SYNOPSIS AND CONCLUDING REMARKS

ODP Leg 208 was conceived primarily 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.

With this objective in mind, Leg 208 was designed to recover an array of sites spanning paleodepths between ~1500 and 3600 m. The northern flank of the Walvis Ridge was selected based on previous drilling (DSDP Leg 74) that revealed the presence of a P/E boundary clay layer in two locations, DSDP Sites 527 and 525, spanning a depth range of nearly 2 km (Moore, Rabinowitz, et al., 1984; Thomas and Shackleton, 1996). Critically, these boundary layers were at subbottom depths within reach of the APC system, an essential requirement for coring with high recovery and minimal disturbance. To optimize the potential of achieving this goal, high-resolution seismic profiles, obtained during Meteor Cruise M49/1 (Spieß et al., 2003), were used to locate sites in areas where the APC system could be deployed, at a minimum, to the level of the P/E boundary. The drilling plan called for double to triple APC and XCB coring, in combination with high-resolution core log measurements to optimize 100% recovery of the sedimentary section, a requisite for construction of composite sections.

Despite a number of minor setbacks, this drilling strategy ultimately proved highly successful. At least one complete P/E boundary layer was recovered by the APC at each of the five sites, spanning depths between 2300 m and 4700 m, with at least one additional copy at four of those 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 predicted by the methane dissociation model. Although postcruise studies are still required to establish the approximate timing of the changes in carbonate content relative to other indicators of changes in ocean chemistry (i.e., carbon isotopes), it appears the primary objective was achieved.

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 (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 the early Eocene Chrons C24n and C22r (Y and X events, respectively). These events may have origins similar to the PETM and thus will be useful for identifying the mechanism(s) behind transient climatic events, as well as for understanding the oceanographic and environmental consequences of these events.

In addition, The K/P boundary interval was recovered at two 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 cyclitiy that clearly changes character across the boundary. As such, these two sections will be critical in establishing the pace of ecological change, particularly during the long recovery following the extinctions at the K/P boundary. The E/O boundary is characterized by a prominent lithologic transition from clays to carbonates that becomes more pronounced at the deepest sites. This transition represents a major shift in the ocean carbon chemistry that has not been well constrained, either in time or space, relative to other environmental changes such as climate. Leg 208 cores, along with those recovered during Legs 198 and 199, will provide the first detailed temporal constraints on this important geochemical event.

Leg 208 also provided important insight into the challenges of coring soft to moderately indurated chalk/clay formations. To recover critical intervals (particularly the short-lived transients) and to establish an orbitally tuned stratigraphy, Leg 208 required 100% recovery of the stratigraphic sections. This was easily achieved with the APC but not with the XCB system in the more deeply buried soft chalk formations. To extend the working range of the APC, the engineers employed a "drillover" method that permitted the use of the APC deeper in the section. In overlapping holes, the quality of the APC cores was significantly better than those recovered by the XCB system. In the case of the P/E boundary layer, we discovered that the upper portion of the clay layers became temporarily lodged in the XCB during coring, effectively blowing away the contact. This necessitated redeployment of the APC in second and third holes to recover this interval.

The crucial scientific contributions of Leg 208 have yet to be realized. Shore-based investigations of the many thousands of samples collected will take years to complete. Nonetheless, the initial results have already answered some fundamental questions concerning the scale of the P/E carbon cycle perturbation. The results of these investigations together with those of other recent legs (207, 198, and 199) focused on the Paleogene will undoubtedly provide a much more detailed and accurate picture of Earth system change during this climatically dynamic period.

NEXT