Radiolarian chronology and fauna across the middle/late Eocene boundary at ODP Hole 171-1052A

Quantitative radiolarian assemblage analysis has been conducted on middle and upper Eocene sediments (Zones RP16 to RP18) from Ocean Drilling Program Site 1052 in order to establish the radiolarian magnetobiochronology and determine the nature of the faunal turnover across the middle/late Eocene boundary in the western North Atlantic Ocean. We recognize and calibrate forty-five radiolarian bioevents to the magneto- and cyclo-stratigraphy from Site 1052 to enhance the biochronologic resolution for the middle and late Eocene. Our data is compared to sites in the equatorial Pacific (Leg 199) to access the diachrony of biostratigraphic events. Eleven bioevents are good biostratigraphic markers for tropical/subtropical locations (south of 30°N). The primary markers (lowest occurrences of Cryptocarpium azyx and Calocyclas bandyca) which are tropical zonal boundary markers for Zones RP17 and RP18 provide robust biohorizons for correlation and age determination from the low to middle latitudes and between the Atlantic and Pacific Oceans. Some other radiolarian bioevents are highly diachronous (<1 million years) between oceanic basins. A significant faunal turnover of radiolarians is recognized within Chron C17n.3n (37.7 Ma) where 13 radiolarian species disappear rapidly in less than 100 kyr and 4 new species originate. The radiolarian faunal turnover coincides with a major extinction in planktonic foraminifera. We name the turnover phase, the Middle/Late Eocene Turnover (MLET). Assemblage analysis reveals the MLET to be associated with a decrease in low-mid latitude taxa and increase in cosmopolitan taxa and radiolarian accumulation rates. The MLET might be related to increased biological productivity rather than to surface-water cooling.本研究针对北大西洋西部海域大洋钻探计划（Ocean Drilling Program，ODP）1052站位的始新统中上部沉积物（RP16至RP18放射虫带）开展了定量放射虫组合分析，旨在建立放射虫磁生物地层年代学，并明确北大西洋西部始新世中-晚世界线处的动物群更替特征。本研究对1052站位的45个放射虫生物事件开展识别与磁地层、旋回地层标定校正工作，以提升始新世中晚期的生物地层年代分辨率。本研究将该数据集与赤道太平洋海域（大洋钻探第199航次）的站位数据进行对比，以探讨生物地层事件的穿时性特征。其中11个生物事件可作为北纬30°以南热带、亚热带海域的优质生物地层标志。作为RP17与RP18带热带区域界线标志的两类首要标志（Cryptocarpium azyx与Calocyclas bandyca的首现事件），可为低-中纬度海域以及大西洋与太平洋之间的地层对比与年代确定提供可靠的生物地层界面。其余部分放射虫生物事件在不同大洋盆地间具有显著的穿时性（时间差小于1百万年）。 本研究在C17n.3n地磁极性带（37.7百万年）中识别出一次显著的放射虫动物群更替事件：13种放射虫在不足10万年的时间内快速灭绝，同时有4个新物种诞生。此次放射虫动物群更替与浮游有孔虫的一次大规模灭绝事件发生时间高度吻合。本研究将此次更替事件命名为“始新世中-晚世更替（Middle/Late Eocene Turnover，MLET）”。组合分析结果显示，MLET事件与低-中纬度放射虫类群占比下降、广布性类群占比及放射虫堆积速率上升存在关联。该更替事件可能与海洋生物生产力提升相关，而非表层海水降温所致。