Ocean Drilling Program Leg 171B was designed to recover a series of 'critical boundaries and intervals' in Earth history in which abrupt changes in climate and oceanography coincide with often drastic changes in the Earth's biota. Some of these events such as the Cretaceous/Paleogene (K/T) extinction and the late Eocene tektite layers are associated with the impacts of extraterrestrial objects, like asteroids or comets. Other events, including the benthic foraminifer extinction in the late Paleocene and the mid-Maastrichtian extinction events, are probably related to intrinsic features of the Earth's climate system. Two intervals of unusually warm climatic conditions, the early Eocene and the late Albian, represent periods when the Earth is thought to have experienced such extreme warmth that they are sometimes described as 'super-greenhouse' periods. The major objectives of Leg 171B were to recover records of these critical boundaries at shallow burial depth where microfossil and lithologic information would be well preserved, and to drill cores along a depth transect where the vertical structure of the oceans during the boundary events could be studied. The recovery of continuous Paleogene or Mesozoic sequences characterized by cyclical changes in lithology would help to establish the rates and timing of major changes in surface and deep water hydrography and microfossil evolution.

Accordingly, five sites were drilled down the spine of the Blake Nose, a salient on the margin of the Blake Plateau where Paleogene and Cretaceous sediments have never been deeply buried by younger deposits. The Blake Nose is a gentle ramp extending from ~1000 to ~2700 m water depth, and is covered by a drape of Paleogene and Cretaceous strata that are largely protected from erosion by a thin veneer of manganiferous sand and nodules. We recovered a record of the Eocene period that is nearly complete except for a few short hiatuses in the middle Eocene. The continuous expanded records show Milankovitch-related cyclicity that provides the opportunity for astronomical calibration of at least parts of the Eocene time scale, particularly when combined with the excellent magnetostratigraphic record and the presence of both calcareous and siliceous microfossils. The chemistry of the well-preserved calcareous microfossils will be used to document climate variability when the Earth's climate switched from a greenhouse to an icehouse state.

Leg 171B successfully recovered a suite of critical events in Earth's history that includes the late Eocene radiolarian extinction, late Paleocene benthic extinction, the K/T boundary, the mid Maastrichtian event, and several episodes of organic-rich sediments in the Albian warm period. The upper Paleocene benthic foraminifer extinction occurs within an expanded interval of calcareous sediments unlike most regions of the Atlantic Ocean where calcareous fossils have been severely dissolved just above the extinction horizon. The K/T boundary was recovered at three sites, each with a biostratigraphically and magnetostratigraphically complete sequence that includes the earliest part of the aftermath of the Late Cretaceous extinctions. We managed to recover three copies of the boundary interval at one site in a section that includes a 10- to 17-cm-thick spherule bed, a rusty brown limonitic layer, a dark gray clay bed (planktonic foraminifer Zone P0), and white ooze that represents planktonic foraminifer Zone Pa.

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