Astronomical time scale for Paleocene and early Eocene sediments

The astronomical-tuned time scale is rapidly extended into the Paleogene but, due to the existence of an Eocene gap, different tuning options had to be presented for the Paleocene. These options differ both in number and tuning of ~405-kyr eccentricity related cycles and are only partially consistent with recalculated 40Ar/39Ar constraints for the Cretaceous/Paleogene (K/Pg) and Paleocene/Eocene (P/E) boundaries. In this paper, we evaluate the cyclostratigraphic interpretation of records from ODP Leg 198 and 208 sites, and the Zumaia section to solve the problem of the different tuning options.
We found that the interval between the K/Pg boundary and the early Late Paleocene biotic event (ELPE) comprises 17 instead of 16 * ~405-kyr eccentricity related cycles as previously proposed, while the entire Paleocene contains 25 * ~405-kyr cycles. Starting from 40Ar/39Ar age constraints for the K/Pg boundary, a new tuning to 405-kyr eccentricity is presented for the Paleocene and earliest Eocene, which results in ages of ~66.0 and ~ 56.0 Ma for the K/Pg and P/E boundaries, respectively. This tuning introduces considerable differences in age for a number of nannofossil events at ODP Sites 1209 and 1262 in the interval between 61 and 63 Ma, but eliminates large and abrupt changes in the seafloor spreading rate. The tuning seems further consistent with recalculated 40Ar/39Ar ages for ash layer -17 of early Eocene age. However, despite this apparent consistency with existing radio-isotopic constraints, an alternative 405-kyr younger or, less likely, older tuning cannot be excluded at this stage.

天文校准年代标尺（astronomical-tuned time scale）已快速延伸至古近纪（Paleogene），但受始新世间断（Eocene gap）的影响，古新世（Paleocene）需采用多种不同的校准方案。这些方案在~40.5万年偏心率相关旋回（~405-kyr eccentricity related cycles）的数量与校准方式上均存在差异，且仅与重新计算得到的白垩纪/古近纪（Cretaceous/Paleogene, K/Pg）及古新世/始新世（Paleocene/Eocene, P/E）界线的氩-氩测年法（40Ar/39Ar）年代约束部分吻合。本文通过对大洋钻探计划（Ocean Drilling Program, ODP）第198、208航次钻井及祖马亚（Zumaia）剖面的旋回地层学（cyclostratigraphic）解译结果进行评估，旨在解决上述不同校准方案的争议问题。
研究发现，K/Pg界线至晚古新世早期生物事件（early Late Paleocene biotic event, ELPE）之间的地层序列包含17个~40.5万年偏心率相关旋回，而非此前提出的16个；整个古新世则包含25个此类旋回。基于K/Pg界线的40Ar/39Ar年代约束，本文为古新世及始新世早期建立了新的~40.5万年偏心率校准方案，据此K/Pg及P/E界线的年代分别为~66.0 Ma与~56.0 Ma。该校准方案使得ODP 1209、1262钻井在61~63 Ma区间内的多个超微化石（nannofossil）事件的年代出现显著差异，但消除了海底扩张速率（seafloor spreading rate）的大幅突变现象。该校准方案还与早始新世-17号火山灰层（ash layer -17）重新计算得到的40Ar/39Ar年代结果吻合良好。尽管该方案与现有放射性同位素（radio-isotopic）年代约束具有较好的一致性，但现阶段仍无法排除另一种~40.5万年偏心率校准方案（年代偏年轻，或可能性更低的偏古老）的合理性。