Abstract:
Light absorption spectroscopy (LAS) is a rapid, nondestructive method for determining mineral concentrations in marine cores. High-resolution mineralogical records can be very useful for investigating paleoclimatic and paleoceanographic variations. The LAS technique measures the absorption spectrum, in the visible and near-infrared bands (350-2500 nm), of light reflected from any surface. Diagnostic absorption bands allow identification of several minerals, including opal, calcite, smectite, and illite--the most common minerals present in sediments drilled during Ocean Drilling Program (ODP) Leg 199. A simple method was developed for calculating mineralogy from Leg 199 sediments, resulting in high-resolution mineralogy concentrations with minimal error (+ or - 5-10%). These data were then used to investigate variations in Paleogene paleoclimate: specifically, the timing of an Asian drying event as recorded by the appearance of illite clay in the Leg 199 area, and paleoceanographic variations recorded by mass accumulation rates (MARs) of biogenic calcite, opal, and terrigenous materials. LAS measurements revealed a downcore transition from illite to smectite in the upper red clay units from ODP Leg 199 sites. The appearance of illite, which was carried into the north Pacific from Asia by the Westerlies, corresponds to an increase in Asian aridity caused by the onset of Northern Hemisphere glaciation that occurred approximately 2.6 Ma. By using multiple regression and LAS mineralogy, the multi-sensor track physical properties data were converted into high-resolution mineralogy logs. These logs, as well as age/depth relations and dry-bulk density, were used to calculate high-resolution carbonate, opal, and terrigenous mass accumulation rates for each Leg 199 site. Opal MARs were at their highest in the middle Eocene, displaying a wide equatorial bulge (northern flank reaching 13 degrees N) centered south of the equator. During the Oligocene and early Miocene, carbonate deposition was dominant with the carbonate equatorial bulge extending to 4 degrees N. This significant change corresponds to the onset of Antarctic glaciation that occurred at the Eocene/Oligocene boundary, which resulted in a dramatic deepening of the carbonate compensation depth. A Pliocene increase in terrigenous accumulations in the north (20 degrees -25 degrees N) may correspond to an increase in the Asian dust flux that occurred approximately 2.6 Ma.