Leg 208 was originally conceived to test a single hypothesis, that the dissociation of methane hydrate of an unprecedented scale at 55 Ma (P/E boundary) initiated a brief but extreme episode of global warming. In theory, such an event would profoundly impact the carbonate saturation state of the ocean, the expression of which should be preserved in deep-sea sediments (Dickens et al., 1997, 1995). In essence, the rapid injection of methane-derived carbon into the atmosphere and ocean should be recorded by distinct spatial and temporal patterns of change in the deposition of carbonate sediments. Such changes should be contemporaneous on a global scale, affecting all major ocean basins and exhibiting diagnostic patterns in carbonate preservation related to gradients in paleodepth. A key test would involve establishing depth-dependent changes in carbonate preservation across the P/E boundary in at least two major ocean basins. To evaluate the gradient of carbonate saturation state, a transect of pelagic sections across a broad paleodepth range would be required for each ocean basin to elucidate both the temporal and spatial evolution of ocean chemistry from its onset through its ultimate recovery.

Leg 208 fulfilled its mission by successfully coring an array of Paleogene sedimentary sections spanning water depths between 2300 and 4700 m and recovering at least one complete P/E boundary layer using the APC at each of five sites. This collection of boundary clay layers shows a clear pattern of change in lithology as a function of depth, which, by most measures, appears consistent with a shoaling of the CCD as caused by the rapid release of a large mass of carbon (>4500 Gt C). This was confirmed with postcruise work, which firmly established the relative timing of this clay layer to the global CIE.

The important scientific contributions of Leg 208 are not limited to the P/E boundary; several other key objectives were achieved. For one, the upper Paleocene and lower Eocene sections that enclose the boundary layer were recovered more or less intact at three sites (Sites 1262, 1263, and 1267). These sections, which are complete and relatively expanded, are characterized by distinct bedding cycles that will provide the basis for establishing a high-resolution, orbitally tuned timescale, a basic requirement for constraining rates of change and for refining the approximate ages of chron boundaries where possible. Moreover, with the composite sections for each site, it became evident that other unusual clay layers and, by association, climatic extremes, occurred during early Eocene Chrons C24r and C24n (Elmo and X events, respectively). Postcruise studies found carbon isotope excursions in these horizons, implying these events may have origins similar to the PETM. Also, the pronounced lithologic cycles have been used to develop an orbital chronology for much of the upper Paleocene and lower Eocene, thus establishing the approximate duration of magnetochrons and the time separating these hyperthermals.

In addition, the K/Pg boundary interval was recovered at two sites (Sites 1262 and 1267) and the E/O boundary was recovered at five sites (1262, 1263, 1265, 1266, and 1267). The former is also characterized by a pronounced cyclicity that clearly changes character across the boundary. Postcruise research has documented changes in biogenic carbonate accumulation and preservation on orbital timescales across the K/Pg boundary along with variations in isotopes, particularly during the long recovery following the extinctions at the K/Pg boundary. The E/O boundary is characterized by a prominent lithologic transition from clays to carbonates that becomes more pronounced at the deepest sites. The new stable isotope records show that this major shift in Atlantic Ocean carbon chemistry coincides with the onset of major glaciation on Antarctica. Other postcruise investigations are establishing in detail important changes in deep-sea circulation and seawater chemical evolution in relation to the major Cenozoic climatic events.

The crucial scientific contributions of Leg 208 are just being realized in numerous postcruise investigations of the many thousands of samples collected. This work will undoubtedly continue to make important advances for years to come.