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

Abstract

We performed high resolution magnetic measurements on about 180 m of continuous U-channel samples from Sites 1012 (East Cortez Basin) and 1013 (San Nicolas Basin) that were recovered during Ocean Drilling Program Leg 167 to the California Margin. Measurements were made at 1-2 cm intervals from near-complete composite stratigraphic sequences at both of the studied sites with the goal of obtaining high spatial resolution of sediment magnetic properties, including detailed characterization of variations in magnetic mineralogy, concentration, and grain size. The decay of a metastable magnetic mineral or the dissolution of fine-grained magnetite by organic matter reduction had strongly affected both cores, reducing the magnetic intensity to 7%-10% of the initial magnetization. Measurements were carried out ca. 4-6 months after the cores were drilled. Despite the reduced magnetic signal we were able to define a stable characteristic remanence for most of the core. Cyclic variations that were observed over short and long stratigraphic intervals are strongly driven at Earth orbital periodicities and have been used to obtain a detailed age model based on comparison with the insolation record.