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Hawaii Institute of Geophysics Contribution No. 1100

More than 50 sediment cores from the Nazca plate have now been analyzed for sedimentation rates and bulk chemical composition. Sedimentation rates determined by direct assessment of unsupported ionium (230Th) and microfossil zonation have been augmented by gamma-ray spectrometry techniques; a comparison of sedimentation rates determined by both direct (alpha) and indirect (gamma) 230Th spectrometric techniques on 6 cores shows a general agreement between the two methods. Measured and calculated dry bulk densities and bulk chemical composition were used to calculate regional patterns of metal accumulation from the sedimentation rates. The regional pattern of Fe accumulation parallels those for Mn and U, whereas those of Cu, biogenic Si, Al, Ti, and Th generally display an inverse relationship. Multivariate factor analysis was employed to include correlation of accumulation rates for Na, K, Mg, Ca, Ba, Ni, Zn, and P, and to distinguish hydrothermal from detrital, hydrogenous, biogenous, and diagenetic associations. A 230Th inventory, defined as the ratio of the 230Th accumulation rate to the 230Th sea-water production rate, was employed as an index of sediment loss or enhanced sediment accumulation. The 230Th inventory is greatest on the East Pacific Rise, which suggests that some sediment ponding occurs there; it is generally lowest in the Bauer Basin, which suggests that sediment has been lost there by either coring disturbance, CaCO3 dissolution, or bottom-current winnowing.

The regional pattern of Fe and Mn accumulation indicates that their rates of accumulation on the crest of the East Pacific Rise (EPR) vary by more than an order of magnitude; values near the equator (6°S) correspond to normal authigenic accumulation and exponentially increase toward 20°S latitude. The lower Fe and Mn accumulation rates in the northern EPR area suggest that (1) there is preferential loss of the metalliferous component to the north because of increased bottom-current activity, or (2) hydrothermal circulation is more intense to the south because of a lack of regional sediment cover or more extensive fracturing, or (3) mantle heterogeneity exists along the rise crest. Bottom-current winnowing is the explanation most consistent with the existing data on the distribution of conductive heat flow and crustal production along the EPR, and can explain both the variation in metal accumulation along the EPR crest and the appearance of a second area of metal enrichment in the Bauer Basin. Global budget calculations indicate that roughly 50% or more of the Mn produced at ridge crests may be lost to the adjacent sea floor by this mechanism, which has implications for the magnitude of hydrothermal input to the ocean and the origin of Mn nodules.

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