Radiolarian events in the eastern Equatorial Pacific

The development of an orbitally tuned time scale for the ODP leg 138 sites provides biostratigraphers a very high resolution chronostratigraphic framework. With this framework we are better able to define which of the first and last appearances of species appear to be synchronous. In addition, the geographic distribution of sites provides the means with which the detailed spatial patterns of invasion of new species and the extinction of older species can be mapped. These maps not only provide information on the process of evolution, migration, and extinction, they can also be related to water mass distributions and near-surface circulation of the ocean. Of 39 radiolarian events studied at 11 sites in the eastern equatorial Pacific, 28 were found to have a minimum range in their estimated age that exceeded 0.15 m.y. The temporal pattern of first and last appearances of these diachronous events have coherent spatial patterns that indicate shifts in the areas of high oceanographic gradients over the past 10 Ma. These changes in the locations of high gradient regions suggest that the South Equatorial Current (SEC) was north of its present position prior to approximately 7 Ma. There was a southward shift in the northern boundary of this current between approximately 6 and 7 Ma, and the development of a relatively strong gradient between the northeastern and northwestern sites. Between approximately 3.7 and 3.4 Ma, there was a very slight northward shift in the northern boundary of the SEC and the steep gradients between the northeastern and northwestern sites may have disappeared. This change is thought to be associated with the closing of the Isthmus of Panama. The temporal-spatial patterns of diachronous events younger than 3.4 Ma are consistent with patterns of circulation in the modern ocean.

为大洋钻探计划（Ocean Drilling Program, ODP）第138航次站位构建轨道调谐时间标尺，可为生物地层学家提供一套高分辨率年代地层框架。借助该框架，我们能够更精准地界定哪些物种的首现与末现事件具有同步性。此外，各站位的地理分布特征，为详细绘制新物种入侵扩散与旧物种灭绝的空间分布格局提供了可行路径。这些空间分布图不仅能够揭示演化、迁移与灭绝的过程，还可与海洋水团分布及近表层海洋环流建立关联。在东赤道太平洋11个站位开展研究的39项放射虫（radiolarian）事件中，有28项的估算年龄最小跨度超过0.15百万年（m.y.）。这些穿时事件的首现与末现的时间格局呈现出清晰的空间分布特征，指示了过去10百万年（Ma）间海洋学高梯度区的位置变迁。上述高梯度区的位置变化表明，南赤道流（South Equatorial Current, SEC）在约7百万年（Ma）前的位置较现今更偏北。该海流的北界在约6 Ma至7 Ma间发生南向迁移，同时东北站位与西北站位之间形成了相对较强的梯度带。在约3.7 Ma至3.4 Ma间，南赤道流的北界出现了极其微弱的北移，东北与西北站位之间的强梯度带可能就此消失。这一变化被认为与巴拿马地峡（Isthmus of Panama）的闭合密切相关。年龄小于3.4 Ma的穿时事件的时空格局，与现代海洋的环流格局完全一致。