Stratigraphy of the Cretaceous-Paleogene transition interval at the Lord Howe Rise, southern Pacific

During Deep Sea Drilling Project (DSDP) Leg 21, Cenozoic and latest Cretaceous pelagic sediments were recovered at Site 208 on the Lord Howe Rise, southwestern Pacific. We provide new geochemical, biostratigraphic and magnetostratigraphic data for cores from Site 208 to constrain stratigraphy around the Cretaceous/Paleogene boundary (K-Pg). The sediment is mainly composed of calcareous chalk interbedded with silicified mudstone and chert. Paleomagnetic polarity and calcareous nannofossil assemblages indicate that the Maastrichtian and Danian chalk between 550 and 590 meters below seafloor (mbsf) was deposited with an average sedimentation rate of 5-7 mm per thousand years. The sedimentation rate significantly decreased in an interval of silicified mudstone and chalk around 576-577 mbsf, which spans the transition between Maastrichtian and Danian. Variations in osmium isotopic composition (¹⁸⁷Os/¹⁸⁸Os) can be used for stratigraphic correlation, as the K-Pg boundary marks a low ¹⁸⁷Os/¹⁸⁸Os value of 0.15 compared to the Maastrichtian ( 0.5-0.6) and Danian ( 0.4). Our ¹⁸⁷Os/¹⁸⁸Os data indicate a similar variation with that of coeval ocean water, with a lowest ¹⁸⁷Os/¹⁸⁸Os value of 0.16 in a silicified mudstone sample at 576.8 mbsf. This suggests that the silicified mudstone represents the K-Pg boundary. Detail investigation on calcareous nannofossil assemblages is also consistent with the Os isotopic data. Our stratigraphic investigation has reveals that while the K-Pg boundary interval at Site 208 may not be as continuous as those preserved at Shatsky Rise, Maud Rise and Walvis Ridge, it nevertheless captures a time interval that is close to the asteroid impact.