During Ocean Drilling Program (ODP) Leg 208, six sites were drilled at water depths between 2500 and 4770 m to recover lower Cenozoic sediments on the northeastern flank of Walvis Ridge. Previous drilling in this region (Deep Sea Drilling Project [DSDP] Leg 74) recovered pelagic oozes and chalk spanning the Cretaceous/Paleogene (K/P), Paleocene/Eocene, and Eocene/Oligocene boundaries. The objective of Leg 208 was to recover intact composite sequences of these "critical" transitions from a wide range of depths. Multichannel seismic data (Meteor Cruise M49/1) along with information from DSDP Leg 74 sites were used to identify sites where continuous sequences of lower Cenozoic sediment should be present. Double to triple advanced hydraulic piston coring, occasional extended core barrel coring to deepen the holes, and high-resolution physical property measurements were employed to construct "composite sections." The composite sections provide a detailed history of paleoceanographic variation associated with several prominent episodes of early Cenozoic climate change, including the K/P boundary, early Eocene Climatic Optimum, the PaleoceneEocene Thermal Maximum (PETM), and the early Oligocene Glacial Maximum.
The PETM interval, the main focus of Leg 208, was recovered at five sites along a depth transect of 2.2 km. The sediment sequence is marked by a red clay layer, which varies in thickness from 20 to 50 cm from site to site, within a thick and uniform sequence of upper Paleocene and lower Eocene foraminifer-bearing nannofossil ooze. The basal color contact is relatively sharp, although magnetic susceptibility data show a more gradual, steplike transition at the deeper Sites 1262 and 1267. The carbonate content drops to 0 wt% at all sites except for Site 1265. The upper contact is gradational in the shallow sites and relatively sharp at the deeper sites. Overlying the clay layer is a sequence of nannofossil ooze, which is slightly richer in carbonate than the unit immediately underlying the clay layer.
The depth transect permits testing of the leading hypothesis for the cause of the PETM: the abrupt dissociation of as much as 2000 Gt of marine methane hydrate. Numerical modeling demonstrates that the injection of such a large mass of carbon to the ocean/atmosphere could have triggered a rapid (~10 k.y.) global shoaling of the calcite compensation depth (CCD) and lysocline, followed by a gradual recovery, and "overcompensation" with the CCD overshooting pre-excursion depths. Based on sediment cores recovered during Leg 208, the estimated magnitude of the shoaling of the CCD is >2 km, considerably more than predicted in present carbon cycle models of the event.
Leg 208 material also documents biotic responses to environmental changes as a result of the methane release and CCD shoaling (e.g., severe dissolution over such a large depth range may well have been an important factor in the benthic foraminiferal extinction event coincident with the base of the clay layer at every site, and nannofossils showed a short-term relative abundance response from Fasciculithus to Zygrhablithus). Planktonic foraminifers are heavily dissolved in the clay layer with only extremely rare specimens of acarinids and morozovellids remaining.
The Leg 208 transect complements a transect drilled on the southern Shatsky Rise during Leg 198, a deep latitudinal transect in the equatorial Pacific drilled during Leg 199, a shallow to bathyal transect drilled on Demarera Rise during Leg 207, and a depth transect proposed for future drilling in the western North Atlantic Ocean (J-Anomaly Ridge and Southeast Newfoundland Rise).
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