Dissolution of Corundum and Andalusite in H2O-Saturated Haplogranitic Melts at 800{degrees}C and 200 MPa: Constraints on Diffusivities and the Generation of Peraluminous Melts

The mechanisms and kinetics of equilibration between peraluminousminerals and granitic melt were investigated experimentallyby the dissolution of corundum and andalusite into H₂O-saturatedmetaluminous haplogranitic melt at 800°C and 200 MPa. Mineraland haplogranitic glass rods were juxtaposed inside platinumcapsules, and then subjected to experimental conditions fortimes ranging from 12 to 2900 h. Upon melting, the mineral –meltinterface retreats with the square root of time. The compositionof the melt at the interface changes with time, but its ASI[aluminum saturation index = molar Al₂O₃/(CaO + Na₂O + K₂O)]remains constant at     

SPRING ICE-JAM FLOODING OF THE PEACE-ATHABASCA DELTA: EVIDENCE OF A CLIMATIC OSCILLATION

A historic record of spring ice-jam floods of the Peace-Athabasca Delta was analyzed for the years 1826–1995. The temporal pattern of flooding is non-random. The likelihood of a flood following a flood, or a non-flood following a non-flood, is greater than expected by chance. Probability analysis of flood occurrence reveals that the period 1860–1880 was a time of unusually few floods, and the period 1915–1950 was a time of unusually frequent floods. The long-term flood frequency is 1 flood in 6.25 years. Changes in flood frequency over the record reveal a pattern of oscillation described by a sine-based model that is correlated with the long-term (Gleissberg) cycle of solar activity. Monte Carlo simulation was used to test a Bennett Dam Model and a Cyclic Model. The Bennett Dam Model is unlikely to have generated the observed flood history (p=0.04). The observed flood history shows a better fit to the Cyclic Model (p=0.65). No correlations between floods and ENSO cold or warm events was detected. The most recent wet period began about 1900 and ended in the early 1960's prior to completion of the W. A. C. Bennett Dam in British Columbia. As independent corroboration of climatically-driven changes in flood frequency we present three additional lines of evidence. The pattern of annual muskrat returns (95 year record) reveals both 10 year cycles and long-term patterns that agree well with the observed flood cycle. The annual area burned in Wood Buffalo National Park is inversely related to flood occurrence. Incised channels and dendritic drainage patterns in the bed of Lake Mamawi provide probable evidence of a previous dry period in the delta. Climatic change or oscillation likely underlies the drying trend observed in recent decades in the Peace-Athabasca Delta.     

τ-p SEISMIC DATA FOR VISCOELASTIC MEDIA - PART 2: LINEARIZED INVERSION1

An approach to extraction of viscoelastic parameters from seismic data is implemented and succesfully tested. Viscoelastic inversion is performed using adaptive damping factors to control the sensitivity of the viscoelastic parameters in relation to the τ-p seismic data. A priori information is incorporated through the damping factors as standard deviations of the data and of the viscoelastic model parameters. The stability of the inversion process is controlled by the variation of the damping factors as a function of the residual errors and parameter updates at each iteration. Tests on synthetic and real data show that P- and S-wave quality factors, Q_p and Q_s, in addition to P- and S-wave velocities and density C_p, C_s and p, can be extracted from τ-p seismic information. Singular value decomposition analysis demonstrates that estimated Q_p and Q_s values are more affected by the presence of data inaccuracies and noise than are those of C_p and p. C_s and Q_s are not uniquely recovered due to the limited contribution of P-S converted waves. Knowledge of the viscoelastic parameters is of particular importance in accurately describing petrophysical properties of rocks and pore fluids existing in the subsurface; this is demonstrated with real data from the Gulf of Mexico.     

Quaternary changes in level of the upper Taylor Glacier, Antarctica: implications for paleoclimate and East Antarctic Ice Sheet dynamics

Glacial drifts perched alongside outlet glaciers that drain through the Transantarctic Mountains constrain inland polar plateau ice elevations. The Taylor Glacier, which heads in the Taylor Dome (a peripheral dome of the East Antarctic Ice Sheet), drains East Antarctic ice into the Dry Valleys sector of Transantarctic Mountains and terminates in central Taylor Valley, about 24 km west of the Ross Sea. Five gravel-rich drifts (including 39 distinct moraine ridges) fringe a lateral lobe of the Taylor Glacier in the lower Arena Valley, Quartermain Mountains, southern Victoria Land. ³He and ¹⁰Be exposure age dating (from Brook et al . 1992), together with Arena Valley stratigraphy and soil morphologic data, provide chronologic control for these drifts and constrain maximum Quaternary thickening of the inland Taylor ice dome to less than 160 m. These minor Quaternary expansions of Taylor Glacier were out-of-phase with outlet glaciers that pass through the Transantarctic Mountains and terminate in the Ross Sea north and south of the Dry Valleys region. Textural analyses suggest that drift deposition occurred from cold-based ice, even though Taylor Glacier advances most likely occurred during global interglaciations. The thermal regime of former Taylor Glacier ice lobes, the character of geomorphic features superimposed on individual drifts, the chemical composition of soils developed on Taylor drifts, and the stability of in situ moraine ridges on steep valley walls suggest that the present cold-desert climate in Arena Valley has persisted for at least the last 2.2 Ma.     

Gould Pond, Maine: late-glacial transitions from marine to upland environments

Gould Pond sediments are unusual in North America in that they include a continuous record of change from marine to freshwater conditions during the late-glacial period, with a wide array of micro- and macrofossils deposited during a period of high sedimentation rate. Marine waters, much colder than those in the present Gulf of Maine, covered the site at the time of deglaciation (c. 13,200 BP). Plants characteristic of modem tundra grew on nearby uplands. Marine recession, due to isostatic rebound of the land, occurred from c . 12,800–12,200 BP. The lake water was completely freshened by 12,000 BP. A sparse shrub-herb tundra became established around Gould Pond as marine waters receded. Subsequent to 11,300 BP, sedges and other herbs became more abundant, and willow and Dryas less abundant, signifying increased warmth and decreased frost action. At least six tree species, all now common in the area, arrived around Gould Pond between c . 10,800 and 10,500 BP. This rapid transition was coincident with the most rapid major non-anthropogenic change of vegetation at sites across eastern North America during the postglacial period.     

EFFECTS OF CLIMATE CHANGE ON THE FRESHWATERS OF ARCTIC AND SUBARCTIC NORTH AMERICA

Region 2 comprises arctic and subarctic North America and is underlain by continuous or discontinuous permafrost. Its freshwater systems are dominated by a low energy environment and cold region processes. Central northern areas are almost totally influenced by arctic air masses while Pacific air becomes more prominent in the west, Atlantic air in the east and southern air masses at the lower latitudes. Air mass changes will play an important role in precipitation changes associated with climate warming. The snow season in the region is prolonged resulting in long-term storage of water so that the spring flood is often the major hydrological event of the year, even though, annual rainfall usually exceeds annual snowfall. The unique character of ponds and lakes is a result of the long frozen period, which affects nutrient status and gas exchange during the cold season and during thaw. GCM models are in close agreement for this region and predict temperature increases as large as 4°C in summer and 9°C in winter for a 2 × CO₂ scenario. Palaeoclimate indicators support the probability that substantial temperature increases have occurred previously during the Holocene. The historical record indicates a temperature increase of > 1°C in parts of the region during the last century. GCM predictions of precipitation change indicate an increase, but there is little agreement amongst the various models on regional disposition or magnitude. Precipitation change is as important as temperature change in determining the water balance. The water balance is critical to every aspect of hydrology and limnology in the far north. Permafrost close to the surface plays a major role in freshwater systems because it often maintains lakes and wetlands above an impermeable frost table, which limits the water storage capabilities of the subsurface. Thawing associated with climate change would, particularly in areas of massive ice, stimulate landscape changes, which can affect every aspect of the environment. The normal spring flooding of ice-jammed north-flowing rivers, such as the Mackenzie, is a major event, which renews the water supply of lakes in delta regions and which determines the availability of habitat for aquatic organisms. Climate warming or river damming and diversion would probably lead to the complete drying of many delta lakes. Climate warming would also change the characteristics of ponds that presently freeze to the bottom and result in fundamental changes in their limnological characteristics. At present, the food chain is rather simple usually culminating in lake trout or arctic char. A lengthening of the growing season and warmer water temperature would affect the chemical, mineral and nutrient status of lakes and most likely have deleterious effects on the food chain. Peatlands are extensive in region 2. They would move northwards at their southern boundaries, and, with sustained drying, many would change form or become inactive. Extensive wetlands and peatlands are an important component of the global carbon budget, and warmer and drier conditions would most likely change them from a sink to a source for atmospheric carbon. There is some evidence that this may be occurring already. Region 2 is very vulnerable to global warming. Its freshwater systems are probably the least studied and most poorly understood in North America. There are clear needs to improve our current knowledge of temperature and precipitation patterns; to model the thermal behaviour of wetlands, lakes and rivers; to understand better the interrelationships of cold region rivers with their basins; to begin studies on the very large lakes in the region; to obtain a firm grasp of the role of northern peatlands in the global carbon cycle; and to link the terrestrial water balance to the thermal and hydrological regime of the polar sea. Overall, there is a strong need for basic research and long-term monitoring. © 1997 John Wiley & Sons, Ltd.     

Contrasting morphodynamics in alluvial fans and fan deltas: effect of the downstream boundary

Alluvial fans and fan deltas can, in principle, have exactly the same upstream conditions, but fan deltas by definition have ponding water at their downstream boundary. This ponding creates effects on the autogenic behaviour of fan deltas, such as backwater adaptation, mouth bars and backward sedimentation, whereas alluvial fans may lack these effects. Hence the present authors hypothesize that morphodynamics on alluvial fans are determined primarily by upstream boundary conditions, whereas morphodynamics on fan deltas are determined by both the upstream and the downstream boundary condition and changes therein. To isolate the effects of the upstream and downstream boundaries, five new alluvial fan experiments are compared with the details of three fan deltas published earlier that were formed under very similar and simple conditions. Similar to the fan deltas, the alluvial fans build up by sheet flow, whilst quasi‐regular periods of incision cause temporary channelized flow. Incision is followed by channel backfilling, after which the fan returns to sheet flow. The channelization and backfilling in alluvial fans is markedly less pronounced and more prone to autogenic disturbance than in fan deltas. The difference is caused by morphodynamics at the downstream boundary. In a fan delta, the flow expansion of the channel causes deposition of all the sediment, which forms a mouth bar and causes strong backfilling. In an alluvial fan, on the other hand, the slope break at the fan perimeter causes some deposition, but transport is not reduced to zero. Consequently, the backfilling in alluvial fans is less pronounced than in fan deltas. Other published experiments support this trend: removal of the mouth bar by a river leads to permanent channelization, whilst pronounced mouth‐bar formation in highly channelized deltas promotes backward sedimentation. The experimental results for this study predict that, when alluvial fans prograde into lakes or deep rivers, they transition to fan deltas with increasingly deeper channels and thicker backfill deposits.     

The character of seismo-turbidites in the S-1 sapropel, Zakinthos and Strofadhes basins, Greece

The early Holocene S-1 sapropelic sequence in the northwest Hellenic Trench has been studied in six piston cores from the Zakinthos and Strofadhes basins. The S-1 sequence, 0.7-3.5 m thick, consists principally of silt to mud turbidites, with rare, thick, disorganized, sandy turbidites. These lithofacies are described and compared with fine-grained turbidites from the literature. Petrographical data, including the abundance of organic carbon and planktonic microfossils, indicate that the principal source of sediment to the turbidites was from the continental slope. On the basis of composition and texture, five turbidite units can be correlated between the two basins. These basins are fed by separate but adjacent drainage systems. The apparently synchronous occurrence of turbidites in the two drainage systems suggests that the turbidity currents were seismically triggered. Some of the turbidites show poorly organized beds which may reflect the slump origin and the short (30 km) distances of travel. Turbidites were deposited more frequently in the S-1 sapropelic interval than in the over- and underlying sediments. Application of slope stability analysis shows that on the 8° slopes above the basins, a 10-cm-thick sapropel would have a factor of safety of about 2, and would fail with earthquake accelerations in excess of 0.08 g. The frequency of earthquakes likely to produce such accelerations is similar to the observed frequency of turbidites. The low strength of the sapropelic sediment makes it particularly susceptible to such failure. Similar thin-skinned slumping may be an important process for the initiation of turbidity currents in other environments where there are steep slopes or high sedimentation rates.