Information on the late Neogene paleoclimatic evolution of the California Borderland is well expressed in the sedimentary record because of the region's high sensitivity to climate change and its known high biological productivity (Emery, 1960; Kennett and Ingram, 1995; Behl and Kennett, 1996; Mortyn and Thunell, 1997). Tanner Basin is located ~150 km west of San Diego (south of Santa Barbara Basin) within the outer band of the California Borderland basins (Fig. 1). This contribution describes the late Quaternary oxygen and carbon isotope stratigraphy and paleoceanography of Tanner Basin, Site 1014 (32°50.046´N; 119°58.879´W) from a depth of 1177 m. The site, being relatively close to the North American continent, experiences high sedimentation rates, which enhance the stratigraphic resolution of paleoclimatic reconstructions. Thus, the site was drilled with the express purpose of examining the late Neogene paleoceanographic evolution of the California Current system in the outer Borderland province.
Tanner Basin is primarily influenced by the southerly flowing California Current (Fig. 1) (Reid et al., 1958). The inner edge of the California Current flows along the outer periphery of the Borderland (except during spring) and turns toward the coast near San Diego. The average surface flow in the central portion of the Borderland is the northerly flowing southern California Countercurrent (Fig. 1). The northern Channel Islands substantially block this flow, and much of the current is diverted to the west, where it merges with the California Current (Lynn and Simpson, 1987). This results in the formation of a counterclockwise-flowing gyre in the southern California bight (except during spring). The net flow beneath these two currents is the northward-flowing California Undercurrent (Lynn and Simpson, 1987; Reid et al., 1958).
The strength and location of these currents are determined by ocean-atmosphere interactions over a broad area of the North Pacific Ocean. Predominant northerly winds associated with the seasonal migration of the North Pacific high pressure cell in spring and summer cause strong flow of the California Current to the south; when the northerly winds weaken in winter, the undercurrent strengthens (Pisias, 1978). This creates two other circulation forces within the southern California margin: wind stress or Ekman transport, which is strongest near Point Conception, and geostrophic flow (pressure gradient) produced by a higher sea level resulting from nearshore warm water in the San Diego region. During spring and summer, the two forces oppose each other, resulting in cyclonic circulation in the region (Winant and Dorman, 1997). Thus, regional sensitivity of sea-surface temperatures (SSTs) to climate change is produced by tight coupling between the atmospheric and surface ocean over the North Pacific.
The southern California margin is composed of the California Borderland and continental slope, viewed as an extension of highly irregular shelf morphology ranging seaward of the true continental slope (Gorsline and Teng, 1989). At depth, the North Pacific Intermediate Water, with sources in the deep Pacific, flows across sills into the basins of the southern California Borderland. Therefore, bottom waters within these basins reflect intermediate water at the depth of these sills and are further altered by internal processes within the basins (Emery, 1954). Recent studies have suggested that intermediate waters in the North Pacific underwent large changes during the late Quaternary and appear to play a role in climate change (Kennett and Ingram, 1995; Behl and Kennett, 1996; van Geen et al., 1996). As a result, it is important to evaluate and understand changes that occurred in intermediate waters in the context of climate change. Ocean Drilling Program (ODP) Leg 167 has provided deep-sea cores from a number of southern California basins. Benthic records from these sites should broaden our understanding of the role of mid-ocean depths in climate change. Because Site 1014 lies near the limit of the Pacific Intermediate Water, it is possible to investigate depths of ventilation switches in the North Pacific during the Quaternary.