Sedimentology and geochemistry of ODP Leg 129 siliceous deposits

Siliceous deposits drilled on Ocean Drilling Program Leg 129 accumulated within a few degrees of the equator during the Jurassic through early Tertiary, as constrained by paleomagnetic data. During the Jurassic and Early Cretaceous, radiolarian ooze, mixed with a minor amount of pelagic clay, was deposited near the equator, and overall accumulation rates were moderate to low. At a smaller scale, in more detail, periods of relatively higher accumulation rates alternated with periods of very low accumulation rates. Higher rates are represented by radiolarite and limestone; lower rates are represented by radiolarian claystone. Our limited data from Leg 129 suggests that accumulation of biogenic deposits was not symmetrical about the equator or consistent over time. In the Jurassic, sedimentation was siliceous; in the Cretaceous there was significant calcareous deposition; in the Tertiary claystone indicates significantly lower accumulation rates at least the northern part of the equatorial zone. Accumulation rates for Leg 129 deposits in the Cretaceous were higher in the southern part of the equatorial zone than in the northern part, and the southern side of this high productivity zone extended to approximately 15°S, while the northern side extended only to about 5°N. Accumulation rates are influenced by relative contributions from various sediment sources. Several elements and element ratios are useful for discriminating sedimentary sources for the equatorial depositional environments. Silica partitioning calculations indicate that silica is dominantly of biogenic origin, with a detrital component in the volcaniclastic turbidite units, and a small hydrothermal component in the basal sediments on spreading ridge basement of Jurassic age at Site 801. Iron in Leg 129 sediments is dominantly of detrital origin, highest in the volcaniclastic units, with a minor hydrothermal component in the basal sediments at Site 801. Manganese concentrations are highest in the units with the lowest accumulation rates. Fe/Mn ratios are >3 in all units, indicating negligible hydrothermal influence. Magnesium and aluminum concentrations are highest in the volcaniclastic units and in the basal sediments at Site 801. Phosphorous is very low in abundance and may be detrital, derived from fish parts. Boron is virtually absent, as is typical of deep-water deposits. Rare earth element concentrations are slightly higher in the volcaniclastic deposits, suggesting a detrital source, and lower in the rest of the lithologic units. Rare earth element abundances are also low relative to "average shale." Rare earth element patterns indicate all samples are light rare earth element enriched. Siliceous deposits in the volcaniclastic units have patterns which lack a cerium anomaly, suggesting some input of rare earth elements from a detrital source; most other units have a distinct negative Ce anomaly similar to seawater, suggesting a seawater source, through adsorption either onto biogenic tests or incorporation into authigenic minerals for Ce in these units. The Al/(Al + Fe + Mn) ratio indicates that there is some detrital component in all the units sampled. This ratio plotted against Fe/Ti shows that all samples plot near the detrital and basalt end-members, except for the basal samples from Site 801, which show a clear trend toward the hydrothermal end-member. The results of these plots and the association of high Fe with high Mg and Al indicate the detrital component is dominantly volcaniclastic, but the presence of potassium in some samples suggests some terrigenous material may also be present, most likely in the form of eolian clay. On Al-Fe-Mn ternary plots, samples from all three sites show a trend from biogenic ooze at the top of the section downhole to oceanic basalt. On Si-Fe-Mn ternary plots, the samples from all three sites fall on a trend between equatorial mid-ocean spreading ridges and north Pacific red clay. Copper-barium ratios show units that have low accumulation rates plot in the authigenic field, and radiolarite and limestone samples that have high accumulation rates fall in the biogenic field.

Chemical and isotopic compositions of rocks and minerals from the Ashadze and Logachev hydrothermal fields, Mid-Atlantic Ridge

This study was aimed at reconstructing a sequence of events in the magmatic and metamorphic evolution of peridotites, gabbroids, and trondhjemites from internal oceanic complexes of the Ashadze and Logachev hydrothermal vent fields. Collections of plutonic rocks from Cruises 22 and 26 of R/V "Professor Logachev", Cruise 41 of R/V "Akademik Mstislav Keldysh", and from the Serpentine Russian-French expedition aboard R/V "Pourquoi pas?" were objects of this study. Data reported here suggest that the internal oceanic complexes of the Ashadze and Logachev fields formed via the same scenario in these two regions of the Mid-Atlantic Ridge. On the other hand, an analysis of petrological and geochemical characteristics of the rocks indicated that the internal oceanic complexes of the MAR axial zone between 12°58'N and 14°45'N show pronounced petrological and geochemical heterogeneity manifested in variations in degree of depletion of mantle residues and in Nd isotopic compositions of rocks from the gabbro-peridotite association. Trondhjemites from the Ashadze hydrothermal field can be considered as partial melting products of gabbroids under influence of hydrothermal fluids. It was supposed that presence of trondhjemites in internal oceanic complexes of MAR can be used as a marker for the highest temperature deep-rooted hydrothermal systems. Perhaps, the region of the MAR axial zone, in which petrologically and geochemically contrasting internal oceanic complexes are spatially superimposed, serves as an area for development of large hydrothermal clusters with considerable ore-forming potential.

Chemical analyses and results of sediments from different DSDP Holes from the North Atlantic

Chemical analyses of North Atlantic D.S.D.P. (Deep Sea Drilling Project) sediments indicate that basal sediments generally contain higher concentrations of Fe, Mn, Mg, Pb, and Ni, and similar or lower concentrations of Ti, Al, Cr, Cu, Zn, and Li than the material overlying them. Partition studies on selected samples indicate that the enriched metals in the basal sediments are usually held in a fashion similar to that in basal sediments from the Pacific, other D.S.D.P. sediments, and modern North Atlantic ridge and non-ridge material. Although, on average, chemical differences between basal sediments of varying ages are apparent, normalization of the data indicates that the processes leading to metal enrichment on the crest of the Mid-Atlantic Ridge appear to have been approximately constant in intensity since Cretaceous times. In addition, the bulk composition of detrital sediments also appears to have been relatively constant over the same time period. Paleocene sediments from site 118 are, however, an exception to this rule, there apparently having been an increased detrital influx during this period. The bulk geochemistry, partitioning patterns, and mineralogy of sediments from D.S.D.P. 9A indicates that post-depositional migration of such elements as Mn, Ni, Cu, Zn, and Pb may have occurred. The basement encountered at the base of site 138 is thought to be a basaltic sill, but the overlying basal sediments are geochemically similar to other metalliferous basal sediments from the North Atlantic. These results, as well as those from site 114 where true oceanic basement was encountered, but where there was an estimated 7 m.y. hiatus between basaltic extrusion and basal sediment deposition, indicate that ridge-crest sediments are not necessarily deposited during active volcanism but can be formed after the volcanism has ceased. The predominant processes for metal enrichment in these deposits and those formed in association with other submarine volcanic features is a combination of shallow hydrothermal activity, submarine weathering of basalt, and the formation of ferromanganese oxides which can scavenge metals from seawater. In addition, it seems as though the formation of submarine metalliferous sediments is not restricted to active-ridge areas.

Chloroflexus spp. and Sulfurihydrogenibium spp. in microbial mats of the Nakabusa hot spring, Japan

Microbial mats develop in a wide range of aquatic habitats, such as geothermal hot springs, hypersaline ponds, marine cold seeps or hydrothermal vents. The Nakabusa hot spring is located in the Nagano Prefecture, Japan (36.3875N, 137.75E), dense olive-green microbial mats develop in regions where the slightly alkaline, sulfidic effluent has cooled to 65°C. The microbial community of such mats was analyzed by focusing on the diversity, as well as the in situ distribution and function of bacteria involved in sulfur cycling. Microbial mat samples were kept in sterile plastic tubes (for molecular analysis) or glass bottles completely filled with hot spring water to avoid oxidation. Samples were transferred to the laboratory on ice and used for physiological experiments within 8h. Quantification of cell biovolumes was carried out based on images of mat sections hybridized with Sulfurihydrogenibium- and Chloroflexi-specific probes, and stained with DAPI. In situ hybridizations (CARD-FISH) of thin matsections showed a heterogeneous vertical distribution of Sulfurihydrogenibium and Chloroflexus. Sulfurihydrogenibium dominated near the mat surface (50% of the total mat biovolume), while Chloroflexus dominated in deeper layers (up to 64% of the total mat biovolume).

Hydrogen consumption by gills of Bathymodiolus spp. collected at the Comfortless cove and Lilliput hydrothermal vent fields in May 2006

The discovery of deep-sea hydrothermal vents in 1977 revolutionized our understanding of the energy sources that fuel primary productivity on Earth. Hydrothermal vent ecosystems are dominated by animals that live in symbiosis with chemosynthetic bacteria. So far, only two energy sources have been shown to power chemosynthetic symbioses: reduced sulphur compounds and methane. Using metagenome sequencing, single-gene fluorescence in situ hybridization, immunohistochemistry, shipboard incubations and in situ mass spectrometry, we show here that the symbionts of the hydrothermal vent mussel Bathymodiolus from the Mid-Atlantic Ridge use hydrogen to power primary production. In addition, we show that the symbionts of Bathymodiolus mussels from Pacific vents have hupL, the key gene for hydrogen oxidation. Furthermore, the symbionts of other vent animals such as the tubeworm Riftia pachyptila and the shrimp Rimicaris exoculata also have hupL. We propose that the ability to use hydrogen as an energy source is widespread in hydrothermal vent symbioses, particularly at sites where hydrogen is abundant.