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Ocean Drilling
Thomas A. Davies

During 2003, JOIDES Resolution carried out the final four legs of the Ocean Drilling Program (ODP), bringing to an end an 18-year odyssey of exploration and discovery. Appropriately, the final legs touched on themes that have run through the duration of the program: paleoceanography, deep crust and mantle, and continental margins.

A principal objective of ODP Leg 207 was to recover expanded, shallowly buried Cretaceous and Paleogene sediments from Demerara Rise off Suriname in South America that could be used for paleoceanographic study of the tropical Atlantic. Drilling five sites in a depth transect from 3,200 to 1,900 meters below sea level, the leg resulted in recovery of multiple sequences of Cenomanian and Turonian black shales, Campanian to Maastrichtian chalk, and Paleocene, lower Eocene and middle Eocene chalk. All sites display pronounced cyclicity in physical properties and sediment color, offering the possibility not only of refining the chronology around critical intervals but also cross-checking results between sites and understanding past climate forcing mechanisms.

ODP Leg 208 drilled six sites at water depths between 2,500 and 4,770 meters to recover lower Cenozoic sediments on the northeastern flank of Walvis Ridge, west of Namibia in the South Atlantic. Previous drilling in this region 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. 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 Paleocene-Eocene Thermal Maximum and the early Oligocene Glacial Maximum.

The goal of Leg 209 was to test the hypothesis that mantle flow and melt extraction from the mantle at slow spreading mid-ocean ridges is focused in three dimensions near the centers of magmatic ridge segments. The Mid-Atlantic Ridge from 14 degrees to 16 degrees north latitude is an ideal region for testing this hypothesis in that igneous crust along this part of the ridge is locally absent and the structure and composition of the mantle can be determined at sites more than 100 kilometers apart along strike. The proportions of gabbro and peridotite recovered from coring the deep open crust during this expedition are consistent with dredging and submersible collections from this region but significantly more gabbro rich than similar collections from ultra-slow spreading ridges.

Based on coring at seven sites along two ridge segments (north and south of the 15 degree 20 minute Fracture Zone), there is no evidence of focused melt accumulation toward the center of ridge segments. Deformation in the peridotite (evidence of mantle flow) is low and appears localized along high temperature shear zones, contacts between gabbro and peridotite, and later brittle faults. These observations indicate that (1) mantle peridotites in this region are incorporated into the lithosphere and are intruded by gabbroic magmas, and (2) the lithosphere cools with little deformation. Ridge extension is accommodated by local shear zones that evolve into brittle faults as the peridotite is exhumed.

ODP Leg 210 took place in the Newfoundland Basin, along a transect conjugate to previous drilling on the Iberia margin (Legs 149 and 173). This was the first time that deep-sea drilling has been conducted on both sides of a nonvolcanic rift in order to understand the structural and sedimentary evolution of the complete rift system. The prime site during Leg 210 (Site 1276) was drilled in "transitional" crust between known continental crust and known oceanic crust, identified by magnetic anomalies M3 to M0 (Barremian-Aptian). On the conjugate Iberia margin, extensive geophysical work and deep-sea drilling have shown that the transition zone crust there is exhumed mantle that is extensively serpentinized in its upper part. Transition zone crust on the Newfoundland side, however, is typically a kilometer or more shallower, has much smoother topography, and may be thin oceanic crust.

A major goal at Site 1276 was to sample basement and the facies responsible for a strong overlying, basin-wide seismic reflector that is poorly developed on the conjugate Iberia margin, together with the intervening section. Site 1276 was cored from 800 to 1,739 meters below seafloor. Before operations ceased due to unstable hole conditions, drilling reached sills more than 10 meters thick that are estimated to be 100 to 200 meters above basement. The sills are alkaline diabases, with sedimentary contacts showing extensive hydrothermal metamorphism. The top of the upper sill is approximately coincident with the basinwide reflector, which correlates with lower Albian fine- to coarse-grained sedimentary gravity flows. The nature of basement at this site remains uncertain.

Following the conclusion of ODP Leg 210 in St. John's, Newfoundland, on Sept. 6, 2003, JOIDES Resolution returned to Galveston, Texas, where demobilization activities continued. The ship then went on to other work. During 18 years of ODP service, JOIDES Resolution drilled 1,803 holes at 673 sites, recovering 222 kilometers of core for study. Approximately 2 million samples have been distributed to over 3,700 investigators worldwide. This is not the end of exploration of the sea floor by drilling, however. On Oct. 1, 2003, the new Integrated Ocean Drilling Program (IODP) came into being (Geotimes, March 2004). IODP seagoing operations are set to begin in June 2004.

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Davies is manager of the Department of Science Operations for the IODP U.S. Implementing Organization (USIO) at Texas A&M University. The Joint Oceanographic Institutions, Lamont Doherty Earth Observatory and Texas A&M University comprise the USIO. E-mail: davies@iodp.tamu.edu.

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