Sediments recovered during drilling of Ocean Drilling Program (ODP) Leg 167 along the California margin reveal important information for paleoclimate and paleoceanographic studies. Because they monitor regions of very high marine primary production, they can record changes in upwelling timing and intensity, track intermediate and deep-water transport pathways, and provide evidence for linkages between continental and marine systems. Shipboard data indicate that Leg 167 sites provide continuous records with generally abundant carbonate and foraminifers available for stable isotopic determination (Shipboard Scientific Party, 1997a, 1997b). Moreover, very high sedimentation rates along the margin make these sites suitable to address questions about the evolution of the California Current system and its link to paleoclimatic conditions from millennial to orbital time scales. Standard piston cores provide very useful records, but the high rate of sedimentation makes it difficult to sample a full glacial cycle even with the longest cores available without drilling. Previous drilling along the California margin occurred during the Deep Sea Drilling Project (DSDP) before the advent of advanced hydraulic piston corer technology and recovered core that was discontinuous and very disturbed, thus unsuitable for modern, high-resolution paleoceanographic studies.
The sediments recovered during Leg 167 represent a mixture of terrigenous clays and silts, biogenic silica, and carbonate. Each of these components can be helpful in deciphering the paleoclimatic history of this region; however, this study will focus on the input of terrigenous material at two sites along the northern California margin, ODP Sites 1018 and 1020. The input of terrigenous sediment carries with it important information about climatic conditions on the continent and about mechanisms and/or processes by which the material is transported to the site of deposition. The amount or mass flux of terrigenous minerals reflects the supply of sediment in the continental source region and can provide an important signal of changes related to precipitation and climate (Griggs and Hein, 1980; Karlin, 1980; Griggs, 1987). Likewise, the grain size of terrigenous mineral material can be used to characterize the process and intensity of transport. Ledbetter (1986) showed the distinct influence abyssal currents have on terrigenous component grain size for the southwest Atlantic. Dowling and McCave (1993) identified changes in bottom current activity since the last glacial maximum based upon the varying importance of size modes in detrital silt along the Fenni Drift. Others used grain-size distributions to discern different transport and depositional processes and assist in the paleoenvironmental interpretation of terrigenous signals (Rea and Hovan, 1995; Joseph et al., 1998; Boven and Rea, 1998).
The provenance and relative importance of continental source regions can be traced by the regional pattern of terrigenous clay mineralogy (Karlin, 1980; Krissek, 1982; Petschick et al., 1996). In the northeast Pacific, Karlin (1980) found that modern terrigenous sediments deposited along the continental margin were supplied by two main sources: (1) a chlorite- and illite-rich source input by streams of the northern California Coastal Range and Klamath Mountains, and (2) a smectite-rich (montmorillonite) component from discharge of the Columbia River Basin. Clay mineral distribution patterns also record dominant dispersal pathways in this region and are strongly influenced by coastal currents (Karlin, 1980; Krissek, 1982). Thus, with the high-resolution records of terrigenous sedimentation recovered during Leg 167 and using modern surface sediment as a reference, we should be able to track changes in the late Pleistocene location of dominant current systems transporting terrigenous sediment in this region.
Here we show the results of our effort to study physical and mineralogical tracers of terrigenous sediment in an attempt to identify regional climatic controls and possible transport pathways during the past few glacial cycles of the late Pleistocene.