Growth rate of the Laurentide Ice Sheet and sea level lowering (with emphasis on the 115,000 BP sea level low)

A physically plausible three-dimensional numerical ice flow model is used to examine the rate at which the Laurentide Ice Sheet could spread and thicken using as input likely values for the rate of fall of snowline and the amount of net mass balance over the growing ice sheet. This provides then both a test of the hypothesis of “instantaneous glacierization” and of the suggested rapid fall of world sea level to between ?20 and ?70 m below present at 115,000 BP. Two experiments are described: The first terminated after 10,050 years of model run with ice sheets centered over Labrador-Ungava and Baffin Island with a total volume of 3.0 × 10⁶ km³ of ice, whereas the second was completed after 10,000 years and resulted in a significantly larger ice sheet (still with two main centers) with a volume of 7.78 × 10⁶ km³ of ice. This latter figure is equivalent to the mass required to lower world sea level by 19.4 m. Our results indicate that large ice sheets can develop in about 10,000 years under optimum conditions.     

Sedimentology of a late Cambrian regressive sequence (bowers group), Northern Victoria land,Antarctica

The late Cambrian Bowers Group is regressive marine to non-marine. On the basis of associations of textures and sedimentary structures (both primary and biotic), and of palaeocurrent measurements, the sediments of the newly defined Mariner Formation appear to have accumulated on a platform or continental shelf of low gradient and narrow tidal range. Trilobite- and brachiopod-bearing fissile mudstone with sparse channels and scattered stratified sandstone (open marine) is succeeded by wavy-bedded and lenticular-bedded mudstone, muddy sandstone, and rippled sandstone with bands rich in brachiopods (shallow open marine), then mudstone and laminated sandstone with lenticular bodies of oosparite (open marine shoals and protected inner shelf), and red and green, ripple- and parallel-laminated sandstone, interbedded with burrowed muddy sandstone and mudstone, with lenticular bedding (tidal flat). The sharply overlying Camp Ridge Quartzite of cross-bedded sandstone, pebbly sandstone, and sandy pebble conglomerate, is regarded as braided channel alluvium.The detritus appears to have been derived from veined low-grade metamorphic rocks and fine-grained detrital sedimentary rocks that lay south of the area.     

The use of a new modifier,Kr6D,in differential sulphide flotation

The suitability of a new reagent, Kr6D, as a depressant for galena and sphalerite in a differential sulphide flotation stage has been tested. Small-scale laboratory flotation tests on pure sulphide minerals indicated that the reagent is capable of depressing both galena and sphalerite when used in small doses. At higher concentrations, chalcopyrite is also depressed. Single-stage batch-scale flotation tests have been carried out on bulk sulphide concentrates containing the three sulphide minerals. Kr6D has been compared with starch as a depressant both in the presence and in the absence of sulphur dioxide. In each case the new modifier has been shown to be more effective. When using sodium dichromate as a differential depressant in similar tests, it has been shown that the quantity necessary to achieve selectivity between chalcopyrite and galena may be considerably reduced if small quantities of Kr6D are added.     

Past and present glaciological responses to climate in eastern Baffin Island

Much of Baffin Island is close to the modern glaciation limit and climatic changes within the last decade are already being reflected in snow cover extent. Statistical analysis of glacierized and ice-free corries indicates that changes in direct solar radiation due to astronomical factors are inadequate to account for glacierization of those at present ice-free. These and other sources of evidence demonstrate the need for augmented winter snowfall in order to increase the extent of glacierization. The pattern of glacial history in this area is for maximum ice extent during the early glacial phase (>68,000, <137,000 BP), folfxlowed by a reduction in ice volume during the cold pleniglacial (>24,000, < 68,000 BP) and then a limited late glacial advance (the Cockburn Stade, ca. 8,000 BP) due to increased precipitation. The Barnes Ice Cap did not disappear in the Holocene as it did in the last interglacial. The area is highly suitable for long-term monitoring of climatic change and glacial response.     

Climatic change recorded by stable isotopes and trace elements in a British Holocene tufa

Combined stable isotope (δ¹⁸O and δ¹³C) and trace element (Mg, Sr) geochemistry from bulk tufa calcite and ostracod shell calcite from an early Holocene British tufa reveal clear records of Holocene palaeoclimatic change. Variation in δ¹⁸O is caused principally by change in the isotopic composition of Holocene rainfall (recharge), itself caused mainly by change in air temperature. The δ¹³C variability through much of the deposit reflects increasing influence of soil‐zone CO₂, owing to progressive woodland soil development. Bulk tufa Mg/Ca and Sr/Ca are controlled by their concentrations in the spring water. Importantly, Mg/Ca ratios are not related to δ¹⁸O values and thus show no temperature dependence. First‐order sympathetic relationships between δ¹³C values and Mg/Ca and Sr/Ca are controlled by aquifer processes (residence times, CO₂ degassing and calcite dissolution/reprecipitation) and probably record intensity of palaeorainfall (recharge) effects. Stable isotope records from ostracod shells show evidence of vital effects relative to bulk tufa data. The ostracod isotopic records are markedly ‘spiky’ because the ostracods record ‘snapshots’ of relatively short duration (years), whereas the bulk tufa samples record averages of longer time periods, probably decades. The δ¹⁸O record appears to show early Holocene warming, a thermal maximum at ca. 8900 cal. yr BP and the global 8200 yr BP cold event. Combined δ¹³C, Mg/Ca and Sr/Ca data suggest that early Holocene warming was accompanied by decreasing rainfall intensity. The Mg/Ca data suggest that the 8200 yr BP cold event was also dry. Warmer and wetter conditions were re‐established after the 8200 yr BP cold event until the top of the preserved tufa sequence at ca. 7100 cal. yr BP. Copyright © 2004 John Wiley & Sons, Ltd.     

Partitioning of Cu, Ni, Au, and platinum-group elements between monosulfide solid solution and sulfide melt under controlled oxygen and sulfur fugacities

We have performed six experiments in which we equilibrated monosulfide solid solution (mss) with sulfide melt in evacuated silica capsules containing solid buffers to fix oxygen and sulfur fugacity, at temperatures of 950°C, 1000°C and 1050°C at bulk concentrations of ∼50 ppm for each of the PGE and Au, 5% Ni, and 7% Cu. Concentrations of O, S, Fe, Ni and Cu were determined by electron microprobe, whereas precious metal concentrations were determined by laser-ablation inductively-coupled mass spectrometry. Partition coefficients of all elements studied show minimal dependences on oxygen fugacity from the IW to the QFM buffers when sulfur fugacity is fixed at the Pt-PtS buffer. Cu, Pt, Pd and Au are strongly incompatible and Ru remains moderately to strongly compatible under all conditions studied. At all oxygen fugacities, at the Pt-PtS sulfur buffer, Ir and Rh remain highly compatible in mss. In the single run at both low oxygen and low sulfur fugacity Ir and Rh were found to be strongly incompatible in mss. At QFM and Pt-PtS the partition coefficient for Ni shows weak temperature dependence, ranging from 0.66 at 1050°C to 0.94 at 950°C. At lower oxygen and sulfur fugacity Ni showed much more incompatible behavior. Comparison with the compositions of sulfide ores from the Lindsley deposit of Sudbury suggests that the sulfide magma evolved under conditions close to the QFM and Pt-PtS buffers. The compatible behavior observed for Ni, Ir and Rh at Lindsley and most other magmatic sulfide deposits hosted by mafic rocks requires equilibration of mss and sulfide liquid at moderately high sulfur fugacity and low temperatures near to the solidus of the sulfide magma. We argue that this constraint requires that the sulfide magma must have evolved by equilibrium crystallization, rather than fractional segregation of mss as is commonly supposed.     

The implications of laboratoryRn flux measurements to the radioactivity in groundwaters: the case of a karstic limestone aquifer

Laboratory time-scale experiments were conducted on Carboniferous Limestone gravels from the Mendip Hills area, England, with the purpose of evaluating the release of²²²Rn to the water phase. The specific surface areas of the samples were 4.14 and 1.69 cm² g⁻¹ , which provided, respectively, values of 50.6 and 12.7 pCi for the released Rn. These results allowed the calculation of the emanation coefficient of this rock matrix with respect to the release of Rn, where completely different values corresponding to 23% and 6% were found, suggesting that the extent to which grain boundaries or imperfections in aggregates of micro-crystals of calcite intersect the particle surface certainly affects the Rn release. They also permitted the evaluation of models for the generation of Rn in rocks and transfer to water, in order to interpret the radioactivity due to this gas in groundwaters from the karstic aquifer of the Mendip Hills area, where the calculated activities in groundwater based on the values of 23% and 6% for the emanation coefficient were about 51 and 15 times higher than actually measured in groundwater. Therefore, the emanation coefficient in nature is considerably smaller than in the lab experiment, and another factork (0 < k < 1) may be introduced into the equations related to the modelling, with the aim of adjusting the theoretical-practical results.     

The geochemistry of formation waters in the molasse basin of upper Austria

The geochemistry of formation waters in the Molasse basin of Upper Austria has been investigated to ascertain the extent of meteoric water replacement of the connate interstitial fluids in these sediments. The chemistry, isotopic composition, and dissolved gas contents of the groundwaters and of oil and gas associated brines have been determined. The most superficial sediments of the basin, the Innviertel (Miocene), have been completely flushed by meteoric waters within the last 200 ka. The underlying Hall and Puchkirchen formations (Miocene/Oligocene) form gas reservoirs for biogenic methane, and the associated formation waters are chemically and isotopically modified connate brines of the original marine deposition. In the northeastern part of the basin, the connate brines of the deeper sediments (Cretaceous/Jurassic) have been partially or completely replaced by meteoric waters, whereas in the south of the basin these sediments contain high salinity fluids which are substantially of connate origin. These conclusions are supported by the stable isotope composition of the various brines. Oil-associated brines from the Eocene sediments contain large amounts of dissolved radiogenic⁴⁰Ar, which suggests that the oils have migrated from high-temperature environments. This is in contrast with the Puchkirchen, for which the observed absence of radiogenic⁴⁰Ar suggests that the biogenic methane has been formed in situ. The⁴He contents of these brines and of the Cretaceous/Jurassic groundwaters are, however, less than those in the overlying Puchkirchen formation and suggest that He has been removed from the deeper sediments as a result of flushing by meteoric water. The ratios of dissolved methane and nitrogen to argon increase with increasing ammonium content of the formation waters. All of these parameters may be used as indices for the maturation of the system. The heavy noble gases, Kr and Xe, are abnormally abundant in the dissolved gases, and this is attributed to geochemical concentration of these gases by adsorption onto shales in the sediment sequence. The overall geochemical situation confirms the existence of separate hydraulic systems with little interconnection in the several overlying geological horizons.     

Land/ocean correlations during the last interglacial/glacial transition, Baffin Bay, northwestern North Atlantic: A review

Evidence from terrestrial sections, ice cores, and marine cores are reviewed and used to develop a scenario for environmental change in the area of the extreme northwest North Atlantic during marine isotope stages 5 and 4. The critical physical link between the landbased glacial chronology and marine events in Baffin Bay is the presence of carbonate rich drift along the Baffin Bay coast of Bylot Island and a detrital carbonate facies (Facies B) in Baffin Bay sediments. Cores from Baffin Bay/Labrador Sea can be dated by means of oxygen isotope variations and by peaks in the abundance of volcanic glass shards. One occurrence of Facies B is dated between late stage 5 and stage 4 and we correlate this event with the Eclipse Glaciation of Bylot Island and the Ayr Lake stade of the Foxe Glaciation of Baffin Island (= Kogalu aminozone). In contrast on West Greenland, amino acid racemization evidence suggests that the Greenland Ice Sheet developed throughout stage 4 and reached a maximum in stage 3 (Svartenhuk advance >40 ka). The oxygen isotope record in the Devon Island Ice Cap (northwest Baffin Bay) indicates that Baffin Bay was largely open during marine isotope stage 5. Analyses of shallow water molluscan and foraminiferal assemblages, deep-water foraminifera, pollen from Iand sections and deep-sea cores, and dinoflagellates from marine cores indicate that interglacial conditions prevailed during much of the stage glaciation.