F. Leo Lynch

Alternative origins for nannobacteria-like objects in calcite

More than 40 calcite-precipitation experiments were performed under sterile conditions in order to investigate the origins of 25–300 nm spherical-, rod-, and ovoid-shaped objects that have been widely interpreted as evidence of nanometer-scale life (i.e., nannobacteria). Individual experiments included the addition of soluble organic compounds, common species of eubacteria, or phage-induced eubacterial lysates. These experiments indicate that many of the nanometer-scale objects have inorganic or nonnannobacterial origins. In the precipitation experiments, calcite formed euhedral crystals 50–800 nm in diameter and smaller (<50 nm) anhedral or rounded particles or protocrystals. The small anhedral or rounded solids resembled nannobacteria. The relative amount of anhedral or rounded calcite was greatest in experiments with a dissolved organic component. These controlled experiments are in accord with observations that rounded nanometer-scale objects are more common in minerals formed in organic-rich environments. Bacterial fragments occur as rounded to irregularly shaped particles that included cell-wall fragments, expulsed cytoplasm, and relict capsules that also closely resembled nannobacteria. Acid etching of the large euhedral crystals produced in the precipitation experiments also resulted in the formation of nanometer-scale features that resembled nannobacteria in natural carbonates. The shapes of the etching artifacts vary as a function of the strength of the acid and the duration of etching. Much caution is advisable in interpreting the origin of rounded features <50 nm.

Muddy waters: temporal variation in sediment discharging from a karst spring

Karst aquifers are capable of transporting and discharging large quantities of suspended sediment, which can have an important impact on water quality. Here we present the results of intensive monitoring of sediment discharging from a karst spring in response to two storm events, one following a wet season and the other following a dry season; we describe temporal changes in total suspended solids (TSS), mineralogy, and particle size distribution. Peak concentrations of suspended sediment coincided with changes in aqueous chemistry indicating arrival of surface water, suggesting that much of the discharging sediment had an allochthonous origin. Concentrations of suspended sediment peaked 14‐16 h after rainfall, and the bulk of the sediment (approximately 1 metric ton in response to each storm) discharged within 24 h after rainfall. Filtered material included brightly colored fibers and organic matter. Suspended sediments consisted of dolomite, calcite, quartz, and clay. Proportions of each mineral constituent changed as the aquifer response to the storm progressed, indicating varying input from different sediment sources. The hydraulic response of the aquifer to precipitation was well described by changes in parameters obtained from the particle size distribution function, and corresponded to changes seen in TSS and mineralogy. Differences between storms in the quantity and mineralogy of sediment transported suggest that seasonal effects on surface sediment supply may be important. The quantity of sediment discharging and its potential to sorb and transport contaminants indicates that a mobile solid phase should be included in contaminant monitoring and contaminant transport models of karst. Temporal changes in sediment quantity and characteristics and differences between responses to the two storms, however, demonstrate that the process is not easily generalized. q 1999 Elsevier Science B.V. All rights reserved.

The possible role of nannobacteria (dwarf bacteria) in clay-mineral diagenesis and the importance of careful sample preparation in high-magnification SEM study

ABSTRACT Bacterial textures are present on clay minerals in Oligocene Frio Formation sandstones from the subsurface of the Corpus Christi area, Texas. In shallower samples, beads 0.05-0.1 µm in diameter rim the clay flakes; at greater depths these beads become more abundant and eventually are perched on the ends of clay filaments of the same diameter. We believe that the beads are nannobacteria (dwarf forms) that have precipitated or transformed the clay minerals during burial of the sediments. Rosettes of chlorite also contain, after HCl etching, rows of 0.1 µm bodies. In contrast, kaolinite shows no evidence of bacterial precipitation. We review other examples of bacterially precipitated clay minerals. A danger present in interpretation of our earlier work (and much work of other ) is the development of nannobacteria-looking artifacts caused by gold coating times in excess of one minute; we strongly recommend a 30-second coating time. Bacterial growth of clay minerals may be a very important process both in the surface and subsurface.

Burial diagenesis of argillaceous sediment, south Texas Gulf of Mexico sedimentary basin: A reexamination

Cuttings from a well through a thick section of Miocene–Oligocene mudrocks from Kenedy County, Texas, spanning a depth range of 2130 to 5490 m (7000 to 18 000 ft), have been studied petrographically and geochemically. On the basis of whole-rock chemical analyses, the deepest samples have lost ≈18 wt% (and approximately vol%), mostly as CaCO3, mineral-bound H2O, and SiO2, but including additional Ca, as well as Sr, light rare earth elements (REE) (La, Ce, Nd, Sm), Fe, and Li. K2O and Rb have been added to the deeper rocks. The large chemical changes are accompanied mineralogically by loss of detrital calcite, kaolinite, K-feldspar, Ca-plagioclase, and muscovite, gain of chlorite and albite, and continued reaction of smectitic illite/smectite (I/S) to more illitic (and K-rich) compositions throughout the entire depth interval of the well. The large chemical changes in this thick mud-rich interval almost certainly require advection of water (free convection?) to accomplish the mass transfer. Initial variation in sediment composition is ruled out as a cause of the observed compositional changes with increasing depth because (1) a variety of “immobile” elements (Al2O3, TiO2, Zr, Hf, heavy REE [Er, Yb], Th, and Sc) remain constant relative to each other despite their uneven distribution across various particle size fractions in the sediments; (2) deep Frio shales are unlike Quaternary Gulf of Mexico sediments or average shales; and (3) unreasonable primary mineralogic compositions would be necessary to explain the chemical composition of the deep samples. These results indicate that burial diagenesis of argillaceous sediment can be a considerably more open chemical process than is conventionally assumed, that it can account for the two major chemical cements (calcite and quartz) in associated sandstones, and that it mirrors secular changes in shales throughout geologic time.

Effects of SEM Preparation Techniques on the Appearance of Bacteria and Biofilms in the Carter Sandstone

Abstract When biofilms (aggregations of bacteria and extracellular polymer secretions) in samples from the Carter Sandstone of Alabama were prepared for scanning electron microscopy (SEM) using different dehydration techniques, the organic material had visibly different textures and distributions. In order to assess whether the variation was attributable to sample preparation or to inherent biofilm heterogeneity, each of five techniques were tested 3 to 10 times on small (1 cm) pieces of the Carter Sandstone containing either a strain of bacteria cultured from and reintroduced into the rock, or an in situ biofilm grown by injection of nutrients through core samples. The techniques tested were (1) air drying alone; (2) fixation in 10% glutaraldehyde with air drying; (3) ethanol dehydration with hexamethyldisilazane (HMDS) drying [2.5% glutaraldehyde, ethanol dehydration, and HMDS]; (4) ethanol dehydration with critical-point drying; and (5) ethanol and acetone dehydration with critical-point drying. Unpreserved control samples were either imaged wet in an environmental scanning electron microscope (ESEM) or vacuum-dried for SEM. Observations were based on SEM microscopy of over 60 samples and study of over 150 photomicrographs. In both experiments, the original morphology of individual bacteria was best preserved by ethanol dehydration with HMDS drying, ethanol dehydration with critical-point drying, or ethanol-acetone dehydration with critical-point drying. Critical-point drying preserved bacteria but stripped away mucilaginous material, revealing filamentous structures within the biofilm. These filaments, along with masses of microspheres (nannobacteria?) and the smooth mucilaginous outer layer, also occur in wet samples studied by ESEM, and are, therefore, not dehydration artifacts. However, different sample preparation techniques accentuated different components of the heterogeneous biofilm, thus resulting in vastly different textures. The cultured bacteria produced a biofilm that had a different surface texture and was more susceptible to sample preparation artifacts than the in situ biofilm. Use of more than one sample preparation technique is recommended in order to avoid bias.

Frio shale mineralogy and the stoichiometry of the smectite-to-illite reaction; the most important reaction in clastic sedimentary diagenesis

Burial diagenesis of shales of the Frio Formation resulted in an increase in the abundance of mixed-layer illite-smectite (I-S), albite and chlorite, and a decrease in the abundance of K-feldspar, illite and kaolinite. Some of the mineralogic trends determined in this study contrast with the results of Hower et al. (1976) and other studies of Frio shales. The differences are due to improvements in laboratory and clay quantification techniques since the time of the earlier research. I-S composition changed from ~20% to ≥80% illite, and mineralogic and chemical reaction of I-S continued throughout burial. Shale diagenesis was an open-system process that required addition of K2O and A12O3, and resulted in loss of SiO2. The amount of SiO2 made available by shale diagenesis is sufficient to be the source of the quartz-overgrowth cements in the associated Frio sandstones. The relationships between I-S diagenesis and fluid flow from shales into sandstones, generation of abnormal formation-water fluid pressure, onset of sandstone diagenesis and distribution of authigenic phases in sandstones indicate that reaction of the I-S in shales is one of the most important components of the sandstone/shale/formation water diagenetic system.

Thermal Conductivity of Wilcox and Frio Sandstones in South Texas (Gulf of Mexico Basin)

Thermal conductivity and petrographic data are presented for verifying mechanistic models of sandstone thermal conductivity. We measured the thermal conductivity of 83 Wilcox and Frio sandstones from south Texas in the Gulf of Mexico sedimentary basin, and correlated conductivity to petrographic variables. Thermal conductivities of water-saturated sandstones at 20°C (68°F) and 3 MPa (435 psi) were measured on core plugs using a divided-bar apparatus. Thermal conductivity ranges from 2.06 to 5.73 W/m/K over a porosity range of 2.4 to 29.6%. Because of a higher quartz content, Wilcox sandstones at a given porosity are more conductive than Frio sandstones. A grain-matrix conductivity of 5.9 W/m/K is estimated for Wilcox sandstones; matrix conductivity is adequately described with an arithmetic mixing model. Thermal conductivities of clean ( 35% of the solids) sandstones, the dependence on quartz content can be dropped and thermal conductivities can be predicted with a linear decrease in conductivity with increasing porosity. These sandstones appear isotropic with respect to thermal conductivity.

δ18O values of mudrocks: More evidence for an 18O-buffered ocean

Abstract The whole-rock δ 18 O of mudrocks, as well as the δ 18 O of the clay-sized silicate fraction, does not change in any systematic manner over Paleozoic and late Proterozoic time. Therefore, the δ 18 O of the ocean has not changed (except a per mil or so as the result of glaciation/deglaciation) over the last billion years of Earth history.

Mineral/Water Interaction, Fluid Flow, and Frio Sandstone Diagenesis: Evidence from the Rocks

The spatial distribution of burial diagenetic cements in sandstones of the Frio Formation is related to permeability and fluid flow. The relationship between sandstone diagenesis and fluid flow implies that the modification of the rocks is largely the outcome of reactions between the detrital and authigenic minerals and the formation water in the pores. During burial, formation water preferentially flowed through the most porous and permeable sandstones. Diagenetic alteration is more extensive in permeable sandstones because the minerals in those rocks were exposed to significantly more pore volumes of reactive water, each of which accomplished a small amount of diagenetic work. Consequently, porous and permeable units were also the preferred sites of diagenetic mineral p ecipitation. The heterogeneous distribution of authigenic minerals in sandstones is a result of changing preferential flow paths, due to diagenetic modification, during burial. Sandstones that are not the most porous or permeable at the time of deposition commonly preserve the best reservoir quality at depth because they are less modified by diagenesis. Petrographic analyses indicate that the SiO2 needed for the quartz overgrowths in the sandstones had an allochthonous source. ^dgr18O values for diagenetic quartz imply that the mineral precipitated from rapidly ascending, hot formation water. Quartz overgrowths are most abundant in distal-shelf facies sandstones, where the isolated nature of the sandstones in a predominately shaly section maximized preferential fluid flow. Mass balance calculations show that thousands of pore volumes of formation water are required to supply the SiO2 needed for the quartz overgrowths in the sandstones. The amount of water required to produce the diagenetic modification of the sandstones is much larger than the amount of water present in the entire Frio Formation, impl ing that reuse of formation water is an important and necessary process during diagenesis.

Diagenesis of Calcite Cement in Frio Formation Sandstones and its Relationship to Formation Water Chemistry

ABSTRACT Calcite cement is one of the most volumetrically important diagenetic minerals formed during burial of Frio Formation sandstones from the Corpus Christi area of Texas. Syndepositional calcite is restricted to shore-zone sandstones, whereas later, post-quartz-overgrowth, burial-diagenetic calcite is present in both shore-zone and shelf sandstones. The d18O of burial-diagenetic calcite becomes depleted with depth. Chemical and textural evidence favors partial dissolution and reprecipitation (recrystallization) with consequent isotopic resetting as being responsible for the change in calcite d18O with progressive burial. The change in calcite 87Sr/86Sr with depth also supports progressive recrystallization of calcite. There is a strong relationship between the 87Sr86Sr and trace element composition of calcite and formation water from individual growth-fault blocks. Significant differences in strontium isotopic and trace element composition exist between adjacent fault blocks, implying that each fault block has behaved as a chemically separate system since the time of calcite precipitation.