Expedition 342 summary

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 (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

Site U1404

“…F2), corresponding to paleodepths at 50 Ma of ~4600 mbsl using the subsidence reconstruction of Tucholke and Vogt (1979) (see Fig. F2 in the "Expedition 342 summary" chapter [Norris et al, 2014a]). In that reconstruction, the contemporaneous North Atlantic CCD is positioned at a paleodepth of ~4400-4300 mbsl and prominent CCD deepening is indicated below 5500 mbsl in the latest Cretaceous and early Paleocene.…”

“…Open and solid symbols represent Holes U1404A and U1404B, respectively. Note that abundance criteria are defined differently for each fossil group (see "Biostratigraphy" in the "Methods" chapter [Norris et al, 2014a]). Abundance: B = barren, P = present, R = rare, F = few, C = common, A = abundant, D = dominant.…”

“…Plots of anisotropy of magnetic susceptibility, bulk density, and porosity vs. depth, Hole U1404A. Separation of eigenvalues is related to the shape and degree of magnetic fabric (see "Paleomagnetism" in the "Methods" chapter [Norris et al, 2014a]). For example, if τ 1 and τ 2 are close or indistinguishable but distinct from τ 3 , then the bulk fabric is oblate.…”

Site U1403

“…Given its current water depth of nearly 5 km and assuming a subsidence history for J-Anomaly Ridge akin to that reconstructed from Deep Sea Drilling Project (DSDP) Leg 43 results (Tucholke and Vogt, 1979) (see Fig. F2 in the "Expedition 342 summary" chapter [Norris et al, 2014a]), Site U1403 should be a sensitive recorder of CCD fluctuations, particularly during intervals when the CCD was depressed to abyssal depths. Of particular interest are CCDdeepening events associated with "overshoots" in carbonate ocean chemistry in the early recovery phases of the most extreme Cenozoic carbon cycle perturbations, such as the Eocene-Oligocene transition, the Paleocene/Eocene Thermal Maximum (PETM), and the Cretaceous/Paleogene (K/Pg) boundary (Figs.…”

“…Tucholke and Vogt (1979) reconstructed the North Atlantic CCD to be positioned at ~ 3 km paleodepth during the Late Cretaceous, to undergo a temporary depression to below 5500 mbsl in the latest Cretaceous and early Paleocene, and to be located around 4400-4300 mbsl in the Eocene (see Fig. F2 in the "Expedition 342 summary" chapter [Norris et al, 2014a]). Compilation of carbonate records from North Atlantic DSDP sites, however, reveals that there is little constraint on the depth of the CCD in the Paleocene and Late Cretaceous North Atlantic Ocean.…”