Sediments in the northeastern Pacific Ocean are almost barren of CaCO3 but relatively rich in Corg. Late Pleistocene marine sediments along the California margin, for example, average only about 3% CaCO3 but ~1% Corg. Despite low average CaCO3 contents, CaCO3 time series have significant structure to them and show potential for paleoceanographic reconstructions (Karlin et al., 1992). Some events in the last 150 k.y. have five times the average CaCO3 content and some sediments >400 ka reach 30% CaCO3. In addition, it is known that numbers of short events can be found in marine isotope Stages (MIS) 3 and 4 (Karlin et al., 1992). These events should be useful to study oceanographic processes at the k.y. scale and to estimate sediment ages, provided that the events are chronostratigraphic. The stratigraphic objectives of this paper are (1) to test whether carbonate and organic carbon time series are chronostratigraphic, (2) to develop a detailed stratigraphy for the last 140 k.y., and (3) to develop a preliminary chronostratigraphy for the last 500 k.y. on the northern California margin. This will be accomplished using radiocarbon dating and oxygen isotope stratigraphy for absolute age control. Sufficient CaCO3 is available above about 3500 m water depth to generate, with some difficulty, an oxygen isotope stratigraphy on benthic foraminifera to test whether these events are truly chronostratigraphic.
The high average sedimentation rates along the California margin have proved to be a mixed blessing for paleoceanographers. Although it is possible to generate records with submillennial resolution, the high sedimentation rates limit sampling to a short time interval. Until Leg 167, it was a rare core that unambiguously penetrated to MIS 5 and even rarer to MIS 6. Core EW9504-17PC from the Leg 167 site survey cruise, for example, reached 140 ka at 15 m below the sediment surface. Long time series of basic paleoceanographic measurements like CaCO3 were simply not available prior to Leg 167. A further objective for this study is to generate a 500-k.y. record of carbon burial on the California margin to compare to other oceanographic regions.
Prior to Leg 167 it was known that k.y.-scale events occurred along the California margin (e.g., Santa Barbara Basin, Behl, and Kennett, 1996; Kennett, Baldauf, Lyle, et al., 1995), and that some of these events could be discerned in the CaCO3 content and benthic stable isotope time series on the northern California margin (Karlin et al., 1992; Gardner et al., 1997; Lund and Mix, 1998). The final objective of this paper is to define the carbon time series sufficiently to compare to other regions, in particular the North Atlantic, and to determine to a first order the cause of the k.y.-scale CaCO3 and Corg events.
Our strategy is to develop detailed age models for 0-140 ka for the CaCO3 and Corg depth series at the eight sites we studied in detail (Table 1; Fig. 1), and to develop less precise age models for about the last 500 k.y. We used the sedimentation rates derived from these models combined with dry bulk density and carbon content to produce mass accumulation rate (MAR) models for the 0-140 ka period. For the northern/central California region we stacked these data to make robust estimates of percentage and MAR time series in the region. These data are used to compare to time series from the North Atlantic. We then briefly examine whether the events represent primarily changes in preservation or production of CaCO3 and Corg along the California margin.
Figure 1 shows the site locations of cores or drill sites used in this study. We chose the northern sites (Sites 1019, 1020, and 1021, and Cores EW9504-17 and W8709-13) to form a depth transect from less than 1000 m below sea level (mbsl) to greater than 4000 mbsl. The other sites (Sites 1018 and 1011, and Core Y74-2-22) were chosen to form a transect along the California margin from ~40°N to the Tropic of Cancer.