Neogene radiolarian datum levels in the equatorial Indian and Pacific Oceans

Fifty radiolarian events of early Pleistocene and Neogene age were identified in an E-W transect of equatorial DSDP sites, extending from the Gulf of Panama to the western Pacific and eastern Indian Oceans. Our objective was to document the degree of synchroneity or time-transgressiveness of stratigraphically-useful datum levels from this geologic time interval. We restricted our study to low latitudes within which morphological variations of individual taxa are minimal, the total assemblage diversity remains high, and stratigraphic continuity is well-documented by an independent set of criteria. Each of the five sites chosen (503, 573, 289/586, 214) was calibrated to an "absolute" time scale, using a multiple of planktonic foraminiferal, nannofossil, and diatom datum levels which have been independently correlated to the paleomagnetic polarity time scale in piston core material. With these correlations we have assigned "absolute" ages to each radiolarian event, with a precision of 0.1-0.2 m.y. and an accuracy of 0.2-0.4 m.y. On this basis we have classified each of the events as either: (a) synchronous (range of ages <0.4 m.y.); (b) time-transgressive (i.e., range of ages >1.0 m.y.); and (c) not resolvable (range of ages 0.4-1.0 m.y.).
Our results show that, among the synchronous datum levels, a large majority (15 out of 19) are last occurrences. Among those events which are clearly time-transgressive, most are first appearances (10 out of 13). In many instances taxa appear to evolve first in the Indian Ocean, and subsequently in the western and eastern Pacific Ocean. This pattern is particularly unexpected in view of the strong east-to-west zonal flow in equatorial latitudes. Three of the time-transgressive events have been used to define zonal boundaries: the first appearances of Spongaster pentas, Diartus hughesi, and D. petterssoni. Our results suggest that biostratigraphic non-synchroneity may be substantial (i.e., greater than 1 m.y.) within a given latitudinal zone; one would expect this effect to be even more pronounced across oceanographic and climatic gradients.
We anticipate that the extent of diachroneity may be comparable for diatom, foraminiferal, and nannofossil datum levels as well. If this proves true, global "time scales" may need to be re-formulated on the basis of a smaller number of demonstrably synchronous events.

在巴拿马湾至西太平洋、东印度洋的赤道区深海钻探计划（DSDP）东西向剖面中，共识别出更新世早期及新近纪时期的50个放射虫事件。本研究旨在明确该地质时段内具有地层学应用价值的标志面的同步性或穿时性程度。我们将研究限定于低纬度区域——在此区域内，单个分类单元的形态变异极小，组合整体多样性维持在较高水平，且地层连续性可通过独立的多套判别标准得到充分验证。所选的5个站位（503、573、289/586、214）均通过浮游有孔虫、超微化石及硅藻标志面的多重组合校准至“绝对”年代标尺；上述标志面已通过活塞岩心材料独立对比至古地磁极性年表。基于这些对比结果，我们为每个放射虫事件赋予了“绝对”年龄，其精度为0.1~0.2百万年，准确度为0.2~0.4百万年。在此基础上，我们将所有事件划分为三类：(a) 同步事件（年龄跨度<0.4百万年）；(b) 穿时事件（即年龄跨度>1.0百万年）；(c) 无法判别事件（年龄跨度0.4~1.0百万年）。

研究结果显示，在同步标志面中，绝大多数（19个中的15个）为类群的末现事件；而在明确属于穿时的事件中，多数为类群的首现事件（13个中的10个）。在诸多案例中，类群似乎首先在印度洋演化，随后依次出现在西太平洋及东太平洋区域。鉴于赤道纬度带存在显著的东向西向流，该分布模式尤为出人意料。其中3个穿时事件曾被用于划定生物地层带边界：五海绵星虫（Spongaster pentas）、休斯双节虫（Diartus hughesi）及彼得松双节虫（D. petterssoni）的首现。本研究结果表明，在同一纬度带内，生物地层标志面的非同步性可能十分显著（即跨度超过1百万年）；而在跨海洋学及气候梯度的区域中，该效应预计会更为突出。

我们推测，硅藻、有孔虫及超微化石标志面的穿时程度或可与之相当。若该推测得到证实，则全球“年代标尺”或许需要基于数量更少、可证明确具同步性的标志面进行重新构建。