Geochemistry of basaltic rocks from ODP Site 129-801

Middle Jurassic basaltic lavas obtained from Site 801 in the western Pacific Pigafetta Basin represent ocean crust from the oldest segment of the present-day Pacific Ocean. A composite 131 m section shows the basement to be composed of an upper alkalic basalt sequence (about 157 Ma) with ocean island basalt chemical features and a lower tholeiitic basalt sequence (about 167 Ma) with typical normal-type mid-ocean ridge basalt features. The basalt sequences are separated by a quartz-cemented, yellow goethite hydrothermal deposit. Most basalts are altered to some degree and exhibit variable, low-grade smectite-celadonite-pyrite-carbonate-zeolite assemblages developed under a mainly hydrated anoxic environment. Oxidation is very minor, later in development than the hydration assemblages, and largely associated with the hydrothermal deposit. The tholeiitic normal-type mid-ocean ridge basalt has characteristically depleted incompatible element patterns and all compositions are encompassed by recent mid-ocean ridge basalt from the East Pacific Rise. Chemically, the normal-type mid-ocean ridge basalt is divided into a primitive plagioclase-olivine +/- spinel phyric group (Mg* = 72-60) and an evolved (largely) aphyric group of olivine tholeiites (Mg* = 62-40). Both groups form a single comagmatic suite related via open-system fractionation of initial olivine-spinel followed by olivine-plagioclase-clinopyroxene. The alkalic ocean island basalt are largely aphyric and display enriched incompatible element abundances within both relatively primitive olivine-rich basalts and evolved olivine-poor hawaiites related via mafic fractionation. In gross terms, the basement lithostratigraphy is a typical mid-ocean ridge basalt crust, generated at a spreading center, overlain by an off-axis seamount with ocean island basalt chemical characters.

Porewater concentrations, pH, single cell number, anaerobic oxidation of methane (AOM) rate, sulfate reduction (SR) rate of samples during METEOR cruise M76/3b at the REGAB cold seep site from the West African margin in 2008

The giant pockmark REGAB (West African margin, 3160 m water depth) is an active methane-emitting cold seep ecosystem, where the energy derived from microbially mediated oxidation of methane supports high biomass and diversity of chemosynthetic communities. Bare sediments interspersed with heterogeneous chemosynthetic assemblages of mytilid mussels, vesicomyid clams and siboglinid tubeworms form a complex seep ecosystem. To better understand if benthic bacterial communities reflect the patchy distribution of chemosynthetic fauna, all major chemosynthetic habitats at REGAB were investigated using an interdisciplinary approach combining porewater geochemistry, in situ quantification of fluxes and consumption of methane, as well bacterial community fingerprinting. This study revealed that sediments populated by different fauna assemblages show distinct biogeochemical activities and are associated with distinct sediment bacterial communities. The methane consumption and methane effluxes ranged over one to two orders of magnitude across habitats, and reached highest values at the mussel habitat, which hosted a different bacterial community compared to the other habitats. Clam assemblages had a profound impact on the sediment geochemistry, but less so on the bacterial community structure. Moreover, all clam assemblages at REGAB were restricted to sediments characterized by complete methane consumption in the seafloor, and intermediate biogeochemical activity. Overall, variations in the sediment geochemistry were reflected in the distribution of both fauna and microbial communities; and were mostly determined by methane flux.

Chemical profiles in sediment and basalt at DSDP Hole 74-525A

Forty sediment and four basement basalt samples from DSDP Hole 525A, Leg 74, as well as nine basalt samples from southern and offshore Brazil, were subjected to instrumental neutron activation analysis. Thirty-two major, minor, and trace elements were determined. The downcore element concentration profiles and regression analyses show that the rare earth elements (REE) are present in significant amounts in both the carbonate and noncarbonate phases in sediments; Sr is concentrated in the carbonate phase, and most of the other elements determined exist mainly in the noncarbonate phase. The calculated partition coefficients of the REE between the carbonate phase and the free ion concentrations in seawater are high and increase with decreasing REE ionic radii from 3.9 x 10**6 for La to 15 x 10**6 for Lu. Calculations show that the lanthanide concentrations in South Atlantic seawater have not been changed significantly over the past 70 Ma. The Ce anomaly observed in the carbonate phase is a redox indicator of ancient seawater. Study of the Ce anomaly reveals that seawater was anoxic over the Walvis Ridge during the late Campanian. As the gap between South America and West Africa widened and the Walvis Ridge subsided from late Campanian to late Paleocene times, the water circulation of the South Atlantic improved and achieved oxidation conditions about 54 Ma that are similar to present seawater redox conditions in the world oceans. The chemical compositions of the basement rocks correspond to alkalic basalts, not mid-ocean ridge basalts (MORBs). The results add more evidence to support the hypothesis that the Walvis Ridge was formed by a series of volcanos moving over a "hot spot" near the Mid-Atlantic Ridge. From the chemical composition and REE pattern, one 112 Ma old basalt on the Brazilian continental shelf has been identified as an early stage MORB. To date, this is the oldest oceanic tholeiite recovered from the South Atlantic. This direct evidence indicates that the continental split between South America and Africa commenced > 112 Ma.

Major and rare earth element contents and their speciations in ferruginous nodules and host sediments from the Barents Sea

Contents of Fe, Mn, Al, P, and rare earth elements (REE) in ferruginous nodules and host sediments of the eastern Barents Sea were studied. A direct Fe-P correlation in reactive components of the sediments and nodules was found. The nodules were shown to be formed through Fe(II) oxidation in the surface layer of sediments and cementation of terrigenous fraction of sediments by Fe(III) oxyhydroxides. The latter accumulate phosphorus due to processes of sorption - co-precipitation, by forming Fe(III) hydrophosphates. REE composition in the sediments and nodules normalized to NASC contents is characterized by increased proportion of light REE that may be caused by regional features of their sources. Due to significant share of terrigenous matter in the Fe nodules (up to 65% for Nd), REE composition of bulk samples is similar to that of host sediments. A negative cerium anomaly in composition of reactive REE may result from REE sorption from seawater. REE bulk composition of a ferruginous crust is closer to that of seawater than one of the ferruginous nodules from the sediments because of essentially lower content of diluent terrigenous matter.

(Table 1) Sulfur isotopes, total sulfur, and degree of oxidation of basalt samples from DSDP Holes 69-504B and 70-504B

Whole-rock basalt samples from the upper half of Deep Sea Drilling Project Hole 504B have oxygen-isotope compositions typical of mid-ocean-ridge basalts which have experienced a moderate degree of low-temperature alteration by sea water. By contrast, d18O values in the lower half of the hole correspond to basalts which have experienced almost no detectable oxygen-isotope alteration. These observations suggest that the overall water/rock ratio was lower in the lower half of the drilled crust. A correlation between d18O values and 87Sr/86Sr ratios suggests that the water/rock ratio, rather than temperature variation, was the main factor determining basalt d18O values. Hydrogen-isotope data appear to be consistent with a low water/rock ratio in the lower part of the crust.