Texas A&M University, United States

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 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 located in the deepest water (Site U1403) was at a paleodepth of approximately 4.5 km 50 m.y. ago, whereas the site located 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. 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 clay-rich sequences with well-preserved microfossils and high rates of accumulation in comparison to the modest rates of accumulation ( approximately 0.5-1 cm/k.y. in the Paleogene) typically encountered at pelagic sites. As anticipated, Expedition 342 recovered sequences with sedimentation rates of as much 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 in the CCD. We find that the thickest central parts of the various sediment drifts typically record similar depositional packages to those recovered in the thin "noses" and "tails" of these drifts, but these central parts are often massively expanded with clay, especially near the CCD. Times of rapid accumulation of drift deposits include the early Eocene to late middle Eocene, the late Eocene to early Oligocene, the late Oligocene and early Miocene, the later Miocene to probable late Pliocene, and the Pleistocene. Widespread hiatuses are present near the Paleocene/Eocene boundary into the middle early Eocene and the middle Oligocene. The Eocene/Oligocene boundary is a period of slow sedimentation at most sites but is expanded at Site U1411. A marked change in the geometry of drift formation is observed in the ?late Pliocene, as has been observed in drift deposits elsewhere....