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Ferromanganese nodules are products of both direct and indirect precipitation in the marine environment; therefore, they potentially posses an important record of the oceanographic conditions under which they have grown. Previous attempts to extract useful paleoceanographic interpretations from ferromanganese nodules have been hampered by the scarcity of data and analytical inconsistencies, which make the separation of locally controlled versus regionally controlled variations in nodule composition difficult. Between 1978 and 1981, the Ocean Minerals Company (OMCO) conducted extensive surveys in the northeastern tropical Pacific Ocean. During this time the average bulk geochemistry of nodules from 4657 grab samples, collected from an area of approximately 1.5 million km2, was determined using highly uniform and closely controlled analytical protocols. Because of its broad areal coverage, large sample size, and the consistency of the analytical techniques employed, this data set is ideally suited to quantitatively examine longstanding hypotheses concerning the factors that control the regional-scale variability of nodule compositions. A combination of geostatistical techniques have been employed to achieve this objective. The results of a Q-mode factor analysis of the geochemical data indicate that the nodules are composed of a two-component mixture of material acquired from both sediment and seawater sources via diagenetic and hydrogenetic accretionary processes, respectively. Linear modeling of these results determined that diagenetic accretion is the predominant accretionary processes in this region of the Pacific. Finally, a kriging analysis was employed to examine the spatial variability of nodule composition. Contour maps of the kriging results suggest that variations in primary productivity levels and sedimentation rates are the primary influences on the regional-scale variability of nodule composition, and that the influence of diagenetic accretion is maximally approaching the equatorial zone of high productivity. The nodules examined in this study are likely Oligocene to Miocene in age. The probable paleolocations of these nodules at that time, as determined by backtracking from the inferred northwesterly movement of the Pacific Plate, would still place the nodules well within the zone of equatorial high productivity. Consequently, while these nodules may have been subject to variations in the magnitudes of primary productivity and sedimentation rate during their active growth phase, the observed compositional trends suggest that the overall spatial pattern of productivity has remained essentially the same. |
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