Miriam Kastner

East pacific rise: hot springs and geophysical experiments.

Hydrothermal vents jetting out water at 380� � 30�C have been discovered on the axis of the East Pacific Rise. The hottest waters issue from mineralized chimneys and are blackened by sulfide precipitates. These hydrothermal springs are the sites of actively forming massive sulfide mineral deposits. Cooler springs are clear to milky and support exotic benthic communities of giant tube worms, clams, and crabs similar to those found at the Gal�pagos spreading center. Four prototype geophysical experiments were successfully conducted in and near the vent area: seismic refraction measurements with both source (thumper) and receivers on the sea floor, on-bottom gravity measurements, in situ magnetic gradiometer measurements from the submersible Alvin over a sea-floor magnetic reversal boundary, and an active electrical sounding experiment. These high-resolution determinations of crustal properties along the spreading center were made to gain knowledge of the source of new oceanic crust and marine magnetic anomalies, the nature of the axial magma chamber, and the depth of hydrothermal circulation.

Glacial to Interglacial Fluctuations in Productivity in the Equatorial Pacific as Indicated by Marine Barite

An empirical correlation between marine barite (BaSO4) accumulation rate in core-top sediment samples from two equatorial Pacific transects (at 140°W and 110°W) and the estimated primary productivity of the overlying water column were used to evaluate glacial to interglacial changes in productivity. Fluctuations in barite accumulation rates down-core indicate that during glacial periods of the past 450,000 years, the productivity in the central and eastern equatorial Pacific was about two times that during intervening interglacial periods. This result is consistent with other evidence that productivity was high in the eastern and central equatorial Pacific during the last glacial.

Diagenesis of siliceous oozes—I. Chemical controls on the rate of opal-A to opal-CT transformation—an experimental study

Abstract Evidence from deep-sea sediments supports the following diagenetic maturation sequence: opal-A (siliceous ooze) → opal-CT (porcelanite) → chalcedony or cryptocrystalline quartz (chert). A solution-redeposition mechanism is involved in the opal-A to opal-CT transformation. Exceptions to the overall maturation sequence are numerous, suggesting that temperature and time are not the only important factors controlling these mineralogical transformations. The rates of the above transformations are strongly affected by the composition of the solution and of the host sediments ; in Mesozoic clayey sediments, opal-CT predominates, while in carbonate sediments quartz is most common. Experiments at 25 and 150°C over a period of one day to six months show that the transformation rate of opal-A to opal-CT is much higher in carbonate than in clay-rich sediments, and that opal-CT lepisphere formation is aided by the precipitation of nuclei with magnesium hydroxide as an important component. The role of carbonate is explained as follows : in carbonate-rich sediments, the dissolution of carbonate provides the necessary alkalinity, and sea water provides the magnesium for the magnesium hydroxide in the nuclei. In contrast, in clay-rich sediments the clay minerals compete with opal-CT formation for the available alkalinity from sea water. As a result, the clays are enriched in Mg, and the rate of opal-CT formation is strongly reduced. This mechanism also bears on the common observation of carbonate replacement by silica.

Hot spring deposits on the East Pacific Rise at 21°N: preliminary description of mineralogy and genesis

Abstract Hydrothermal sulfide-sulfate deposits were sampled from eight active and inactive vent sites along the East Pacific Rise at 21°N during the RISE expedition of April, 1979. The mineralogy of the samples has been determined by X-ray diffractometry, scanning electron microscopy, and X-ray energy dispersive analysis. Mounds of Zn, Fe, and Cu sulfides, dominated by sphalerite, pyrite, and lesser chalcopyrite, are topped by inactive and active chimneys, spouting fluids ≤350°C. The outer zones of active chimneys bear abundant anhydrite precipitated from heated ambient seawater, in addition to hydrothermal pyrite and sphalerite. Mg-hydroxysulfate-hydrate, a phase identified in seawater heating experiments, but previously not observed in nature, is intimately intergrown with anhydrite. Hottest chimneys contain massive chalcopyrite±bornite in their interior zones and belch fluids blackened by a presumably non-equilibrium assemblage of pyrrhotite plus minor sphalerite and pyrite. The early, outer walls of chimneys form from sulfates and the sulfide minerals in black smoke, but metastable pyrrhotite in outer zones is rapidly recrystallized to pyrite or marcasite. Reduction in the permeability of the outer walls permits a high-temperature (>∼250°C), low-pH environment within chimneys that enhances precipitation of Cu-Fe sulfides in the central zones. Cooler, worm-covered chimneys emit white fluids bearing particulates of amorphous silica, barite, and pyrite. Amorphous silica and barite are also widely associated with fossilized worm-tubes. Two inactive chimneys are filled with sphalerite, wurtzite, sulfur, pyrite, and marcasite. Anhydrite has been dissolved from these dead chimneys, and the sulfate assemblage is dominated by barite and alteration products such as jarosite and natrojarosite. Silicates other than amorphous silica are not abundant in these deposits, although talc forms in hot chimneys from seawater Mg and hydrothermal silica, and nontronite is found in sediments on the crest of the East Pacific Rise. Other accessory phases identified include copper-rich sulfides such as cubanite, chalcocite, covellite, and digenite; galena; Fe-oxyhydroxides, including goethite; and gypsum. Chimney debris accumulates to form basal mounds, and the mineralogical differences between mounds and chimneys are attributable to weathering of mounds. Mn-oxyhydroxides form crusts within a few meters of the vents, but are not coprecipitating with the sulfide/sulfate minerals.

Fluids in convergent margins: what do we know about their composition, origin, role in diagenesis and importance for oceanic chemical fluxes?

The nature and origin of fluids in convergent margins can be inferred from geochemical and isotopic studies of the venting and pore fluids, and is attempted here for the Barbados Ridge, Nankai Trough and the convergent margin off Peru. Venting and pore fluids with lower than seawater Cl- concentrations characterize all these margins. Fluids have two types of source: internal and external. The three most important internal sources are: (1) porosity reduction; (2) diagenetic and metamorphic dehydration ; and (3) the breakdown of hydrous minerals. Gas hydrate formation and dissociation, authigenesis of hydrous minerals and the alteration of volcanic ash and/or the upper oceanic crust lead to a redistribution of the internal fluids and gases in vertical and lateral directions. The maximum amount of expelled water calculated can be ca. 7 m3 a-1 m-1, which is much less than the tens to more than 100 m3 a-1 m-1 of fluid expulsion which has been observed. The difference between these figures must be attributed to external fluid sources, mainly by transport of meteoric water enhanced by mixing with seawater. The most important diagenetic reactions which modify the fluid compositions, and concurrently the physical and even the thermal properties of the solids through which they flow are: (1) carbonate recrystallization, and more importantly precipitation; (2) bacterial and thermal degradation of organic matter; (3) formation and dissociation of gas hydrates; (4) dehydration and transformation of hydrous minerals, especially of clay minerals and opal-A; and (5) alteration, principally zeolitization and clay mineral formation, of volcanic ash and the upper oceanic crust.

Water column methane oxidation adjacent to an area of active hydrate dissociation, Eel River Basin

Abstract The role of methane clathrate hydrates in the global methane budget is poorly understood because little is known about how much methane from decomposing hydrates actually reaches the atmosphere. In an attempt to quantify the role of water column methanotrophy (microbial methane oxidation) as a control on methane release, we measured water column methane profiles (concentration and δ13C) and oxidation rates at eight stations in an area of active methane venting in the Eel River Basin, off the coast of northern California. The oxidation rate measurements were made with tracer additions of 3H-CH4. Small numbers of instantaneous rate measurements are difficult to interpret in a dynamic, advecting coastal environment, but combined with the concentration and stable isotope measurements, they do offer insights into the importance of methanotrophy as a control on methane release. Fractional oxidation rates ranged from 0.2 to 0.4% of ambient methane per day in the deep water (depths >370 m), where methane concentration was high (20–300 nM), to near-undetectable rates in the upper portion of the water column (depths

ORGANIC MATTER PRESERVATION ON CONTINENTAL SLOPES: IMPORTANCE OF MINERALOGY AND SURFACE AREA

Theoretical considerations, calculations, and data reported by Keil et al. (1994a) were used to assess the recent hypotheses that mineral surface roughness controls the specific surface area of continental margin sediments and that the matrix-linked organic carbon content of continental margin sediments occurs as a monolayer or ‘monolayer-equivalent’ coating of organic compounds on the surfaces of detrital minerals and other detrital nonorganic grains (Mayer, 1994). Results of our analysis indicate that it is not the surface roughness of the terrigenous detrital framework grains that controls the specific surface area of most continental margin sediments, even in the sand and silt fractions, but rather the presence of nonspherical, high surface area-to-volume particles, primarily clays but also oxy-hydroxides and ultra-structured nonorganic bioclasts such as diatom frustules. Analysis of the mineralogy, organic carbon content, surface area, and in situ microfabric of continental margin sediments off California in the Mendocino and San Luis Obispo areas and off the state of Washington indicate a strong correlation between the amount of matrix-linked organic matter present and the suite of clay minerals. Data indicate that organic carbon appears to be preferentially sequestered in smectite-rich sediments compared to those whose clay fractions are dominated by chlorite. We suggest that this association is a function of differences in the site density and chemistry of the clays and differences in their flocculation behavior. Our data also indicate that organic carbon preservation on the three California transects, all of which have a suboxic oxygen minimum zone that impinges on the sea floor, is not significantly influenced by differences in bottom water oxygen concentration; and carbon stable isotope data suggest no preferential preservation of continental organic matter over that of marine origin.

Lithium isotopic compositions of pore fluids and sediments in the Costa Rica subduction zone: implications for fluid processes and sediment contribution to the arc volcanoes

Abstract Pore fluid and sediment Li concentrations and isotopic ratios provide important insights on the hydrology, sediment contribution to the arc volcanoes and fluid–sediment reactions at the dominantly non-accretionary Costa Rica subduction zone. Ocean Drilling Program Site 1039 in the trench axis provides a reference section of 400 m of the incoming sediments, and Site 1040, situated arcward from the trench, consists of a deformed sedimentary wedge and apron sediments, the decollement, and the partially dewatered underthrust sediment section. At the reference site, pore fluids show important isotopic variations (δ6Li=−21.7 to −37.8‰), reflecting the interplay of in situ alteration of volcanic material and ion exchange with clay minerals. In the basal section, a reversal of Li concentration and δ6Li toward seawater values is observed, providing supporting evidence for a lateral seawater flow system in the upper oceanic basement underlying this sediment section. At Site 1040, pore fluid of the lower deformed wedge sediments and within the decollement is enriched in Li and the isotopic compositions are relatively light, suggesting infiltration of a deep-seated fluid. The δ6Li value of −22‰ of this Li-enriched fluid (261 μM), when compared with the δ6Li value of the subducted sediment section (−11‰), suggests that the deep source fluid originates from mineral fluid dehydration and transformation reactions at temperatures of 100 to 150°C, consistent with the temperature range of the up-dip seismogenic zone and of transformation of smectite to illite. The distribution of Li and its isotopes in the underthrust section are similar to those at the reference site, indicating near complete subduction of the incoming sediments and that early dewatering of the underthrust sediments occurs predominantly by lateral flow into the ocean. The hemipelagic clay-rich sediment section of the subducting plate carries most of the Li into this subduction zone, and the pelagic diatomaceous and nannofossil calcareous oozes contain little Li. The Li isotopes of both the clay-rich hemipelagic sediments and of the pelagic oozes are, however, similar, with δ6Li values of −9 to −12‰. The observations that (1) the δ6Li values of the underthrust sediments are distinctly lower than that of the mantle, and (2) the lavas of the Costa Rican volcanoes are enriched in Li and 7Li, provide an approximation of the contribution of the subducted sediments to the arc volcanoes. A first order mass balance calculation suggests that approximately half of the Li flux delivered by subducted sediments and altered oceanic crust into the Middle American Trench is recycled to the Costa Rican arc and at most a quarter of sedimentary Li is returned into the ocean through thrust faults, primarily the decollement thrust.

New insights into deformation and fluid flow processes in the Nankai Trough accretionary prism: Results of Ocean Drilling Program Leg 190

Moore, G. F., Taira, A., Klaus, A., Becker, L., Boeckel, B., Cragg, B. A., Dean, A., Fergusson, C. L., Henry, P., Hirano, S., Hisamitsu, T. et al. (2001). New insights into deformation and fluid flow processes in the Nankai Trough accretionary prism: Results of Ocean Drilling Program Leg 190. Geochemistry, Geophysics, Geosystems, 2, Article No: 2001GC000166.

A high-temperature hydrothermal deposit on the seabed at a gulf of California spreading center

Abstract A submersible dive on a turbidite-covered spreading axis in Guaymas Basin photographed and sampled extensive terraces and ledges of talc. The rock contains siliceous microfossils, smectite, and euhedral pyrrhotite as well as rather pure iron-rich talc. Sulfur and oxygen isotopes indicate precipitation around a hydrothermal vent, at about 280°C.