Evaluation of heterogeneity and field-scale groundwater flow regime in a leaky till aquitard

Recent work in southern Ontario, Canada, demonstrates anomalously high vertical groundwater flow velocities (>1 m/year) through a thick (as much as 60 m), sandy silt till aquitard (Northern till), previously assumed to be of very low permeability (hydraulic conductivity <10^–10 m/s). Rapid recharge is attributed to the presence of fractures and sedimentary heterogeneities within the till, but the field-scale flow regime is poorly understood. This study identifies the nature of physical groundwater pathways through the till and provides estimates of the associated groundwater fluxes. The aquitard groundwater flow system is characterized by integrating details of the outcrop and subsurface sedimentary characteristics of the till with field-based hydrogeologic investigation and numerical modeling. Outcrop and subsurface data identify a composite internal aquitard stratigraphy consisting of tabular till beds (till elements) separated by laterally continuous sheet-like sands and gravels (interbeds) and boulder pavements. Individual till elements contain sedimentary heterogeneities, including discontinuous sand and gravel lenses, vertical sand dikes, and zones of horizontal and vertical fractures. Hydrogeologic field investigations indicate a three-layer aquitard flow system, consisting of upper and lower zones of more hydraulically active and heterogeneous till separated by a middle unit of relatively lower hydraulic conductivity. Groundwater pathways and fluxes in the till were evaluated using a two-dimensional aquitard/aquifer flow model which indicates a step-wise flow mechanism whereby groundwater moves alternately downward along vertical pathways (fractures, sedimentary dikes) and laterally along horizontal sand interbeds within the till. This model is consistent with observed hydraulic-head and isotope profiles, and the presence of tritiated pore waters at various depths throughout the till. Simulations suggest that a bulk aquitard vertical hydraulic conductivity on the order of 1×10^–9 m/s is required to reproduce observed hydraulic-head and tritium profiles. Electronic Publication     

Diatom, Pollen, and Chemical Evidence of Postglacial Climatic Change at Big Lake, South-Central British Columbia, Canada

Postglacial climatic conditions were inferred from cores taken from Big Lake in southern British Columbia. Low concentrations of nonarboreal pollen and pigments near the base of the core suggest that initial conditions were cool. Increases in both aquatic and terrestrial production suggest warmer and moister conditions until 8500 cal yr B.P. Hyposaline diatom assemblages, increases in nonarboreal pollen, and increased concentrations of pigments suggest the onset of arid conditions from 8500 to 7500 cal yr B.P. Slightly less arid conditions are inferred from 7500 until 6660 cal yr B.P. based on the diatoms, small increases and greater variability in biogenic silica and pigments, and higher percentages of arboreal pollen. At 6600 cal yr B.P., changes in diatoms, pigments, biogenic silica, and organic matter suggest that Big Lake became fresh, deep, and eutrophic until 3600 cal yr B.P., when water levels and nutrients decreased slightly. Our paleoclimatic inferences are similar to pollen-based studies until 6600 cal yr B.P. However, unlike these studies, our multiple lines of evidence from Big Lake imply large changes in effective moisture since 6000 cal yr B.P.     

Asymmetric spreading across the Juan de Fuca and Gorda rises as obtained from a detailed magnetic survey

Marine magnetic data collected along 37 east-west lines off the Washington and Oregon coast between 42°N and 48°N westward to 144°W have been interpreted. Our interpretation shows that decrease in the offsets of the anomalies across the Surveyor fracture zone and the presence of undisturbed north-south lineations east of it are the result of continuous asymmetric spreading along the Surveyor fracture zone. The survey has delineated the extension of the Blanco fracture zone north-westward to about 133°W. Movement along the Blanco fracture zone was initiated about 15 million years ago. The undisturbed north-south trend of the magnetic anomalies between latitudes 42°N and 48°N and longitudes 133°W and 136°W is interpreted as the interval (22 to 15 my) during which the Juan de Fuca and Gorda rises were one continuous structure. West of 137° the Surveyor, Sedna and three minor fracture zones are mapped.     

Aragonite crystallization from strained calcite at reduced pressures and its bearing on aragonite in low-grade metamorphism

Aragonite, the dense form of CaCO₃, grew hydrothermally at 100–300° C and dry at 300–400° C at very low pressures from calcite strained by grinding. Nearly complete inversion to aragonite occurred in some runs with Ca-Mg chloride solutions at 0–2.4 kb and 100–200° C on strained calcite having a (10¯14) reflection with a half-width of 0.48° 2 Cu K. A little aragonite grew dry at one atm. from the ground calcite at 300–400° C in a few hrs. Simultaneous shear during recrystallization of calcite in a rotating squeezer resulted in significant aragonite at 300–400° C several kb. below the stability field. No inversion occurred in any ground calcite when previously annealed in CO₂ at 500° C for a few hrs. Thermochemical data show that at least 200 cal/mole of strain energy can be produced in calcite by mild deformation. This much stored energy would lower the pressure requirements of aragonite, relative to the strained calcite by more than 3 kb, and our observation that aragonite growth was faster than strain recovery of calcite indicates that aragonite can grow in nature at reduced pressures from strained calcite.Some experiments were also carried out on highly magnesian calcites with the thought that aragonite might also form at the expense of this metastable material. No aragonite was produced, but the possibility that this mechanism could be operative in nature cannot be discounted.The microtexture of aragonitic deformed marbles from NW Washington (prehnite-pumpellyite facies rocks, courtesy of J. A. Vance) as well as electron probe microanalysis of these rocks indicates that aragonite selectively replaced highly strained calcite. The calcite-aragonite transition is thus a questionable indicator of high-pressure in certain metamorphic rocks.     

Observations of the diurnal oscillation of the inversion over the Israeli coast

Vertical profiles of temperature and humidity were measured over the sea in two series of 48 hours each, during the summer near the Israeli coast of the Mediterranean. A prominent inversion was observed in the temperature profiles. In the first series the average height of the inversion base was 350m and in the second, 600m. In the inversion a very sharp decrease of the absolute humidity was found. Below the inversion down to the sea surface the atmosphere was well mixed. A significant diurnal oscillation was observed at the height of the inversion base. During the day the inversion moved up and during the night it moved down. This movement was 250m in the first series and 450m in the second. The movement of the inversion base was almost adiabatic. It is suggested that the fluctuation in the height of the inversion base is mainly due to the developing breeze.     

Atmospheric input of inorganic nitrogen to Delaware Bay

The coastal waters of the mid-Atlantic region of the United States receive inputs of atmospheric pollutants as a consequence of being located downwind from major industrial and urban emissions. These inputs are potentially the largest received by any marine area of the country. Of current interest is the atmospheric input of dissolved inorganic nitrogen (DIN = NO₃ ^?+NH₄ ⁺). We have conducted a first-order examination of the magnitude of atmospheric DIN deposition relative to other large-scale inputs for Delaware Bay, a partially urbanized mid-Atlantic coastal plain estuary. The following loading terms: direct atmospheric deposition, indirect atmospheric loading, urban point discharges, fluvial input, benthic flux, and salt marsh export were evaluated. On an annual basis, municipal-industrial effluents provide a dominant source (ca. 40%) of the DIN inputs to the estuary. Total (wet plus dry) atmospheric deposition accounts for about 15% of the total annual DIN inputs. However, during summer, which is characterized by low river-flow and seasonally maximum atmospheric loading, this figure increases to around 25%. Although atmospheric input can satisfy only a fraction of the primary production demands, this summer flux may represent an ecologically important source of external DIN, half of which is directly deposited to surface photic zones where it is readily available for biological uptake.     

Criteres d'analyse géochimique des gisements de fluorine basés sur l'étude de la distribution des lanthanides — application au gite de Maine (71—Cordesse,France)

59 fluorite samples have been analysed by neutron activation for La, Ce, Sm, Eu, Tb, Dy, Yb and Lu. Two properties of the partition coefficient of the R. E. E. between fluorite and poorly complexing solutions (L. Marchand, 1976) have been used for interpretations: (R. E. E.) CaF₂/(R. E. E.)_solution are always very high so that CaF₂ records with an important magnification and without major distortion in the R. E. E. pattern from the mother solution; the partition coefficients are higher for the heavy earths than for the lighter ones. Consequently, in finite reservoirs where CaF₂ precipitates, the trend for solutions will be a gradual enrichment in ceric earths. Interpretations at different scales of the Maine hydrothermal deposit (D. Joseph, 1974) permit to emphasize: — a constant geochemical signal at the scale of a growth band of homogenous colour; — a possible characterization of the ore-forming solutions dynamic (infinite vs finite reservoirs) by comparison of successive growth bands in a same sample; — distinct source rocks for the formation blanche and formation verte.     

Maximum carbon isotope fractionation in photosynthesis by blue-green algae and a green alga

The maximum carbon isotope fractionation occurring in photosynthetic fixation of carbon dioxide in pure cultures of blue-green algae was ?23.9%. and for a green alga was ?22.6%., Maximum fractionations were obtained where cell densities were low and carbon dioxide concentrations were greater than 0.5%. Fractionation was reduced at higher temperatures using a thermophilic blue-green alga. For filamentous blue-green algae wherein clumping occurs and localized cell concentrations were high, fractionation was also lower. Fractionations reported in literature for Precambrian organic materials are comparable to the maximum fractionations reported here. This suggests that the early photosynthetic organisms developed under conditions of high carbon dioxide availability, i.e. slow growth rates and/or low population densities.     

Potassium-argon dating of polyhalite in southeastern New Mexico

Polyhalite, K₂Ca₂Mg (SO₄)₄-2H₂O, is an important mineral in many evaporites. Although its use for K-Ar dating has never been investigated, our results indicate that it is a very useful mineral for dating events ranging from the time of potash mineralization to any younger events which may have affected the evaporite. Five K-Ar dates on pure polyhalite, including two from included material and from beds distorted by the formation of a rubble chimney, yield dates between 198 and 216 Myp, in good agreement with Rb-Sr dates and the diagenetic age of the potash deposits from the same rocks. Two polyhalites mixed with sylvite gave lower dates (154 and 174 Myr) which is to be expected because of radiogenic ⁴⁰Ar loss from the sylvite phase. One polyhalite, formed after the intrusion of a 31 Myr lamprophyre dike, yielded 21 Myr. Collectively our results indicate that pure polyhalite is satisfactory for K-Ar dating and may provide critical age information in studies of the geologic history of the evaporite sequences.