Composition of ferromanganese crusts from the Atlantic, Pacific and Indian Oceans

A collection of layered ferromanganese ores (27 samples) from the Atlantic and Pacific oceans was studied. Trace element and PGE contents were determined layer-by-layer (up to 10 microlayers) in 13 of these samples. The trace, rare earth, and platinum group element distributions, including their layer-to-layer variations, were compared in hydrogenic and hydrothermal crusts from different regions. It was found that the main PGE variations (by a factor of 10-50) are related to their layer-to-layer variations within a given ore field. The distributions of PGE and trace elements are strongly heterogeneous, which is related, first, to different contents of the elements in the layers of different age in ferromanganese crusts (FMC) and, second, to the observed regional heterogeneity and influence of hydrothermal fluids. Geochemical data indicate that CFC formation was mainly caused by the hydrochemical precipitation of material from seawater. This process was accompanied by diagenetic phenomena, water-rock interaction, and influence of volcanic and hydrothermal sources.

Carbon and Lead chemistry of sediments from the Middle Atlantic Bight

A mass budget was constructed for organic carbon on the upper slope of the Middle Atlantic Bight, a region thought to serve as a depocenter for fine-grained material exported from the adjacent shelf. Various components of the budget are internally consistent, and observed differences can be attributed to natural spatial variability or to the different time scales over which measurements were made. The flux of organic carbon to the sediments in the core of the depocenter zone, at a water depth of 1000 m, was measured with sediment traps to be 65 mg C m**-2 day**-1, of which 6-24 mg C m**-2 day**-1 is buried. Oxygen fluxes into the sediments, measured with incubation chambers attached to a free vehicle lander, correspond to total carbon remineralization rates of 49-70 mg C m**-2 day**-1. Carbon remineralization rates estimated from gradients of Corg within the mixed layer, and from gradients of dissolved ammonia and phosphate in pore waters, sum to only 4-6 mg C m**-2 day**-1. Most of the Corg remineralization in slope sediments is mediated by bacteria and takes place within a few mm of the sediment-water interface. Most of the Corg deposited on the upper slope sediments is supplied by lateral transport from other regions, but even if all of this material were derived from the adjacent shelf, it represents <2% of the mean annual shelf productivity. This value is further lowered by recognizing that as much as half of the Corg deposited on the slope is refractory, having originated by reworking from older deposits. Refractory Corg arrives at the sea bed with an average 14C age 600-900 years older than the pre-bomb 14C age of DIC in seawater, and has a mean life in the sediments with respect to biological remineralization of at least 1000 years. Labile carbon supplied to the slope, on the other hand, is rapidly and (virtually) completely remineralized, with a mean life of < 1 year. Carbon-14 ages of fine-grained carbonate and organic carbon present within the interstices of shelf sands are consistent with this material acting as a source for the old carbon supplied to the slope. Winnowing and export of reworked carbon may contribute to the often-described relationship between organic carbon preservation and accumulation rate of marine sediments.

Sedimentology and Lead and Strontium isotope ratios of Miocene sediments from the Kerguelen Plateau

Characterization of sediment from Ocean Drilling Program Site 745, representing the East Kerguelen Ridge sediment drift, addresses important issues surrounding the timing of Miocene to present East Antarctic ice sheet stability and oceanic environmental change. Our results show three periods of greatly enhanced accumulation of Antarctic-derived sediment, at 6.4-5.9 Ma, 4.9-4.4 Ma and 1.1-0.8 Ma, potentially indicative of warmer, less stable ice sheets at these times. Conversely, the accumulation of Antarctic-derived material is comparatively less during the middle of the Pliocene warm epoch (4.8-3.2 Ma). The deep flow forming the Kerguelen drift was stronger during the latest Miocene and earliest Pliocene and has decreased in intensity continuously since then.

Gotland Deep (Baltic Sea) independent age model based on Palaeomagnetic Secular Variation (PSV) and lead (Pb) deposition profiles

Dating of sediment cores from the Baltic Sea has proven to be difficult due to uncertainties surrounding the 14C reservoir age and a scarcity of macrofossils suitable for dating. Here we present the results of multiple dating methods carried out on cores in the Gotland Deep area of the Baltic Sea. Particular emphasis is placed on the Littorina stage (8 ka ago to the present) of the Baltic Sea and possible changes in the 14C reservoir age of our dated samples. Three geochronological methods are used. Firstly, palaeomagnetic secular variations (PSV) are reconstructed, whereby ages are transferred to PSV features through comparison with varved lake sediment based PSV records. Secondly, lead (Pb) content and stable isotope analysis are used to identify past peaks in anthropogenic atmospheric Pb pollution. Lastly, 14C determinations were carried out on benthic foraminifera (Elphidium spec.) samples from the brackish Littorina stage of the Baltic Sea. Determinations carried out on smaller samples (as low as 4 µg C) employed an experimental, state-of-the-art method involving the direct measurement of CO2 from samples by a gas ion source without the need for a graphitisation step - the first time this method has been performed on foraminifera in an applied study. The PSV chronology, based on the uppermost Littorina stage sediments, produced ten age constraints between 6.29 and 1.29 cal ka BP, and the Pb depositional analysis produced two age constraints associated with the Medieval pollution peak. Analysis of PSV data shows that adequate directional data can be derived from both the present Littorina saline phase muds and Baltic Ice Lake stage varved glacial sediments. Ferrimagnetic iron sulphides, most likely authigenic greigite (Fe3S4), present in the intermediate Ancylus Lake freshwater stage sediments acquire a gyroremanent magnetisation during static alternating field (AF) demagnetisation, preventing the identification of a primary natural remanent magnetisation for these sediments. An inferred marine reservoir age offset (deltaR) is calculated by comparing the foraminifera 14C determinations to a PSV & Pb age model. This deltaR is found to trend towards younger values upwards in the core, possibly due to a gradual change in hydrographic conditions brought about by a reduction in marine water exchange from the open sea due to continued isostatic rebound.

Cadmium and lead in sea ice and seawater of the Amur Bay, Sea of Japan

Distribution of Cd and Pb in sea ice and in under-ice water of the Amur Bay at the end of February 1998 is considered. The metals were determined by technique of inversion voltammetry. Contribution of Cd and Pb from atmospheric precipitation and from under-ice water to sea ice examined is discussed. On the basis of analysis of vertical distribution in ice, atmospheric fluxes supplying metals to the aquatic area of the bay are estimated at 100 and 2000 µg/m**2/year for Cd and Pb, respectively. Concentrations of Cd and Pb found in middle and lower parts of ice cores allow to suggest that their accumulation relative to main ions of seawater occurs in the ice. Estimated enrichment factors of Cd and Pb in sea ice relative to seawate are ~9 and ~5. A possible mechanism of relative metal accumulation in sea ice is considered.

Composition of sediments, rocks, and ores from the Northern and Equatorial Indian Ocean

The monograph has been written on the base of data obtained from samples and materials collected during the 19-th cruise of RV ''Akademik Vernadsky'' to the Northern and Equatorial Indian Ocean. Geological features of the region (stratigraphy, tectonic structure, lithology, distribution of ore-forming components in bottom sediments, petrography of igneous rocks, etc.) are under consideration. Regularities of trace element concentration in Fe-Mn nodules, nodule distribution in bottom sediments, and engineering-geological properties of sediments within the nodule fields have been studied. Much attention is paid to ocean crust rocks. The wide range of ore mineralization (magnetite, chromite, chalcopyrite, pyrite, pentlandite, and other minerals) has been ascertained.

Lead and strontium isotopic composition of some metalliferous and pelagic sediments and basalts at DSDP Leg 70 Holes

In recent years, metalliferous sediments have been discovered overlying newly generated oceanic crust in the East Pacific, North Atlantic, Indian Ocean, Red Sea, Gulf of Aden, and elsewhere (e.g., Boström, 1973; Lalou et al., 1977; Bischoff, 1969; Boström and Fisher, 1971; Cann et al., 1977, respectively). Such material has also been recovered by drilling from sediments lying upon older oceanic crust (Boström et al., 1972, 1976; Horowitz and Cronan, 1976). Hydrothermal circulation of seawater at a spreading ridge results in the leaching of Fe, Mn, and possibly other elements from the basaltic volcanic layer and their transport and discharge into ocean bottom waters, whereupon fine-grained Fe-Mn-rich precipitates form and settle into the ambient sediment (cf. Corliss, 1971; Dasch et al., 1971; Spooner and Fyfe, 1973; Bischoff and Dickson, 1975; Heath and Dymond, 1977; Corliss et al., 1979, Edmond et al., 1979). Mn-rich crusts have also been recovered from active ridges and are inferred to have formed in the vicinity of hydrothermal discharge areas (Scott et al., 1974; Moore and Vogt, 1976; Corliss et al., 1978; Hoffert et al., 1978). The source of the trace elements in the metalliferous deposits is generally not clear. They may be derived from seawater by adsorption onto the precipitates or crusts, or from hydrothermal solutions which have leached them from the basalts. Pb, however, can be used as a geochemical tracer because of the known isotopic compositional differences between oceanic basalts and seawater. Isotopic investigations of Pb in ferruginous sediments from the East Pacific have shown that it has been derived partly or mostly from a basaltic source (Bender et al., 1971; Dasch et al., 1971; Dymond et al., 1973). In the present study, Pb isotopic analyses have been made of a suite of metalliferous sediments (nontronite, Mn-oxide crust, Mn-Fe-oxide mud), pelagic sediments, and basalts from the Galapagos mounds area. The main purposes of the Pb study were to determine the source or sources of Pb in the metalliferous sediments, and whether or not stratigraphic variations exist in the isòtopic composition of Pb in the sediments.

Lead 210 and silicate profiles from sediments of the equatorial Pacific and South Atlantic

Particle mixing rates have been determined for 5 South Atlantic/Antarctic and 3 equatorial Pacific deep-sea cores using excess 210Pb and 32Si measurements. Radionuclide profiles from these siliceous, calcareous, and clay-rich sediments have been evaluated using a steady state vertical advection diffusion model. In Antarctic siliceous sediments210Pb mixing coefficients (0.04-0.16 cm**2/y) are in reasonable agreement with the 32Si mixing coefficient (0.2 or 0.4 cm**2/y, depending on 32Si half-life). In an equatorial Pacific sediment core, however, the 210Pb mixing coefficient (0.22 cm**2/y) is 3-7 times greater than the 32Si mixing coefficient (0.03 or 0.07 cm**2/y). The difference in 210Pb and 32Si mixing rates in the Pacific sediments results from: (1) non-steady state mixing and differences in characteristic time and depth scales of the two radionuclides, (2) preferential mixing of fine-grained clay particles containing most of the 210Pb activity relative to coarser particles (large radiolaria) containing the 32Si activity, or (3) the supply of 222Rn from the bottom of manganese nodules which increases the measured excess 210Pb activity (relative to 226Ra) at depth and artificially increases the 210Pb mixing coefficient. Based on 32Si data and pore water silica profiles, dissolution of biogenic silica in the sediment column appears to have a minor effect on the 32Si profile in the mixed layer. Deep-sea particle mixing rates reported in this study and the literature do not correlate with sediment type, sediment accumulation rate, or surface productivity. Based on differences in mixing rate among three Antarctic cores collected within 50 km of each other, local variability in the intensity of deep-sea mixing appears to be as important as regional differences in sediment properties.

Lead isotopes inthe Eastern Equatorial Pacific

The influence of atmospheric dust on climate and biogeochemical cycles in the oceans is well understood but poorly quantified. Glacial atmospheric dust loads were generally greater than those during the Holocene, as shown, for example, by the covariation of dust fluxes in the Equatorial Pacific and Antarctic ice cores. Nevertheless, it remains unclear whether these increases in dust flux were associated with changes in sources of dust, which would in turn suggest variations in wind patterns, climate or paleo-environment. Such questions can be answered using radiogenic isotope tracers of dust provenance. Here, we present a 160-kyr high-precision lead isotope time-series of dust input to the Eastern Equatorial Pacific (EEP) from core ODP Leg 138, Site 849 (0°11.59' N, 110°31.18' W). The Pb isotope record, combined with Nd isotope data, rules out contributions from Northern Hemisphere dust sources, north of the Intertropical Convergence Zone, such as Asia or North Africa/Sahara; similarly, eolian sources in Australia, Central America, the Northern Andes and Patagonia appear insignificant based upon the radiogenic isotope data. Fluctuations in Pb isotope ratios throughout the last 160 kyr show, instead, that South America remained the prevailing source of dusts to the EEP. There are two distinct South American Pb isotope end-members, constrained to be located in the south Central Volcanic Zone (CVZ, 22° S - 27.5° S) and the South Volcanic Zone (SVZ, 33° S - 43° S), with the former most likely originating in the Atacama Desert. Dust availability in the SVZ appears to be related to the weathering of volcanic deposits and the development of ash-derived Andosols, and influenced by local factors that might include vegetation cover. Variations in the dust fluxes from the two sources are in phase with both the dust flux and temperature records from Antarctican ice cores. We show that the forcing of dust provenance over time in the EEP overall is influenced by high-southerly-latitude climate conditions, leading to changes in the latitudinal position and strength of the South Westerlies as well as the coastal winds that blow northward along the Chilean margin. The net result is a modulation of dust emission from the Atacama Desert and the SVZ via a northward migration of the South Westerlies during cold periods and southward retreat during glacial terminations.

Strontium- and lead-isotope composition of some basalts at DSDP Hole 69-504B

Seventeen whole-rock samples, generally taken at 25- to 50-meter intervals from 5 to 560 meters sub-basement in Deep Sea Drilling Project Hole 504B, were analyzed for 87Sr/86Sr ratios, and rubidium and strontium concentrations. Ten of these samples also were analyzed for Pb-isotope composition. Strontium-isotope ratios for eight samples from the upper 260 meters of the hole range from 0.70287 to 0.70377, with a mean of 0.70320. In the interval 330 to 560 meters, five samples have a restricted range of 0.70259 to 0.70279, with a mean of 0.70266, almost identical to the average value of fresh mid-ocean-ridge basalts. In the interval 260 to 330 meters, approximately intermediate strontium- isotope ratios are found. The higher 87Sr/86Sr ratios in the upper part of the hole can be interpreted in terms of strontium-isotope alteration during basalt-sea-water interaction. Relative to average fresh mid-ocean ridge basalts, the upper 260 meters of basalts are enriched by an average of about 9% in sea-water strontium 87Sr/86Sr = 0.7091). This Sr presumably is located in the smectites, which, as the main secondary minerals throughout the hole, replace olivine and matrix glass and locally fill vesicles (analyzed samples contained no veins). The strontium-isotope data strongly suggest that the integrated flux of sea water through the upper part of the Hole 504B crust has been greater than through the lower part. This is also suggested by (1) the common occurrence of Feoxide- hydroxide minerals as alteration products above 270 meters, but their near absence below 320 meters, (2) the presence of vein calcite above 320 meters, but its near absence below this level, and (3) the occurrence of vein pyrite only below a depth of 270 meters. Sea-water circulation in the lower basalts may have been partly restricted by the greater number of relatively impermeable massive lava flows below 230 meters sub-basement. Although sufficient sea water was present within the lower part of the hole to produce smectitic alteration products, the overall water /rock ratio was low enough to prevent significant modification of strontium-isotope ratios. Lead-isotope ratios of Hole 504B basalts form approximately linear arrays in plots of 208Pb/204Pb and 207Pb/204Pb versus 206Pb/204Pb. The arrays are similar to those reported for basalts from other mid-ocean ridges. There is no trend in Hole 504B lead-isotope ratios with vertical position in the basement. The arrays indicate that the lead-isotope composition of the upper mantle from which the Hole 504B basaltic melts were derived was inhomogeneous.