Mineral, chemical and isotopic compositions of hydrothermally altered sediments from the Middle Valley (Juan de Fuca Ridge), ODP Site 139-858

Li and Li isotopes have been measured in the clay fraction of sediments recovered from the Middle Valley hydrothermal site on the Juan De Fuca Ridge. The Li content of pure detrital clays is 51 ppm while hydrothermal clays and carbonates have lower Li (22+/-11 ppm). However, there is no clear relationship between the mineralogy of the hydrothermal alteration products and their Li content. The d7Li value of the detrital clays is +5.8?. Hydrothermal clays and carbonates have d7Li in the range of -3.9? to +7.8?; these values do not seem to be dependent on the temperature at which they formed. Modelling of the Li and Li isotope systematics indicates that the fluid from which the alteration products form is significantly enriched in Li (higher than 10000 µmol/kg) relative to pore fluids recovered from within the sediments (up to 589 µmol/kg; [Wheat, C.G., M.J. Mottl, 1994. Data report: trace metal composition of pore water from Sites 855 through 858, Middle valley, Juan De Fuca Ridge. In Mottl, M.J., Davis, E.E., Fisher, A.T., Slack, J.F. (Eds.), Proc. ODP, Sci. Res. 139: 749-755; doi:10.2973/odp.proc.sr.139.269.1994]), and that this Li is derived from sediment. Thus, the alteration products are not in equilibrium with their conjugate pore fluids; rather, the alteration minerals formed at lower water/sediment ratios. This suggests that fluid flow pathways at Middle Valley were more diffuse in the past than they are today.

Strontium chemistry of anhydrite from DSDP/ODP Hole 504B

A unique record of the chemical evolution of seawater during hydrothermal recharge into oceanic crust is preserved by anhydrite from the volcanic sequences and sheeted dike complex in ODP Hole 504B. Chemical and isotopic analyses 87Sr/86Sr, delta18O, delta34S of anhydrite constrain the changing composition of fluids due to reaction with basalt. There is a general trend of decreasing 87Sr/86Sr of anhydrite, corresponding to the minor incorporation of basaltic strontium with depth in the volcanic rocks. 87Sr/86Sr ratios decrease rapidly with depth in the dikes to values identical to host basalt (0.7029). Sr/Ca ratios (<0.1 mmol/mol) suggest that recharge fluids have very low Sr concentrations and fluids evolve by first precipitating Sr-bearing phases before extensive exchange of Sr with the host basalt. There is a background trend of decreasing sulfate delta18O with depth from +12-13? in the lower volcanics to +7? in the lower sheeted dikes recording an increase in recharge fluid temperature from c. 150° to c. 250°C, and confirming the presence of sulfate in hydrothermal fluids at elevated temperatures. From the amount of anhydrite recovered from Hole 504B and the amount of seawater sulfur that has been reduced to sulfide, a minimum seawater recharge flux can be calculated. This value is 4-25 times lower than estimates of high-temperature fluid fluxes based on either thermal constraints or global chemical budgets and suggests that there is significant deficit of seawater-derived sulfur in the oceanic crust. Only a minor proportion of the seawater that percolates into the crust near the axis is heated to high temperatures and exits as black smoker-type fluids. A significant proportion of the axial heat loss must be advected at 200-250°C by sulfate-bearing hydrothermal solutions that egress diffusely from the crust. These fluids penetrate into the dikes and exchange both heat and chemical tracers without the extensive clogging of porosity by anhydrite precipitation, which would halt hydrothermal circulation for any reasonable fluid flux. The heating of the major proportion of hydrothermal fluids to only moderate temperatures (c. 250°C) reconciles estimates of hydrothermal fluxes derived from thermal models and global geochemical budgets. The flux of hydrothermal sulfate would be of a magnitude similar to the riverine input, and oxygen-isotopic exchange at 200-250°C between dissolved sulfate and recharge fluids during hydrothermal circulation provides a mechanism to continuously buffer seawater sulfate oxygen to the light isotopic composition observed.

Petrography, minerals and isotopic composition at DSDP Leg 66 Holes

We collected 20 carbonate nodules from the inner trench slope deposits of the Middle America Trench area off Mexico. Carbonate nodules are found only within the methane-rich layer beneath the mixed layer of methane and hydrogen sulfide. They have been investigated by microscopic, scanning electron microscopic (SEM), X-ray diffraction, and stable isotopic analytical methods. Calcite, magnesian calcite, dolomite, and rhodochrosite were recognized as carbonate minerals. Each carbonate nodule is usually represented by single species of carbonate minerals. Carbonate nodules are subdivided into micrite nodules and recrystallized nodules according to textural features. The carbonate crystallites in each micrite nodule are equidimensional. Their sizes range from several to 30 µm, as revealed by SEM micrographs. The chemical composition of calcite is changed from pure calcite to high magnesian calcite, as shown by the shift of the (104) reflection in X-ray diffraction patterns. Fe substitution for Ca in dolomite was also observed. Carbon isotopic composition shows an unusually wide range - from -42.9 to +13.5 per mil - in PDB scale, whereas oxygen isotopic compositions of almost all the carbonate nodules are constantly enriched in 18O from +3.4 to +7.60 per mil in PDB scale. These wide variations in carbon isotopic composition indicate several sources for the carbon in carbonate nodules. Carbon with a negative d13C value was derived from biochemical oxidation of methane with a negative d13C value. On the other hand, carbon with positive d13C value was probably formed during methane production in an anoxic condition.

Mineralogy and geochemistry of alteration products in DSDP Leg 81 Holes

Alteration products of basalts from the four holes drilled during Leg 81 were studied and found to be characterized by the widespread occurrence of trioctahedral clay minerals (Mg smectite to chlorite). In some cases zeolites (analcite, chabazite) are associated with the saponite. A more oxidizing stage is marked by a saponite-celadonite association, presenting the geochemical characteristics of hydrothermal processes. Later stages of alteration are represented by palagonitization and subaerial weathering at two sites. These different alteration processes of basalts from Leg 81 record the paleoenvironment during the first opening stages of the Northeast Atlantic Ocean in the Paleocene-Eocene periods.

Trace elements in the aerosol at Neumayer Station, Antarctica

Atmospheric trace element concentrations were measured from March 1999 through December 2003 at the Air Chemistry Observatory of the German Antarctic station Neumayer by inductively coupled plasma - quadrupol mass spectrometry (ICP-QMS) and ion chromatogra-phy (IC). This continuous five year long record derived from weekly aerosol sampling re-vealed a distinct seasonal summer maximum for elements linked with mineral dust entry (Al, La, Ce, Nd) and a winter maximum for the mostly sea salt derived elements Li, Na, K, Mg, Ca, and Sr. The relative seasonal amplitude was around 1.7 and 1.4 for mineral dust (La) and sea salt aerosol (Na), respectively. On average a significant deviation regarding mean ocean water composition was apparent for Li, Mg, and Sr which could hardly be explained by mir-abilite precipitation on freshly formed sea ice. In addition we observed all over the year a not clarified high variability of element ratios Li/Na, K/Na, Mg/Na, Ca/Na, and Sr/Na. We found an intriguing co-variation of Se concentrations with biogenic sulfur aerosols (methane sul-fonate and non-sea salt sulfate), indicating a dominant marine biogenic source for this element linked with the marine biogenic sulfur source.

Barium concentrations und stable isotope ratios of late Paleocene deep-sea sediments

Deep-sea sediment Ba* (Ba/Al2O3(sample) * 15% - Ba(aluminosilicate) records show increasing values synchronous with the evolution of the late Paleocene global d13C maximum, reflecting an increase in marine surface primary production and biogenic barite formation at this time. At two oligotrophic locations, Deep Sea Drilling Project (DSDP) Sites 384 and 527 in the North and South Atlantic, respectively, Ba* increases from 160-360 ppm in the early Paleocene to 1100-3000 ppm during the d13C maximum. At equatorial DSDP Site 577, positioned within or near the high-productivity zone, Ba* increases from ~15,500 ppm in the early Paleocene to ~25,400 ppm in conjunction with late Paleocene maximum d13C values. Linear fitted correlation plots of sediment Ba* content versus surface water d13C in all three regions support barite originating in the euphotic zone. The early to late Paleocene relative increase in Ba* illustrates how burial rates of Corg (relative to Al2O3) accelerated by a factor of ~1.8 and ~6.0 in the eutrophic and oligotrophic areas, respectively. A tentative estimate, weighing our result for the entire ocean, suggests that accumulation rates of organic carbon increased by a factor of 2 during the late Paleocene d13C maximum.

Geochemistry, SiO2 minerals and isotopes at DSDP Hole 72-516F

Cherts recovered during DSDP Leg 72 from Rio Grande Rise sediments (Site 516) consist of both cristobalite and quartz, and contain ghosts of foraminifers and (more rare) radiolarians. Porcelanite made of disordered cristobalite is found in most old enclosing sediments. Local dissolution of siliceous microfossils during diagenesis is the most likely source of the silica required for the chert formation. As sediment age increases, the proportion of biogenic silica decreases and authigenic silica increases.

Stable isotope geochemistry of pore waters and marine sediments from the New Jersey shelf: Methane formation and fluid origin

Interstitial water and sediment samples of the Integrated Ocean Drilling Program (IODP) expedition 313 "New Jersey Shallow Shelf" were analyzed for chemical composition and stable isotope ratios. A total of 222 water samples were collected from the cores by Rhizon samplers and squeezing of fresh core material. Water was analyzed for its stable oxygen and hydrogen isotope geochemistry (d2H and d18O) at sites M0027A and M0029A, and the carbon isotope composition of the dissolved inorganic carbon (d13CDIC) (all sites). In addition, organic material (Corg) and inorganic carbonates from sediments were analyzed for their carbon ratios (d13Corg and d13Ccarb), and in case of the carbonates also for oxygen (d18Ocarb). Carbon isotopes were also analyzed in samples containing enough methane gas (d13Cmeth). Pore fluids from site M0027A were analyzed for the sulfur isotope composition of dissolved sulfate (d34S). The combination of isotope analyses of all phases (interstitial water, sediment, and gas) with pore water chemistry is expected to enable a better understanding of processes in the sediment and will help to identify the origin of fluids under the New Jersey shelf.

Stable oxygen isotope ratios of pore waters from ODP Leg 166 sites

This study investigates the d18O of pore waters from Sites 1003 through 1007, drilled along the western margin of the Great Bahama Bank during Leg 166 of the Ocean Drilling Program. These pore waters generally show a positive correlation between d18O and the concentration of chloride. The exception to this trend is Site 1006, where the pore waters exhibit nonlinear behavior with respect to chloride. The correlation between the concentration of Cl- and d18O at most of the sites appears to be a coincidence because although the increase in Cl- is a result of diffusion from an underlying source, the increases in d18O result from the recrystallization of metastable carbonates in the presence of a geothermal gradient. The difference in behavior in the d18O of the pore water at Site 1006 is probably a result of the relative reduced rate of carbonate recrystallization at this site. The d18O of the pore waters in the upper portion of the cores shows a pattern similar to the concentration of chloride in that there is an interval of 30-50 m in which neither the d18O nor the concentration of Cl- changes. This interval is consistent with either an interval of very rapid deposition of sediment or the advection of fluid through the platform. Both the d18O and the concentration of Cl- increase toward the platform, suggesting an input of saline and isotopically heavy water from the platform surface.