The cause of the end-Cretaceous mass extinction is vigorously debated, owing to the occurrence of a very large bolide impact and flood basalt volcanism near the boundary. Disentangling their relative importance is complicated by uncertainty regarding kill mechanisms and the relative timing of volcanogenic outgassing, impact, and extinction. We used carbon cycle modeling and paleotemperature records to constrain the timing of volcanogenic outgassing. We found support for major outgassing beginning and ending distinctly before the impact, with only the impact coinciding with mass extinction and biologically amplified carbon cycle change. Our models show that these extinction-related carbon cycle changes would have allowed the ocean to absorb massive amounts of carbon dioxide, thus limiting the global warming otherwise expected from postextinction volcanism.

Integrated Ocean Drilling Program Expedition 342 was designed to recover Paleogene sedimentary sequences with unusually high deposition rates across a wide range of water depths (Sites U1403-U1411). The drilling area is positioned to capture sedimentary and geochemical records of ocean chemistry and overturning circulation beneath the flow of the Deep Western Boundary Current in the northwest Atlantic Ocean. In addition, two operational days were dedicated to a sea trial of the Motion Decoupled Hydraulic Delivery System (MDHDS) developmental tool (Site U1402).The expedition was primarily targeted at reconstructing the Paleogene carbonate compensation depth (CCD) in the North Atlantic for reference to recently obtained high-fidelity records of the CCD in the equatorial Pacific. The site in the deepest water (Site U1403) was at a paleodepth of ~4.5 km 50 m.y. ago, whereas the site in the shallowest water (Site U1408) can be backtracked to a paleodepth of 2.5 km at the same time. The combination of sites yields a record of the history of CCD change over a 2 km depth range from the ocean abyss to middle-range water depths. Notable findings include the discovery of intermittent calcareous sediments in the Cretaceous, Paleocene, and early to middle Eocene at 4.5 km paleodepth, suggesting a deep Atlantic CCD during these times. We find evidence of carbonate deposition events following the Cretaceous/Paleogene (K/Pg) boundary mass extinction, the Paleocene/Eocene Thermal Maximum, and the Eocene-Oligocene transition (EOT). These deposition events may reflect the rebalancing of ocean alkalinity after mass extinctions or abrupt global climate change. Intervals during which the CCD appears to have been markedly shallow in the North Atlantic include the Early Eocene Climatic Optimum, the late Eocene, and the middle Oligocene.A second major objective of Expedition 342 was to recover clayrich sequences with well-preserved microfossils and high rates of accumulation in comparison to the modest rates of accumulation (~0.5-1 cm/k.y. in the Paleogene) typically encountered at pelagic sites. As anticipated, Expedition 342 recovered sequences with sedimentation rates as high as 10 cm/k.y.-high enough to enable studies of the dynamics of past abrupt climate change, including both transitions into "greenhouse" and "icehouse" climate states, the full magnitudes of hyperthermal events, and rates of change Chapter contents

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X-ray diffraction analyses Bulk sample XRD analyses were performed using a Bruker D-4 Endeavor X-ray diffractometer with a Vantec detector using Ni-filtered CuKα radiation. Instrument settings were Voltage = 40 kV. Current = 40 mA. Goniometer scan = 2°-70°2θ (air-dried samples). Step size = 0.01°2θ. Scan speed = 1.2°2θ/min. Count time = 0.5 s. Biostratigraphy Preliminary age assignments were based on biostratigraphic analyses of calcareous nannofossils, planktonic foraminifers, and radiolarians. Paleodepth interpretations were based on benthic foraminifers. The biostratigraphy is tied to the geomagnetic polarity timescale (GPTS) used for Expedition 342, which is based upon the timescale of Gradstein et al. (2012) (Fig. F5). The age of events based on the previous