Description: We revisited the stratigraphy at Deep Sea Drilling Project (DSDP) Site 296, which has promise as a reference sequence for the mid-latitude western North Pacific. We constructed the biostratigraphy (calcareous nannofossils, planktic foraminifera, and radiolaria) and chemostratigraphy (strontium isotope ratios, and stable oxygen and carbon isotope ratios) to reveal continuous sedimentation at Site 296 through the past 20 million years (Myr). The biostratigraphy of calcareous nannofossils comprised 27 biohorizons from biozones NN2 to NN21 (early Miocene to Pleistocene), which are consistent with the biohorizons of planktic foraminifera and radiolaria. The uninterrupted sedimentation throughout the past 20 Myr was further supported by strontium isotope stratigraphy aligned to nannofossil datums and by correlation of stable isotope data from benthic foraminifera with an isotopic compilation from the Pacific Ocean. The refined age–depth model showed low sedimentation rates (〈2 cm/kyr) through most of the Miocene and higher sedimentation rates (2–4 cm/kyr) during the Plio–Pleistocene, and potentially identified the “biogenic bloom” event from the late Miocene to early Pliocene. The continuity of sedimentation through the middle Miocene contrasts with the shipboard biostratigraphy, which inferred a hiatus (erosion or non-deposition) during the middle Miocene. Thus, the revised stratigraphy at Site 296 provides a key to correlation with other deep-sea sites in the North Pacific. Our revised age–depth model provides a framework for future studies of important climatic events during the Miocene, including the Miocene Climatic Optimum, the middle Miocene Climatic Transition, and the late Miocene global cooling under the influence of the past Kuroshio Current.

Description: Knowledge of the equatorial thermocline is essential for understanding climate changes in the tropical Pacific. Multispecies planktic foraminiferal analyses provide a way to examine temperature distributions and thus the structure of the thermocline. Although the secular thermocline development has been documented back to the late Miocene, the early to middle Miocene interval has rarely been examined. In addition, relationships with the dynamic Antarctic ice sheets remain unclear. Here we investigate the vertical thermal gradient in the upper water column at Integrated Ocean Drilling Program Site U1337 in the eastern equatorial Pacific (EEP) throughout the early to middle Miocene (23.1 to 11.7 Ma). The gradient increased over the Miocene Climatic Optimum, whereas it decreased during the East Antarctic Ice Sheet Expansion (EAIE). Comparison of the EEP record with its western equatorial Pacific (WEP) counterpart suggests that sea surface temperature was more stable in the WEP than in the EEP. We further estimated equatorial thermocline from two diagonal gradients between the EEP and the WEP: thermocline shoaled from 16.7 to 15.7 Ma and tilt weakened between 16.5 and 13.8 Ma. The onset of the "Monterey Excursion" and the reduced Antarctic ice sheet volume would have affected thermocline depth and tilt, respectively. Thermocline depth was likely much deeper compared to Pliocene‐to‐modern conditions. Furthermore, a 4‐point‐based distribution of isotherms (4DI index) was used as a metric of the evenness or unevenness of the isotherm distributions. The 4DI index considerably reduced at around the EAIE and other Mi‐events, reflecting the evenly distributed isotherms under a more glaciated Antarctica.

Description: During the Paleocene, the plankton community recovered its diversity after a severe diversity loss at the Cretaceous/Paleogene (K/Pg) boundary. In general, changes in Paleocene deep-sea ostracode fauna are poorly understood. It is still unclear how deep-sea ostracode fauna changed during compelling events such as the K/Pg boundary (66.04 Ma) and the Latest Danian Event (LDE: ~62.18 Ma), a transient Paleocene warming event. In this report, we describe a study on the latest Cretaceous-late Paleocene ostracodes from Integrated Ocean Drilling Program Site U1407, off Newfoundland, in the northwestern Atlantic. Thirty-four ostracode taxa were obtained from 202 discrete samples that were discretely collected from the interval between 66.3 and 57.3 Ma. Near the K/Pg boundary, 8% (2 of 24) of the Maastrichtian taxa became extinct. The extinction rate is estimated to be at most 0.12 E MSY-1, which is in the same range as the background-level extinction rate. Comparing the extinction rates between different ocean basins, the ostracode extinction at the K/Pg boundary displays striking geographical heterogeneity. Between the Maastrichtian and Danian, the ostracode fauna change from a biofacies dominated by shallow bathyal taxa to one dominated by deep bathyal taxa. During the LDE, ostracode fauna display an increase in their benthic ostracode accumulation rates and taxonomic diversity as well as an increase in the relative abundance of Krithe crassicaudata simultaneously with a decline in carbonate content. A similar change is commonly documented in fauna from the northeastern Atlantic during the Paleocene-Eocene Thermal Maximum (PETM; 56.0 Ma). In contrast to the PETM fauna, an extinction event is not recognized. After the Danian, the ostracode fauna indicate changes in water mass and minor environmental disturbances with possible development of corrosive water.