Chemical weathering of a marine terrace chronosequence, Santa Cruz, California I: Interpreting rates and controls based on soil concentration-depth profiles

The spatial and temporal changes in element and mineral concentrations in regolith profiles in a chronosequence developed on marine terraces along coastal California are interpreted in terms of chemical weathering rates and processes. In regoliths up to 15 m deep and 226 kyrs old, quartz-normalized mass transfer coefficients indicate non-stoichiometric preferential release of Sr > Ca > Na from plagioclase along with lesser amounts of K, Rb and Ba derived from K-feldspar. Smectite weathering results in the loss of Mg and concurrent incorporation of Al and Fe into secondary kaolinite and Fe-oxides in shallow argillic horizons. Elemental losses from weathering of the Santa Cruz terraces fall within the range of those for other marine terraces along the Pacific Coast of North America.Residual amounts of plagioclase and K-feldspar decrease with terrace depth and increasing age. The gradient of the weathering profile b_s is defined by the ratio of the weathering rate, R to the velocity at which the profile penetrates into the protolith. A spreadsheet calculator further refines profile geometries, demonstrating that the non-linear regions at low residual feldspar concentrations at shallow depth are dominated by exponential changes in mineral surface-to-volume ratios and at high residual feldspar concentrations, at greater depth, by the approach to thermodynamic saturation. These parameters are of secondary importance to the fluid flux q_h, which in thermodynamically saturated pore water, controls the weathering velocity and mineral losses from the profiles. Long-term fluid fluxes required to reproduce the feldspar weathering profiles are in agreement with contemporary values based on solute Cl balances (q_h = 0.025-0.17 m yr⁻¹).During saturation-controlled and solute-limited weathering, the greater loss of plagioclase relative to K-feldspar is dependent on the large difference in their respective solubilities instead of the small difference between their respective reaction kinetics. The steady-state weathering rate under such conditions is defined as

     

Fracture flow modelling based on satellite images of the Wajid Sandstone, Saudi Arabia

Large-scale geological features have been identified by satellite imagery and global positioning system data in the Wajid Sandstone in Saudi Arabia. The main objective is to evaluate the importance of fractures for the overall flow behaviour in this fractured rock aquifer and to estimate in-situ hydraulic apertures. Data on fractures and lineaments were available for three outcrops. By applying a “cut-out” routine on the fracture endpoint data of these fracture trace windows, three deterministic discrete fracture networks (DFN), with an area of 100 m?×?100 m, could be generated. These were used to simulate the fracture flow and to determine the hydraulic conductivity tensors. Using additional data on hydraulic pumping tests and matrix conductivities, in-situ hydraulic apertures could be determined. Average in-situ hydraulic apertures range from 1,300 to 1,700 µm. Observations from the field support these results. In addition, a hydraulic conductivity ratio between the matrix and fracture system was used to identify the contribution of the DFN to the overall fluid transport. A ratio of 10.4 was determined, which indicates that the effective flow behaviour in the Wajid Sandstone aquifer is not entirely dominated by the fracture system, though evidently strongly controlled by it.     

A Semi-Evolutive Partially Local Filter for Data Assimilation

The singular evolutive extended Kalman (SEEK) filter has been proposed recently by Pham et al. (1997) for data assimilation into numerical oceanic models. This filter has been applied in different realistic ocean frameworks and has provided satisfactory results ( Pham et al., 1997; Verron et al., 1998). However, the SEEK filter remains expensive in real operational assimilation. To reduce cost and obtain a better representativity, we introduce the idea ‘local correction basis'. Such basis however cannot be made to evolve according to the model without destroying its locality property. Therefore we shall keep this basis fixed and we augment it by a few global basis vectors which evolve. The resulting semi-evolutive partially local filter is much less costly to implement than the SEEK filter and yet can yield better results. In the first application, validation twin experiments are conducted in a realistic setting of the OPA model over the tropical Pacific Ocean.     

Ca/Sr and Sr isotope systematics of a Himalayan glacial chronosequence: carbonate versus silicate weathering rates as a function of landscape surface age

We explored changes in the relative importance of carbonate vs. silicate weathering as a function of landscape surface age by examining the Ca/Sr and Sr isotope systematics of a glacial soil chronosequence located in the Raikhot watershed within the Himalaya of northern Pakistan. Bedrock in the Raikhot watershed primarily consists of silicate rock (Ca/Sr ≈ 0.20 μmol/nmol, ⁸⁷Sr/⁸⁶Sr ≈ 0.77 to 1.2) with minor amounts of disseminated calcite (Ca/Sr ≈ 0.98 to 5.3 μmol/nmol, ⁸⁷Sr/⁸⁶Sr ≈ 0.79 to 0.93) and metasedimentary carbonate (Ca/Sr ≈ 1.0 to 2.8 μmol/nmol, ⁸⁷Sr/⁸⁶Sr ≈ 0.72 to 0.82). Analysis of the exchangeable, carbonate, and silicate fractions of seven soil profiles ranging in age from ∼0.5 to ∼55 kyr revealed that carbonate dissolution provides more than ∼90% of the weathering-derived Ca and Sr for at least 55 kyr after the exposure of rock surfaces, even though carbonate represents only ∼1.0 wt% of fresh glacial till. The accumulation of carbonate-bearing dust deposited on the surfaces of older landforms partly sustains the longevity of the carbonate weathering flux. As the average landscape surface age in the Raikhot watershed increases, the Ca/Sr and ⁸⁷Sr/⁸⁶Sr ratios released by carbonate weathering decrease from ∼3.6 to ∼0.20 μmol/nmol and ∼0.84 to ∼0.72, respectively. The transition from high to low Ca/Sr ratios during weathering appears to reflect the greater solubility of high Ca/Sr ratio carbonate relative to low Ca/Sr ratio carbonate. These findings suggest that carbonate weathering controls the dissolved flux of Sr emanating from stable Himalayan landforms comprising mixed silicate and carbonate rock for tens of thousands of years after the mechanical exposure of rock surfaces to the weathering environment.     

Nd,Sr and O isotopic study of the petrogenesis of two syntectonic members of the New Hampshire Plutonic Series

 Nd, Sr and O isotope systematics were used to investigate the petrogenesis of two adjacent plutons of the Bethlehem Gneiss (BG) and the Kinsman Quartz Monzonite (KQM), exposed within the Central Maine Terrane (CMT) of New England. Both are Acadian-aged (≈413 Ma) synmetamorphic and syntectonic members of the New Hampshire Plutonic Series (NHPS). Potential source rocks analyzed for this study include Silurian and Devonian metasedimentary rocks of the CMT, and Ordovician metasedimentary rocks and granitic gneisses of the Bronson Hill Anticlinorium (BHA), which border the CMT to the west. The ɛ_Sr(413),  ɛ_Nd(413) and δ¹⁸O values for the KQM range from 56.3 to 120.0, 2.8 to −6.4, and 7.6‰ to 12.9‰, respectively; values for the BG range from 7.4 to 144.7, 0.6 to −9.3, and 8.3‰ to 11.3‰, respectively; and values for possible source rocks range from 38.1 to 654.2, −10.7 to 5.4, and 6.2‰ to 14.1‰, respectively. Both the BG and KQM have extremely heterogeneous initial isotopic compositions consistent with mixing of multiple crustal source rocks, and neither contains a volumetrically significant (i.e., ≥10%) mantlederived component. Overlapping values of ɛ_Nd(413),  ɛ_Sr(413) and δ¹⁸O values for both the BG and KQM samples resemble values for metasedimentary host rocks of the CMT and BHA. We observe no systematic correlations between ɛ_Nd and ɛ_Sr values for either the BG or the KQM. The ɛ_Sr and δ¹⁸O values for the BG do not form any simple mixing trends, nor is there any direct correlation between the isotopic compositions of contact BG samples and their adjacent host rocks, in contrast to our observations for the KQM (Lathrop et al. 1994). We propose that the KQM and BG magmas were generated through anatexis of metasedimentary rocks from both the BHA and CMT in response to crystal thickening during the Acadian orogeny. Melting may have been initiated within CMT metasediments in response to high heat production in these mid-crustal rocks combined with crustal thickening, whereas melting of BHA rocks with normal crustal heat production, which were located at lower-crustal levels than CMT rocks, is likely to have been driven by crustal thickening alone. Following upward advection of mobile BHA magmas, BHA- and CMT-derived magmas may have mingled during complex Acadian deformation in the CMT, thus accounting for the isotopic similarities we observe between the BG and the KQM. Received: 13 September 1994/Accepted: 31 January 1996     

Formation and decay of a modern massive sulfide deposit in the Indian Ocean

In December 1993, the first massive sulfides were recovered from the Indian Ocean floor, north of the Rodrigues Triple Junction. The polymetallic deposit is situated in the fourth Central Indian Ridge segment, close to the rift axis; it is hydrothermally no longer active. The deposit appears to be typical of mid-ocean ridge massive sulfide occurrences but is in a phase of disintegration and about to be buried by sediment. The chimney structures were formed by multiple hydrothermal events and are now degraded by mass wasting showing various stages of weathering. Later-stage, low-temperature hydrothermal mineralization processes led to copper and gold enrichment. Here we report on the geological setting, mineral zonation, different sulfide types and stages of formation of the “Sonne Sulfide Field”, which is part of a larger mineralized zone. Received: 20 October 1997 / Accepted: 4 December 1997     

Fluage du sel gemme sous très faibles charges

Long-term creep tests have been performed under very small uniaxial loadings ($\sigma =0.02$ to 0.1 MPa). Testing devices were set in an underground gallery, where temperature fluctuations are in the order of one hundredth of a Celsius degree. The mechanical loading was provided by dead weights. The displacements were measured through special sensors whose resolution is $\mathrm{\Delta }?={10}^{\mathrm{?8}}$. Strain rates as small as $\dot{?}={10}^{\mathrm{?12}}{\text{s}}^{\mathrm{?1}}$ were measured. The tests prove that constitutive laws deduced from tests performed on rock salt samples under standard mechanical loadings cannot be extrapolated to very small loadings. To cite this article: P. Bérest et al., C. R. Geoscience 336 (2004).     

A zonal-averaged dynamical model for the middle atmosphere including gravity wave mean flow interaction: Solstice conditions

Using a set of transformed Eulerian equations the zonal-averaged circulation of the middle atmosphere (10–110 km) is calculated on a global scale for solstice conditions. The emphasis lies on an improved modelling of the zonal momentum balance of the mesophere and lower thermosphere. For this purpose an internal gravity wave mean flow interaction model suggested by T. Matsuno has been incorporated in a slightly modified version. With this model the observed reversal of the zonal wind with height in the summer upper mesosphere and lower thermosphere can be reproduced. The coefficient of eddy momentum diffusion and the Rayleigh friction coefficient used in this model have been made temperature dependent by describing them as a function of the local static stability parameter.