Geology topics

Jeffrey C. Alt

Publications and source records attributed to Jeffrey C. Alt.

5 recordsLinked to original sources

USGS2013

Evidence for microbial carbon and sulfur cycling in deeply buried ridge flank basalt

Sediment-covered basalt on the flanks of mid-ocean ridges constitutes most of Earth's oceanic crust, but the composition and metabolic function of its microbial ecosystem are largely unknown. By drilling into 3.5-million-year-old subseafloor basalt, we demonstrated the presence of methane- and sulfur-cycling microbes on the eastern flank of the Juan de Fuca Ridge. Depth horizons with functional genes indicative of methane-cycling and sulfate-reducing microorganisms are enriched in solid-phase sulfur and total organic carbon, host δ 13 C- and δ 34 S-isotopic values with a biological imprint, and show clear signs of microbial activity when incubated in the laboratory. Downcore changes in carbon and sulfur cycling show discrete geochemical intervals with chemoautotrophic δ 13 C signatures locally attenuated by heterotrophic metabolism.

Science↗

USGS2012

Recycling of water, carbon, and sulfur during subduction of serpentinites: A stable isotope study of Cerro del Almirez, Spain

We use the concentrations and isotope compositions of water, carbon, and sulfur in serpentinites and their dehydration products to trace the cycling of volatiles during subduction. Antigorite serpentinites from the Cerro del Almirez complex, Spain, contain 9–12 wt.% H2O and 910 ± 730 ppm sulfur, and have bulk δ18O values of 8.6 ± 0.4‰, δD = − 54 ± 5‰, and δ34S = 5.0‰, consistent with serpentinization at temperatures of ~ 200 °C by seawater hydrothermal fluids in a seafloor setting. The serpentinites were dehydrated to chlorite–harzburgite (olivine + orthopyroxene + chlorite) at 700 °C and 1.6–1.9 GPa during subduction metamorphism, resulting in loss of water, and sulfur. The chlorite–harzburgites contain 5.7 ± 1.9 wt.% H2O, and have bulk δ18O = 8.0 ± 0.9‰, and δD = − 77 ± 11‰. The rocks contain 650 ± 620 ppm sulfur having δ34S = 1.2‰. Dehydration of serpentinite resulted in loss of 5 wt.% H2O having δ18O = 8–10‰ and δD = − 27 to − 65‰, and loss of 260 ppm sulfur as sulfate, having δ34S = 14.5‰. The contents and δ13C of total carbon in the two rock types overlap, with a broad trend of decreasing carbon contents and δ13C from ~ 1300 to 200 ppm and − 9.6 to − 20.2‰. This reflects mixing between reduced carbon in the rocks (210 ppm, δ13C ≈ − 26‰) and seawater-derived carbonate (δ13C ≈ − 1‰). Our results indicate: 1) Serpentinized oceanic peridotites carry significant amounts of isotopically fractionated water, carbon and sulfur into subduction zones; 2) Subduction of serpentinites to high P and T results in loss of water, and sulfur, which can induce melting and contribute to 18O, D, and 34S enrichments and oxidation of the sub-arc mantle wedge; and 3) Isotopically fractionated water, carbon, and sulfur in serpentinite dehydration products are recycled deeper into the mantle where they can contribute to isotope heterogeneities and may be significant for volatile budgets of the deep Earth.

Cerro Del Almirez;Betic Cordillera In Southern Spa↗

USGS2011

Sulfur geochemistry and microbial sulfate reduction during low-temperature alteration of uplifted lower oceanic crust: Insights from ODP Hole 735B

Sulfide petrography plus whole rock contents and isotope ratios of sulfur were measured in a 1.5 km section of oceanic gabbros in order to understand the geochemistry of sulfur cycling during low-temperature seawater alteration of the lower oceanic crust, and to test whether microbial effects may be present. Most samples have low SO 4 /ΣS values (≤ 0.15), have retained igneous globules of pyrrhotite ± chalcopyrite ± pentlandite, and host secondary aggregates of pyrrhotite and pyrite laths in smectite ± iron-oxyhydroxide ± magnetite ± calcite pseudomorphs of olivine and clinopyroxene. Compared to fresh gabbro containing 100–1800 ppm sulfur our data indicate an overall addition of sulfide to the lower crust. Selection of samples altered only at temperatures ≤ 110 °C constrains microbial sulfate reduction as the only viable mechanism for the observed sulfide addition, which may have been enabled by the production of H 2 from oxidation of associated olivine and pyroxene. The wide range in δ 34 S sulfide values (− 1.5 to + 16.3‰) and variable additions of sulfide are explained by variable ε sulfate-sulfide under open system pathways, with a possible progression into closed system pathways. Some samples underwent oxidation related to seawater penetration along permeable fault horizons and have lost sulfur, have high SO 4 /ΣS (≥ 0.46) and variable δ 34 S sulfide (0.7 to 16.9‰). Negative δ 34 S sulfate –δ 34 S sulfide values for the majority of samples indicate kinetic isotope fractionation during oxidation of sulfide minerals. Depth trends in sulfide–sulfur contents and sulfide mineral assemblages indicate a late-stage downward penetration of seawater into the lower 1 km of Hole 735B. Our results show that under appropriate temperature conditions, a subsurface biosphere can persist in the lower oceanic crust and alter its geochemistry.

Chemical Geology↗

USGS2011

Microbial sulfate reduction and the sulfur budget for a complete section of altered oceanic basalts, IODP Hole 1256D (eastern Pacific)

Sulfide mineralogy and the contents and isotope compositions of sulfur were analyzed in a complete oceanic volcanic section from IODP Hole 1256D in the eastern Pacific, in order to investigate the role of microbes and their effect on the sulfur budget in altered upper oceanic crust. Basalts in the 800m thick volcanic section are affected by a pervasive low-temperature background alteration and have mean sulfur contents of 530ppm, reflecting loss of sulfur relative to fresh glass through degassing during eruption and alteration by seawater. Alteration halos along fractures average 155ppm sulfur and are more oxidized, have high SO 4 /ΣS ratios (0.43), and lost sulfur through oxidation by seawater compared to host rocks. Although sulfur was lost locally, sulfur was subsequently gained through fixation of seawater-derived sulfur in secondary pyrite and marcasite in veins and in concentrations at the boundary between alteration halos and host rocks. Negative δ34S sulfide-S values (down to -30 °) and low temperatures of alteration (down to ~40 °C) point to microbial reduction of seawater sulfate as the process resulting in local additions of sulfide-S. Mass balance calculations indicate that 15-20% of the sulfur in the volcanic section is microbially derived, with the bulk altered volcanic section containing 940ppm S, and with δ34S shifted to -6.0&permil;) from the mantle value (0 &permil;). The bulk volcanic section may have gained or lost sulfur overall. The annual flux of microbial sulfur into oceanic basement based on Hole 1256D is 3-4 X10 10 molSyr-1, within an order of magnitude of the riverine sulfate source and the sedimentary pyrite sink. Results indicate a flux of bacterially derived sulfur that is fixed in upper ocean basement of 7-8 X 10 -8 molcm -2 yr -1 over 15m.y. This is comparable to that in open ocean sediment sites, but is one to two orders of magnitude less than for ocean margin sediments. The global annual subduction of sulfur in altered oceanic basalt lavas based on Hole 1256D is 1.5-2.0 X 10 11 moly -1 , comparable to the subduction of sulfide in sediments, and could contribute to sediment-like sulfur isotope heterogeneities in the mantle.

Earth and Planetary Science Letters↗

USGS2005

Hydrothermal vent fluids, siliceous hydrothermal deposits, and hydrothermally altered sediments in Yellowstone Lake

Stable isotopic (dD and d18O) data indicate about 13% total evaporative concentration has occurred in Yellowstone Lake, yet lake waters are enriched in dissolved As, B, Cl, Cs, Ge, Li, Mo, Sb, and W by at least an order-of-magnitude relative to the flow-weighted composition of inflowing streams. We conclude that lake water is a mixture of inflowing surface water and hydrothermal source fluid that is strongly enriched in Cl and other elements. We estimate that ~10% of the total hydrothermal flux in Yellowstone National Park (YNP) occurs in Yellowstone Lake. Geochemical and mineralogical studies of hydrothermal deposits and hydrothermally altered lake sediments (vent muds) from the active or recently active vent sites on the floor of Yellowstone Lake indicate that their formation is due to hydrothermal fluid quenching during flow through shallow conduits, or to mixing upon egress into cold bottom waters. Siliceous precipitates form conduits within the uppermost sediments, tabular deposits along sedimentary layers, and spires up to 8 m tall. These deposits are enriched in As, Cs, Hg, Mo, Sb, Tl and W. Spires, vent deposits, and conduits contain filamentous microstructures that probably represent silicified bacteria. Partly recrystallized and silicified diatoms are abundant in deposits below the sediment-water interface. Vent muds and some outer conduit walls show pervasive leaching of silica, which explains the occurrence of most sublacustrine vents in craters. Systematics of dD and Cl variations, as well as silica and cation geothermometry for hydrothermal fluids, suggest that ascending fluids boil due to depressurization to a temperature of ~220°C and then mix with pore waters prior to venting on the lake bottom. Depositional temperatures for sublacustrine silica deposits, calculated using oxygen isotope fractionation, range from 78°C to 164°C. The amorphous silica-saturated vent fluids precipitate silica to form spires or conduits largely by conductive cooling. Bacterial accumulations may have inhibited the mixing of vent fluids and bottom waters, and provided a site for silica deposition.

Wyoming↗