Integrated Paleocene calcareous plankton magnetobiochronology of DSDP Hole 43-384 in the Northwest Atlantic Ocean

At Deep Sea Drilling Site 384 (J-Anomaly Ridge, Grand Banks Continental Rise, NW Atlantic Ocean) Paleocene nannofossil chalks and oozes (~70 m thick) are unconformably/disconformably underlain (~168 m; upper Maastrichtian) and overlain (~98.7 m; upper lower Eocene) by sediments of comparable lithologies. The chalks are more indurated in stratigraphically higher levels of the Paleocene reflecting increasing amounts of biosiliceous (radiolarians and diatoms) components. This site serves as an excellent location for an integrated calcareous and siliceous microfossil zonal stratigraphy and stable isotope stratigraphy. We report the results of a magnetostratigraphic study which, when incorporated with published magnetostratigraphic results, reveals an essentially complete magnetostratigraphic record spanning the interval from Magnetochron C31n (late Maastrichtian) to C25n (partim) (late Paleocene, Thanetian). Integrated magnetobiochronology and stable isotope stratigraphy support the interpretation of, and constrain the estimated duration of, a short hiatus (~0.9 my) within the younger part of Chron C29r (including the K/P boundary) and an ~6 my hiatus separating upper Paleocene (Magnetozone C25n) and upper lower Eocene (Magnetozone C22r) sediments. Some 30 planktonic foraminiferal datum levels [including the criteria used to denote the Paleocene planktonic foraminiferal (sub)tropical zonal scheme of Berggren and Miller, Micropaleontology 34 (4) (1988) 362-380 and Berggren et al., SEPM Spec. Publ. 54 (1995) 129-212, Geol. Soc. Am. Bull. 107 (11) (1995) 1272-1287], and nearly two dozen calcareous nannoplankton datum levels have been recognized and calibrated to the magnetochronology. Planktonic foraminiferal Subzones P4a and P4b of (upper Paleocene) Zone P4 are emended/redefined based on the discovery of a longer stratigraphic extension of Acarinina subsphaerica (into at last Magnetozone C25n). Stable isotope stratigraphies from benthic foraminifera and fine fraction (<38 µm) carbonate have been calibrated to the biochronology and magnetostratigraphy. A minimum in benthic foraminifer delta13C was reached near the Danian/Selandian boundary (within Chron C26r, planktonic foraminiferal Zone P3a and calcareous nannoplankton Zone NP4) and is followed by the rise to maximum delta13C values in the late Thanetian (near the base of C25n, in Zone P4c and NP9a, respectively) that can be used for global correlation in the Paleocene.

深海钻探站位（Deep Sea Drilling Site）384（位于西北大西洋大岸隆J异常岭）的古新世（Paleocene）钙质超微化石（nannofossil）白垩岩与软泥（ooze），厚度约70米，以不整合/假整合接触关系被上下两套岩性相近的沉积物覆盖：其下伏沉积物厚度约168米，对应马斯特里赫特阶（Maastrichtian）上部；其上覆沉积物厚度约98.7米，对应早始新世上部。该白垩岩在古新世地层序列中随层位升高越发致密，这反映了生物硅质组分（放射虫radiolarians与硅藻diatoms）占比的不断提升。该站位是开展钙质与硅质微化石带地层学以及稳定同位素地层学（stable isotope stratigraphy）综合研究的理想区域。本研究报道了该站位的磁性地层学（magnetostratigraphic）研究结果，结合已发表的磁性地层数据后，得到了一条近乎完整的地磁极性序列，其覆盖时限从地磁极性带（Magnetochron）C31n（马斯特里赫特阶晚期）至C25n（局部段）（古新世晚期，塞南阶Thanetian）。综合磁性生物年代学与稳定同位素地层学的结果，我们证实并限定了两处沉积间断（hiatus）的估算时长：一处位于C29r地磁极性带的较年轻段（包含白垩纪-古近纪K/P边界），持续时长约0.9百万年；另一处间隔了古新世上部（C25n地磁极性亚带（Magnetozone））与早始新世上部（C22r地磁极性亚带（Magnetozone））的沉积物，时长约6百万年。本研究共识别出约30个浮游有孔虫（planktonic foraminifera）化石基准面（datum level），其中包括Berggren与Miller在《Micropaleontology》34卷第4期（1988年，第362-380页）以及Berggren等人在《SEPM Special Publication》54卷（1995年，第129-212页）、《美国地质学会通报》107卷第11期（1995年，第1272-1287页）中提出的古新世浮游有孔虫（亚）热带带划分标准；同时还识别出近24个钙质超微化石（calcareous nannoplankton）基准面，并将这些基准面与地磁年代学进行了校准。基于球状阿卡林虫（Acarinina subsphaerica）的地层分布范围被证实延伸至至少C25n地磁极性亚带的新发现，我们对（古新世上部）P4带的浮游有孔虫亚带P4a与P4b进行了修订与重新定义。研究人员对底栖有孔虫（benthic foraminifera）与<38微米细粒级碳酸盐的稳定同位素地层序列进行了校准，使其与生物年代学和磁性地层学结果相匹配。底栖有孔虫的碳同位素δ¹³C（delta13C）值在达宁阶（Danian）/塞兰阶（Selandian）边界附近（对应C26r地磁极性带、浮游有孔虫P3a带以及钙质超微化石NP4带）达到最小值，随后在塞南阶晚期（分别对应C25n带底部、P4c带与NP9a带）升至最大值，该同位素序列可用于古新世的全球地层对比。