Barbara Ransom

ORGANIC MATTER PRESERVATION ON CONTINENTAL SLOPES: IMPORTANCE OF MINERALOGY AND SURFACE AREA

Theoretical considerations, calculations, and data reported by Keil et al. (1994a) were used to assess the recent hypotheses that mineral surface roughness controls the specific surface area of continental margin sediments and that the matrix-linked organic carbon content of continental margin sediments occurs as a monolayer or ‘monolayer-equivalent’ coating of organic compounds on the surfaces of detrital minerals and other detrital nonorganic grains (Mayer, 1994). Results of our analysis indicate that it is not the surface roughness of the terrigenous detrital framework grains that controls the specific surface area of most continental margin sediments, even in the sand and silt fractions, but rather the presence of nonspherical, high surface area-to-volume particles, primarily clays but also oxy-hydroxides and ultra-structured nonorganic bioclasts such as diatom frustules. Analysis of the mineralogy, organic carbon content, surface area, and in situ microfabric of continental margin sediments off California in the Mendocino and San Luis Obispo areas and off the state of Washington indicate a strong correlation between the amount of matrix-linked organic matter present and the suite of clay minerals. Data indicate that organic carbon appears to be preferentially sequestered in smectite-rich sediments compared to those whose clay fractions are dominated by chlorite. We suggest that this association is a function of differences in the site density and chemistry of the clays and differences in their flocculation behavior. Our data also indicate that organic carbon preservation on the three California transects, all of which have a suboxic oxygen minimum zone that impinges on the sea floor, is not significantly influenced by differences in bottom water oxygen concentration; and carbon stable isotope data suggest no preferential preservation of continental organic matter over that of marine origin.

TEM study of in situ organic matter on continental margins: occurrence and the “monolayer” hypothesis

Abstract TEM photomicrographs show that organic matter in the continental margin sediments that we have examined is primarily patchy in distribution and occurs as: (1) discrete, discontinuous blebs and micro-blebs of differentiated and undifferentiated protoplasm; (2) bacterial cells and associated muco-polysaccharide networks and (3) localized smears generally associated with clay minerals and domain junctions in clay-rich flocs, not as thin uniform grain coatings or as infillings of nicks or etch pits on sediment grains with high degrees of surface roughness. This occurrence contradicts recent assertions that most organic matter in continental margins occurs as monolayer or “monolayer equivalent” coatings on mineral grains and inorganic bioclastic particles. Our results are based on TEM photomicrographs of samples from the northern California continental slope that fall within the “monolayer equivalent” envelope defined by Mayer (1994), marine snow from the overlying nepheloid layer, and sediments from Eckernforde Bay in the Baltic Sea. These samples were prepared specifically to preserve and image in situ sediment fabric and organic constituents. Textural evidence suggests that low permeabilities caused by the interaction between the organic, inorganic and biologic components of the clay size fraction play a major role in determining organic matter preservation on continental margins.

Stable Cl isotopes in subduction-zone pore waters: Implications for fluid-rock reactions and the cycling of chlorine

Stable Cl isotope ratios, measured in marine pore waters associated with the Barbados and Nankai subduction zones, extend significantly (to ∼−8‰) the range of δ 37 Cl values reported for natural waters. These relatively large negative values, together with geologic and chemical evidence from Barbados and Nankai and recent laboratory data showing that hydrous silicate minerals (i.e., those with structural OH sites) are enriched up to 7.5‰ in 37 Cl relative to seawater, strongly suggest that the isotopic composition of Cl in pore waters from subduction zones reflects diagenetic and metamorphic dehydration and transformation reactions. These reactions involve clays and/or other hydrous silicate phases at depth in the fluid source regions. Chlorine therefore cannot be considered geochemically conservative in these systems. The uptake of Cl by hydrous phases provides a mechanism by which Cl can be cycled into the mantle through subduction zones. Thus, stable Cl isotopes should help in determining the extent to which Cl and companion excess volatiles like H 2 O and CO 2 cycle between the crust and mantle.

PRECISE DETERMINATION OF STABLE CHLORINE ISOTOPIC RATIOS IN LOW-CONCENTRATION NATURAL SAMPLES

Investigation of stable chlorine isotopes in geological materials has been hindered by large sample requirements and/or lack of analytical precision. Here we describe precise methods for the extraction, isolation, and isotopic analysis of low levels of chlorine in both silicate and aerosol samples. Our standard procedure uses 2 μg of Cl for each isotopic analysis. External reproducibility (1 σ) is 0.25%. for the 37Cl35Cl measurements. Chlorine is extracted from silicate samples (typically containing at least 20 μg of Cl) via pyrohydrolysis using induction heating and water vapor as the carrier, and the volatilized chlorine is condensed in aqueous solution. Atmospheric aerosols collected on filters are simply dissolved in water. Prior to isotopic measurement, removal of high levels of SO42−, F−, and organic compounds is necessary for the production of stable ion beams. Sulfate is removed by BaSCO4 precipitation, F− by CaF2 precipitation, and organic compounds are extracted with activated carbon. Chlorine is converted to stoichiometric CsCl by cation exchange, and isotopic ratios are determined by thermal ionization mass spectrometry of Cs2Cl+. We demonstrate that the sensitivity and precision of this method allow resolution of natural variations in chlorine isotopic composition, and thereby provide insight to some fundamental aspects of chlorine geochemistry.

Porosity corrections for smectite-rich sediments: Impact on studies of compaction, fluid generation, and tectonic history

Porosity is a fundamental parameter that must be correctly determined in order to relate physical property, hydrologic, and chemical flux studies to natural systems. Traditional porosity determinations generated from physical property or seismic data can greatly overestimate the true porosity of sediments in which hydrated minerals such as smectite are abundant. To produce a true porosity distribution such data must be corrected to account for the H 2 O residing in smectite interlayers which can make up to 25% of the total hydrated mineral mass. Such H 2 O is easily removed from the mineral by oven drying and/or exposure to low humidities. Standard physical property measurements can be corrected, provided the weight percent smectite in the sediment is known and the interlayer H 2 O content of the mineral can be estimated. We illustrate the significant consequences of this correction by comparing profiles of reported and corrected porosities for smectite-rich Barbados abyssal plain and accretionary wedge sediments from Ocean Drilling Program Leg 110, Sites 671 and 672.

Comparison of pelagic and nepheloid layer marine snow: implications for carbon cycling

Abstract Marine snow from upper and mid-water (i.e., pelagic) depths on the California margin is texturally and compositionally different from that traveling in the nepheloid layer. Transmission electron microscopy shows that pelagic marine snow consists primarily of bioclasts (e.g., diatom frustules, foram tests), organic matter, and microbes. These components are entrained as discrete particles or small aggregates (≤10 μm in diameter) in a loose network of exocellular, muco-polysaccharide material. Clays are infrequent but, when present, are constituents of comparatively compact organic-rich microaggregates. Microbes are abundant and appear to decrease in number with increasing water depth. In contrast, marine snow aggregates collected from just above the sea floor in the nepheloid layer are assemblages of clay particles, clay flocs, and relatively dense clay–organic-rich microaggregates in an exocellular organic matrix. Bioclasts and microorganisms occur only rarely. The prevalence of clay–organic-rich aggregates in the nepheloid layer suggests that, prior to final deposition and burial, marine snow from the pelagic zone is subject to disaggregation and recombination with terrigenous detrital material near or at the sea floor. Results have significant implications for the accumulation and burial rates of organic carbon on continental margins and the aging and bioavailability of sedimentary organic matter. Samples examined were collected offshore of northern and central California.

In situ conditions and interactions between microbes and minerals in fine-grained marine sediments; a TEM microfabric perspective

Abstract Microbes, their exocellular secretions, and their impact on the mineralogy and microfabric of fine-grained continental margin sediments were investigated by transmission electron microscopy. Techniques were used that retained the in situ spatial relations of both bio-organic and mineralogical constituents. Photomicrographs were taken of characteristic mineral-microbe associations in the first meter of burial at conditions ranging from aerobic to anaerobic. Single-celled prokaryotes, prokaryotic colonies, and eukaryotic organisms were observed as were motile, sessile, and predatory species. Bacterial cells dominate the assemblage. The most commonly observed mineral-biological interaction was the surrounding, or close association, of isolated heterotrophic bacterial cells by clay minerals. Almost without exception, the external surfaces of the bacteria were covered with secreted exocellular slimes composed of cross-linked polysaccharide fibrils. These fibrils act to bind sediment grains into relatively robust microaggregates, roughly ≤ 25 μm in diameter. These exocellular polymers can significantly impact the interaction between microbes and minerals, as well as the chemical and physical transport of fluids and dissolved aqueous species through the sediment. Although pore water chemical profiles from the field sites studied have dissolved Fe and Mn, no close association was found between the microbes imaged and precipitated metal oxyhydroxides or other authigenic minerals, such as is commonly reported from laboratory cultures.

COMPOSITIONAL END MEMBERS AND THERMODYNAMIC COMPONENTS OF ILLITE AND DIOCTAHEDRAL ALUMINOUS SMECTITE SOLID SOLUTIONS

Consideration of XRD, TEM, AEM, and analytical data reported in the literature indicates that dioctahedral aluminous smectite and illite form two separate solid solutions that differ chemically from one another primarily by the extent of Al substitution for Si, the amount of interlayer K, and the presence of interlayer H2O. The data indicate that limited dioctahedral-trioctahedral and dioctahedral-vacancy compositional variations occur in both minerals. Excluding interlayer H2O and based on a half unit cell [i.e., O10(OH)2], natural dioctahedral smectite and illite solid solutions fall within the compositional limits represented by A0.3

Estimation of the standard molal heat capacities, entropies and volumes of 2:1 clay minerals

{\rm{R}}_{1.9}^{3 + }

Marine biogeochemistries of Be and Al: A study based on cosmogenic10Be, Be and Al in marine calcite, aragonite, and opal

Si4O10(OH)2-AR2+ R3+ Si4O10(OH)2-A0.25