Abstract

A high-resolution record of sediment magnetic properties and stable oxygen isotopes has been determined from marine isotope stage (MIS) 6 to the Holocene (170 ka to present day) for hemipelagic sediments (Site 1014 of Ocean Drilling Program Leg 167) from a subbasin west of the Tanner Basin, off the coast of southern California. The spacing between samples is 10 cm. The sedimentation rate in the top 20 m of the sequence was determined to be 12 cm/k.y. We have taken the intensity of anhysteretic remanent magnetization (ARM) as a measure of the concentration of magnetic minerals. The quantity of magnetic material increased during transitions from warm to cool periods; this change is inferred to have resulted from increased shelf erosion during lower sea levels. During interglacials and possibly interstadials, a reduction occurred in the quantity of magnetic material in the sediment, and magnetic susceptibility therefore dropped. This decrease seems to have resulted from dilution of the magnetic fraction due to increased biogenic deposition during these warm intervals. The ratio of ARM to ARM after 15 mT alternating-field demagnetization (ARM/ARM[15]) is used as a coercivity indicator. The magnetic properties of the sediments from the subbasin west of the Tanner Basin are largely determined by two climatically controlled fluxes of magnetic material. Large and magnetically softer titanohematite grains were deposited during times of decreasing sea level when the concentration of magnetic particles was elevated. Small magnetite grains (<2 μm) control the magnetic properties in sediments deposited during transgressions and times of high sea levels, when detrital titanohematite content was low. The ARM/ARM[15] ratio leads the δ18O curve by 0.3–0.8 m. This offset between the rock magnetic coercivity parameter and intervals of climatic change represents evidence for authigenic growth of magnetite grains near the Fe-redox boundary. The Fe-redox boundary appears to have been at 0.8 m depth during MIS 3 and at 0.3 m during MIS 5. Authigenic growth of magnetite is associated with rapid climate events as recorded by planktonic foraminiferal assemblage shifts. These relationships may have resulted from changes in oxygen concentrations in intermediate-depth waters associated with the climate shifts. Therefore the ratio of ARM/ARM[15] is suggested to be a new indicator (proxy) of past changes in oxygen levels of intermediate water.

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