Diagrammatic theory of effective hydraulic conductivity

This work presents a stochastic diagrammatic theory for the calculation of the effective hydraulic conductivity of heterogeneous media. The theory is based on the mean-flux series expansion of a log-normal hydraulic conductivity medium in terms of diagrammatic representations and leads to certain general results for the effective hydraulic conductivity of three-dimensional media. A selective summation technique is used to improve low-order perturbation analysis by evaluating an infinite set of diagrammatic terms with a specific topological structure that dominates the perturbation series. For stochastically isotropic media the selective summation yeilds the anticipated exponential expression for the effective hydraulic conductivity. This expression is extended to stochastically anisotropic media. It is also shown that in the case of non homogeneous media the uniform effective hydraulic conductivity is replaced by a non-local tensor kernel, for which general diagrammatic expressions are obtained. The non-local kernel leads to the standard exponential behavior for the effective hydraulic conductivity at the homogeneous limit.     

Diagrammatic theory of effective hydraulic conductivity

This work presents a stochastic diagrammatic theory for the calculation of the effective hydraulic conductivity of heterogeneous media. The theory is based on the mean-flux series expansion of a log-normal hydraulic conductivity medium in terms of diagrammatic representations and leads to certain general results for the effective hydraulic conductivity of three-dimensional media. A selective summation technique is used to improve low-order perturbation analysis by evaluating an infinite set of diagrammatic terms with a specific topological structure that dominates the perturbation series. For stochastically isotropic media the selective summation yeilds the anticipated exponential expression for the effective hydraulic conductivity. This expression is extended to stochastically anisotropic media. It is also shown that in the case of non homogeneous media the uniform effective hydraulic conductivity is replaced by a non-local tensor kernel, for which general diagrammatic expressions are obtained. The non-local kernel leads to the standard exponential behavior for the effective hydraulic conductivity at the homogeneous limit.     

Bioerosion and carbonate mud production on high-latitude shelves

Low-latitude carbonate muds often are composed either of entire units of skeletons (e.g., algal muds) or of precipitates, whereas high-latitude carbonate muds are bioerosional or result from maceration. Bioerosion at high latitudes is most intense in the photic zone, particularly down to 25 m depth. Shelly substrata may be crushed, bitten, drilled, bored or scraped. Clionid sponges, endolithic algae, acmaeid gastropods and regular echinoids are the most significant agents. Clionids produce distinctive facetted carbonate silt chips when boring, which have been described from both high- and low-latitudes. Faecal pellets break down to yield mud-sized carbonate particles that are more irregular than those produced by maceration. Exhumed infaunal bivalves are often preferred to epifaunal organisms as substrata. Bioerosion occurs very rapidly; shells may be totally infested with boring algae in three months. A “moth-eaten” appearance therefore does not denote a relict grain. Reliable rates of fine sediment production are not yet available.

The mud fraction of northwest European shelf sediment generally contains 10–20% CaCO₃, though an inshore and offshore belt with higher values may be identified. Some Holocene supratidal mud-flats exceed 50% CaCO₃. Much of the shelf represents a modern-day equivalent of the “calcareous shale” facies common in the geological record. Instances of synsedimentary cementation are not uncommon, particularly in association with heavily burrowed muds.     

Vapour loss (“boiling”) as a mechanism for fluid evolution in metamorphic rocks

The calculation of fluid evolution paths during reaction progress is considered for multicomponent systems and the results applied to the ternary system, CO₂-H₂O-NaCl. Fluid evolution paths are considered for systems in which a CO₂-rich phase of lesser density (vapour) is preferentially removed from the system leaving behind a saline aqueous phase (liquid). Such boiling leads to enrichment of the residual aqueous phase in dissolved components and, for certain reaction stoichiometries, to eventual saturation of the fluids in salt components. Distinctive textures, particularly radiating growths of prismatic minerals such as tremolite or diopside, are associated with saline fluid inclusions and solid syngenetic salt inclusions at a number of field localities. The most thoroughly studied of these localities is Campolungo, Switzerland, where metasomatic rocks have developed in association with fractures and veins at 500° C and 2,000 bars of pressure. The petrography of these rocks suggests that fluid phase separation into liquid and vapour has been an important process during metasomatism. Fracture systems with fluids at pressure less than lithostatic may facilitate the loss of the less dense vapour phase to conditions of the amphibolite facies.     

Geologic and atmospheric input factors affecting watershed chemistry in upper michigan

The relationships between watershed variables and lakewater chemistry were examined for 53 lakes in the Upper Peninsula of Michigan to identify factors influencing lake sensitivity to atmospheric inputs. The lakes lie in three distinct geologic/geomorphic regions. Acid neutralization capacity (ANC), sulfate, and color were correlated with parameters related to atmospheric loading, watershed area and relief, hydrology, geology, and land use for the entire 53-lake set and for lower alkalinity subsets. Acid-neutralizing capacity was related to atmospheric acidic inputs and, in the southern portion of the Upper Peninsula, to the presence of mineralized groundwater inputs. In the north, ANC is correlated with hydrologic lake type and surficial deposits. Results show the highest density of acidified lakes in the northern region, which is underlain by noncalcareous sedimentary rocks. Color was related to lake size and the presence of organic soils in the watershed, whereas lake sulfate concentration was mainly influenced by atmospheric or groundwater inputs, surficial deposits, and soil type.     

Loess stratigraphy in central China

The loess deposits in central China record world-wide climate changes of the last 2.5 Ma. Numerous climatic oscillations are marked by alternating loess and soil units with continuous coverage of the Matuyama and Brunhes Epochs. Magnetic susceptibility of the deposits correlates closely with the oxygen isotope record of the deep-sea sediments and provides an independent measure of climate and time.The key marker bed of the chinese loess sequence is the paleosol S5, the time equivalent of the mid-Brunhes oxygen isotope stages 13, 14 and 15. It marks a prolonged interval of warm humid climate lasting from approximately 615 to 470 thousand years ago. Several episodes of river downcutting coincide with deposition of the exceptionally thick loess units L1 (oceanic oxygen isotope stages 2 to 4) L2 (stage 6), L5 (stage 12) L6, (stage 16) L9 (stage 22) L15 (stage 38 about 1.15 Ma) and WS4 (about 2.3 Ma). These erosional events are interpreted as a result of episodic uplift of the Loess Plateau.The deposition of the earliest loess layers between 2.5 and 2.3 Ma ago marks a first order shift from warm and humid environments toward harsh continental steppes comparable to those of the Middle and Late Pleistocene. Little, if any lithologic or paleontologic changes were noted within or above the Olduvai magnetozone, so that the proposed Plio/Pleistocene boundary at approximately 1.65 Ma has no lithostratigraphic and biostratigraphic representation in the chinese loess series.Comparison with the Deep Sea Drilling Program core 552A in the North Atlantic and with the Santerno River section near Bologna shows that the occurrence of the earliest loess in China coincides with the timing of the first significant ice rafting in the North Atlantic and with the appearance of cold water foraminifers in the marine deposits of northern Italy. An extension of the stage system of the oxygen isotope signal extended back to the Gauss-Matuyama boundary is proposed.