New composite bio- and isotope stratigraphies spanning the Middle Eocene Climatic Optimum at tropical ODP Site 865 in the Pacific Ocean
New composite bio- and isotope stratigraphies spanning the Middle Eocene Climatic Optimum at tropical ODP Site 865 in the Pacific Ocean
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DOI：
10.31223/osf.io/m64gk
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发表时间：
2020-02
影响因子：
2
通讯作者：
K. Edgar;S. Bohaty;H. Coxall;P. Bown;S. Batenburg;C. Lear;P. Pearson
中科院分区：
文献类型：
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作者：
K. Edgar;S. Bohaty;H. Coxall;P. Bown;S. Batenburg;C. Lear;P. Pearson
抽象的。中始新世气候最适期（MECO）。40 Ma是始新世最大的短暂全球变暖事件之一。然而，它是相对鲜为人知的热带环境，因为很少有网站跨越整个MECO事件和/或主机钙质微体化石，这是古海洋重建的主要代理载体。大洋钻探计划（ODP）太平洋865号站点位于低纬度北太平洋（Allison Guyot），有可能提供有用的热带MECO参考，但以前缺乏评估其完整性所需的详细地层和年代限制。我们通过在865号站点的两个钻孔中生成新的高分辨率生物地层学、稳定同位素和X射线荧光（XRF）记录来解决这一缺陷，这些记录跨越了MECO间隔（38.0-43.0 Ma）。XRF衍生的锶闪烁钙（Sr scinCa）和钡闪烁锶（Ba scinSr）的比率和铁计数记录允许孔之间的相关性，并揭示了明显的韵律性，使我们能够开发第一个复合部分的孔865 B和865 C和初步的旋回地层MECO。利用这一新的框架，沉积记录被解释为在整个事件中是连续的，如由显著的瞬时底栖有孔虫δ 18 O偏移1.08 ‰所确定的。从广泛使用的分区方案中识别出钙质微体化石生物地层事件，两个孔之间的一致性通常良好，突出了新复合剖面的稳健性，并使我们能够识别南极有孔虫带E10-E15和钙质超微化石带NP 15 -18。然而，在相对位置和顺序的几个主要和次要的生物事件相对于已公布的计划的差异。具体而言，地层最高发生率的南极有孔虫，蜱螨目，Guembelitrioides nuttalli，和Morozovella aragonensis，和钙质超微化石，Chiasmolithus solitus和Sphenolithus furcatolithoides，和最低发生率的Reticulofenestra reticulata都出现在该节比预测相对于其他生物事件。我们还注意到，在我们的研究间隔内，较老的微体化石（来自南极有孔虫带E5-E9和E13）明显地改造成较年轻的沉积物（南极有孔虫带E14-15），这与MECO间隔以上的改造一致。无论是否返工，高质量的XRF记录都可以实现孔之间的分米级相关性，并突出了865号站点限制热带环境和生物变化的潜力，不仅在MECO，而且在整个古新世和早至中始新世间隔。
Abstract. The Middle Eocene Climatic Optimum (MECO) at ca. 40 Ma is one of the largest of the transient Eocene global warming events. However, it is relatively poorly known from tropical settings since few sites span the entirety of the MECO event and/or host calcareous microfossils, which are the dominant proxy carrier for palaeoceanographic reconstructions. Ocean Drilling Program (ODP) Pacific Ocean Site 865 in the low-latitude North Pacific (Allison Guyot) has the potential to provide a useful tropical MECO reference, but detailed stratigraphic and chronological constraints needed to evaluate its completeness were previously lacking. We have addressed this deficit by generating new high-resolution biostratigraphic, stable isotope, and X-ray fluorescence (XRF) records spanning the MECO interval (∼38.0–43.0 Ma) in two holes drilled at Site 865. XRF-derived strontium ∕ calcium (Sr∕Ca) and barium ∕ strontium (Ba∕Sr) ratios and Fe count records allow correlation between holes and reveal pronounced rhythmicity, enabling us to develop the first composite section for Holes 865B and 865C and a preliminary cyclostratigraphy for the MECO. Using this new framework, the sedimentary record is interpreted to be continuous across the event, as identified by a pronounced transient benthic foraminiferal δ18O shift of ∼0.8 ‰. Calcareous microfossil biostratigraphic events from widely used zonation schemes are recognized, with generally good agreement between the two holes, highlighting the robustness of the new composite section and allowing us to identify planktic foraminiferal Zones E10–E15 and calcareous nannofossil Zones NP15–18. However, discrepancies in the relative position and ordering of several primary and secondary bioevents with respect to published schemes are noted. Specifically, the stratigraphic highest occurrences of planktic foraminifera, Acarinina bullbrooki, Guembelitrioides nuttalli, and Morozovella aragonensis, and calcareous nannofossils, Chiasmolithus solitus and Sphenolithus furcatolithoides, and the lowest occurrence of Reticulofenestra reticulata all appear higher in the section than would be predicted relative to other bioevents. We also note conspicuous reworking of older microfossils (from planktic foraminiferal Zones E5–E9 and E13) into younger sediments (planktic foraminiferal Zones E14–15) within our study interval consistent with reworking above the MECO interval. Regardless of reworking, the high-quality XRF records enable decimetre-scale correlation between holes and highlight the potential of Site 865 for constraining tropical environmental and biotic changes, not just across the MECO but also throughout the Palaeocene and early-to-middle Eocene interval.