Paleogene calcareous nannofossil record of the Tasmanian gateway

The opening of the Tasmanian Gateway between Australia and Antarctica has long been considered a critical element in the initiation of the Antarctic Circumpolar Current, thermal isolation of Antarctica, and Cenozoic global cooling. The timing for the opening of the gateway to shallow-water circulation and subsequently to deep-water circulation was poorly known, however, and the dating of these events was a major objective of ODP Leg 189. Nannofossil data from Leg 189 sites and DSDP Site 281 in the Tasmanian Gateway suggest a 41-42 Ma age for the initiation of widespread glauconite deposition in the region, which coincided with a sharp drop in sedimentation rate. This is interpreted to be the opening of the gateway to shallow-water circulation, which occurred within the middle of the 51-33 Ma long-term cooling. The change from siliciclastic sediments to pelagic carbonates, the most conspicuous sedimentological and paleontological change in the region for the last 70 Ma and presumably the indicator for the opening of the gateway to deep-water circulation, is dated at about 31 Ma. This event is more than 2 my younger than the major high-latitude cooling in the earliest Oligocene, and thus cannot be the cause for the latter.

长期以来，澳大利亚与南极洲之间的塔斯曼海道（Tasmanian Gateway）的开启，被视为南极绕极流（Antarctic Circumpolar Current）形成、南极洲热隔离以及新生代全球降温的关键要素。然而，该海道开启浅水环流乃至后续深水环流的时间节点长期以来尚不明确，因此确定这两类事件的发生年代，成为大洋钻探计划（Ocean Drilling Program, ODP）第189航次的核心目标之一。来自该航次各站位以及塔斯曼海道内深海钻探计划（Deep Sea Drilling Program, DSDP）281号站位的超微化石（Nannofossil）数据显示，该区域广泛发育海绿石沉积的起始时间为4100万至4200万年前，这一时间恰好对应沉积速率的急剧下降。这一现象被解读为塔斯曼海道开启浅水环流的标志，该事件发生于5100万至3300万年的长期降温过程的中期阶段。区域内近7000万年以来最显著的沉积学与古生物学变化——从陆源碎屑沉积物向远洋碳酸盐沉积物的转变，被认为是塔斯曼海道开启深水环流的指示标志，其定年结果约为3100万年前。该事件比渐新世早期的主要高纬度降温事件晚了逾200万年，因此并非后者的成因。