SEDIMENTATION AT MANOP SITE H (EASTERN EQUATORIAL PACIFIC) OVER THE PAST 400,000 YEARS: CLIMATICALLY INDUCED REDOX VARIATIONS AND THEIR EFFECTS ON TRANSITION METAL CYCLING

Gravity cores recovered from Manganese Nodule Project site H (6 degrees 33'N, 92 degrees 49'W) show marked downcore variations in the abundance of calcium carbonate organic carbon, opal, manganese and other component deposited over the past 400,000 years. Variations in the downcore abundance of organic carbon, which ranges from 0.2 to 1.0%, can be used to hindcast redox conditions in the surface sediments over this time. The results indicate that the depth to the manganese redox boundary varied from about 5 to 25 cm below the seafloor during four major cycles. Downcore variations in solid phase Mn, Ni, and Cu can be produced by such changes in redox conditions. A model which predicts that solid phase Mn can be trapped and buried when the Mn redox boundary migrates rapidly upward is consistent with the observed organic carbon and Mn records and supports the reconstructed redox variations. The history of redox variations at site H can be explained by changes with time in surface water productivity. Major productivity variations at the site occur over 100-kyr cycles, with relatively higher productivity occurring during glacial stages. Thus Quaternary climate changes influence surface water productivity redox conditions in sediments and the cycling of transition metals.