G. Ross Heath

Origin of Metalliferous Sediments from the Pacific Ocean

Sediments from near the basement of a number of Deep Sea Drilling Project (DSDP) sites, from the Bauer Deep, and from the East Pacific Rise have unusually high transition metal-to-aluminum ratios. Similarities in the chemical, isotopic, and mineralogical compositions of these deposits point to a common origin. All the sediments studied have rare-earth-element (REE) patterns strongly resembling the pattern of sea water, implying either that the REEs were coprecipitated with ferromanganese hydroxyoxides (hydroxyoxides denote a mixture of unspecified hydrated oxides and hydroxides), or that they are incorporated in small concentrations of phosphatic fish debris found in all samples. Oxygen isotopic data indicate that the metalliferous sediments are in isotopic equilibrium with sea water and are composed of varying mixtures of two end-member phases with different oxygen isotopic compositions: an iron-manganese hydroxyoxide and an iron-rich montmorillonite. A low-temperature origin for the sediments is supported by mineralogical analyses by x-ray diffraction which show that goethite, iron-rich montmorillonite, and various manganese hydroxyoxides are the dominant phases present. Sr87/Sr86 ratios for the DSDP sediments are indistinguishable from the Sr87/Sr86 ratio in modern sea water. Since these sediments were formed 30 to 90 m.y. ago, when sea water had a lower Sr87/Sr86 value, the strontium in the poorly crystalline hydroxyoxides must be exchanging with interstitial water in open contact with sea water. In contrast, uranium isotopic data indicate that the metalliferous sediments have formed a closed system for this element. The sulfur isotopic compositions suggest that sea-water sulfur dominates these sediments with little or no contribution of magmatic or bacteriologically reduced sulfur. In contrast, ratios of lead isotopes in the metalliferous deposits resemble values for oceanic tholeiite basalt, but are quite different from ratios found in authigenic marine manganese nodules. Thus, lead in the metalliferous sediments appears to be of magmatic origin. The combined mineralogical, isotopic, and chemical data for these sediments suggest that they formed from hydrothermal solutions generated by the interaction of sea water with newly formed basalt crust at mid-ocean ridges. The crystallization of solid phases took place at low temperatures and was strongly influenced by sea water, which was the source for some of the elements found in the sediments.

Genesis and transformation of metalliferous sediments from the East Pacific Rise, Bauer Deep, and Central Basin, northwest Nazca plate

Analytical data for northwest Nazca plate sediments can be described in terms of a mixture of hydrothermal, detrital, hydrogenous, and biogenous material. Fe, Mn, Cu, Zn, Ni, Ba, Si, and Al are more than 50 percent hydrothermal in East Pacific Rise samples from lat 10° to 25 °S. The first four elements are dominantly hydrothermal in the Bauer Deep and Central Basin as well. Seventy to 80 percent of the Ni, 60 to 80 percent of the Ba, and 30 to 60 percent of the Cu and Zn in Bauer Deep and Central Basin sediments are hydrogenous. Si, Ba, and Zn are dominantly biogenous on the northern East Pacific Rise crest, where more than one-third of the Cu also is derived from this source. Detrital Al and Si are dominant away from the rise crest, particularly in the Central Basin, where about 40 percent of the Fe and 15 percent of the Zn may also be detrital. Much of the hydrothermal Fe and biogenous Si have been transformed to an iron-rich smectite. The proportion of total Fe bound in this phase varies from less than 20 percent on the southern rise crest to about 40 percent in the Bauer Deep. The distribution of each element is governed by (1) supply from the four basic sources; (2) lateral transport by bottom currents moving east and then south across the northern East Pacific Rise and Bauer Deep to the Central Basin and moving west from the Peru Basin to the Central Basin; and (3) transformation of the unstable metalliferous hydroxides into more stable smectite and ferromanganese oxyhydroxides.

Tectonics of the Panama Basin, Eastern Equatorial Pacific

The Panama Basin includes portions of the Nazca, Cocos and South America lithospheric plates and borders the Caribbean plate. The complex interactions of these units have largely determined the topography, pattern of faulting, sediment distribution, and magnetic character of the basin. Only heat flow data fail to correlate with major structural features related to these units. The topographic basin appears to have been created by rifting of an ancestral Carnegie Ridge. The occurrence of a distinctive smooth acoustic basement and a characteristic overlying evenly stratified sedimentary sequence on virtually all elevated blocks in the basin suggest that they all once formed part of this ancestral ridge. The present Carnegie Ridge is the relatively undeformed southern half of this feature, while the Cocos Ridge is the northern half fragmented by left-lateral north-south transcurrent faulting. As blocks of the Cocos Ridge reach the Middle America Trench, they appear to clog the subduction zone and become welded to the Nazca plate. Thus, the active transform fault at the eastern edge of the Cocos plate has episodically shifted west as segments of the trench were deactivated. Such a shift appears to be occurring at the present time.

Distribution of biogenic silica and quartz in recent deep-sea sediments

All available quartz and biogenic silica concentrations from deep-sea surface sediments were intercalibrated, plotted, and contoured on a calcium-carbonate-free basis. The maps show highest concentrations of biogenic silica (opal) along the west African coast, along equatorial divergences in all oceans, and at the Polar Front in the southern Indian Ocean. These are all areas where upwelling is strong and there is high biological productivity. Quartz in pelagic sediments deposited far from land is generally eolian in origin. Its distribution reflects dominant wind systems in the Pacific, but in much of the Atlantic and Indian oceans the distribution pattern is strongly modified by turbidite deposition and bottom current processes.

Sedimentary indicators of atmospheric activity in the Northern Hemisphere during the Cenozoic

Abstract The concentrations and accumulation rates of eolian phases and the mineralogy of sediments from a North Pacific core (LL44-GPC3) have been used to evaluate changes in eolian sedimentation and atmospheric activity during the Cenozoic. The quartz content, and clay mineralogy of the sediments reflect changing source areas for eolian material whereas the accumulation rates of eolian phases reflect climatic variation and wind intensity. Eolian sedimentation decreased from 70 to 50 m.y. ago at the site as it moved north out of the equatorial Pacific where it was supplied with eolian debris from North America and Africa. Low rates of eolian sediment accumulation 50 to 25 m.y. ago reflect the temperate, humid environment of the early Tertiary and the lack of vigorous atmospheric circulation at that time. From 25 to 7 m.y. ago eolian accumulation increased as the site moved into the influence of the westerlies. From 7 to 3 m.y. ago there was a more rapid increase in accumulation of eolian material at the site which was not accompanied by changes in mineralogy or quartz content and was, therefore, related to climatic change and more vigorous atmospheric circulation. About 2.5 m.y. ago a dramatic increase in eolian sedimentation accompanied the onset of dry Northern Hemisphere glaciation.

Remobilization of transition metals in surficial pelagic sediments from the eastern Pacific

Abstract The top thirty centimeters of sediment at two sites in the eastern equatorial Pacific contain evidence of post-depositional remobilization of Mn, Fe, Co, Ni, Cu, and Zn. Remobilization takes place as Mn and Fe oxyhydroxides are released to the pore water during the microbially-mediated decomposition of organic matter. Precipitation of the dissolved metals in near-surface more oxic strata controls the solid-phase distribution of Mn, Fe, and Zn. The solid-phase redistribution of Co and Ni requires only suitable material for readsorption. Comparison of pore water fluxes with solid-phase metal distributions in the solid sediment indicates no loss of dissolved metal to the overlying water column at the present time. Loss of Mn during the Quaternary is indicated by the composition of the sediments, however. Leaching experiments suggest that portions of the mobile Mn, Fe, Co, Ni, and Cu are fixed by incorporation in authigenic smectite in the surficial sediments.

Calcite: Degree of Saturation, Rate of Dissolution, and the Compensation Depth in the Deep Oceans

The hydrographic features that appear to control the distribution of calcite in deep-sea sediments are the level at which the water passes from super- to undersaturation, and the level at which the rate of dissolution increases abruptly (the lysocline). The compensation depth results from a roughly linear increase in the rate of solution of calcite with depth below the lysocline. It does not correspond to a unique physico-chemical change within the water column.

Biogenic Sediments of the Panama Basin

The Panama Basin is a miniature ocean basin in which the effect of variable rates of supply, dilution, dissolution, and lateral transport of biogenous sediments can be studied in detail. The rate of input inferred from rates of biologic productivity in surface waters does not resemble the distribution of either carbonate (foraminifera and calcareous nannofossils) or opal (diatoms and radiolaria) in sediments at the sea floor. The distribution of carbonate is primarily controlled by dissolution ; at any depth, the rate is highest adjacent to the continent and decreases offshore in the more pelagic areas. The rate of dissolution increases rapidly with depth at about 1,500 m. Winnowing and lateral transport from ridges into the basin is the second most important factor controlling the distribution of carbonate and the dominant factor governing the distribution of opal. In pelagic sediments of the western basin, about half the carbonate fraction appears to be derived from the surrounding ridges. Dilution by terrigenous material is important only in the eastern Panama Basin, where the concentration of opal is significantly reduced by fine-grained debris derived from Central America.

Transport and release of transition elements during early diagenesis: Sequential leaching of sediments from MANOP Sites M and H. Part I. pH 5 acetic acid leach

Sediments from MANOP sites M and H in the eastern tropical Pacific Ocean can be partitioned into operationally defined phases by means of a sequential leach procedure. This paper reports the results of the first leach in the sequence, an acetic acid solution buffered as p H 5 with sodium acetate. This leach is designed to remove carbonate-bound and sorbed cations. The only cation bound in a consistent ratio to calcium in the carbonate is strontium. The molar ratio is 2 × 10−3. In contrast, transition metals are sorbed onto the surfaces of other sedimentary particles. The proportions sorbed range from 2 to 10% of the total manganese, about 10% of the total nickel and copper, and less than 1% of the total iron. The pool of sorbed metals in surface sediments is sufficiently large and the rate of biological stirring is sufficiently rapid for this metal reservoir potentially to dominate the porewater reservoir in supplying metals to ferromanganese nodules. A simple model for nodule growth based upon transfer of metals through this reservoir suggests that only 1% of the sedimentary sorbed metals within a radius of 2 to 9 cm is required to support typical nodule growth rates.

Isotopic analysis of metalliferous sediment from the East Pacific Rise

Analysis of iron-rich sediments from the East Pacific Rise suggest that they originate as precipitates from hydrothermal solutions injected into seawater. Ultra-slow-scan X-ray diffraction and K-Ar data point to only trace amounts of biotic and detrital components. The sediment, which consists largely of discrete granules of siliceous and of ferruginous material, contains unusually high concentration of Pb, Zn, and Ni among other elements. An untreated sample and several water-rinsed, size-fractionated samples contain 669–880 ppm Sr with identical87Sr/86Sr ratios of 0.70905 ± 0.0001 - indistinguishable from Sr in modern seawater. After leaching a <20μ fraction of the sample with acetic acid, it retains 356 ppm Sr with87Sr/86Sr = 0.7088. Further leaching with HCl results in the removal of practically all of the ferruginous component; the degraded amorphous silica residue contains 34 ppm Sr with87Sr/86Sr = 0.7065. Volcanogenic Sr (87Sr/86Sr ∼ 0.7035) apparently contributes less than 3% of the total Sr. The sediment contains about 178 ppm Pb with isotopic ratios that are within the ranges of values for Pb from oceanic, tholeiitic basalts; the sediment Pb is markedly less radiogenic than Pb from marine manganese nodules, particularly with respect to207Pb/204Pb. Thus, the bulk of the Sr is derived from seawater, probably by absorption onto reactive Fe-Mn surfaces and incorporation in biogenic phosphate and carbonate, whereas most of the Pb ultimately is of magmatic origin.