Latest Quaternary rock magnetic record of climatic and oceanic change, Tanner Basin, California borderland

A high-resolution record of sediment magnetic properties and stable oxygen isotopes has been determined from marine isotope stage (MIS) 6 to the Holoc ene (170 ka to present day) for hemipelagic sediments (Site 1014 of Ocean D rilling 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 sed imentation 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 (AR M) as a measure of the concentration of magnetic minerals. The quantity of magnetic material increased during transitions from warm to cool periods; t his change is inferred to have resulted from increased shelf erosion during lower sea levels. During interglacials and possibly interstadials, a reduc tion occurred in the quantity of magnetic material in the sediment, and mag netic susceptibility therefore dropped. This decrease seems to have resulte d from dilution of the magnetic fraction due to increased biogenic depositi on during these warm intervals, The ratio of ARM to ARM after 15 mT alterna ting field demagnetization (ARM/ARM[15]) is used as a coercivity indicator. The magnetic properties of the sediments from the subbasin west of the Tan ner Basin are Largely determined by two climatically controlled fluxes of m agnetic material. Large and magnetically softer titanohematite grains were deposited during times of decreasing sea level when the concentration of ma gnetic particles was elevated, Small magnetite grains (<2 <mu>m) control th e magnetic properties in sediments deposited during transgressions and time s of high sea levels, when detrital titanohematite content was low. The ARM /ARM[15] ratio leads the delta O-18 curve by 0.3-0.8 m, This offset between the rock magnetic coercivity parameter and intervals of climatic change re presents evidence for authigenic growth of magnetite grains near the Fe-red ox boundary, The Fe-redox boundary appears to have been at 0.8 m depth duri ng MIS 3 and at 0.3 m during MIS 5, Authigenic growth of magnetite is assoc iated with rapid climate events as recorded by planktonic foraminiferal ass emblage shifts. These relationships may have resulted from changes in oxyge n concentrations in intermediate-depth waters associated with the climate s hifts. Therefore the ratio of ARM/ARM[15] is suggested to be a new indicato r (proxy) of past changes in oxygen levels of intermediate water.