Jane S. Tribble

Christmas Island lagoonal lakes, models for the deposition of carbonate{evaporite{organic laminated sediments

The atoll of Christmas Island (now known as Kiritimati) in the Kiribati Republic (Central Pacific) lies at about 28N in the intertropical convergence zone. Much of the surface area of the atoll (ca. 360 km 2 ) is occupied by numerous lakes in which carbonate, evaporite (calcium sulfate, halite) and organic layers are deposited. Observations suggest that deposition of these different laminae is controlled by climatic and biologic factors. It is thought that periodic climatic variations, such as El Nino- Southern Oscillations (ENSO) events which bring heavy rainfall to the atoll, result in the succession of the precipitation of carbonate minerals (during periods after dilution of hypersaline waters by heavy rains), followed by evaporitic minerals (carbonate, calcium sulfate, halite) when salinity increases through evaporation. Thick (up to 5 cm) microbial (essentially cyanobacterial) mats develop continuously on the lake bottom surfaces providing the sediment with an important (total organic carbon 2-5%) organic contribution in the form of an internal, geometrically structured, network in which the authigenic minerals precipitate. The high bioproductivity of these microbial populations is reflected in low d 13 C values of sedimentary organic carbon (214 to 217‰), interpreted as being the result of high atmospheric CO2 demand (Geochim. Cosmochim. Acta, 56 (1992) 335). The well-laminated organic layers present in the sediment profile result from the death and burial of microbial populations at the time of severe climatic events (storms, heavy rainfall). These lagoonal lakes provide a model for the deposition of carbonate and organic matter in an evaporitic environment. The high ratio of deposited carbonate vs. sulfate 1 chloride, when compared to low ratio in evaporitic salinas, results from both a lack of limitation of calcium, magnesium and carbonate ions (in a carbonate reef environment) and active processes of high-Mg calcite precipitation (organomineralization). q 2001 Elsevier Science B.V. All rights reserved.

Crystal chemistry, and thermodynamic and kinetic properties of calcite, dolomite, apatite, and biogenic silica: applications to petrologic problems

Abstract Sedimentary minerals are generally metastable phases that, given time and changing environmental conditions, recrystallize to more stable phases. The actual pathway of stabilization is governed by a host of kinetic factors. Unfortunately, much of the theoretical and experimental work on thermodynamic and kinetic behavior of sedimentary minerals either has not reached field practitioners in sedimentary petrology, or has been conducted under conditions that are difficult to extrapolate to natural sedimentary environments. In this paper we review and present new data on the basic crystal chemistry, thermodynamic and kinetic properties of calcite, dolomite, apatite, and biogenic silica, and discuss the relevance of these data to the solution of geological and geochemical problems. The crystal chemistry and structure of a given magnesian calcite exert a fundamental control on its solubility and solid solution behavior, and this control can be seen most clearly through comparison of synthetic and biogenic phases. Thus, variations in crystal chemistry and structure, through solubility control during diagenesis, yield a range of possible stabilization pathways, whose documentation is the domain of much field-based study. Experimental work involving dolomite has focused on delineation of phase relations in dry and aqueous systems at moderate to high temperatures, determination of reaction pathways followed during dolomitization of calcium carbonate, and measurement of reaction rate. Uncertainties reside in the relevance of these data to the classic problem of low-temperature dolomite formation. We suggest that the effort must now focus on designing experimental systems that effectively mimic natural environments, and yield reaction rate data as a function of temperature and solution composition. Such an example is presented. A primary goal in experimental work involving carbonate fluorapatite has been an understanding of the mechanism of formation of this mineral. We review the state of this knowledge, and also present the results of ongoing dissolution rate experiments. The importance of this work is that it bears directly on the understanding of the role carbonate fluorapatite plays in the biogeochemical cycle of P in the oceans. Many factors influence the solubilities of opaline silica and the silica polymorphs, and solubility plays an important role in controlling silica diagenesis. A model is presented that relates changes in sediment properties including density and acoustic velocity to the stages of silica diagenesis. The model is applied to sediments of Deep Sea Drilling Project Leg 63. The discussions of the sedimentary phases calcite, dolomite, apatite, and biogenic silica in this paper point to several directions for future experimental research on sedimentary mineral-solution reactions. These include emphasis on: (1) experimental studies of synthetic sedimentary mineral-solution reactions to form a foundation for an understanding of natural mineral-solution reactions; (2) experimental investigations of mineral reactions in aqueous solutions under conditions of composition, temperature, and pressure similar to the natural conditions of mineral formation; and (3) studies of the surface (as opposed to bulk) properties of sedimentary minerals in aqueous solutions and their role in reactions governing precipitation and dissolution of sedimentary phases.

Recrystallization of Magnesian Calcite Overgrowths on Calcite SeedsSuspended in Seawater

The formation and subsequent reactions of magnesiancalcite overgrowths on calcite were investigated bymeans of closed system seeded precipitationexperiments. These experiments demonstrated that(1) thin overgrowths of magnesian calcite are precipitatedon calcite seeds suspended in seawater;(2) the solubilities of the coatings increase outward from theseed crystals as a linear function of the log ofrelative coating thickness;(3) during the period ofthese experiments (up to 5.5 months), the magnesiancalcite coatings continued to increase in thickness,but became less soluble in composition. Thestabilization reaction, referred to asrecrystallization, can be described by the followingequation:Cax Mg(1-x) CO3 + [z + y(x + z)]Ca2++ 2yHCO3- ]= (1 + y)Ca(x + z)Mg(1 - x -z)CO3 + [z + y(x + z - 1)]Mg2++ y CO2 + yH20;]4) recrystallization rate is dependent on solutionsaturation state, with a reaction order of 3.2 forartificial seawater and 4.0 for natural seawater; and(5) by the cessation of the closed system experiments,overgrowth compositions approached that of the stablecalcite (a few mol % MgCO3).Armoring of suspended carbonate particles in thesurface oceans with magnesian calcite overgrowthswould provide an effective barrier to release of theoceanic supersaturation with respect to calcite. Thicknesses of such coatings would be limited by therecrystallization rate of the magnesian calcite. Estimates based on the recrystallization ratesdetermined in this work indicate coatings on the orderof 0.02 µm in thickness could form on particles asthey sink through the mixed layer. According to thesecalculations, the total amount of carbon precipitatedannually in magnesian calcite overgrowths iscomparable to the riverine flux of dissolved carbon tothe oceans. Field observations of severalinvestigators indicate the likely presence ofmagnesian calcite coatings on planktonic particles,and provide evidence for possible recrystallization ofbiogenic magnesian particles in the marineenvironment.

Origin of smectite and illite-smectite in the Barbados accretionary complex: Oxygen isotopic evidence

Oxygen isotopic and mixed-layer compositional data are used to infer modes of formation of smectite and illite-smectite from the Barbados accretionary complex. Nearly pure smectites have δ 18 O values between 26‰ and 30‰ and apparently formed authigenically by alteration of volcanic ash. Illite-smectites are thought to have formed in situ at temperatures less than 40 δC, although a detrital origin cannot be ruled out for several samples. The unusually low temperature of transition of smectite to illite indicates that fluid generation by mineral dehydration and consequent structural effects can occur at shallow depths in accretionary complexes.

Irregular trends of physical properties in homogeneous clay-rich sediments of DSDP Leg 87 Hole 584, midslope terrace in the Japan Trench

Abstract Compressional velocity ( V p ), attenuation (in terms of its inverse, the compressional quality factor Q p ), electrical resistivity (σ), bulk ( ρ b ) and grain ( ρ g ) densities, porosity (o), and bulk mineralogy were measured on diatomaceous hemipelagic sediments from DSDP Leg 87, Hole 584. Although the sediment lithology and the bulk mineralogy determined by X-ray diffraction are relatively homogeneous, the physical, acoustic and electrical properties of the sediments show irregular trends with depth, with variations in the magnitude and sign of property gradients. Between subbottom depths of 500 and 800 m, V p , ρ b , ρ g , and σ all sharply increase and then sharply decrease, while porosity sharply decreases, and then sharply increases. The changes in gradients are due to abrupt changes in the opal-A content of the sediment, as determined from smear-slide data. Q p decreases with depth to about 650 m and then increases. Such a trend has not been previously documented in laboratory measurements. However, it is assumed that the “attenuation maximum” observed here is due to the increased grain density and decreased porosity caused by the reduction in opal-A content around this depth. The anisotropies for V p and Q p undergo sign inversions with depth. The V p anisotropies change from positive ( V ph > V pv ) at the top to negative at the bottom of the hole, whereas Q p anisotropies change from negative ( Q pv > Q ph ) to positive. The change in sign with depth is caused by a change in sediment bedding dip from near 0° at the top of the hole to about 65° toward the bottom.

Preferred Orientation and Velocity Anisotropy in Marine Clay-Bearing Calcareous Sediments

In order to better understand the role of preferred orientation of calcite in the compressional velocity (V p ) anisotropy of calcareous marine sediments, ultrasonic V p and x-ray pole figure goniometry measurements were made on selected laminated calcareous claystones, laminated clay-bearing limestones, and nonlaminated limestones from various DSDP sites. Although all samples exhibit V p anisotropy (up to 20%), none exhibit calcite-preferred orientation. Thus, V p anisotropy in these calcareous sediments is not caused by calcite-preferred orientation, in agreement with findings of other researchers. Pole figures and thin section observations of the laminated carbonate samples indicate that poles to (001) of kaolinite and illite are strongly aligned normal to bedding. Clay-preferred orientation is probably responsible for some of the observed V p anisotropy. The V p anisotropy in calcareous claystones is found to be correlated to calcite content, in contrast to the relation found by others for pelagic chalks and limestones, suggesting a dependence upon lithology. Most of the anisotropy in laminated calcareous claystones appears to be controlled by flat pores oriented parallel to bedding, which could slow acoustic waves traveling perpendicular to bedding. Pelagic chalks and limestones tend to have irregularly shaped pores that do not affect anisotropy in the same way as in calcareous claystones.

Clay mineral and zeolite diagenesis in the Toa Baja Well, Puerto Rico

Several diagenetic reactions typical of volcaniclastic sediments have been identified in the Toa Baja well. The zeolites heulandite and (less commonly) analcite, present in the upper half of the drilled section, give way to laumontite at depth. A decrease with depth in the expandability of mixed layer chlorite/smectite was observed. Corrensite was detected over the depth range 1,676-2,286 m. Temperatures implied by the depth ranges of diagenetic and indicator minerals are consistent with a paleothermal gradient of about 30-50C/km, assuming little or no erosion of section.

Physical Property Changes Accompanying Deep Burial of Clay-Rich Sediments, Barbados Convergent Margin

Clay-rich sediments undergo a variety of compositional and physical changes related to the processes of compaction and lithification during transformation from mud to shale. Well-documented aspects of the lithification process include the chemical and mineralogical alteration of mud as it undergoes deep burial diagenesis (e.g., Boles and Franks, 1979 and references therein). In addition, curves describing changes in physical properties such as bulk density, porosity, and acoustic velocity have been derived for various stages of compaction and lithification of mud-rich sediments (e.g., Hamilton, 1978; Hamilton and Bachman, 1982). Changes in sediment fabric during the early stages of compaction have been documented using SEM and TEM techniques (e.g., Bowles et al., 1969; Bennett et al., 1977; Bohlke and Bennett, 1980; Bryant and Bennett, 1988), but there have been few similar studies of deep burial of clay-rich sediments.