David Z. Piper

Inorganic geochemical indicators of glacial-interglacial changes in productivity and anoxia on the California continental margin

Abstract Evidence from sediments in cores collected from within the present oxygen-minimum zone (OMZ; 600–1200 m) on the central and northern California margins record several episodes during the last interstadial (OIS-3, ca. 60-24 ka) of deposition of laminated sediments containing elevated concentrations of several trace elements indicative of anoxic conditions (e.g., Mo, Ni, Zn, and Cu). The presence of abundant well-preserved organic matter, as well as lack of bioturbation and the presence of elevated concentrations of Mo and other trace elements, all support the theory that the OMZ in the northeastern Pacific Ocean was more intense, possibly anoxic, at several times during the late Pleistocene. Sediments of all ages in cores from the southern California margin contain elevated concentrations of Mo, suggesting that this area has always had higher rates of sulfate reduction than either the central or northern California areas. Most of the Ba in sediments in all cores collected on the upper continental slope (200–2700 m) off California and southern Oregon is derived from detrital clastic material, and this source did not change much in time. However, the amount of biogenic Ba did vary with time, and these variations closely follow the temporal variations in organic C (Corg) mass accumulation rate. Using Ba and Corg mass accumulation rates as proxy variables for productivity, all cores show that organic productivity under the California Current upwelling system was highest during OIS-3 and the Holocene, and lowest during the last glacial interval (LGI, ca. 24-10 ka). All paleoproductivity proxy variables indicate that the southern California area has always experienced higher productivity than other areas under the California Current, at least over the last 50 ky.

Ferromanganese nodules from MANOP Sites H, S, and R-Control of mineralogical and chemical composition by multiple accretionary processes

Abstract The chemical composition of ferromanganese nodules from the three nodule-bearing MANOP sites in the Pacific can be accounted for in a qualitative way by variable contributions of distinct accretionary processes. These accretionary modes are: 1. (1) hydrogenous, i.e., direct precipitation or accumulation of colloidal metal oxides in seawater, 2. (2) oxic diagenesis which refers to a variety of ferromanganese accretion processes occurring in oxic sediments; and 3. (3) suboxic diagenesis which results from reduction of Mn+4 by oxidation of organic matter in the sediments. Geochemical evidence suggests processes (1) and (2) occur at all three MANOP nodule-bearing sites, and process (3) occurs only at the hemipelagic site, H, which underlies the relatively productive waters of the eastern tropical Pacific. A normative model quantitatively accounts for the variability observed in nearly all elements. Zn and Na, however, are not well explained by the three end-member model, and we suggest that an additional accretionary process results in greater variability in the abundances of these elements. Variable contributions from the three accretionary processes result in distinct top-bottom compositional differences at the three sites. Nodule tops from H are enriched in Ni, Cu, and Zn, instead of the more typical enrichments of these elements in nodule bottoms. In addition, elemental correlations typical of most pelagic nodules are reversed at site H. The three accretionary processes result in distinct mineralogies. Hydrogenous precipitation produces δMnO2. Oxic diagenesis, however, produces Cu-Ni-rich todorokite, and suboxic diagenesis results in an unstable todorokite which transforms to a 7 A phase (“birnessite”) upon dehydration. The presence of Cu and Ni as charge-balancing cations influence the stability of the todorokite structure. In the bottoms of H nodules, which accrete dominantly by suboxic diagenesis, Na+ and possibly Mn+2 provide much of the charge balance for the todorokite structure. Limited growth rate data for H nodules suggest suboxic accretion is the fastest of the three processes, with rates at least 200 mm/106 yr. Oxic accretion is probably 10 times slower and hydrogenous 100 times slower. Since these rates predict more suboxic component in bulk nodules than is calculated by the normative analysis, we propose that suboxic accretion is a non-steady-state process. Variations in surface water productivity cause pulses of particulate flux to the sea floor which result in transient Mn reduction in the surface sediments and reprecipitation on nodule surfaces.

Seawater as the source of minor elements in black shales, phosphorites and other sedimentary rocks

Many of the minor elements in seawater today have a concentration-depth profile similar to that of the biologically essential nutrients, NO−3 and PO3−4. They show a relative depletion in the photic zone and enrichment in the deep ocean. The difference between their surface- and deep-ocean values, normalized to the change in PO3−4, approaches the average of measured minor-element: P ratios in marine plankton, although individual analyses of the latter show extreme scatter for a variety of reasons. Despite this scatter in the minor-element analyses of plankton, agreement between the two sets of data shows unequivocally that an important marine flux of many minor elements through the ocean is in the form of biogenic matter, with a composition approaching that of plankton. This interpretation is further supported by sediment studies, particularly of sediments which accumulate in shelf-slope environments where biological productivity in the photic zone is exceptionally high and organic carbon contents of the underlying sediment elevated. The interelement relations observed for some of these sediments approach the average values of plankton. These same interelement relations are observed in many marine sedimentary rocks such as metalliferous black shales and phosphorites, rocks which have a high content of marine fractions (e.g., organic matter, apatite, biogenic silica and carbonates). Many previous studies of the geochemistry of these rocks have concluded that local hydrothermal activity, and/or seawater with an elemental content different from that of the modern ocean, was required to account for their minor-element contents. However, the similarity in several of the minor-element ratios in many of these formations to minor-element ratios in modern plankton demonstrates that these sedimentary rocks accumulated in environments whose marine chemistry was virtually identical to that seen on continental shelf-slopes, or in marginal seas, of the ocean today. The accumulation of the marine fraction of minor elements on these ancient sea floors was determined largely by the accumulation of organic matter, settling from the photic zone and with a composition of average plankton. A second marine fraction of minor elements in these rocks accumulated through precipitation and adsorption from seawater. The suite of elements in this fraction reflects redox conditions in the bottom water, as determined by bacterial respiration. For example, high Mn, high Cr+V and high Mo concentrations, above those which can be attributed to the accumulation of planktonic matter, characterize accumulation under bottom-water oxidizing, denitrifying and sulfate-reducing conditions, respectively.

Geochemistry of ferromanganese nodules from DOMES site a, Northern Equatorial Pacific: Multiple diagenetic metal sources in the deep sea

Abstract The major and minor element composition of ferromanganese nodules from DOMES Site A has been determined by X-ray fluorescence methods. Three phases appear to control the bulk compositions: Mn and Fe oxyhydroxides and aluminosilicates. Relatively wide compositional variations are evident throughout the area. Nodules with high Mn/Fe ratios, high Cu, Mg, Mo, Ni and Zn concentrations and high todorokite/δ-MnO 2 ratios have granular surface textures and are confined to an east-west trending depression with thin Quaternary sediment cover. Nodules with low Mn/Fe ratios, high concentrations of As, Ca, Ce, Co, La, P, Sr, Ti, V, Y and Zr and low todorokite/δ-MnO 2 ratios have smooth surfaces and are confined to shallower areas with relatively thick Quaternary sediment to the north and south of the depression. All nodules in the area have compositions which are influenced by diagenesis, but those with the most marked diagenetic signature (high Mn/Fe and Cu/Ni ratios, low Ce/La ratios and more todorokite) are found in areas of very slow or non-existent sedimentation; many of these nodules are actually in contact with outcropping Tertiary sediment. This paradox may be resolved by postulating, by analogy with some shallow-water occurrences, that the nodules accrete from bottom waters which have enhanced particulate and dissolved metal contents derived from diagenetic reaction in areas remote from the site of nodule formation. The metals are supplied in a bottom flow (probably Antarctic Bottom Water) which also erodes, or prevents modern sedimentation in, the depression. Nodules on the flanks of the depression are not evidently affected by this flow and derive at least pan of their constituent metals from diagenetic reaction in the underlying Quaternary sediment. Apparently, abyssal diagenetic nodules can have an immediate and a remote diagenetic metal source. Metal fluxes derived from pore water dissolved metal gradients may not be relevant to particular accreting nodules if a significant fraction of their metals is derived from outside the area in which they form.

Fluxes of metals to a manganese nodule radiochemical, chemical, structural, and mineralogical studies

Fluxes of metals to the top and bottom surfaces of a manganese nodule were determined by combining radiochemical (230Th,231Pa,232Th,238U,234U) and detailed chemical data. The top of the nodule had been growing in its collected orientation at 4.7 mm Myr−1 for at least 0.5 Myr and accreting Mn at 200 μg cm−2 kyr−1. The bottom of the nodule had been growing in its collected orientation at about 12 mm Myr−1 for at least 0.3 Myr and accreting Mn at about 700 μg cm−2 yr−1. Although the top of the nodule was enriched in iron relative to the bottom, the nodule had been accreting Fe 50% faster on the bottom.232Th was also accumulating more rapidly in the bottom despite a 20-fold enrichment of230Th on the top. The distribution of alpha-emitting nuclides calculated from detailed radiochemical measurements matched closely the pattern revealed by 109-day exposures of alpha-sensitive film to the nodule. However, the shape and slope of the total alpha profile with depth into the nodule was affected strongly by226Ra and222Rn migrations making the alpha-track technique alone an inadequate method of measuring nodule growth rates. Diffusion of radium in the nodule may have been affected by diagenetic reactions which produce barite, phillipsite and todorokite within 1 mm of the nodule surface; however, our sampling interval was too broad to document the effect. We have not been able to resolve the importance of nodule diagenesis on the gross chemistry of the nodule.

Molybdenum accumulation in Cariaco basin sediment over the past 24 k.y.: A record of water-column anoxia and climate

Molybdenum (Mo) concentrations in a sediment core from the Cariaco basin on the Venezuelan continental shelf can be partitioned between a marine fraction and a terrigenous fraction. The accumulation rate of the marine fraction of Mo increased abruptly 15 000 calendar years ago (15 ka), from −2 ṁyr −1 to >4 μgṁcm −2 ṁyr −1 , and then decreased abruptly at 9 ka. The accumulation rate remained high throughout this 6 k.y. period, but exhibited maxima at 15–14 and 12.5 ka, corresponding in time to meltwater pulse IA into the Gulf of Mexico and the onset of the Younger Dryas cold event, respectively. The marine fraction of Mo is interpreted in terms of redox conditions of bottom water, as dictated by both the flux of settling organic matter and bottom-water residence time. Correspondence between geochemical extremes in this core with changes in sea level and global climate demonstrates the high degree to which this ocean-margin basin has responded to the paleoceanographic regime throughout the past 24 k.y.

Oxidation state of marine manganese nodules

Analyses of the bulk oxidation state of marine manganese nodules indicates that more than 98% of the Mn in deep ocean nodules is present as Mn(IV). The samples were collected from three quite different areas: the hemipelagic environment of the Guatemala Basin, the pelagic area of the North Pacific, and seamounts in the central Pacific. Results of the study suggest that todorokite in marine nodules is fully oxidized and has the following stoichiometry: (K, Na, Ca, Ba).33(Mg, Cu, Ni).76Mn5O22(H2O)3.2.

Minor elements in Quaternary sediment from the Sea of Japan: a record of surface-water productivity and intermediate-water redox conditions

Sediment of Quaternary age from Oki Ridge (903 m depth) in the Sea of Japan (∼3500 m deep) records six episodes of high accumulation rates of Cd, Cr, Cu, Mo, Ni, U, V, and Zn. The high rates correspond to periods of sulfate reduction in the water column at the intermediate depth of Oki Ridge; the intervening low values correspond to periods of denitrification and oxygen respiration. The maxima have a period of 41 k.y., the youngest having an age of 1.10 Ma. The 41 k.y. cycle is similar to the cycle of δ 18 O values of open-ocean plankton of the same age. The similarity between the cycles of minor-element accumulation in Sea of Japan sediment and δ 18 O values of Atlantic Ocean foraminifera indicates that redox changes in the water column of the Sea of Japan during the Quaternary, forced by major shifts in water-column advection and minor shifts in photic-zone productivity, reflect global events.

Rare earth elements in the phosphatic-enriched sediment of the Peru shelf

Abstract Apatite-enriched materials from the Peru shelf have been analyzed for their major oxide and rare earth element (REE) concentrations. The samples consist of (1) the fine fraction of sediment, mostly clay material, (2) phosphatic pellets and fish debris, which are dispersed throughout the fine-grained sediment, (3) tabular-shaped phosphatic crusts, which occur within the uppermost few centimeters of sediment, and (4) phosphatic nodules, which occur on the seafloor. The bulk REE concentrations of the concretions suggest that these elements are partitioned between the enclosed detrital material and the apatite fraction. Analysis of the fine-grained sediment with which the samples are associated suggested that this detrital fraction in the concretions should have shale REE values; the analysis of the fish debris suggested that the apatite fraction might have seawater values. The seawater contribution of REEs is negligible in the nodules and crust, in which the apatite occurs as a fine-grained interstitial cement. That is, the concentration of REEs and the REE patterns are predominantly a function of the amount of enclosed fine-grained sediment. By contrast, the REE pattern of the pelletal apatite suggests a seawater source and the absolute REE concentrations are relatively high. The REE P 2 O 5 ratios of the apatite fraction of these samples thus vary from approximately zero (in the case of the crust and nodules) to as much as approximately 1.2 × 10−3 (in the case of the pellets). The range of this ratio suggests that rather subtle variations in the depositional environment might cause a significant variation in the REE content of this authigenic fraction of the sediment. Pelletal glauconite was also recovered from one sediment core. Its REE concentrations closely resemble those of the fish debris.

Instability of bottom‐water redox conditions during accumulation of Quaternary sediment in the Japan Sea

The concentrations of Cd, Cr, Cu, Mo, Ni, Sb, U, V, and Zn were measured in early Quaternary sediment (1.32 to 1.08 Ma) from the Oki Ridge in the Japan Sea. The elements were partitioned between a detrital fraction, composed of terrigenous and volcaniclastic aluminosilicate debris, and a marine fraction, composed of biogenic and hydrogenous debris derived from seawater. The most important factors controlling minor-element accumulation rates in the marine fraction were (1) primary productivity in the photic zone, which largely controlled the flux of particulate organic-matter-bound minor elements settling through the water column and onto the seafloor, and (2) bottom-water redox, which determined the suite of elements that accumulated directly from seawater. This marine fraction of minor elements on Oki Ridge recorded six periods of high minor-element abundance. Assuming a constant bulk sediment accumulation rate, each period lasted roughly 5,000 to 10,000 years with a 41,000-year cycle. Accumulation rates of individual elements such as Cd, Mo, and U suggest sulfate-reducing conditions were established in the bottom water during the 10,000-year periods; accumulation rates of Cr and V during the intervening periods are indicative of less reducing, denitrifying conditions. Interelement ratios, for example, Cu:Mo, V:Cr, and Sb:Mo, further reflect bottom-water instability, such that bottom-water redox actually varied from sulfate reducing to denitrifying during the periods of highest minor-element accumulation rates; it varied from denitrifying to oxidizing during the intervening periods. Sediment lithology supports these interpretations of the minor-element distributions; the sediment is finely laminated for several of the periods represented by Cd, Mo, and U maxima and weakly laminated to bioturbated for the intervening periods. The geochemistry of this sediment demonstrates the unambiguous signal of Mo, principally, but of several other minor elements as well in recording sulfate-reducing conditions in bottom water. The forcing function that altered their accumulation, that is, that altered primary productivity and bottom water redox conditions, is problematic. Currently held opinion suggests that O2 depletion was most strongly developed during glacial advances. Low sea level during such times is interpreted to have enhanced primary productivity and restricted bottom-water advection.