Water and sediment outbursts from advanced Franz Josef Glacier,New Zealand

The Franz Josef Glacier, Westland, New Zealand, has a history of catastrophic sediment‐laden outburst ?oods associated with extreme rainfall events when the glacier toe is advanced over its own sediments. Consideration of these events and inspection of recent sediment deposits suggest that there are three distinct modes of outburst. The ?rst is associated with fans fed by over?ow along the glacier margin. As the glacier has advanced across its own fore?eld gravels, it is inferred that the primary drainage conduit has developed a reach of negative slope. In high ?ows massive boulders can block the conduit, trapping lesser clasts. The resulting backup of water causes over?ows through marginal moulins, producing the fan type of deposit. The second type of outburst deposits massive imbricated boulders at a greater or lesser distance from the glacier portal. In this case, pressure buildup drives the blockage out of the portal where the boulders deposit. Smaller materials are generally carried away. The third type consists of very shallow ?ows, and produces massive gravel deposits of uncertain provenance. In this condition, the excess pressure in the conduit results in slight uplift of the glacier and widespread discharge of water and sediment below the glacier snout; gravels and smaller sediments are laid down in a massive deposit across the fore?eld. The massive, boulder‐veneered deposit from the December 1995 outburst is interpreted in the light of the above mechanisms as a hyperconcentrated ?ow deposit from hydraulic jacking, overlain by boulders emplaced by a subsequent conduit outburst. A possible association of outbursts with the present advanced position of the glacier is suggested. Copyright © 2003 John Wiley & Sons, Ltd.     

The Mobility of Rare Earth Elements and Redox Sensitive Elements in the Groundwater/Seawater Mixing Zone of a Shallow Coastal Aquifer

The concentrations of Rare Earth Elements (REE) and Redox Sensitive Elements (RSE) were measured in groundwaters along a transect of the forest-marsh interface of a surficial aquifer system in North Inlet, SC. The well transect extended from a forest recharge area across the marsh and tidal creek to a tidal recharge area of beach ridge. The concentrations of the RSE (Fe, Mn, and U) were consistent with reducing conditions through the transect. Fe was present at concentrations ranging from a few micromolar to greater than one hundred micromolar in most wells. U was depleted with respect to salinity predicted concentrations, indicating removal with respect to the seawater endmember. Dissolved Mn concentrations were generally low in all wells, indicating no significant solid source of Mn (as MnOx) in this system. When extrapolated to a global scale, estimates of U removal during seawater exchange with the aquifer solids equaled 10–20% of the total riverine dissolved U input flux. REE concentrations in the forest recharge area were high in shallow wells, and showed a light enriched fractionation pattern, characteristic of soil leaching by Natural Organic Matter (NOM) rich waters. A decrease in REE concentration with depth in the forest wells coupled with a trend towards Heavy REE (HREE) enriched fractionation pattern indicated removal of the REE coincident with NOM and Dissolved Organic Carbon (DOC) removal. The saline waters of the beach ridge wells show a Light REE (LREE) enriched fractionation pattern and have the highest overall concentrations of the REE, indicating a significant REE source to the seawater endmember waters. The concentration gradients along the beach ridge flow path indicate a large source in the deep wells, and net export of dissolved REE to the tidal creek system and the coastal ocean. Ultrafiltration experiments indicate a transition from a colloidal dominated reservoir for the REE in the forest wells to a colloidal and dissolved reservoir in the beach ridge wells. The ultrafiltration data coupled with a correlation with Dissolved Inorganic Carbon (DIC) release suggest that there is diagenetic mobilization of an REE rich organic carbon phase in the saline endmember wells. We suggest here that degradation of this relic terrestrial organic carbon and REE rich phase results in the export of dissolved REE equal to or exceeding river inputs in this region.     

A late quaternary record of eolian silt deposition in a maar lake, St. Michael Island, western Alaska

Recent stratigraphic studies in central Alaska have yielded the unexpected finding that there is little evidence for full-glacial (late Wisconsin) loess deposition. Because the loess record of western Alaska is poorly exposed and not well known, we analyzed a core from Zagoskin Lake, a maar lake on St. Michael Island, to determine if a full-glacial eolian record could be found in that region. Particle size and geochemical data indicate that the mineral fraction of the lake sediments is not derived from the local basalt and is probably eolian. Silt deposition took place from at least the latter part of the mid-Wisconsin interstadial period through the Holocene, based on radiocarbon dating. Based on the locations of likely loess sources, eolian silt in western Alaska was probably deflated by northeasterly winds from glaciofluvial sediments. If last-glacial winds that deposited loess were indeed from the northeast, this reconstruction is in conflict with a model-derived reconstruction of paleowinds in Alaska. Mass accumulation rates in Zagoskin Lake were higher during the Pleistocene than during the Holocene. In addition, more eolian sediment is recorded in the lake sediments than as loess on the adjacent landscape. The thinner loess record on land may be due to the sparse, herb tundra vegetation that dominated the landscape in full-glacial time. Herb tundra would have been an inefficient loess trap compared to forest or even shrub tundra due to its low roughness height. The lack of abundant, full-glacial, eolian silt deposition in the loess stratigraphic record of central Alaska may be due, therefore, to a mimimal ability of the landscape to trap loess, rather than a lack of available eolian sediment.     

A methane fuse for the Cambrian explosion: carbon cycles and true polar wander

The dramatic diversification of animal groups known as the Cambrian Explosion (evolution's ‘Big Bang’) remains an unsolved puzzle in Earth Science. The Vendian–Cambrian interval is characterized by anomalously high rates of apparent plate motion, interpreted as True Polar Wander (TPW), and by more than a dozen large, high-frequency perturbations in carbon isotopes that dwarf all others observed through the past 65 million years. We suggest that these biological, tectonic, and geochemical events are intimately related in the following fashion. First, tropical continental margins and shelf-slopes which formed during fragmentation of the supercontinent Rodinia accumulated massive quantities of isotopically-light organic carbon during Late Neoproterozoic time, as indicated by strikingly heavy isotope ratios in inorganic carbon during interglacial intervals. Second, an initial phase of Vendian TPW moved these organic-rich deposits to high latitude, where conditions favored trapping biogenic methane in layers of gas hydrate and perhaps permafrost. Continued sedimentation during Late Vendian time added additional hydrate/gas storage volume and stabilized underlying units until the geothermal gradient moved them out of the clathrate stability field, building up deep reservoirs of highly pressurized methane. Finally, a burst of TPW brought these deposits back to the Tropics, where they gradually warmed and were subjected to regional-scale thermohaline eddy variation and related sedimentation regime changes. Responding to the stochastic character of such changes, each reservoir reached a critical failure point independently at times throughout the Cambrian. By analogy with the Late Paleocene Thermal Maximum event, these methane deposits yield transient, greenhouse-induced pulses of global warming when they erupt. Temperature correlates powerfully with biodiversity; the biochemical kinetics of metabolism at higher temperature decrease generation time and maintain relatively rich and dense invertebrate populations. Repeated thermal pulses along with progressive disruption and alteration of global ocean circulation patterns by TPW could cause the increase in diversity that accompanied the radiation of metazoans. We suggest that a methane ‘fuse’ ignited the Cambrian Evolutionary Explosion.     

Measurement of Carbon Dioxide Emissions Plumes from Prudhoe Bay, Alaska Oil Fields

Large carbon dioxide plumes with concentrations up to 45 ppm aboveambient levels were measured about 15 km downwind of the Prudhoe Bay, Alaskamajor oil production facilities, located at 70° N Lat. above the ArcticCircle. The measured emissions were 1.3 × 10³ metrictons (C) hour^-1 (11.4× 10⁶ metric tons(C) year^-1), six times greater than the combustion emissionsassumed by Jaffe and coworkers in J. Atmos. Chem. 20 (1995), 213–227,based on 1989 reported Prudhoe Bay oil facility fuel consumption data, andfour times greater than the total C emissions reported by the oil facilitiesfor the same months as the measurement time periods. Variations in theemissions were estimated by extrapolating the observed emissions at a singlealtitude for all tundra research transect flights conducted downwind of theoil fields. These 30 flights yielded an average emission rate of1.02 × 10³ metric tons (C) hour^-1 with astandard deviation of 0.33 × 10³. These quantity ofemissions are roughly equivalent to the carbon dioxide emissions of7–10 million hectares of arctic tussock tundra (Oechel and Vourlitis,Trends in Ecol. Evolution 9 (1994), 324–329).     

Structure of the Mount Raymond transverse zone at the southern end of the Wyoming salient, Sevier fold-thrust belt, Utah

The Mount Raymond transverse zone (MRTZ) forms the east-west-trending boundary between the Wyoming salient of the Sevier fold-thrust belt and the Uinta/Cottonwood arch in north-central Utah. Major faults in the zone dip 40° to 45° north. Our structural analysis indicates that the MRTZ contains both contractional and extensional structures. The contractional structures (thrusts and related folds) initially formed as part of a southeast-verging, northeast-trending thrust system. This system gradually curved and merged to the north with the east-verging Absaroka thrust system in the apex of the Wyoming salient. The contrast in trend between structures in the MRTZ and those in the apex of the Wyoming salient reflects the initial curvature of the salient. This curve formed because the stratigraphic sequence involved in thrusting thinned gradually southward, toward the axis of a proto-Uinta arch. The present east-west trend of the MRTZ developed during Laramide uplift of the Unita/Cottonwood arch, an event which tilted the faults into their present position. Cenozoic crustal extension subsequently reactivated segments of the MRTZ. In sum, the evolution of the MRTZ illustrates how post-thrusting processes can affect the map-view geometry of thrust belts to create transverse zones.     

Magmatic processes that generated the rhyolite of Glass Mountain, Medicine Lake volcano, N. California

Glass Mountain consists of a 1 km³, compositionally zoned rhyolite to dacite glass flow containing magmatic inclusions and xenoliths of underlying shallow crust. Mixing of magmas produced by fractional crystallization of andesite and crustal melting generated the rhyolite of Glass Mountain. Melting experiments were carried out on basaltic andesite and andesite magmatic inclusions at 100, 150 and 200 MPa, H₂O-saturated with oxygen fugacity controlled at the nickel-nickel oxide buffer to provide evidence of the role of fractional crystallization in the origin of the rhyolite of Glass Mountain. Isotopic evidence indicates that the crustal component assimilated at Glass Mountain constitutes at least 55 to 60% of the mass of erupted rhyolite. A large volume of mafic andesite (2 to 2.5 km³) periodically replenished the magma reservoir(s) beneath Glass Mountain, underwent extensive fractional crystallization and provided the heat necessary to melt the crust. The crystalline residues of fractionation as well as residual liquids expelled from the cumulate residues are preserved as magmatic inclusions and indicate that this fractionation process occurred at two distinct depths. The presence and composition of amphibole in magmatic inclusions preserve evidence for crystallization of the andesite at pressures of at least 200 MPa (6 km depth) under near H₂O-saturated conditions. Mineralogical evidence preserved in olivine-plagioclase and olivine-plagioclase-high-Ca clinopyroxene-bearing magmatic inclusions indicates that crystallization under near H₂O-saturated conditions also occurred at pressures of 100 MPa (3 km depth) or less. Petrologic, isotopic and geochemical evidence indicate that the andesite underwent fractional crystallization to form the differentiated melts but had no chemical interaction with the melted crustal component. Heat released by the fractionation process was responsible for heating and melting the crust. Received: 26 March 1996 / Accepted: 14 November 1996     

Torngat ultramafic lamprophyres and their relation to the North Atlantic Alkaline Province

Geological mapping and diamond exploration in northern Quebec and Labrador has revealed an undeformed ultramafic dyke swarm in the northern Torngat Mountains. The dyke rocks are dominated by an olivine-phlogopite mineralogy and contain varying amounts of primary carbonate. Their mineralogy, mineral compositional trends and the presence of typomorphic minerals (e.g. kimzeyitic garnet), indicate that these dykes comprise an ultramafic lamprophyre suite grading into carbonatite. Recognized rock varieties are aillikite, mela-aillikite and subordinate carbonatite. Carbonatite and aillikite have in common high carbonate content and a lack of clinopyroxene. In contrast, mela-aillikites are richer in mafic silicate minerals, in particular clinopyroxene and amphibole, and contain only small amounts of primary carbonate. The modal mineralogy and textures of the dyke varieties are gradational, indicating that they represent end-members in a compositional continuum.

The Torngat ultramafic lamprophyres are characterized by high but variable MgO (10–25 wt.%), CaO (5–20 wt.%), TiO₂ (3–10 wt.%) and K₂O (1–4 wt.%), but low SiO₂ (22–37 wt.%) and Al₂O₃ (2–6 wt.%). Higher SiO₂, Al₂O₃, Na₂O and lower CO₂ content distinguish the mela-aillikites from the aillikites. Whereas the bulk rock major and trace element concentrations of the aillikites and mela-aillikites overlap, there is no fractional crystallization relation between them. The major and trace element characteristics imply related parental magmas, with minor olivine and Cr-spinel fractionation accounting for intra-group variation.

The Torngat ultramafic lamprophyres have a Neoproterozoic age and are spatially and compositionally closely related with the Neoproterozoic ultramafic lamprophyres from central West Greenland. Ultramafic potassic-to-carbonatitic magmatism occurred in both eastern Laurentia and western Baltica during the Late Neoproterozoic. It can be inferred from the emplacement ages of the alkaline complexes and timing of Late Proterozoic processes in the North Atlantic region that this volatile-rich, deep-seated igneous activity was a distal effect of the breakup of Rodinia. This occurred during and/or after the rift-to-drift transition that led to the opening of the Iapetus Ocean.     

Fifty-thousand-year vegetation and climate history of Noel Kempff Mercado National Park, Bolivian Amazon

Pollen and charcoal records from two large, shallow lakes reveal that throughout most of the past 50,000 yr Noel Kempff Mercado National Park, in northeastern lowland Bolivia (southwestern Amazon Basin), was predominantly covered by savannas and seasonally dry semideciduous forests. Lowered atmospheric CO₂ concentrations, in combination with a longer dry season, caused expansion of dry forests and savannas during the last glacial period, especially at the last glacial maximum. These ecosystems persisted until the mid-Holocene, although they underwent significant species reassortment. Forest communities containing a mixture of evergreen and semideciduous species began to expand between 6000 and 3000 ¹⁴C yr B.P. Humid evergreen rain forests expanded to cover most of the area within the past 2000 ¹⁴C yr B.P., coincident with a reduction in fire frequencies. Comparisons between modern pollen spectra and vegetation reveal that the Moraceae-dominated rain forest pollen spectra likely have a regional source area at least 2-3 km beyond the lake shore, whereas the grass- and sedge-dominated savanna pollen spectra likely have a predominantly local source area. The Holocene vegetation changes are consistent with independent paleoprecipitation records from the Bolivian Altiplano and paleovegetation records from other parts of southwestern Amazonia. The progressive expansion in rain forests through the Holocene can be largely attributed to enhanced convective activity over Amazonia, due to greater seasonality of insolation in the Southern Hemisphere tropics driven by the precession cycle according to the Milankovitch Astronomical Theory.