Rare earth element contents and sources in phosphorites from Pacific seamounts

Distribution of rare earth elements (REE) was studied in phosphorites collected from seamounts at depths from about 400 to 3600 m. In general phosphorites are characterized by high REE con¬tent, by a strong negative Ce anomaly, by a slight positive Gd anomaly, and by slight enrichment in heavy REE, which is also characteristic of seawater, where, to certain extent, composition of REE depends on depth. Comparison of REE composition in phosphorites and in seawater from the Northwest Pacific by means of Q-mode factor analysis revealed that REE have been transported into the phosphorites from various water depths following submergence of the seamounts. This corresponds to paleotectonic reconstructions, but is only partially consistent with age determinations of phosphorites.

Halogene concentrations and iodine isotopic composition in pore water on Hydrare Ridge, Cascadia continental margin

We report iodine and bromine concentrations in a total of 256 pore water samples collected from all nine sites of Ocean Drilling Program Leg 204, Hydrate Ridge. In a subset of these samples, we also determined iodine ages in the fluids using the cosmogenic isotope 129I (T1/2 = 15.7 Ma). The presence of this cosmogenic isotope, combined with the strong association of iodine with methane, allows the identification of the organic source material responsible for iodine and methane in gas hydrates. In all cores, iodine concentrations were found to increase strongly with depth from values close to that of seawater (0.0004 mM) to concentrations >0.5 mM. Several of the cores taken from the northwest flank of the southern summit show a pronounced maximum in iodine concentrations at depths between 100 and 150 meters below seafloor in the layer just above the bottom-simulating reflector. This maximum is especially visible at Site 1245, where concentrations reach values as high as 2.3 mM, but maxima are absent in the cores taken from the slope basin sites (Sites 1251 and 1252). Bromine concentrations follow similar trends, but enrichment factors for Br are only 4-8 times that of seawater (i.e., considerably lower than those for iodine). Iodine concentrations are sufficient to allow isotope determinations by accelerator mass spectrometry in individual pore water samples collected onboard (~5 mL). We report 129I/I ratios in a few samples from each core and a more complete profile for one flank site (Site 1245). All 129I/I ratios are below the marine input ratio (Ri = 1500x10**-15). The lowest values found at most sites are between 150 and 250x10**-15, which correspond to minimum ages between 40 and 55 Ma, respectively. These ages rule out derivation of most of the iodine (and, by association, of methane) from the sediments hosting the gas hydrates or from currently subducting sediments. The iodine maximum at Site 1245 is accompanied by an increase in 129I/I ratios, suggesting the presence of an additional source with an age younger than 10 Ma; there is indication that younger sources also contribute at other sites, but data coverage is not yet sufficient to allow a definitive identification of sources there. Likely sources for the older component are formations of early Eocene age close to the backstop in the overriding wedge, whereas the younger sources might be found in recent sediments underlying the current locations of the gas hydrates.

Niche differentiation among mat-forming, sulfide-oxidizing bacteria in chemosynthetic ecosystems of the Nile Deep Sea Fan (Eastern Mediterranean Sea)

Sulfidic muds of cold seeps on the Nile Deep Sea Fan are populated by different types of mat-forming sulfide-oxidizing bacteria. The predominant sulfide oxidizers of three different mats were identified by microscopic and phylogenetic analyses as (i) Arcobacter species producing cotton-ball-like sulfur precipitates, (ii) large filamentous sulfur bacteria including Beggiatoa species, or (iii) single, spherical cells resembling Thiomargarita species. High resolution in situ microprofiles revealed different geochemical settings selecting for different mat types. Arcobacter mats occurred where oxygen and sulfide overlapped at the bottom water interface. Filamentous sulfide oxidizers were associated with non-overlapping, steep gradients of oxygen and sulfide. A dense population of Thiomargarita was favored by temporarily changing supplies of oxygen and sulfide. These results indicate that the decisive factors in selecting for different mat-forming bacteria within one deep-sea province are spatial or temporal variations in energy supply. Furthermore, the occurrence of Arcobacter spp.-related 16S rRNA genes in the sediments below all three types of mats, as well as on top of brine lakes of the Nile Deep Sea Fan, indicates that this group of sulfide oxidizers can switch between different life modes depending on the geobiochemical habitat setting.

Minerals of bulk samples and clay fraction at DSDP Leg 80 Holes

At the request of the Leg 80 scientific party, selected samples of Cretaceous age were processed by X-ray diffraction at the mineralogy laboratories at the Ecole des Mines (Albian to Late Cretaceous samples) and at the Institut de Géologie at Dijon (Barremian samples). The results were used in developing the lithostratigraphy and sedimentology discussed in this volume by Rat et al. 1985 (doi:10.2973/dsdp.proc.80.140.1985) in their study of Barremian-Albian paleoenvironment, by Graciansky and Gillot in their study of Albian and Cenomanian limestones, and by Graciansky and Bourbon in their paleoenvironmental reconstructions for the Late Cretaceous chalks.

Chemical and isotopic compositions of basaltic glasses and crusts from the Mid-Atlantic Ridge rift valley

Data presented in the paper suggest significant differences between thermodynamic conditions, under which magmatic complexes were formed in MAR at 29°-34°N and 12°-18°N. Melts occurring at 29°-34°N were derived by melting of a mantle source with homogeneous distribution of volatile components and arrived at the surface without significant fractionation, likely, due to their rapid ascent. The MAR segments between 12° and 18°N combine contrasting geodynamic environments of magmatism, which predetermined development of a large plume region with widespread mixing of melting products of geochemically distinct mantle sources. At the same time, this region is characterized by conditions favorable for origin of localized zones of anomalous plume magmatism. These sporadic magmatic sources were spatially restricted to MAR fragments with the Hess crust, whose compositional and mechanical properties were, perhaps, favorable for focusing and localization of plume magmatism. The plume source between 12° and 18°N beneath MAR may be geochemically heterogeneous.