Mineralogy and interstitial-water chemistry of ODP Leg 101 sites

Concentrations of dissolved Ca2+, Sr2+, Mg2+, SO4[2-], and alkalinity were measured in pore waters squeezed from sediments taken from ODP Holes 626C and 626D in the Florida Straits; Holes 627A and 627B, 628A, and 630A and 630C north of Little Bahama Bank; Holes 631 A, 632A and 632B, and 633A in Exuma Sound; and Holes 634A and 635A and 635B in Northeast Providence Channel. These data are compared with the mineralogy and strontium content of the sediments from which the waters were squeezed. Contrasts in the geochemical profiles suggest that significantly different processes govern pore-water signatures at each group of sites. In Little Bahama Bank, strong positive Ca2+ gradients are correlated with weak negative Mg2+ profiles. These trends are analogous to those seen at DSDP sites where carbonate deposits immediately overlie mafic basement, but as the depth to basement may be in excess of 5000 m, we suggest that diffusion gradients are initiated by an underlying sedimentary unit. In contrast, Ca2+ and Mg2+ gradients in Exuma Sound are not developed to any appreciable extent over similar thicknesses of sediment. We suggest that the pore-water chemistry in these deposits is principally controlled by diagenetic reactions occurring within each sequence. The location and extent of carbonate diagenesis can be estimated from dissolved Sr2+ profiles. In Little Bahama Bank and Exuma Sound, Sr2+ concentrations reach a maximum value of between 700 and 1000 µmol/L. Although the depths at which these concentrations are achieved are different for the two areas, the corresponding age of the sediment at the dissolved Sr2+ maximum is similar. Consequently, the diffusive flux of Sr2+ and the calculated rates of recrystallization in the two areas are likewise of a similar magnitude. The rates of recrystallization we calculate are lower than those found in some DSDP pelagic sites. As the waters throughout most of the holes are saturated with respect to SrSO4, celestite precipitation may cause erroneously low Sr2+ production rates and, consequently, low calculated rates of recrystallization. We therefore encourage only the discriminate use of Sr2+ profiles in the quantification of diagenetic processes.

Sediment composition in NE Greenland Fjords, Shelf, and Slope

In order to document changes in Holocene glacier extent and activity in NE Greenland (~73° N) we study marine sediment records that extend from the fjords (PS2631 and PS2640), across the shelf (PS2623 and PS2641), to the Greenland Sea (JM07-174GC). The primary bedrock geology of the source areas is the Caledonian sediment outcrop, including Devonian red beds, plus early Neoproterozoic gneisses and early Tertiary volcanics. We examine the variations in colour (CIE*), grain size, and bulk mineralogy (from X-ray diffraction of the <2 mm sediment fraction). Fjord core PS2640 in Sofia Sund, with a marked red hue, is distinct in grain size, colour and mineralogy from the other fjord and shelf cores. Five distinct grain-size modes are distinguished of which only one is associated with a coarse ice-rafting signal - this mode is rare in the mid- and late Holocene. A sediment unmixing program (SedUnMixMC) is used to characterize down-core changes in sediment composition based on the upper late Holocene sediments from cores PS2640 (Sofia Sund), PS2631 (Kaiser Franz Joseph Fjord) and PS2623 (south of Shannon Is), and surface samples from the Kara Sea (as an indicator of transport from the Russian Arctic shelves). Major changes in mineral composition are noted in all cores with possible coeval shifts centred c. 2.5, 4.5 and 7.5 cal. ka BP (±0.5 ka) but are rarely linked with changes in the grain-size spectra. Coarse IRD (>2 mm) and IRD-grain-size spectra are rare in the last 9-10 cal. ka BP and, in contrast with areas farther south (~68° N), there is no distinct IRD signal at the onset of neoglaciation. Our paper demonstrates the importance of the quantitative analysis of sediment properties in clarifying source to sink changes in glacial marine environments.

Organic geochemistry and sedimentology of lower to mid-Cretaceous at DSDP Leg 77 Holes

Analyses of 40 carbonate core samples - 27 from Site 535, 12 from Site 540, and 1 from Site 538A - have confirmed many of the findings of the Shipboard Scientific Party. The samples, all but one Early to mid-Cretaceous in age (Berriasian to Cenomanian), reflect sequences of cyclically anoxic and oxic depositional environments. They are moderately to very dark colored, dominantly planar-parallel, laminated lime mudstones. Most show the effects of intense mechanical compaction. Visual kerogen characteristics and conventional Rock-Eval parameters indicate that these deep basinal carbonates contain varying mixtures of thermally immature kerogen derived from both marine and terrigenous precursors. However, variations in kerogen chemistry are evident upon analysis of the pyrolysis mass spectral data in conjunction with the other geochemical analyses. Particularly diagnostic is the reduction index, Rl, a measure of H2S produced during pyrolysis. Total organic carbon, TOC, ranges from 0.6 to 6.6%, with an overall average of 2.4%. Average TOCs for these fine-grained mudstones are: late Eocene 2.5% (1 sample), Cenomanian 2.2% (6), Albian 2.0% (10), Aptian 1.3% (1), Barremian-Hauterivian 2.8% (11), late Valanginian 4.8% (3), Berriasian-early Valanginian 1.6% (7). Most of the carbonates have source-potential ratings of fair to very good of predominantly oil-prone to mixed kerogen, with only a few gas-prone samples. The ratings correlate well with the inferred depositional environments, i.e., whether oxic or anoxic. Several new organic-geochemical parameters, especially Rl, based on pyrolysis mass spectrometry of powdered whole-rock samples, support this view. Tar from fractures in laminated to bioturbated limestones of Unit IV (late Valanginian) at 535-58-4, 19-20 cm (530 m sub-bottom) appears to be mature, biodegraded, and of migrated rather than on site indigenous origin.

Stable carbon and oxygen isotope ratios of carbonates of ODP Site 103-639

A Tithonian sequence of shallow-water limestones, intercalated with siliciclastics and overlain by dolomite, was recovered during drilling at ODP Site 639 on the edge of a tilted fault block. The carbonates were strongly affected by fracturing, dolomitization, dedolomitization, and compaction. The chronology and nature of the fractures, fracture infilling, and diagenesis of the host rock are established and correlated for both the limestone and the dolomite. A first phase of dolomitization affected limestone that was already, at least partially, indurated. In the limestone unit, fractures were filled by calcite and dolomite; most of the dolomite was recrystallized into calcite, except for the upper part. In the dolomitic unit, the first-formed dolomite was progressively recrystallized into saddle dolomite, as fractures were simultaneously activated. The dolomitic textures become less magnesian (the molar ratio mMg/mCa goes from 1.04-0.98 to 0.80), and the d18O (PDB) ranges from -10 per mil to -8 per mil. The varying pores and fissures are either cemented by a calcic saddle dolomite (mMg/mCa ranging from 0.95 to 0.80) or filled with diverse internal sediments of detrital calcic dolomite, consisting of detrital dolomite silt (d18O from -9 per mil to -7 per mil) and laminated yellow filling (with different d18O values that range from -4 per mil to +3 per mil). These internal sediments clearly contain elements of the host rock and fragments of saddle crystals. They are covered by marls with calpionellids of early Valanginian age, which permits dating of most of the diagenetic phases as pre-Valanginian. The dolomitization appears to be related to fracturing resulting from extensional tectonics; it is also partially related to an erosional episode. Two models of dolomitization can be proposed from the petrographic characteristics and isotopic data. Early replacement of aragonite bioclasts by sparite, dissolution linked to dolomitization, and negative d18O values of dolomite suggest a freshwater influence and 'mixing zone' model. On the other hand, the significant presence of saddle dolomite and repeated negative d18O values suggest a temperature effect; because we can dismiss deep burial, hydrothermal formation of dolomite would be the most probable model. For both of these hypotheses, the vadose filling of cavities and fractures by silt suggests emersion, and the different, and even positive, d18O values of the last-formed yellow internal sediment could suggest dolomitization of the top of the sequence under saline to hypersaline conditions. Fracturing resulting in the reopening of porosity and the draining of dolomitizing fluids was linked to extensional tectonics prior to the tilting of the block. These features indicate an earlier beginning to the rifting of the Iberian margin than previously known. Dolomitization, emersion, and erosion correspond to eustatic sea-level lowering at the Berriasian/Valanginian boundary. Diagenesis, rather than sedimentation, seems to mark this global event and to provide a record of the regional tectonic history.

Isotopic survey of diagenetic carbonates, at DSDP Leg 63 Holes

Authigenic carbonates, principally calcium-rich dolomites, with extremely variable isotopic compositions were recovered in organic-rich marine sediments during Leg 63 drilling off southern California and Baja California. These carbonates occur as thin layers in fine-grained, diatomaceous sediments and siliceous rocks, mostly deposited during the Neogene. A combination of textural, geochemical, and isotopic evidence indicates these dolomites formed as cements and precipitates in shallow subsurface zones of high alkalinity spawned by abundant CO2 and methane production during progressive microbial decay of organic matter. Depths and approximate temperatures of formation estimated from oxygen isotopes are 87 to 658 meters and 10°C to 50°C, respectively. Within any sedimentary section, dolomites may form simultaneously at several depths or at different times within the same interval. Highly variable carbon isotopes (-30 to +16 per mil) reflect the isotopic reservoir in which the carbonates formed. Oxidation of organic matter through microbial reduction of sulfate at shallow depths favors light-carbon carbonates such as those at Sites 468 and 471; heavy-carbon carbonates at Site 467 most likely formed below this zone where HC**12O3**- is preferentially removed by reduction of CO2 to methane during methanogenesis. An important controlling factor is the sedimentation rate, which dictates both the preservation of organic matter on the sea floor and depth distribution of subsurface zones of organic-matter decay.

Minerals of bulk samples and the fine-grained (<2-µm) fractions from DSDP Leg 67 Holes

Leg 67 sample sediments are mainly composed of biogenic carbonate and biogenic siliceous materials. Apart from calcite, crystallized minerals are scarce; however, they are better represented in sites near the continent and in all Quaternary sediments. These minerals are: quartz, feldspars, smectite, and, rarely, chlorite, dolomite, and zeolite.

Strontium isotope dating of ODP Leg 133 sediments

The strontium-isotope dating method, based on the strontium-isotope seawater curve, was used to date stratigraphic events recognized in carbonate sediments drilled during Leg 133 on the Queensland and Marion plateaus. The strontium isotope ages of these events are used to correlate paleoceanographic changes, delineated from oxygen isotope signals, and paleoenvironmental or facies changes recorded in the lithostratigraphy. Results indicate that a strong connection exists between prevailing paleoenvironmental conditions and the developmental style of a carbonate platform. Also, the strontium-isotope ages of discrete dolomite intervals within the sequences were determined, indicating that multiple dolomitization events took place and that a hydrodynamically driven process may be currently active within the modern carbonate platform.