Published:January 01, 1981
Strong compositional gradients in Nazca plate sediments result from variability in the sedimentary sources in different geographic regions. At the western margin of the plate, precipitates from hydrothermal sources associated with the East Pacific Rise dominate the sediment budget. In the northern part of the plate, biogenic tests that reflect the high productivity along the equator are important. Near the continent the sediments are largely detrital aluminosilicates. The basins, which lie in the central portions of the plate, have strong enrichments in the transition metals whose source is either hydrothermal precipitates or authigenic ferromanganese deposition from normal sea water.
This study develops and applies a normative composition model that converts bulk chemical composition data into quantitative estimates of the weight percent of five distinct components: (1) hydrothermal precipitates, (2) biogenic tests, (3) detrital aluminosilicates, (4) hydrogenous ferromanganese precipitates, and (5) the insoluble residue of organisms. The model assumes that each component has a constant composition anywhere on the plate and uses linear programming to arrive at a mixture of components that best accounts for the measured composition of any sample. The chemical compositions of 425 samples were determined and analyzed using the normative model.
Samples from within 100 km of the rise crest generally have >80% hydrothermal components. The geographic distribution of the hydrothermal components indicates that north of approximately 30°S bottom currents transport hydrothermal precipitates to the west, whereas south of 30°S bottom currents carry the hydrothermal precipitates eastward into the Roggeveen Basin. The biogenic contents of the sediments decrease latitudinally from approximately 80% near the equator to values less than 10% south of 20°S latitude. The detrital abundance is generally >80% in regions within 1,000 km of the South American Continent; however, between 20° and 25°S unusually high concentrations of the detrital component extend away from the continent and into the Yupanqui Basin. These data suggest the importance of bottom nepheloid-layer transport in this part of the plate. The deep basins lying east of the East Pacific Rise have the highest contents of hydrogenous component, presumably caused by a constant deposition of ferromanganese material into a region with small contributions from other sources. Elements remaining after the dissolution of biogenic matter are termed “dissolution residue” in the model. This component reaches a maximum abundance in the basins on the southern edge of the equatorial high productivity zone, a pattern caused by decreased preservation of biogenic matter in regions of lower biogenic deposition.
Published sedimentation rates have been used to calculate rates of the accumulation of the 5 components. Although far fewer in number of samples, these rate data support the conclusions based solely on the relative abundances of the sources.
Except for an obvious dilution by biogenic opal in the more northern samples, compositional range of rise-crest sediments is small. This observation is remarkable, considering the broad range in compositions previously reported for hydrothermal precipitates recovered from the oceans, and confirms the validity of the assumed constant composition of the hydrothermal source. These data suggest that either the Nazca plate metalliferous sediments are the result of large-scale homogenization of precipitates from a broad range of hydrothermal vent types or the Nazca plate portion of the East Pacific Rise has a magmatic and tectonic regime which produces a consistent hydrothermal fluid composition and precipitation conditions.