Association of phosphate with rhyolite glass in marine Neogene tuffs from Patagonia,Argentina

The precipitation/replacement of Ca‐phosphate is a complex process that commonly takes place during the early diagenesis in marine sediments. The unusual occurrence of shallow marine, early diagenetic phosphatic deposits associated with glassy tuffs in the Neogene Gaiman Formation, in the Chubut Province, Patagonia, Argentina, constitutes a good case example for the study of replacement and precipitation of Ca‐phosphate on an unstable substrate. Isocon diagrams illustrate that chemical changes during glass diagenesis include gains in loss on ignition and Ca, and losses of K. These changes are the result of glass hydration during sea water–glass interaction, together with adsorption and diffusion of ions into the bulk shard; combined, these represent an incipient process of volcanic glass replacement by Ca‐phosphate. Subsequent early diagenetic P enrichment in the pore solutions led to phosphate precipitation, associated with pitting on the glass shards and pumice. The associated development of a reactive surface promoted the incorporation of P and Ca into their margins. Lastly, precipitation of calcium phosphate filled the vesicles and other open cavities, inhibiting further glass dissolution. The high porosity and reactivity of the volcanic glass provided an appropriate substrate for phosphate precipitation, leading to the development of authigenic apatite concretions in the volcanic‐glass bearing strata of the Gaiman Formation. This research is of significance for those concerned with marine phosphatic deposits and sheds light on the processes of early diagenetic phosphate precipitation by replacement of an atypical, unstable substrate like hydrated volcanic glass.     

Formation of refractory inclusions by evaporation of condensate precursors

Berman’s (1983) activity-composition model for CaO-MgO-Al₂O₃-SiO₂ liquids is used to calculate the change in bulk chemical and isotopic composition during simultaneous cooling, evaporation, and crystallization of droplets having the compositions of reasonable condensate precursors of Types A and B refractory inclusions in CV3 chondrites. The degree of evaporation of MgO and SiO₂, calculated to be faithfully recorded in chemical and isotopic zoning of individual melilite crystals, is directly proportional to evaporation rate, which is a sensitive function of P_H2, and inversely proportional to the droplet radius and cooling rate. When the precursors are partially melted in pure hydrogen at peak temperatures in the vicinity of the initial crystallization temperature of melilite, their bulk chemical compositions evolve into the composition fields of refractory inclusions, mass-fractionated isotopic compositions of Mg, Si, and O are produced that are in the range of the isotopic compositions of natural inclusions, and melilite zoning profiles result that are similar to those observed in real inclusions. For droplets of radius 0.25 cm evaporating at P_H2 = 10⁻⁶ bar, precursors containing 8 to 13 wt.% MgO and 20 to 23% SiO₂ evolve into objects similar to compact Type A inclusions at cooling rates of 2 to 12 K/h, depending on the precise starting composition. Precursors containing 13 to 14 wt.% MgO and 23 to 26% SiO₂ evolve into objects with the characteristics of Type B1 inclusions at cooling rates of 1.5 to 3 K/h. The relatively SiO₂-poor members of the Type B2 group can be produced from precursors containing 14 to 16 wt.% MgO and 27 to 33% SiO₂ at cooling rates of <1 K/h. Type B2’s containing 27 to 35 wt.% SiO₂ and <12% MgO require precursors with higher SiO₂/MgO ratios at MgO > 15% than are found on any condensation curve. The characteristics of fluffy Type A inclusions, including their reversely zoned melilite, can only be understood in the context of this model if they contain relict melilite.     

A chemical survey of the Mississippi estuary

A “snap shot” survey of the Mississippi estuary was made during a period of low river discharge, when the estuarine mixing zone was within the deltaic channels. Concentrations of H⁺, Ca²⁺, inorganic phosphorus and inorganic carbon suggest that the waters of the river and the low salinity (<5‰) portion of the estuary are near saturation with respect to calcite and sedimentary calcium phosphate. An input of oxidized nitrogen species and N₂O was observed in the estuary between 0 and 4‰ salinity. The concentrations of dissolved NH₄ ⁺ and O₂, over most of the estuary, appeared to be influenced by decomposition of terrestrial organic matter in bottom sediments. The estuarine bottom also appears to be a source of CH₄ which has been suggested to originate from petroleum shipping and refining operations. Estuarine mixing with offshore Gulf waters was the dominant influence on distributions of dissolved species over most of the estuary (i.e., from salinities >5‰). The phytoplankton abundance (measured as chlorophylla) increased as the depth of the mixed layer decreased in a manner consistent with that expected for a light-limited ecosystem. Fluxes of NO₃ ^?+NO₂ ^? and soluble inorganic phosphorus to the Gulf of Mexico were estimated to be 3.4±0.2×10³ g N s^?1 and 1.9±0.2 g P s^?1 respectively, at the time of this study.     

The present day formation of apatite in Mexican continental margin sediments

Results of pore water and sediment analyses from the western Mexican continental margin strongly suggest the present day formation of apatite. The interstitial water phosphate and fluoride profiles indicate chemical removal at a depth which corresponds to a large maximum in the phosphorus content of the sediments. Apatite is identified within this maximum via X-ray diffraction but is elsewhere undetectable in the core. Radioisotopic thorium, uranium, and radium data support the conclusion that this deposit is modern. The present day depositional environment is consistent with those reported by other workers for phosphorite formation with the exception that pore water magnesium is not depleted below its seawater value.     

Evaluation of biogeochemical prospecting methods in the search for sulfide deposits in the Appalachian piedmont, Virginia, U.S.A.

Soil and stream sediment sampling have been the primary geochemical exploration tools in the Appalachian piedmont to date. However, the great thicknesses of soil and saprolite found in the region coupled with the dense vegetation frequently encountered favor biogeochemistry as an alternative or supplemental method since deep-rooted plants sample closer to bedrock. To evaluate this method, an orientation survey was performed in which soils and vegetation at 17 sites north of Mineral, Virginia, were sampled and analyzed for Ag, Cd, Cu, Pb and Zn. The traverse included stations over the host rocks of massive sulfide mineralization, as well as over apparent “barren” country rock. Samples were analyzed by atomic absorption spectrophotometry using standard digestion and analytical techniques.Both A- and B-horizon soil metals generally appear to be reliable indicators of mineralization, with soils developed over sulfides showing up to three-fold enrichment in metal content relative to the average soils developed on the country rock. Correlation of metal concentrations in vegetation to soil metal concentrations reveal plant concentrations expressed on a dry-weight basis correlate stronger and more frequently to soil metals than do ash-weight concentrations. Copper shows some promise in selected organs and species, Ag appears fair but data are limited to one organ of one species, and plant Pb seems totally unresponsive to soil metal concentrations perhaps because foliar absorption is an important plant uptake mechanism here. However, Zn and Cd in organs of the oak group, especially mature leaves and twigs of the current year's growth show the greatest promise as prospecting tools. They correlate well with soil metals and when compared directly to the geology they reliably reflect mineralization. Although results using White oak were slightly less profound than those obtained from the Black-Red oak group, White oak may be preferred as it is a single, more widespread, easily-identifiable species. Copper and especially Zn although essential elements to plants, do not appear to be “difficult” elements for biogeochemical prospecting in the Appalachian piedmont.     

Stability and dynamics of the continental tectosphere

Continental cratons overlie thick, high-viscosity, thermal and chemical boundary layers, where the chemical boundary layers are less dense than they would be due to thermal effects alone, perhaps because they are depleted in basaltic constituents. If the continental tectosphere is the same age as the overlying Archaean crust, then the continental tectosphere must be able to survive for several billion years without undergoing a convective instability, despite being both cold and thick. Since platforms and shields correlate only weakly with Earth's gravity and geoid anomalies, acceptable models of the continental tectosphere must also satisfy this gravity constraint. We investigate the long-term stability of the continental tectosphere by carrying out a number of numerical convection experiments within a two-dimensional Cartesian domain. We initiate our experiments with a tectosphere (thermal and chemical boundary layers) immersed in a region of uniform composition, temperature, and viscosity, and consider the effects on the stability of the tectosphere of (1) activation energy (used to define the temperature dependence of viscosity), (2) compositional buoyancy, and (3) linear or non-linear rheology. The large lateral thermal gradients required to match oceanic and tectosphere structures initiate the dominant instability, a “drip” which develops at the side of the tectosphere and moves to beneath its center. High activation energies and high background viscosities restrict the amount and rate of entrainment. Compositional buoyancy does not significantly change the flow pattern. Rather, compositional buoyancy slows the destruction process somewhat and reduces the stress within the tectosphere. With a non-Newtonian rheology, this reduction in stress helps to stiffen the tectosphere. In these experiments, dynamical systems that adequately model the present ocean-continent structures have activation energy E*≥180 kJ mole⁻¹ — a value about one third the estimate of activation energy for olivine, E*≈520 kJ mole⁻¹. Although for E*≈520 kJ mole⁻¹, compositional buoyancy is not required for the tectosphere to survive, the joint application of longevity and gravity constraints allows us to reject all models not containing compositional buoyancy, and to predict that the ratio of compositional to thermal buoyancy within the continental tectosphere is approximately unity.     

Tectonic and eustatic control on deposition and preservation of Upper Cretaceous ooidal ironstone and associated facies: Peace River Arch area, NW Alberta, Canada

The Late Coniacian, shallow-marine Bad Heart Formation of the Western Canada foreland basin is very unusual in that it contains economically significant ooidal ironstone. Deposition of shallow-water and iron-rich facies appears to have been localized over the crest and flanks of a subtle intrabasinal arch, in part interpreted as a forebulge and partly attributed to reactivation of the long-lived Peace River Arch. The formation comprises two upward-shoaling allomembers, typically 5–10 m thick, that are bounded by regionally mappable ravinement surfaces. The lower unit, allomember 1, grades up from laminated mudstone to bioturbated silty sandstone, which is abruptly overlain by bioturbated ooidal silty sandstone grading into an almost clastic-free ooidal ironstone up to 7 m thick. Ooidal ironstone was concentrated into NW- to SE-trending ridges, kilometres wide and tens of kilometres long. Ironstone formation appears to have been promoted by: (a) drowning of the arch, which progressively curtailed sediment supply; and (b) enhanced reworking over the shallowly submerged arch and over a fault-bounded block that underwent episodic vertical movement of 10–20 m during Bad Heart deposition. Allomember 2 also shoals upwards from mudstone to bioturbated and laminated silty sandstone but lacks ooids, apparently reflecting a rejuvenated supply of detrital sediment from the arch. The marine ravinement surface above allomember 2 is a Skolithos firmground, above which is developed a regional blanket of ooidal sediment. In the east, ooids are dispersed in a bioturbated silty sandstone with abundant evidence of repeated reworking and early siderite and phosphate cements. Westwards, this facies grades, over about 40 km, into almost clastic-free ooidal ironstone about 5 m thick; the lateral facies change may reflect progressive clastic starvation distal to a low-relief source area. The two allomembers are interpreted to reflect eustatic oscillations of about 10 m, superimposed on episodic tectonic warping and block-faulting events. The development of ooidal ironstone immediately above initial marine flooding surfaces indicates a close relationship to marine transgression, reflecting sediment-starved conditions. Ironstone does not appear to be related to either sequence boundaries or maximum flooding surfaces. The Bad Heart Formation is blanketed by marine mudstone deposited in response to major flexural subsidence and rejuvenation of clastic sources in the Cordillera to the SW.     

Kepone®: Toxicity and bioaccumulation in blue crabs

Two long-term studies were conducted to determine toxicity, uptake and depuration of Kepone in blue crabs (Callinectes sapidus). In the first, Kepone was administered to crabs in seawater (0.03 or 0.3 μg Kepone/I) or food (eastern oyster,Crassostrea virginica, containing 0.25 μg/g Kepone). Uptake of Kepone in 28 days was primarily through the contaminated oysters. When these crabs were held in Kepone-free seawater and fed Kepone-free oysters for 28 days, no loss of the insecticide was evident. There were adverse effects on molting and survival in crabs fed oysters that contained 0.25 μg/g Kepone. A second study was conducted to determine: (1) the depuration of Kepone over a 90-day period in blue crabs fed oysters from the James River, Virginia (containing 0.15 μg/g Kepone); and (2) the effects of Kepone on molting and survival of blue crabs fed James River oysters or laboratory-contaminated oysters that contained 0.15 or 1.9 μg/g Kepone. Crabs fed Kepone-contaminated oysters followed by a diet of Kepone-free oysters for 90 days had detectable concentrations of the insecticide in tissues. Also, blue crabs that ate oysters containing Kepone in concentrations similar to those found in oysters from the James River, died or molted less frequently than crabs fed Kepone-free oysters meats.     

A skeletal assemblage classification system for non-tropical carbonate deposits based on New Zealand Cenozoic limestones

Cenozoic limestones in New Zealand are mainly skeletal grainstones and packstones formed under non-tropical climatic conditions in open marine shelf or ramp environments. Following petrographic analysis of the nature and abundance of the skeletal components in nearly 500 samples of these limestones, a complete linkage cluster analysis identified seven major skeletal assemblages that may be regarded as subdivisions of the single foramol skeletal association defined by Lees and Buller (1972) for temperate-region carbonate deposits. The seven assemblages are given contracted names, as follows: (a) BARNAMOL = barnacle/bivalve-dominated; (b) BIMOL = bivalve-dominated; (c) BRYOMOL = bryozoan/bivalve-dominated; (d) ECHINOFOR = echinoderm/benthic foraminiferal-dominated; (e) NANNOFOR = nannofossil/planktonic foraminiferal-dominated; (f) RHODALGAL = calcareous red algal-dominated; and (g) RHODECHFOR = calcareous red algal/echinoderm/benthic foraminiferal-dominated. A composite triangular classification diagram has been devised for naming the skeletal assemblage of an unknown sample on the basis of its three main skeletal components. The diagram successfully characterises more than 85% of the New Zealand Cenozoic limestone samples and also appears to be applicable for the skeletal assemblage designation of many overseas examples of non-tropical carbonate deposits. Limitations relate mainly to locally common skeletal types (e.g. serpulids, brachiopods) that are presently not incorporated into the New Zealand-based scheme. The general ecological preferences of the main skeletal contributors in each of the seven skeletal assemblages form a basis for relating the assemblages to broad shelf habitats. Consequently, as well as the benefits of providing a more consistent skeletal assemblage terminology for comparative studies between different workers, the scheme can assist with the paleoenvironmental interpretation of non-tropical skeletal carbonate facies.