Geochemistry on ferromanganese crusts and nodules from Green Bay, Lake Michigan

The differential solubility of ferromanganese oxides can lead to stratigraphic separation of iron and manganese. Results of chemical analysis of a sequence of ferromanganese nodules overlying iron-rich crusts in northern Green Bay show that selec¬tive ion transport is important in concentrating manganese and associated trace elements near the oxygenated water-sediment interface. Manganese carbonate, which cements ferromanganese nodules, occurs in dark-gray silty sands that are located adjacent to the organic-rich muds of southern Green Bay. These muds contain an average of approximately 3.5 ppm (6x10-5M) interstitial Mn with 2.8 meq/l carbonate alkalinity. Thermodynamic calculation shows that interstitial water approaches equilibrium with MnCO3 in the upper 10 cm of sediment. This carbonate has a composition (Mn73Ca22Fe5)CO3 and has been identified as rhodochrosite.

Composition of hydrothermal massive sulfide deposits from ODP Site 158-957 and Leg 169 sites

Sulfide mineral major and trace element analyses were performed on more than 50 polished slabs representing mineralization from three seafloor hydrothermal massive sulfide deposits. Samples from the Bent Hill and ODP Mound massive sulfide deposits, both on the Juan de Fuca Ridge, can be contrasted with samples from the Trans-Atlantic Geotraverse (TAG) hydrothermal mound on the Mid-Atlantic Ridge. The massive sulfide at Bent Hill is predominantly pyrite and pyrrhotite, with increasing amounts of copper-bearing sulfide minerals at the base of the massive sulfide body and through the stockwork to an interval 200 m below seafloor that hosts high copper mineralization (Deep Copper Zone). ODP Mound contains much more abundant sphalerite and copper-bearing sulfides as compared to either Bent Hill or TAG, which are predominantly pyrite with much less abundant chalcopyrite. Copper-bearing sulfides from the Deep Copper Zone beneath Bent Hill and the lowest sampled interval of ODP Mound are petrographically and chemically similar, but distinct from copper-bearing minerals higher in either sequence.

Geochemistry and mineralogy of variably altered dacite volcanic rocks from ODP Leg 193 sites, PACMANUS field

A geochemical, mineralogical, and isotopic database comprising 75 analyses of Ocean Drilling Program (ODP) Leg 193 samples has been prepared, representing the variable dacitic volcanic facies and alteration types observed in drill core from the subsurface of the PACMANUS hydrothermal system (Table T1. The data set comprises major elements, trace and rare earth elements (REE), various volatiles (S, F, Cl, S, SO4, CO2, and H2O), and analyses of 18O and 86Sr/87Sr for bulk rock and mineral separates (anhydrite). Furthermore, normative mineral proportions have been calculated based on the results of X-ray diffraction (XRD) analysis (Table T2) using the SOLVER function of the Microsoft Excel program. Several of the samples analyzed consist of mesoscopically distinctive domains, and separate powders were generated to investigate these hand specimen-scale heterogeneities. Images of all the samples are collated in Figure F1, illustrating the location of each powder analyzed and documenting which measurements were performed.

Geology, geochemistry, and mineralogy of sediments from the Trans Indian Ocean Geotraverse

The monograph is devoted to the main results of research on the Trans Indian Ocean Geotraverse from the Maskarene Basin to the north-western margin of Australia. These results were obtained by Russian specialists and together with Indian specialists during 15 years of cooperation in investigation of geological structure and mineral resources of the Indian Ocean. The monograph includes materials on information support of marine geological and geophysical studies, composition and structure of information resources on the Indian Ocean, bathymetry and geomorphology, structure and geological nature of the magnetic field, gravity field, plate tectonics, crustal structure and sedimentary cover, seismic stratigraphy, perspectives for detecting oil and gas, solid minerals, sediment composition, composition and properties of clay minerals, stratigraphy and sediment age, chemical composition of sediments, composition of and prospects for solid minerals.

Chemical composition of phosphorites, Fe-Mn nodules and crusts, and basalts from the West Pacific

The monograph gives the first systematic description of ore-bearing guyots from the West Pacific. It is mostly based on data obtained in numerous expeditions of Russian vessels during 1984-1992. Ore deposits located on upper parts of all slopes and tops of the guyots include phosphorites associated with cobalt- and platinum-rich ferromanganese crusts. Location, origin and prospecting of mineral deposits are discussed on the base of new data on metallogenic factors (geodynamics, tectonics, magmatism, sedimentation and morphostructures).

Description and geochemical composition of manganese nodules retrieved from Lake Ontario, Canada

A large deposit of ferromanganese oxide coated sands and scattered manganese nodules occurs in the northern portion of Lake Ontario. The Mn and Fe contents of the concretions are similar to those in concretions from other environments, while their Ni, Cu, and Co contents are lower than in deep-sea nodules, but higher than in most previously described lacustrine concretions. Pb and Zn are high in the coatings and exceed the concentrations found in many previously analyzed Mn deposits. Within the deposit, Mn, Ni, Co, and Zn contents are correlated, and they vary inversely with Fe. Mn, Fe, Ni, Cu, and Pb are present in the interstitial waters of the sediments underlying the deposit in higher concentrations than in the overlying lake waters, thus providing a potential source of metals for concretion formation.The origin and compositional variations in the deposit possibly can be explained in terms of the fractionation and precipitation of Fe and Mn as a result of redox variations in the lake sediments. Eh increases from south to north across the deposit in such a way that iron may be selectively oxidized and precipitated in the south and manganese, in the north. The upward diffusion of Mn, Fe, and associated elements from the underlying sediments probably provides the principal source of the metals in the south of the deposit, while metal-enriched bottom waters are probably the principal source in the north.

Geochemistry and position of turbidites in the Cap Timiris Canyon off Mauritania

This study of sediments from the Cap Timiris Canyon demonstrates that geochemical data can provide reliable age-depth correlation even of highly turbiditic cores and attempts to improve our understanding of how turbidite emplacement is linked to climatic-related sea-level changes. The canyon incises the continental margin off NW Africa and is an active conduit for turbidity currents. In sediment cores from levee and intrachannel sites turbidites make up 6-42% of sediment columns. Age models were fitted to all studied cores by correlating downcore element data to dated reference cores, once turbidite beds had been removed from the dataset. These age models enabled us to determine turbidite emplacement times. The Cap Timiris Canyon has been active at least over the last 245 kyr, with turbidite deposition seemingly linked to stage boundaries and glacial stages. The highly turbiditic core from the intrachannel site postdates to ~15 kyr and comprises Holocene and late Pleistocene sediments. Turbidite deposition at this site was associated especially with the rapid sea-level rise at the Pleistocene/Holocene transition. During the Holocene, turbidity current activity decreased but did not cease.

Microfossils and chemical elements in bottom sediments of the Ashadze-1 Hydrothermal Field (tropical Mid-Atlantic Ridge)

The first thorough analysis of microfossils from ore-bearing sediments of the Ashadze-1 Hydrothermal Field in the Mid-Atlantic Ridge sampled during Cruise 26 of R/V Professor Logachev in 2005 revealed substantial influence of hydrothermal processes on preservation of planktonic calcareous organisms as well as on preservation and composition of benthic foraminifera. From lateral and vertical distribution patterns and secondary alterations of microfossils it is inferred that the main phase of hydrothermal mineralization occurred in Holocene. Heavy metals (Cu, Co, Cr, and Ag) were accumulated by foraminiferal tests and in their enveloping Fe-Mn crusts. Distribution of authigenic minerals replacing foraminiferal tests demonstrates local zoning related to hydrothermal activity. There are three mineral-geochemical zones defined: sulfide zone, zone with elevated Mg content, and zone of Fe-Mn crusts.

Microsedimentological characterization as indicators of sedimentary processes and climate changes during Lateglacial at Laguna Potrok Aike, Santa Cruz, Argentina

Palaeoclimatic and paleoenvironmental high latitude records in the Southern Hemisphere are scarce compared to the northern counterpart. However, understanding global evolution of environmental systems during sudden climate changes is inseparable from an equivalent knowledge of both Hemispheres. In this context, a high-resolution study of lacustrine sediments from Laguna Potrok Aike, Santa Cruz province, Patagonia, Argentina, was conducted for the Lateglacial period using concurrent X-Ray Fluorescence (XRF) and Scanning electron microscope analyses. Peaks of Ca/Si and Mn, and occurrences of the green alga Phacotus lenticularis have been interpreted as variations in ventilation of the water column from 13.6 to 11.1 ka cal. BP. During this interval, mild climate conditions during the Younger Dryas are characterized by relatively weak westerlies favouring the formation of a stratified water body as indicated by preserved manganese and Ca/Si peaks and high Total Organic Carbon (TOC) values. In this environment, water in the epilimnion can reach sufficiently high temperature to allow P. lenticularis to grow. Colder conditions are marked by peaks in Ca without P. lenticularis and occur during the Antarctic Cold Reversal (ACR). In this Lateglacial interval, micropumices were also detected in large amount. Image analysis of thin sections allowed the counting and size measurement of detrital particles and micropumices separately. Micropumices significantly influence the iron and titanium content, hence preventing to use them as proxies of detrital input in this interval.