Neogene benthic foraminifera in the Southwest Pacific

Early Miocene to Quaternary benthic foraminifers have been quantitatively studied (>63 ?m size fraction) in a southwest Pacific traverse of DSDP sites at depths from about 1300 to 3200 m down the Lord Howe Rise (Site 590,1299 m; Site 591, 2131 m; Site 206, 3196 m). Benthic foraminiferal species smaller than 150 µm are by far dominant in the samples, averaging from 78 to 89% of the total benthic foraminiferal assemblages in the three sites examined. Although about 150 benthic foraminiferal species or taxonomic groups have been identified, only a few species dominate the assemblages. These dominant species include Epistominella exigua, E. rotunda, and Globocassidulina subglobosa, which prevail in the three sites, and Oridorsalis umbonatus, E. umbonifera, and Cassidulina carinata, which occur usually in frequencies of between 10 and 30%. Faunal changes in Neogene benthic foraminiferal assemblages are not similar in each of the three sites, but faunal successions are most similar between the two shallowest sites. The deepest site differs in composition and distribution of dominant species. There are three intervals during which the most important changes occur in benthic foraminiferal assemblages: the early middle Miocene (14 Ma; the Orbulina suturalis Zone and the Globorotalia fohsi s.l. Zone); the late Miocene (6 Ma; the Globigerina nepenthes Zone) and near the Pliocene/Pleistocene boundary at about 2 Ma. A Q-mode factor analysis of the faunal data has assisted in recognizing assemblage changes during the Neogene at each of the sites. Early Miocene assemblages were dominated by Globocassidulina subglobosa at Site 590 (1299 m), by G. subglobosa and Oridorsalis umbonatus at Site 591 (2131 m), and by G. subglobosa, E. exigua, and Bolivina pusilla at Site 206 (3196 m). In the early middle Miocene at Sites 590 and 591, a marked increase occurred in the frequencies of E. exigua. Epistominella exigua reached maximum abundance in the early Miocene in the deeper Site 206, and in the middle and early late Miocene in the shallower Sites 590 and 591. In the late Miocene, a spike occurred in the frequencies of E. umbonifera in Site 206, whereas the dominant species changed from E. exigua to E. rotunda at Site 590. Latest Miocene to late Pliocene assemblages were dominated by E. rotunda at Site 590, by E. exigua at Site 591, and by G. subglobosa-E. exigua (early Pliocene) and E. rotunda-E. exigua (late Pliocene) at Site 206. At the Pliocene/Pleistocene boundary, E. exigua temporarily diminished in importance at Sites 591 and 206. Quaternary assemblages were dominated by E. rotunda and Cassidulina carinata at Site 590, by E. rotunda at Site 591, and by E. exigua at Site 206. These major faunal changes are all associated with known major paleoceanographic events-the middle Miocene development of the Antarctic ice sheet; the latest Miocene global cooling and increased polar glaciation; and the onset of quasiperiodic glaciation of the Northern Hemisphere. These major paleoceanographic events undoubtedly had a profound effect on the intermediate and deep water mass structure of the Tasman Sea as recorded by changes in benthic foraminiferal assemblages.本研究对西南太平洋豪勋爵海隆（Lord Howe Rise）沿线深海钻探计划（Deep Sea Drilling Project, DSDP）3个站位（站位590，水深1299m；站位591，水深2131m；站位206，水深3196m）采集的、粒径大于63μm粒级的早中新世至第四纪底栖有孔虫（benthic foraminifers）开展了定量分析。本次研究的样品中，粒径小于150μm的底栖有孔虫物种占绝对优势，在3个观测站位中，其占底栖有孔虫总组合的比例平均为78%至89%。尽管已鉴定出约150个底栖有孔虫物种或分类类群，但仅有少数物种构成组合的优势类群。这些优势物种包括细小表皮虫（*Epistominella exigua*）、圆形表皮虫（*Epistominella rotunda*）与近球形盖状虫（*Globocassidulina subglobosa*），三者在3个站位中均占主导地位；此外还有脐盖虫（*Oridorsalis umbonatus*）、脐部表皮虫（*Epistominella umbonifera*）与隆背盖状虫（*Cassidulina carinata*），其通常出现频率介于10%至30%之间。 3个站位的新近纪底栖有孔虫动物群组合变化并不一致，但两个较浅站位的动物群演替模式最为相似。水深最大的站位的优势物种组成与分布模式均存在差异。底栖有孔虫动物群组合发生重大变化的时段共有三期：中中新世早期（14 Ma，对应拱孔球虫带（Orbulina suturalis Zone）与佛氏抱球虫复合种带（Globorotalia fohsi s.l. Zone））；晚中新世（6 Ma，对应猪笼草抱球虫带（Globigerina nepenthes Zone））；以及约2 Ma的上新世/更新世边界附近。 对动物群数据开展的Q型因子分析，有助于识别各站位新近纪期间的底栖有孔虫组合变化。早中新世的组合中，站位590（1299m）以近球形盖状虫为优势类群；站位591（2131m）的优势类群为近球形盖状虫与脐盖虫；站位206（3196m）的优势类群则为近球形盖状虫、细小表皮虫与小型双折虫（*Bolivina pusilla*）。在中中新世早期，站位590与591的细小表皮虫出现频率显著升高。细小表皮虫的丰度峰值在水深较深的站位206出现于早中新世，而在较浅的站位590与591则出现于中中新世至晚中新世早期。晚中新世期间，站位206的脐部表皮虫出现频率出现峰值；而站位590的优势物种则从细小表皮虫转变为圆形表皮虫。晚中新世至晚上新世期间，站位590的优势类群为圆形表皮虫，站位591为细小表皮虫；站位206的优势类群在早上新世为近球形盖状虫-细小表皮虫组合，晚上新世则为圆形表皮虫-细小表皮虫组合。在上新世/更新世边界处，站位591与206的细小表皮虫的优势地位暂时下降。第四纪的组合中，站位590以圆形表皮虫与隆背盖状虫为优势类群，站位591以圆形表皮虫为优势类群，站位206则以细小表皮虫为优势类群。 上述重大动物群演替事件均与已知的重大古海洋学事件相关：中中新世南极冰盖的形成；晚中新世全球降温与极地冰川作用增强；以及北半球准周期性冰川作用的启动。这些重大古海洋学事件无疑对塔斯曼海的中层与深层水团结构产生了深远影响，这一结论可由底栖有孔虫组合的变化得到印证。