Mineralogic, elemental, and stable isotopic composition of bulk carbonate sediments from ODP Leg 166 sites

An intensive mineralogic and geochemical investigation was conducted on sediments recovered during Ocean Drilling Program Leg 166 from the western Great Bahama Bank at Sites 1006, 1008, and 1009. Pleistocene through middle Miocene sediments recovered from Site 1006, the distal location on the Leg 166 transect, are a mixture of bank-derived and pelagic carbonates with lesser and varying amounts of siliciclastic clays. A thick sequence of Pleistocene periplatform carbonates was recovered near the platform edge at Sites 1008 and 1009. Detailed bulk mineralogic, elemental (Ca, Mg, Sr, and Na), and stable isotopic (d18O and d13C) analyses of sediments are presented from a total of 317 samples from all three sites.

Pliocene stable isotope and carbonate stratigraphy at DSDP Holes 85-572C and 85-573A

Holes 572C and 573A provide high resolution (about 5000-yr. sampling interval) records of oxygen and carbon isotope stratigraphy (Globigerinoides sacculifera) and carbonate stratigraphy for the Pliocene of the equatorial Pacific. These data enable detailed correlation of carbonate events between sites and provide additional resolution to the previous carbonate stratigraphy. Comparison of calcium carbonate and d18O data reveal a "Pacific-type" carbonate stratigraphy throughout the Pliocene. The d18O data have two modes of variability with a boundary at 2.9 Ma. The planktonic d18O record does not have a steplike enrichment at 3.2 Ma, which is observed in benthic records elsewhere, suggesting that this event does not represent the proposed initiation of northern hemispheric glaciation. Hole 572C does record a distinct d18O enrichment event at about 2.4 Ma, which has been previously associated with the onset of major ice rafting in the North Atlantic.

Major and trace element analyses from DSDP Hole 26-253 at the Ninetyeast Ridge in the Indian Ocean

A basaltic sequence of Eocene submarine-erupted pyroclastic sediments totals at least 388 m at DSDP Site 253 on the Ninetyeast Ridge. These fossiliferous hyaloclastic sediments have been erupted and fragmented by explosive volcanism (hydroexplosions) in shallow water. The occurrence of interbedded basaltic ash-fall tuffs within the younger horizons of the hyaloclastic sequence marks the emergence of some Ninetyeast Ridge volcanic vents above sea level. Considerable textural variation allows subdivision of the sequence into six informal lithostratigraphic units. Hydrothermal and diagenetic alteration has caused the complete replacement of all original glass by smectites, and the introduction of abundant zeolite and calcite cements. The major and trace element contents of the hyaloclastites vary due to the alteration, and the admixture of biogenous calcite. On a calcium carbonate-free basis systematic variations are recognisable. Mg, Ni, Cr and Cu are enriched, and Li and Zn depleted in the three older units relative to the younger three. The chemical variability is reflected by the development of saponite in the older part of the sequence and montmorillonite in the younger; and by the presence of a quartz-normative basalt flow occurring in Unit II, in contrast to the Mg-rich highly olivine-normative basalt at the base of the sequence. The younger and older parts of the sequence therefore appear to have been derived from magmas of different chemistry. The sequence, like other basaltic rocks recovered from the Ninetyeast Ridge, is enriched in the light relative to the heavy rare earth elements (REE) although the REE contents vary unsystematically with depth, probably because of the high-temperature subaqueous alteration and the presence of biogenous calcite. This REE data indicates that the Ninetyeast Ridge volcanism was different from that which produces mid-ocean ridge basalts.

Carbonate content in lower Cretaceous sediments of ODP Hole 123-765C

Sediments recovered during Ocean Drilling Program (ODP) Leg 123 from the Argo Abyssal Plain (AAP) consist largely of turbidites derived from the adjacent Australian continental margin. The oldest abundant turbidites are Valanginian-Aptian in age and have a mixed (smarl) composition; they contain subequal amounts of calcareous and siliceous biogenic components, as well as clay and lesser quartz. Most are thin-bedded, fine sand- to mud-sized, and best described by Stow and Piper's model (1984) for fine-grained biogenic turbidites. Thicker (to 3 m), coarser-grained (medium-to-coarse sand-sized) turbidites fit Bouma's model (1962) for sandy turbidites; these generally are base-cut-out (BCDE, BDE) sequences, with B-division parallel lamination as the dominant structure. Parallel laminae most commonly concentrate quartz and/or calcispheres vs. lithic clasts or clay, but distinctive millimeter- to centimeter-thick, radiolarian-rich laminae occur in both fine- and coarse-grained Valanginian-Hauterivian turbidites. AAP turbidites were derived from relatively deep parts of the continental margin (outer shelf, slope, or rise) that lay below the photic zone, but above the calcite compensation depth (CCD). Biogenic components are largely pelagic (calcispheres, foraminifers, radiolarians, nannofossils); lesser benthic foraminifers are characteristic of deep-water (abyssal to bathyal) environments. Abundant nonbiogenic components are mostly clay and clay clasts; smectite is the dominant clay species, and indicates a volcanogenic provenance, most likely the Triassic-Jurassic volcanic suite exposed along the northern Exmouth Plateau. Lower Cretaceous smarl turbidites were generated during eustatic lowstands and may have reached the abyssal plain via Swan Canyon, a submarine canyon thought to have formed during the Late Jurassic. In contrast to younger AAP turbidites, however, Lower Cretaceous turbidites are relatively fine-grained and do not contain notably older reworked fossils. Early in its history, the northwest Australian margin provided mainly contemporaneous slope sediment to the AAP; marginal basins adjacent to the continent trapped most terrigenous detritus, and pronounced canyon incisement did not occur until Late Cretaceous and, especially, Cenozoic time.

Chemical and isotopic compositions of ocean authigenic carbonates

Oceanic authigenic carbonates are classified according to origin of the carbonate carbon source using a complex methodology that includes methods of sedimentary petrography, mineralogy, isotope geochemistry, and microbiology. Mg-calcite (protodolomite) and aragonite predominate among the authigenic carbonates. All authigenic carbonates are depleted in 13C and enriched in 18O (in PDB system) that indicates biological fractionation of isotopes during carbonate formation. Obtained results show that authigenic carbonate formation is a biogeochemical (microbial) process, which involves carbon from ancient sedimentary rocks, abiogenic methane, and bicarbonate-ion of hydrothermal fluids into the modern carbon cycle.

Sm-Nd isotopic investigation for rocks of Prince Charles Mountains, East Antarctica

The Prince Charles Mountains have been subject to extensive geological and geophysical investigations by former Soviet, Russian and Australian scientists from the early 1970s. In this paper we summarise, and review available geological and isotopic data, and report results of new isotopic studies (Sm-Nd, Pb-Pb, and U-Pb SHRIMP analyses); field geological data obtained during the PCMEGA 2002/2003 are utilised. The structure of the region is described in terms of four tectonic terranes. Those include Archaean Ruker, Palaeoproterozoic Lambert, Mesoproterozoic Fisher, and Meso- to Neoproterozoic Beaver Terranes. Pan-African activities (granite emplacement and probably tectonics) in the Lambert Terrane are reported. We present a summary of the composition of these terranes, discuss their origin and relationships. We also outline the most striking geological features, and problems, and try to draw attention to those rocks and regional geological features which are important in understanding the composition and evolution of the PCM and might suggest targets for further investigations.

Thermal conductivity at DSDP Leg 60 Holes

A total of 1547 thermal conductivity values were determined by both the NP (needle probe method) and the QTM (quick thermal conductivity meter) on 1319 samples recovered during DSDP Leg 60. The NP method is primarily for the measurement of soft sedimentary samples, and the result is free from the effect of porewater evaporation. Measurement by the QTM method is faster and is applicable to all types of samples-namely, sediments (soft, semilithified, and lithified) and basement rocks. Data from the deep holes at Sites 453, 458, and 459 show that the thermal conductivity increases with depth, the rate of increase ranging from (0.18 mcal/cm s °C)/100 m at Site 459 to (0.72 mcal/cm s °C)/100 m at Site 456. A positive correlation between the sedimentary accumulation rate and the rate of thermal conductivity increase with depth indicates that both compaction and lithification are important factors. Drilled pillow basalts show nearly uniform thermal conductivity. At She 454 the thermal conductivity of one basaltic flow unit was higher near the center of the unit and lower toward the margin, reflecting variable vesicularity. Hydrothermally altered basalts at Site 456 showed higher thermal conductivity than fresh basalt because secondary calcite, quartz, and pyrite are generally more thermally conductive than fresh basalt. The average thermal conductivity in the top 50 meters of sediments correlates inversely with water depth because of dissolution of calcite, a mineral with high thermal conductivity, from the sediments as the water depth exceeds the lysocline and the carbonate compensation depth. Differences between the Mariana Trench data and the Mariana Basin and Trough data may reflect different abundances of terrigenous material in the sediment. There are remarkable correlations between thermal conductivity and other physical properties. The relationship between thermal conductivity and compressional wave velocity can be used to infer the ocean crustal thermal conductivity from the seismic velocity structure.

(Table 2) Mineralogy, stable isotopes, and mineral percentages of the carbonate fraction of samples from the South China Sea

Chemoherm carbonates, as well as numerous other types of methane seep carbonates, were discovered in 2004 along the passive margin of the northern South China Sea. Lithologically, the carbonates are micritic containing peloids, clasts and clam fragments. Some are highly brecciated with aragonite layers of varying thicknesses lining fractures and voids. Dissolution and replacement is common. Mineralogically, the carbonates are dominated by high magnesium calcites (HMC) and aragonite. Some HMCs with MgCO3 contents of between 30-38 mol%-extreme-HMC, occur in association with minor amounts of dolomite. All of the carbonates are strongly depleted in d13C, with a range from -35.7 to -57.5 per mil PDB and enriched in d18O (+ 4.0 to + 5.3 per mil PDB). Abundant microbial rods and filaments were recognized within the carbonate matrix as well as aragonite cements, likely fossils of chemosynthetic microbes involved in carbonate formation. The microbial structures are intimately associated with mineral grains. Some carbonate mineral grains resemble microbes. The isotope characteristics, the fabrics, the microbial structure, and the mineralogies are diagnostic of carbonates derived from anaerobic oxidation of methane mediated by microbes. From the succession of HMCs, extreme-HMC, and dolomite in layered tubular carbonates, combined with the presence of microbial structure and diagenetic fabric, we suggest that extreme-HMC may eventually transform into dolomites. Our results add to the worldwide record of seep carbonates and establish for the first time the exact locations and seafloor morphology where such carbonates formed in the South China Sea. Characteristics of the complex fabric demonstrate how seep carbonates may be used as archives recording multiple fluid regimes, dissolution, and early transformation events.