[M-IS18] Drilling Earth Science

[MIS18-P08] Stratigraphy of the Cretaceous/Paleogene boundary interval at the Lord Howe Rise, Southwest Pacific

*Junichiro Kuroda^1,3, Kyoko Hagino², Yoichi Usui³, Paul R. Bown⁴, KanHsi Hsiung³, Saneatsu Saito³, Saburo Sakai³, Ron Hackney⁶, Masafumi Murayama², Takuto Ando⁵, Naohiko Ohkouchi³ (1.Department of Ocean Floor Geoscience, Atmosphere and Ocean Research Institute, the University of Tokyo, 2.Center for Advanced Marine Core Research, Kochi University, 3.Japan Agency for Marine-Earth Science and Technology, 4.Department of Earth Sciences, University Collage London, 5.Estuary Research Center, Shimane University, 6.Minerals, Energy, Groundwater Division, Geoscience Australia)

Keywords:Cretaceous/Paleogene boundary, Os isotope, Lord Howe Rise

During Deep Sea Drilling Project (DSDP) Leg 21, Cenozoic and latest Cretaceous sediments were recovered at Site 208 on the Lord Howe Rise, Southwest Pacific. We provide new biostratigraphic, magnetostratigraphic and chemostratigraphic data from Site 208 to constrain the stratigraphy around the Cretaceous/Paleogene (K/Pg) boundary. Biostratigraphic data from calcareous nannofossils indicate a near-continuous succession of sediments from the mid-Maastrichtian (Late Cretaceous) to lowermost Thanetian (Paleocene) at depths of 540-590 meters below seafloor (mbsf). The biostratigraphic data suggest that the K/Pg boundary corresponds to a siliceous claystone at the base of an interval of silicified sediments at the depths of 576.0-576.8 mbsf. Carbonate carbon isotopic composition (δ¹³C_carb) reveals a negative shift across this interval, which is consistent with global patterns of δ¹³C across the K/Pg boundary. Osmium concentration and Os isotopic composition (¹⁸⁷Os/¹⁸⁸Os) can also be used to identify the K/Pg boundary interval, as it is marked by a peak in Os concentration and a drop in ¹⁸⁷Os/¹⁸⁸Os values to 0.12-0.15, caused by the Chicxulub impact event. Our ¹⁸⁷Os/¹⁸⁸Os data show similar trends to those of coeval global seawater, with the lowest value of 0.12-0.16 in the siliceous claystone (576.8 mbsf). However, the concentration of Os is low (<80 pg g^-1) in this sample, suggesting that this siliceous claystone was deposited around the K/Pg boundary, but may not include the boundary itself. This study shows that the sedimentary record across the K/Pg boundary interval at Site 208 may not be completely continuous, it nevertheless captures a time interval that is close to the Chicxulub impact event.