Magnetic intensity loss and core diagenesis in long-core samples from the East Cortez Basin and the San Nicolas Basin (California Borderland)

We performed high resolution magnetic measurements on about 180 m of contin uous U-channel samples from Sites 1012 (East Cortez Basin) and 1013 (San Ni colas Basin) that were recovered during Ocean Drilling Program Leg 167 to t he California Margin. Measurements were made at 1-2 cm intervals from near- complete composite stratigraphic sequences at both of the studied sites wit h the goal of obtaining high spatial resolution of sediment magnetic proper ties, including detailed characterization of variations in magnetic mineral ogy, concentration, and grain size. The decay of a metastable magnetic mine ral or the dissolution of fine-grained magnetite by organic matter reductio n had strongly affected both cores, reducing the magnetic intensity to 7%-1 0% of the initial magnetization, Measurements were carried out ca. 4-6 mont hs after the cores were drilled. Despite the reduced magnetic signal we wer e able to define a stable characteristic remanence for most of the core. Cy clic variations that were observed over short and long stratigraphic interv als are strongly driven at Earth orbital periodicities and have been used t o obtain a detailed age model based on comparison with the insolation recor d.