Physico-chemical environment of pedogenic carbonate formation in Devonian vertic palaeosols, central Appalachians, USA

The morphology and geochemistry of pedogenic carbonate found in vertic claystone palaeosols in the Devonian Catskill Formation in central Pennsylvania preserve a record of the physical and chemical environment of carbonate precipitation. The carbonate is characterized by three distinct petrographic generations. Pedogenic rhizoliths and nodules are the earliest precipitated generation, and typically consist of dull red-brown luminescent micrite. Clear, equant calcite spar cement fills voids in the centres of rhizoliths, as well as circumgranular cracks and septarian voids in nodules. Early spar cements are non-luminescent to dull luminescent, whereas later spar cements exhibit bright yellow-orange luminescence. Late stage pedogenic fractures are always occluded with very bright yellow-orange luminescent spar cements. The incorporation of progressively higher concentrations of Mn (up to 34000 ppm) into successively younger calcite spar cements, without concomitant increases in Fe, suggests carbonate precipitation from an evolving meteoric water in which Mn²⁺ became increasingly mobile over time. The increased mobility is possibly due to decreasing Eh, resulting from oxidation of organic matter after rapid soil burial on the floodplain. The amount of Fe²⁺ available for incorporation into calcite was limited because most iron was immobile, having been earlier oxidized and bound to the palaeosol clay matrix as a poorly crystallized ferric oxide or oxyhydroxide mineral. Carbon isotope compositions of pedogenic carbonate correlate with the inferred depth of carbonate precipitation. Rhizoliths preserved below the lowest stratigraphic occurrences of pedogenic slickensides are consistently depleted in ¹³C relative to nodules, which formed stratigraphically higher, within the zone of active soil shrink and swell processes. Nodular carbonate, precipitated in proximity to deep cracks in the soil, is enriched due to increased gas exchange with isotopically heavy atmospheric CO₂. Accordingly, rhizolith compositions will most accurately estimate palaeoatmospheric levels of CO₂; the use of nodule compositions may result in overestimation of P_CO2 by as much as 30%.     

MODELLING OF SOFT SEDIMENTS AND LIQUID-SOLID INTERFACES: MODIFIED WAVENUMBER SUMMATION METHOD AND APPLICATION1

Alekseev and Mikhailenko have developed a wavenumber-summation method which combines a finite integral transformation with a finite-difference calculation and involves no approximations other than numerical ones. However, numerical anisotropy causes velocity errors for shear waves which are unacceptable if Poisson's ratios are larger than 0.4 and unless the number of grid points per wavelength is chosen considerably higher than the value generally regarded as sufficient in finite-difference computations. To overcome this limitation in the applicability of the otherwise very powerful modelling scheme, the method is applied to the elastodynamic equations for the velocity vector. Thus, instead of solving a second-order hyperbolic system as in the case of the wave equation, solutions to a first-order hyperbolic system are computed. The finite-difference iteration is performed in a staggered grid. In addition to mastering the numerical difficulties in cases where the Poisson's ratio is unusually high, this approach results in a code which can be used for the modelling of liquid layers. With the new scheme, water reverberations are investigated in terms of normal modes. It is found that for realistic sea-bottom velocities the critical and supercritical cases exist only for P-waves. It means that compressional waves are trapped within the water layer but energy leaks into the substratum through converted shear waves. These leaky compressional normal modes attain properties similar to those of shear normal modes or Pseudo-Love waves. Due to their origin from conversion of dispersed multi-modal compressional waves the shear waves generated at the sea-bottom form a long complex wavetrain. They were found to mask the reflections from the target horizon in an offset-VSP field section.     

Physical compaction of vertic palaeosols: implications for burial diagenesis and palaeo-precipitation estimates

Burial compaction is one of several major obstacles to estimating palaeoprecipitation from depth to pedogenic carbonate in favourably preserved palaeosols. Palaeosols must be decompacted and the preburial depth to the pedogenic carbonate obtained. Vertic palaeosols may be particularly good candidates for palaeoprecipitation estimates, because of their increased likelihood of preserving clastic dykes, one of the best features for estimating burial compaction. Compaction estimates from clastic dykes and literature-based depth of burial estimates suggest vertic palaeosols undergo significantly less burial compaction than may be commonly assumed. Late Carboniferous vertic palaeosols, buried to 2·5–3·0 km, compacted to 93% of their original thickness. In contrast, clastic dykes in a nonpedogenic shale directly underlying one of the Late Carboniferous palaeosols records compaction to 70% of original thickness. Similarly obtained burial compaction and burial depth estimates for Early Carboniferous, Ordovician, and Proterozoic vertic palaeosols were used to test a burial compaction curve and equation specific to vertic palaeosols. Results suggest this ‘vertic-calibrated’curve and equation can be used to estimate burial compaction for vertic palaeosols lacking clastic dykes, but additional testing is needed. Naturally high bulk densities may have limited the compactibility of vertic palaeosols. Likewise, high initial bulk density and an abundance of swelling clays may have severely limited the transmissivity of some vertic palaeosols as they passed from pedogenic to burial environments. Upon burial these vertic palaeosols may have behaved as closed systems, which has implications for understanding their diagenetic modification. Additional efforts to understand burial compaction of vertic palaeosols also promises to improve our understanding of aquifer/aquiclude and hydrocarbon reservoir/seal relationships in sedimentary basins containing intercalated palaeosols.     

Magmatic Differentiation and Alteration in Isofacial Greenschists and Blueschists, Shuksan Suite, Washington: Statistical Analysis of Major-Element Variation

A multivariate statistical technique separates compositionalvariability in metaigneous rocks attributed to magmatic differentiationfrom compositional effects of alteration. Magmatic differentiationin metabasalts from the Shuksan Suite, Washington, is intermediate(mean MD index=2.6) between fresh ocean-floor basalts from theFAMOUS area (MD=0.0) and eastern Galapagos-rift (MD=3.8). Identificationand removal of a magmatic differentiation trend from the Shuksansamples reveals the effects of sea-floor weathering and hydrothermalalteration, which caused significant decreases in Si and Ca,increases in Na and K, and oxidation of iron. Comparison tomodern altered MORBs indicates that the Shuksan protolith rockswere probably hydrothermally altered at a sea-water/rock ratioless than 15. The intercalated metabasaltic greenschists and blueschists fromthe Shuksan Suite differ in composition. Stepwise discriminantanalysis finds Fe₂O₃ and CaO to be the only variables necessaryto completely separate compositions of greenschists and blueschistsfrom widely distributed Shuksan localities. Pressure and/ortemperature gradients of metamorphism in the Shuksan Suite mustbe too small to cause major shifts in the compositional boundarybetween greenschist and blueschist.     

Fibrous calcite from the Ordovician of Tennessee: preservation of marine oxygen isotopic composition and its implications

Three categories of fibrous calcite from early to middle Caradoc platform-marginal buildups in east Tennessee can be delineated using cathodoluminescent microscopy, minor element chemistry and stable C-O isotopic composition. Bright luminescent fibrous cement has elevated Mn (>1000 p.p.m.), negative δ¹³C and intermediate δ¹⁸O values relative to other types of fibrous calcite. This cement reflects fibrous calcite that interacted with reducing Mn-rich fluids. Dully luminescent fibrous cement has elevated Fe (>400 p.p.m.), positive δ¹³C and negative δ¹⁸O values relative to other fibrous cements. This cement was stabilized by burial fluids. Nonluminescent fibrous cement has low Mn and Fe (generally below 400 p.p.m.) and positive δ¹³C and δ¹⁸O values relative to other types of fibrous calcite. The latter cement is interpreted to be the best material for determining the isotopic composition of calcite precipitated in equilibrium with early to middle Caradoc seawater, which is δ¹³C=1% PDB and δ¹⁸O=?4 to ?5‰ PDB. Results from this study and Ashgillian brachiopods indicate that the average δ¹⁸O composition of the Ordovician ocean, during nonglacial periods, was probably never more negative than ?3‰ SMOW. Assuming an Ordovician seawater δ¹⁸O value of ?1‰ SMOW, Holston Formation fibrous cements would have precipitated at temperatures between 27 and 36 °C, which is near the upper temperature limit for metazoans. A seawater δ¹⁸O value of ?2‰ SMOW yields temperatures ranging from 23 to 31 °C, while a ?3‰ SMOW value yields temperatures of 18–26 °C.     

Physical characterization of aerosol particles during nucleation events

Particle concentrations and size distributions have been measured from different heights inside and above a boreal forest during three BIOFOR campaigns (14 April–22 May 1998, 27 July–21 August 1998 and 20 March–24 April 1999) in Hyytiälä, Finland. Typically, the shape of the background distribution inside the forest exhibited 2 dominant modes: a fine or Aitken mode with a geometric number mean diameter of 44 nm and a mean concentration of 1160 cm⁻³ and an accumulation mode with mean diameter of 154 nm and a mean concentration of 830 cm⁻³. A coarse mode was also present, extending up to sizes of 20 μm having a number concentration of 1.2 cm⁻³, volume mean diameter of 2.0 μm and a geometric standard deviation of 1.9. Aerosol humidity was lower than 50% during the measurements. Particle production was observed on many days, typically occurring in the late morning. Under these periods of new particle production, a nucleation mode was observed to form at diameter of the order of 3 nm and, on most occasions, this mode was observed to grow into Aitken mode sizes over the course of a day. Total concentrations ranged from 410–45 000 cm⁻³, the highest concentrations occurring on particle production days. A clear gradient was observed between particle concentrations encountered below the forest canopy and those above, with significantly lower concentrations occurring within the canopy. Above the canopy, a slight gradient was observed between 18 m and 67 m, with at maximum 5% higher concentration observed at 67 m during the strongest concentration increases.     

Clay-mineral distribution in surface sediments of the Eurasian Arctic Ocean and continental margin as indicator for source areas and transport pathways — a synthesis

Clay-mineral distributions in the Arctic Ocean and the adjacent Eurasian shelf areas are discussed to identify source areas and transport pathways of terrigenous material in the Arctic Ocean. The main clay minerals in Eurasian Arctic Ocean sediments are illite and chlorite. Smectite and kaolinite occur in minor amounts in these sediments, but show strong variations in the shelf areas. These two minerals are therefore reliable in reconstructions of source areas of sediments from the Eurasian Arctic. The Kara Sea and the western part of the Laptev Sea are enriched in smectite, with highest values of up to 70% in the deltas of the Ob and Yenisey rivers. Illite is the dominant clay mineral in all the investigated sediments except for parts of the Kara Sea. The highest concentrations with more than 70% illite occur in the East Siberian Sea and around Svalbard. Chlorite represents the clay mineral with lowest concentration changes in the Eastern Arctic, ranging between 10 and 25%. The main source areas for kaolinite in the Eurasian Arctic are Mesozoic sedimentary rocks on Franz-Josef Land islands. Based on clay-mineral data, transport of the clay fraction via sea ice is of minor importance for the modern sedimentary budget in the Arctic basins.