REGIONAL VARIABILITY IN FERROMANGANESE NODULE COMPOSITION - NORTHEASTERN TROPICAL PACIFIC-OCEAN

Ferromanganese nodules are products of both direct and indirect precip itation in the marine environment; therefore, they potentially posses an important record of the oceanographic conditions under which they h ave grown. Previous attempts to extract useful paleoceanographic inter pretations from ferromanganese nodules have been hampered by the scarc ity of data and analytical inconsistencies, which make the separation of locally controlled versus regionally controlled variations in nodul e composition difficult. Between 1978 and 1981, the Ocean Minerals Com pany (OMCO) conducted extensive surveys in the northeastern tropical P acific Ocean. During this time the average bulk geochemistry of nodule s from 4657 grab samples, collected from an area of approximately 1.5 million km(2), was determined using highly uniform and closely control led analytical protocols. Because of its broad areal coverage, large s ample size, and the consistency of the analytical techniques employed, this data set is ideally suited to quantitatively examine longstandin g hypotheses concerning the factors that control the regional-scale va riability of nodule compositions. A combination of geostatistical tech niques have been employed to achieve this objective. The results of a e-mode factor analysis of the geochemical data indicate that the nodul es are composed of a two-component mixture of material acquired from b oth sediment and seawater sources via diagenetic and hydrogenetic accr etionary processes, respectively. Linear modeling of these results det ermined that diagenetic accretion is the predominant accretionary proc esses in this region of the Pacific. Finally, a kriging analysis was e mployed to examine the spatial variability of nodule composition. Cont our maps of the kriging results suggest that variations in primary pro ductivity levels and sedimentation rates are the primary influences on the regional-scale variability of nodule composition, and that the in fluence of diagenetic accretion is maximally approaching the equatoria l zone of high productivity. The nodules examined in this study are li kely 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 no dules well within the zone of equatorial high productivity. Consequent ly, while these nodules may have been subject to variations in the mag nitudes of primary productivity and sedimentation rate during their ac tive growth phase, the observed compositional trends suggest that the overall spatial pattern of productivity has remained essentially the s ame. (C) 1998 Elsevier Science B.V. All rights reserved.