Calcium concentrations and isotopic composition in carbonates and porefluids from ODP Hole 130-807A

The calcium isotopic compositions (d44Ca) of 30 high-purity nannofossil ooze and chalk and 7 pore fluid samples from ODP Site 807A (Ontong Java Plateau) are used in conjunction with numerical models to determine the equilibrium calcium isotope fractionation factor (a_s-f) between calcite and dissolved Ca2+ and the rates of post-depositional recrystallization in deep sea carbonate ooze. The value of a_s-f at equilibrium in the marine sedimentary section is 1.0000+/-0.0001, which is significantly different from the value (0.9987+/-0.0002) found in laboratory experiments of calcite precipitation and in the formation of biogenic calcite in the surface ocean. We hypothesize that this fractionation factor is relevant to calcite precipitation in any system at equilibrium and that this equilibrium fractionation factor has implications for the mechanisms responsible for Ca isotope fractionation during calcite precipitation. We describe a steady state model that offers a unified framework for explaining Ca isotope fractionation across the observed precipitation rate range of ~14 orders of magnitude. The model attributes Ca isotope fractionation to the relative balance between the attachment and detachment fluxes at the calcite crystal surface. This model represents our hypothesis for the mechanism responsible for isotope fractionation during calcite precipitation. The Ca isotope data provide evidence that the bulk rate of calcite recrystallization in freshly-deposited carbonate ooze is 30-40%/Myr, and decreases with age to about 2%/Myr in 2-3 million year old sediment. The recrystallization rates determined from Ca isotopes for Pleistocene sediments are higher than those previously inferred from pore fluid Sr concentration and are consistent with rates derived for Late Pleistocene siliciclastic sediments using uranium isotopes. Combining our results for the equilibrium fractionation factor and recrystallization rates, we evaluate the effect of diagenesis on the Ca isotopic composition of marine carbonates at Site 807A. Since calcite precipitation rates in the sedimentary column are many orders of magnitude slower than laboratory experiments and the pore fluids are only slightly oversaturated with respect to calcite, the isotopic composition of diagenetic calcite is likely to reflect equilibrium precipitation. Accordingly, diagenesis produces a maximum shift in d44Ca of +0.15? for Site 807A sediments but will have a larger impact where sedimentation rates are low, seawater circulates through the sediment pile, or there are prolonged depositional hiatuses.

Element concentrations, and Sr and Os isotope ratios of DSDP Leg 92 metalliferous carbonates

The Os concentration and isotopic composition of metalliferous carbonates deposited on the East Pacific Rise over the past 28 Ma are reported with complimentary Sr isotope data. Variations in the Os isotopic composition of these samples are interpreted as a record of past changes in the Os isotopic composition of seawater. These results are consistent with isotopic analyses of leachable Os in pelagic clays which have also been interpreted as a record of the 187Os/186Os ratio of seawater through time (Pegram et al., 1992, doi:10.1016/0012-821X(92)90132-F). The metalliferous carbonate record clearly shows that seawater Os and Sr isotope systems are partially decoupled from one another over the past 28 Ma. Accelerated weathering of ancient organic-rich sediments is suggested as a possible mechanism to account for this decoupling and the rapid increase in the 187Os//186Os ratio of seawater over the past 15 Ma. This rapid increase suggests that the seawater Os record can potentially be used as a stratigraphic tool in some Neogene marine deposits.

Summary of shipboard physical properties of mudstones, carbonates, and clastic at DSDP Holes 72-515B and 72-516F

Compressional-wave velocity, wet-bulk density, and porosity were measured on sediments and rocks recovered from Deep Sea Drilling Project Holes 515B and 516F. Wet-bulk densities were measured by both gravimetric and GRAPE methods. Velocities were measured on trimmed samples with the Hamilton frame velocimeter. The shipboard measurement techniques are discussed in the explanatory notes chapter (Coulbourn, this volume) and are described in detail by Boyce (1976a). Only the shipboard measurements are reported here.

Sedimentology and origin of lower Cretaceous pelagic carbonates and redeposited clastics at DSDP Hole 76-534A

Drilling at Site 534 in the Blake-Bahama Basin recovered 268 m of Lower Cretaceous, Berriasian to Hauterivian, pelagic carbonates, together with volumetrically minor intercalations of claystone, black shales, and terrigenous and calcareous elastics. Radiolarian nannofossil pelagic carbonates accumulated in water depths of about 3300 to 3650 m, below the ACD (aragonite compensation depth) but close to the CCD (calcite compensation depth). Radiolarian abundance points to a relatively fertile ocean. In the Hauterivian and Barremian, during times of warm, humid climate and rising sea level, turbiditic influxes of both terrigenous and calcareous sediments, and minor debris flows were derived from the adjacent Blake Plateau. The claystones and black shales accumulated on the continental rise, then were redeposited onto the abyssal plain by turbidity currents. Dark organic-rich and pale organic-poor couplets are attributed to climatic variations on land, which controlled the input of terrigenous organic matter. Highly persistent, fine, parallel lamination in the pelagic chalks is explained by repeated algal "blooms." During early diagenesis, organic-poor carbonates remained oxygenated and were cemented early, whereas organic-rich intervals, devoid of burrowing organisms, continued to compact later in diagenesis. Interstitial dissolved-oxygen levels fluctuated repeatedly, but bottom waters were never static nor anoxic. The central western Atlantic in the Lower Cretaceous was thus a relatively fertile and wellmixed ocean basin.

Neptunian dikes and their fillings in carbonates of the Warstein area (northeastern Rhenish massif)

Neptunian dikes and cavities as weil as their fillings are described from Middle to Upper Devonian carbonates of the Warstein area. The genesis of the pre-Upper Carboniferous dikes is due to pre-orogenic synsedimentary tensional movements. Lifting, subsidence and tilting caused joints and cracks, which are enlarged to dikes and cavities on submarine conditions. The post-Upper Carboniferous dikes are based on the orogenesis during Upper Carboniferous time, causing numerous tectonical divisional planes in the sediments. Along these planes a far-reaching karstification took place since mesozoic time. According to their size the cavities are subdivided into macro-, mega- and microdikes. With the exception of one macrodike all the others are limited to the massive limestone. Megadikes especially occur in Upper Devonian cephalopod limestone and in the Erdbach limestone, microdikes can be found in all carbonatic rocks. The dikes follow pre-orogenic, tectonical and sedimentary divisional planes and are orientated to ac-, bc- as well as bedding planes and diagonal directions. The fillings happened down from above either in a solitary event or repeatedly in long-lived dikes during a span of several ten millions of years. More seldom the fillings took place laterally or upside from beneath. The dikes contain - without regard to autochthonous conodont faunas - older and/or younger mixed faunas, too. Occasionally they were used as life district by a trilobite fauna adapted to the dikes. The dikes represent sedimentary pitfalls and conserve sediments eroded in other places. Therefore, by aid of the fillings, it can be demonstrated, that stratigraphic gaps are not absolutely due to primary interruptions of sedimentation, but were caused by reworking. Some dikes contain the distal offsets of slides and suspension streams. Relations between condensation and development of dikes could not be derived in the Warstein area. However, an increase of the frequency of dikes towards east to the eastern margin of the Warstein carbonate platform could be pointed out. This margin is a slope, persisting more than 10 millions of years, between a block and a basin. Evidently cracks and dikes, which were caused by settlements, slides and earth quakes, occured there frequently. The Warstein dikes and cavities, caused by karstification, are filled with terrestrial Lower Cretaceous, marine Upper Cretaceous and terrestrial Pleistocene to Holocene sediments. Tertiary sediments could not be detected.

Mineral and chemical compositions of authigenic carbonates from the northern Deryugin Basin, Sea of Okhotsk

Mineral and chemical compositions of authigenic carbonates are studied by several methods in a sediment core collected in the axial zone of the Deryugin riftogenic basin. Manganese carbonates (kutnahorite, rhodochrosite) associated with manganiferous calcite, manganiferous pyrite, and nontronite are firstly identified in the Sea of Okhotsk. Manganese carbonates in Holocene diatomaceous ooze were presumably formed due to diagenetic transformation of sedimentary manganese hydroxides, organic matter, and biogenic silica, while those found in the underlying turbidites precipitated owing to the intermittent influx of endogenic fluids migrating along sand interbeds.

(Table 1) Geochemistry of ODP Hole 130-807C carbonates across the K/T boundary

Paleontological, stable isotopic, trace elemental abundance, and magnetostratigraphic studies have been performed on limestones spanning the Cretaceous/Tertiary boundary transition at Ocean Drilling Program (ODP) Hole 807C. Paleontological evidence exists for considerable resedimentation, which we attribute to the fact that Hole 807C is located in a basement graben. Age estimates based on planktonic foraminiferal biostratigraphy, as well as magnetostratigraphy, indicate that sedimentation rates could have been on the order of 12-14 m/m.y. This is significantly higher than those documented in other important Deep Sea Drilling Project (DSDP) and ODP Cretaceous/Tertiary boundary sections using the same age control points (e.g., DSDP Hole 577 and ODP Hole 690B), although not as high as those documented from DSDP Hole 524. The expanded nature of this succession has resulted in the Cretaceous/Tertiary boundary d13C decrease occurring over approximately a 9-m interval. Ir analysis of these sediments do not show a single large anomaly, as has been found in other Cretaceous/Tertiary boundary sections, but trivial background levels instead. Ce data support the hypothesis that this section has been expanded by secondary sedimentological processes.

Stable carbon and oxygen isotope record of diagenetic carbonates from the New Jersey shelf

Carbon cycling is an important but poorly understood process on passive continental margins. In this study, we use the ionic and stable isotopic composition of interstitial waters and the petrology, mineralogy, and stable isotopic composition of authigenic carbonates collected from Ocean Drilling Program (ODP) Leg 174A (Sites 1071 and 1072) to constrain the origin of the carbonates and the evolution of methane on the outer New Jersey shelf. The pore fluids of the New Jersey continental shelf are characterized by (1) a fresh-brackish water plume, and (2) organic matter degradation reactions, which proceed through sulfate reduction. However, only minor methanogenesis occurs. The oxygen isotopic composition of the pore fluids supports a meteoric origin of the low salinity fluids. Authigenic carbonates are found in nodules, thin (~1-cm) layers, and carbonate cemented pavements. Siderite is the most common authigenic carbonate, followed by dolomite and calcite. The oxygen isotopic composition of the authigenic carbonates, i.e. 1.3-6.5 per mil PeeDee Belemnite (PDB), indicates an origin in marine pore fluids. The carbon isotopic composition of dolomite cements range from -16.4 to -8.8 per mil PDB, consistent with formation within the zone of sulfate reduction. Siderite d13C values show a greater range (-17.67-16.4 per mil), but are largely positive (mean=2.8 per mil) and are interpreted to have formed throughout the zone of methanogenesis. In contrast, calcite d13C values are highly negative (as low as -41.7 per mil)and must have formed from waters with a large component of dissolved inorganic carbon derived from methane oxidation. Pore water data show that despite complete sulfate reduction, methanogenesis appears not to be an important process presently occurring in the upper 400 m of the outer New Jersey shelf. In contrast, the carbon isotopic composition of the siderites and calcites document an active methanogenic zone during their formation. The methane may have been either oxidized or vented from shelf sediments, perhaps during sea-level fluctuations. If this unaccounted and variable methane flux is an areally important process during Neogene sea-level fluctuations, then it likely plays an important role in long-term carbon cycling on passive continental margins

Geochemical composition and atomic formulas of saponites, celadonites and carbonates from ODP Leg 168 sites

With this study, we investigate the mineralogical variations associated with the low-temperature (<100°C) alteration of normal tholeiitic pillow basalts varying in age from 0.8 to 3.5 Ma. Their alteration intensity varies systematically and is related to several factors, including (1) the aging of the igneous crust, (2) the increase of temperatures from the younger to the older sites, measured at the sediment/basement interface, (3) the local and regional variations in lithology and primary porosity, and (4) the degree of pillow fracturing. Fractures represent the most important pathways that allow significant penetration of fluids into the rock and are virtually the only factor controlling the alteration of the glassy rim and the early stages of pillow alteration. Three different alteration stages have been recognized: alteration of glassy margin, oxidizing alteration through fluid circulation in fracture systems, and reducing alteration through diffusion. All the observed mineralogical and chemical variations occurring during the early stages of alteration are interpreted as the result of the rock interaction with "normal," alkaline, and oxidizing seawater, along preferential pathways represented by the concentric and radial crack systems. The chemical composition of the fluid progressively evolves while moving into the basalt, leading to a reducing alteration stage, which is initially responsible for the precipitation of Fe-rich saponite and minor sulfides and subsequently for the widespread formation of carbonates. At the same time, the system evolved from being "water dominated" to being "rock dominated." No alteration effects in pillow basalts were observed that must have occurred at temperatures higher than those measured during Leg 168 at the basement/sediment interface (e.g., between 15° and 64°C).

Chemical and isotopic compositions and age of carbonates from the Lost City hydrothermal field, Mid-Atlantic Ridge

Two genetically different types of authigenic carbonate mounds are studied: (1) from an active hydrothermal field related to serpentinite protrusions in a zone of intersection of a transform fracture zone with the Mid-Atlantic Ridge, (2) from an active field of methane seepings in the Dnieper canyon of the Black sea. General geochemical conditions, under which authigenic carbonate formation occurs within these two fields, were found. They include: presence of reduced H2S, H2, and CH4 gases at absence of free oxygen; high alkalinity of waters participating in carbonate formation; similarity of textural and structural features of authigenic aragonite, which represents the initial carbonate mineral of the mounds; paragenesis of aragonite with sulfide minerals; close relation of carbonate mounds with communities of sulfate-reducing and methane-oxidizing microorganisms. A new mechanism of formation of hydrothermal authigenic carbonates is suggested. It implies their microbial sulfate reduction over hydrogen from fluid in the subsurface mixing zone of hydrothermal solution and adjacent seawater.