Eocene-Oligocene planktonic foraminifera of ODP Hole 135-841B

A middle Eocene to lower Oligocene sedimentary sequence was drilled at Site 841 in the Tonga forearc region during Ocean Drilling Program Leg 135. A 56-m-thick sequence of volcanic sandstone, spanning from Cores 135-841B-4IR to -47R (549.1 to 605 mbsf), unconformably overlies rhyolitic volcanic basement. The middle Eocene planktonic foraminifer assemblages (P Zone?), which occur in association with larger benthic foraminifers, include spinose species of Acarinina, Morozovella, and Truncorotaloides, but their abundance is low. Late Eocene and early Oligocene faunas are abundant and show the highest diversity of the Paleogene sequence drilled at this site. They have been assigned to Zones P15-16 and P18, respectively. The Eocene/Oligocene boundary was not recognized because of a hiatus in which Zone P17 (37.2-36.6 Ma) was missing. Another hiatus is recorded in the interval between the middle and late Eocene, spanning at least 1.8 Ma. Paleogene assemblages of Site 841 contain equal numbers of warm- and cool-water species, an attribute of the warm middle-latitude Paleogene fauna of the Atlantic Ocean. In particular, common to high abundances of cool-water taxa, such as Globorotaloides, Catapsydrax, Tenuitella, and small globigerinids, may be related to the opening of a shallow seaway south of Tasmania permitting the influx of cool Indian Ocean waters into the South Pacific before the late Eocene (approximately 37 Ma).大洋钻探计划（Ocean Drilling Program）第135航次在汤加弧前区域的841站位钻取了中始新世至早渐新世的沉积序列。一套厚度达56米的火山砂岩序列，取样岩心涵盖135-841B-4IR至-47R号岩心（井段549.1至605 mbsf），以不整合接触关系覆于流纹质火山基底之上。中始新世浮游有孔虫（planktonic foraminifer）组合（P带？）与大型底栖有孔虫伴生，包含Acarinina、Morozovella及Truncorotaloides的具刺种属，但该组合的丰度较低。晚始新世及早渐新世的动物群丰度较高，为本站位钻取的古近纪沉积序列中多样性最高的类群，二者分别被归为P15-16带与P18带。由于存在沉积间断，缺失P17带（37.2~36.6 Ma），因此未能识别出始新世/渐新世界线。在中始新世与晚始新世之间的井段还存在另一处沉积间断，间断时长至少达1.8 Ma。 841站位的古近纪生物组合中，暖水种与冷水种的数量占比相当，这一特征与大西洋中纬度古近纪暖水动物群的属性一致。尤为值得注意的是，Globorotaloides、Catapsydrax、Tenuitella及小型抱球虫类等冷水类群普遍达到较高丰度，这可能与塔斯马尼亚南部浅海通道的开启有关——该通道允许冷水印度洋海水在晚始新世之前（约37 Ma）涌入南太平洋。