INORGANIC GEOCHEMICAL INDICATORS OF GLACIAL-INTERGLACIAL CHANGES IN PRODUCTIVITY AND ANOXIA ON THE CALIFORNIA CONTINENTAL-MARGIN

Evidence from sediments in cores collected from within the present oxy gen-minimum zone (OMZ: 600-1200 m) on the central and northern Califor nia margins record several episodes during the last interstadial (OIS- 3, ca. 60-24 ka) of deposition of laminated sediments containing eleva ted concentrations of several trace elements indicative of anoxic cond itions (e.g., Mo, Ni, Zn, and Cu). The presence of abundant well-prese rved organic matter, as well as lack of bioturbation and the presence of elevated concentrations of Mo and other trace elements, all support the theory that the OMZ in the northeastern Pacific Ocean was more in tense, possibly anoxic, at several times during the late Pleistocene. Sediments of all ages in cores from the southern California margin con tain elevated concentrations of Mo, suggesting that this area has alwa ys had higher rates of sulfate reduction than either the central or no rthern California areas. Most of the Ba in sediments in all cores coll ected on the upper continental slope (200-2700 m) off California and s outhern Oregon is derived from detrital elastic material, and this sou rer did not change much in time. However, the amount of biogenic Ba di d vary with time, and these variations closely follow the temporal var iations in organic C (Corg) mass accumulation rate. Using Ba and Corg mass accumulation rates as proxy variables for productivity, all cores show that organic productivity under the California Current upwelling system was highest during OIS-3 and the Holocene, and lowest during t he last glacial interval (LGI, ca. 24-10 ka). All paleoproductivity pr oxy variables indicate that the southern California area has always ex perienced higher productivity than other areas under the California Cu rrent, at least over the last 50 ky. Copyright (C) 1997 Elsevier Scien ce Ltd.