MIDDLE MIOCENE TO HOLOCENE : A SYNTHESIS OF LEG 167 RESULTS 1

During Ocean Drilling Program Leg 167, the California continental margin was drilled from about 30°N to 42°N to sample high-resolution paleoceanographic and paleoclimatic records in the California Current system. Because of typically high sedimentation rates along the margin (80 to >200 m/m.y.), drilling has proved necessary to collect late Pleistocene sedimentary records that usually are sampled by standard piston cores in other oceanic regions. Triple piston coring on Leg 167 enabled us to construct continuous submillennial paleoceanographic records to about 2 Ma. In the offshore drill sites, continuous records were constructed into the Miocene. The oldest sediments recovered on Leg 167 have an age of ~14 Ma. The California margin has an active diagenetic system driven by the degradation of organic matter. Leg 167 drilling provided a means to quantify the diagenetic processes within the deep sediment column and to study the links between sediment diagenesis and primary productivity. This synthesis chapter also documents the oceanographic variability along the California margin at all time scales, from 103 to 106 yr. Millennial-scale variability is found in Santa Barbara Basin (drilled during Leg 146) and in nearby basins drilled during Leg 167. Leg 167 cores also captured millennial-scale variability in the northern and central California margin. Orbitally forced insolation changes invoke a strong response throughout the California margin. Sea-surface temperature (SST) measured by the alkenone paleothermometer is highly coherent with the oxygen isotope record, being cold in glacials and much warmer in interglacials. Faunal and floral plankton assemblages vary strongly on the glacial–interglacial scale. Coastal plant communities show a glacial–interglacial variability that is most pronounced in the north, near the Cordilleran Ice Sheet. Major changes in the sediments prior to the Pleistocene mark major late Neogene oceanographic events. Opaline silica burial in the middle and upper Miocene sections have step-like drops from high opal deposition in the middle Miocene. One major drop occurs at ~11 Ma and is roughly correlative with the eastern equatorial Pacific Miocene carbonate crash. A second major drop occurs at about 8 Ma, equivalent in age to the end of the Monterey Formation. A third drop occurs slightly younger than the end of the Miocene. A lower Pliocene interval, roughly from 5 to 4.2 Ma, is low in all biogenic components. It separates the Miocene high-opal sediments from upper Pliocene high-carbonate sediments. High CaCO3 deposition occurred all along the entire California margin in the late Pliocene, but CaCO3 burial dropped abruptly with the beginning of Northern Hemisphere glaciation (2.6 Ma). U ′ 37 1Lyle, M. , Koizumi, I. , Richter, C. , and Moore, T.C. , Jr. (Eds.), 2000. Proc. ODP, Sci. Results, 167: College Station TX (Ocean Drilling Program). 2Center for Geophysical Investigation of the Shallow Subsurface, Boise State University, Boise ID 83725, USA. mlyle@cgiss.boisestate. edu 3Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Sapporo 060, Japan. 4U.S. Geological Survey, MS 910, Menlo Park CA 94025, USA. 5Ocean Sciences/Institute of Marine Sciences, University of California, Santa Cruz, Santa Cruz CA 95064, USA. INTRODUCTION The modern California Current system has well-known physical dynamics, chemical structure, biological standing stocks, and biogeochemical fluxes, but on paleoceanographic time scales (102–107 yr) its dynamics are poorly understood. Drilling during Ocean Drilling Program (ODP) Leg 167 was designed to explore the evolution of the California Current system and to study how the North Pacific Ocean has interacted with the global climate system from about 12 Ma to the present (Fig. 1). Many of the Leg 167 scientific party are also studying the effect of northeast Pacific paleoceanographic change on the regional climate of western North America, and how productivity along the California margin has evolved with changes in the California Current system. Other Leg 167 priorities are to improve regional stratigraphy of the sediment column, to understand how paleoceanographic signals can be preserved or modified by diagenesis in a highly productive sedimentary system, and to understand how paleomagnetic signals are preserved on a rapidly sedimenting continental margin. In this synthesis we will describe briefly what we have accomplished toward the Leg 167 objectives and then describe further work that should prove fruitful. With the immediate postcruise studies we have already profoundly improved our understanding of the California Current system and its variability on paleoceanographic time scales. The true synthesis work is just beginning, however. We are only just beginning to compare data between drill sites to study water column structure and are only just beginning detailed studies of the sedimentary record prior to the last 150 k.y. We have noted major changes in sediment deposition since the end of the middle Miocene but so far have only done reconnaissance-scale study of Neogene events (Ravelo et al., 1997). The prospects for future study are bright, however. The high quality of the Leg 167 sedimentary records will make it possible to study important climate transitions since 12 Ma in very high resolution. Description of the Modern Environment The California Current system is the eastern expression of the North Pacific subtropical gyre (Fig. 2). It carries cold, relatively fresh surface water out of the subarctic North Pacific and mixes it into warmer more saline water of the subtropical and tropical regions. The northern edge of the Leg 167 California margin drilling is roughly aligned with the modern position of the northern edge of the North Pacific subtropical gyre. Here the North Pacific Current flows across the Pacific, separating the cold nutrient-rich waters of the subarctic Alaska Gyre from the warm nutrient-depleted waters to the south. The southern Leg 167 drill sites reach into the southern reaches of the California current to permit latitudinal study of changes in strength of the California Current through time. Because the California Current is an eastern boundary current, the region of southward flow is diffuse, especially when compared to the