faunas.本研究对采自南非以南亚南极带（Subantarctic Zone）中部的大洋钻探计划（Ocean Drilling Program, ODP）第177航次1090站位的浮游有孔虫组合（planktic foraminiferal assemblages）进行分析，构建了一套厚度达330米、地层跨度覆盖中始新世至早上新世的地质年代序列。一系列离散生物事件（bioevents）可用于实现中晚始新世（Middle-Late Eocene）区间南极古近纪带（Antarctic Paleogene Zonation, AP Zonation）与低纬度生物带方案（lower latitude biozonal schemes）的地层校准。尽管浮游有孔虫组合的回收率较低，研究仍尝试对整个调查层位与低纬度标准浮游有孔虫生物带进行对比。已识别的生物事件参照伯格伦等人（Berggren et al., 1995）的生物年代学框架，初步标定至地磁极性年表（geomagnetic polarity time scale）中。除浮游有孔虫生物事件外，底栖有孔虫（benthic foraminifera）Nuttallides truempyi的消失也被用于近似限定中/晚始新世界线。在综合深度（composite depth）78米与71米层位之间存在至少11.7百万年的沉积间断（hiatus），其时间跨度为早中新世至晚中新世末-早上新世。中始新世的浮游有孔虫组合表现出温带生态亲和性，而中始新世晚期至晚始新世早期多个浮游有孔虫物种的绝灭反映了当时的气候变冷过程。晚始新世-渐新世（Late Eocene-Oligocene）期间强烈的溶解作用（dissolution）导致浮游有孔虫组合趋于贫乏，这可能与寒冷腐蚀性底水的形成有关。不过，研究识别出两次升温峰值（warming peaks）：中始新世晚期（约40.5 Ma）出现暖水种Acarinina spinuloinflata与Hantkenina alabamensis的入侵；晚始新世中期（约34.3 Ma）则有包括Globigerinatheka luterbacheri及Globigerinatheka cf. semiinvoluta在内的球盖虫类（globigerinathekids）迁入。早中新世以及晚中新世-早上新世的地层记录更为连续，其中的浮游有孔虫组合与南半球中高纬度的动物群具有显著的生态亲和性。