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doi:10.2204/iodp.pr.342.2012

Abstract

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 ~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 (~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.

An unexpected finding was the recovery of a number of Cretaceous “critical boundaries.” These include the K/Pg boundary, the Campanian–Coniacian interval, the Cenomanian–Turonian boundary and oceanic anoxic event (OAE) 2, and the Albian/Cenomanian boundary OAE 1d. These intervals were drilled opportunistically when they were encountered near or above our target depth for a given site. The K/Pg boundary was recovered at Site U1403, where it proved to have a well-preserved, normally graded spherule bed and unusually well preserved earliest Danian planktonic foraminifer community. The Campanian–Coniacian interval was cored at Site U1407 and is unusual mainly for the relative biostratigraphic completeness of a sequence that elsewhere commonly shows hiatuses in the early Campanian. The Cenomanian–Turonian transition was also cored at Site U1407 and consists of a series of organic black shales in nannofossil chalk with as much as 11 wt% total organic carbon (TOC). The Cenomanian–Turonian sequence at Site U1407 is broadly similar in biostratigraphy, sequence of black shales, and sediment color to classic Italian and northern German outcrop sections. Finally, coring at Site U1407 also recovered a lower Cenomanian nannofossil chalk and nannofossil claystone record that extends into the biozones associated with OAE 1d. The Albian–Cenomanian sequence is notable for the generally high quality of microfossil preservation and its gradational contact with underlying Albian shallow-marine carbonate grainstone and packstone.

We created high-quality spliced records of most of the sites on Southeast Newfoundland Ridge that penetrate sequences with carbonate-rich lithologies. As anticipated, the task of creating spliced records in the more clay-rich lithologies in some sites was not straightforward due to intervals of low-amplitude change in some physical properties data sets. All sites also proved to have differences in stratigraphy between adjacent holes showing that there is often considerable local variation within the drift sequences. Particularly expanded sedimentary sequences were drilled in Miocene–Oligocene, Eocene–Oligocene, and middle Eocene sequences. Comparison of our drilling results with seismic stratigraphy from the Southeast Newfoundland Ridge indicates large-scale lateral changes in age and thickness of sediment drift packages, regionally meaning that the area presents an attractive target for future expeditions aimed at recovering high deposition–rate records from many parts of the Cenozoic.

On 6–8 June 2012, before the main leg of Expedition 342, a sea trial of the Motion Decoupled Hydraulic Delivery System (MDHDS) with the temperature-dual-pressure probe (T2P) was conducted at Site U1402 on the New Jersey margin. This deployment was the culmination of the testing and development phase of a delivery system designed to remotely deploy and recover penetrometers. The benefit of this system over its predecessor is the complete decoupling of the penetrometer from the drill string, negating the effect of ship heave on the quality of data. The second of two deployments of this system fulfilled the mission goals by successfully deploying and acquiring in situ data with complete decoupling from the ship. The in situ pore pressure measured at Site U1402 was 7.536 MPa, just slightly above hydrostatic pressure. This is the first in situ pressure measurement made on the Atlantic margin in scientific ocean drilling and confirms previous indirect pressure estimates. Accelerometer data collected within the T2Ps electronic housing showed perfect decoupling from the drill string. We now have a dependable method to deploy pore pressure penetrometers successfully, which will allow the rapid measurement of in situ pressure in sediment. This new capability opens an exciting range of future science for the drilling program.

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