167 Preliminary Report


The California Current system is probably the best investigated eastern boundary current system in the world, with well-known physical dynamics, chemical structure, biological standing stocks, and biogeochemical fluxes. Nevertheless, the response of the California Current system and associated coastal upwelling systems to climate change is poorly documented. Climate models and available paleoceanographic data indicate that the California Current system changed dramatically with the growth and decay of the North American ice sheets. The paleoceanographic records, however, remain too sketchy to test the models.

Ocean Drilling Program (ODP) Leg 167 (Fig. 1) represents the first time since 1978 that the Pacific margin of North America was drilled to study ocean history. The leg collected both high-resolution records appropriate for studying events with durations of a few thousand years or less within the Pleistocene and Pliocene and lower-resolution records to examine much of the Neogene interval. Sites were drilled to collect sediments needed to study the links between the evolution of North Pacific climate and the development of the California Current system. The same material will also be used to study the climate links between the North Pacific Ocean and North America.

Only three other drilling legs, all part of the Deep Sea Drilling Project (DSDP), have sampled the historical sediment record along the California Margin. A single advanced hydraulic piston core (APC) site in the Santa Barbara Basin (Site 893) represents all of ODP drilling before Leg 167. The last major drilling effort, DSDP Leg 63, occurred immediately before the first deployment of the APC. Recovered core from the DSDP drilling is discontinuous and very disturbed, so it is impossible to use this material for modern, high-resolution, paleoceanographic studies. Reconnaissance studies using DSDP cores have shown, however, that the Leg 167 drilling region is highly sensitive to climate change and that new ODP drilling would collect a detailed record of this variability.

Significant contrasts in sedimentary environments occur along the California Margin, including the unique tectonic and sedimentary environments of the basins in the California Borderland. Site selection exploited these opportunities, constructing latitudinal, longitudinal, and depth transects. There are three east-west transects, with at least two sites in each transect, one located in the coastal upwelling zone (from 50 to 90 km offshore, 1000 to 2000 m water depth) and one in the core of the California Current proper (from 150 to 360 km offshore, 3500 to 4200 m water depth). The Gorda Transect (~40°N; Sites 1019, 1022, 1020, and 1021) is in a region of strong summer upwelling. The Conception Transect (~35°N; Sites 1017 and 1016) is influenced by year round upwelling with relatively cool surface waters. The Baja Transect (~30°N; Sites 1011 and 1010) is influenced by year-round upwelling with warmer surface waters. The oldest sediments from the inshore sites are Pleistocene in age, and from the offshore sites, middle or late Miocene. Coastal upwelling processes at the margin will be reflected in the nearshore sites, whereas nutrient supply by California Current processes will be reflected in the deeper sites.

The north-south Coastal Transect covers the latitude range from 31º to 42°N (in order from north to south: Sites 1019, 1022, 1018, 1017, 1014, 1013, 1012, and 1011). All sites are within <160 km offshore, in water depths of 1000-2500 m. The oldest sediments from all sites are at least Pleistocene in age, and some range to late Miocene. The coastal transect will detail the history of coastal upwelling and of continent-ocean interaction.

Sites also constitute two depth transects. The Northern Depth Transect (~37° to 42°N, from 60 to 360 km offshore; Sites 1019, 1022, 1018, 1020, and 1021) covers 1000-4200 m water depth. Oldest sediments from Site 1019 are Pleistocene, whereas the sediments from the deepest-water site (Site 1022) range in age from latest middle Miocene to Quaternary in age. The Southern Depth Transect (~30° to 35°N, from 30 to 210 km offshore; Sites 893 [Leg 146], 1015, 1017, 1013, 1014, 1012, 1011, 1010, and 1016) covers 475-3850 m water depth. This transect takes advantage of the different sill depths within the California Borderlands for detailed study of the shallow intermediate water column. The oldest sediments from sites with the shallower water depths are at least late Pleistocene in age, whereas basal sediments from the sites with deeper water depths are typically late Miocene. These depth transects will provide sediments to investigate hypotheses about water-depth control of sedimentation processes and water column structure and its evolution through time.

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