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At least 1,495 records · Page 83Linked to original sources
USGS2015

The comparative limnology of Lakes Nyos and Monoun, Cameroon

Lakes Nyos and Monoun are known for the dangerous accumulation of CO 2 dissolved in stagnant bottom water, but the shallow waters that conceal this hazard are dilute and undergo seasonal changes similar to other deep crater lakes in the tropics. Here we discuss these changes with reference to climatic and water-column data collected at both lakes during the years following the gas release disasters in the mid-1980s. The small annual range in mean daily air temperatures leads to an equally small annual range of surface water temperatures (ΔT ~6–7 °C), reducing deep convective mixing of the water column. Weak mixing aids the establishment of meromixis, a requisite condition for the gradual buildup of CO 2 in bottom waters and perhaps the unusual condition that most explains the rarity of such lakes. Within the mixolimnion, a seasonal thermocline forms each spring and shallow diel thermoclines may be sufficiently strong to isolate surface water and allow primary production to reduce P CO2 below 300 μatm, inducing a net influx of CO 2 from the atmosphere. Surface water O 2 and pH typically reach maxima at this time, with occasional O 2 oversaturation. Mixing to the chemocline occurs in both lakes during the winter dry season, primarily due to low humidity and cool night time air temperature. An additional period of variable mixing, occasionally reaching the chemocline in Lake Monoun, occurs during the summer monsoon season in response to increased frequency of major storms. The mixolimnion encompassed the upper ~40–50 m of Lake Nyos and upper ~15–20 m of Lake Monoun prior to the installation of degassing pipes in 2001 and 2003, respectively. Degassing caused chemoclines to deepen rapidly. Piping of anoxic, high-TDS bottom water to the lake surface has had a complex effect on the mixolimnion. Algal growth stimulated by increased nutrients (N and P) initially stimulated photosynthesis and raised surface water O 2 in Lake Nyos, but O 2 removal through oxidation of iron was also enhanced and appeared to dominate at Lake Monoun. Depth-integrated O 2 contents decreased in both lakes as did water transparency. No dangerous instabilities in water-column structure were detected over the course of degassing. While Nyos-type lakes are extremely rare, other crater lakes can pose dangers from gas releases and monitoring is warranted.

Book chapter↗
USGS1991

Assessing the solubilities and reaction kinetics of aluminous minerals in soils

The use of chemical thermodynamics and reaction kinetics is necessary to quantitatively model the transformation of aluminous minerals and their dissolved constituents in soils and other geochemical systems. Soils are thermodynamically open systems subject to atmospheric and biological forces and do not attain overall thermodynamic equilibrium with respect to either mass or time. However, local or partial equilibrium conditions may persist for particular minerals and their dissolved constituents. Igneous and metamorphic primary minerals break down chemically to yield disordered gels or colloids and constituent ions, which can then reorganize or precipitate to form more stable hydrous oxides, silicates, carbonates or other mineral species. Naturally-occurring iron and aluminum hydrous oxides and kaolin clays, abundant in highly weathered soils, are commonly believed to be the ultimate, stable end products of weathering reactions, but usually are thermodynamically metastable with respect to more perfectly ordered, synthetic specimens. Thermodynamic stability is no guaranty of mineral persistence; with sufficient time, even the most perfectly crystallized, stable mineral will yield to the solubilizing assault of undersaturated surface waters. All of the dissolution and precipitation reactions of soil minerals are driven by energy differences in the thermodynamic stabilities of reactants and products, and the velocities (or kinetics) of such reactions are regulated by variables of the hydrogeochemical environment.

Book chapter↗
USGS1995

Hydrological processes and the water budget of lakes

Lakes interact with all components of the hydrological system: atmospheric water, surface water, and groundwater. The fluxes of water to and from lakes with regard to each of these components represent the water budget of a lake. Mathematically, the concept of a water budget is deceptively simple: income equals outgo, plus or minus change in storage. In practice, however, measuring the water fluxes to and from lakes accurately is not simple, because understanding of the various hydrological processes and the ability to measure the various hydrological components are limited.

Book chapter↗
USGS2010

The effects of land cover and land use change on the contemporary carbon balance of the arctic and boreal terrestrial ecosystems of northern Eurasia

Recent changes in climate, disturbance regimes and land use and management systems in Northern Eurasia have the potential to disrupt the terrestrial sink of atmospheric CO 2 in a way that accelerates global climate change. To determine the recent trends in the carbon balance of the arctic and boreal ecosystems of this region, we performed a retrospective analysis of terrestrial carbon dynamics across northern Eurasia over a recent 10-year period using a terrestrial biogeochemical process model. The results of the simulations suggest a shift in direction of the net flux from the terrestrial sink of earlier decades to a net source on the order of 45 Tg C year −1 between 1997 and 2006. The simulation framework and subsequent analyses presented in this study attribute this shift to a large loss of carbon from boreal forest ecosystems, which experienced a trend of decreasing precipitation and a large area burned during this time period.

Book chapter↗
USGS2012

Global Change and the Function and Distribution of Wetlands

The Global Change Ecology and Wetlands book series will highlight the latest research from the world leaders in the field of climate change in wetlands. Global Change and the Function and Distribution of Wetlands highlights information of importance to wetland ecologists. The chapters include syntheses of international studies on the effects of drought on function and regeneration in wetlands, sea level rise and the distribution of mangrove swamps, former distributions of swamp species and future lessons from paleoecology, and shifts in atmospheric emissions across geographical regions in wetlands. Overall, the book will contribute to a better understanding of the potential effects of climate change on world wetland distribution and function.

Book↗
USGS1985

Geomorphologic evidence for ground ice on Mars

For ground ice to exist on Mars, two conditions have to be met. One is the presence of permafrost; the second is the availability of water. Because the mean temperature of Mars’surface is − 80 C., permafrost 1–3 km thick occurs over the entire planet. Remote-sensing measurements suggest that water presently exists in the atmosphere and in the polar caps; frost has been observed at the Viking landing sites. Landforms that support the contention that ground ice exists on Mars include lobate ejecta from craters, small valley networks, and numerous features that may be attributed to thermokarst. Chaotically collapsed terrain, chain craters, irregular depressions, and valleys tributary to canyons all appear to be related to loss of ground ice. Patterned ground has dimensions similar to that on Earth as well as dimensions of giant size. Masswasting features are common on scarps Even though individual features may be explained as results of other processes, the combined evidence indicates that a large reservoir of ground ice has existed on Mars.

Book chapter↗
USGS2014

Identifying sources of aeolian mineral dust: Present and past

Aeolian mineral dust is an important component of the Earth’s environmental systems, playing roles in the planetary radiation balance, as a source of fertilizer for biota in both terrestrial and marine realms and as an archive for understanding atmospheric circulation and paleoclimate in the geologic past. Crucial to understanding all of these roles of dust is the identification of dust sources. Here we review the methods used to identify dust sources active at present and in the past. Contemporary dust sources, produced by both glaciogenic and non-glaciogenic processes, can be readily identified by the use of Earth-orbiting satellites. These data show that present dust sources are concentrated in a global dust belt that encompasses large topographic basins in low-latitude arid and semiarid regions. Geomorphic studies indicate that specific point sources for dust in this zone include dry or ephemeral lakes, intermittent stream courses, dune fields, and some bedrock surfaces. Back-trajectory analyses are also used to identify dust sources, through modeling of wind fields and the movement of air parcels over periods of several days. Identification of dust sources from the past requires novel approaches that are part of the geologic toolbox of provenance studies. Identification of most dust sources of the past requires the use of physical, mineralogical, geochemical, and isotopic analyses of dust deposits. Physical properties include systematic spatial changes in dust deposit thickness and particle size away from a source. Mineralogy and geochemistry can pinpoint dust sources by clay mineral ratios and Sc-Th-La abundances, respectively. The most commonly used isotopic methods utilize isotopes of Nd, Sr, and Pb and have been applied extensively in dust archives of deep-sea cores, ice cores, and loess. All these methods have shown that dust sources have changed over time, with far more abundant dust supplies existing during glacial periods. Greater dust supplies in glacial periods are likely due to greater production of glaciogenic dust particles from expanded ice sheets and mountain glaciers, but could also include dust inputs from exposed continental and insular shelves now submerged. Future dust sources are difficult to assess, but will likely differ from those of the present because of global warming. Global warming could bring about shifts in dust sources by changes in degree or type of vegetation cover, changes in wind strength, and increases or decreases in the size of water bodies. A major uncertainty in assessing dust sources of the future is related to changes inhuman land use, which could affect land surface cover, particularly due to increased agricultural endeavors and water usage.

Book chapter↗
USGS2015

Effects and empirical critical loads of Nitrogen for ecoregions of the United States

Human activity in the last century has increased nitrogen (N) deposition to a level that has caused or is likely to cause alterations to the structure and function of many ecosystems across the United States. We synthesized current research relating atmospheric N deposition to effects on terrestrial and freshwater ecosystems in the United States, and estimated associated empirical critical loads of N for several receptors: freshwater diatoms, mycorrhizal fungi, lichens, bryophytes, herbaceous plants, shrubs, and trees. Biogeochemical responses included increased N mineralization and nitrification, increased gaseous N losses, and increased N leaching. Individual species, population, and community responses included increased tissue N, physiological and nutrient imbalances, increased growth, altered root-shoot ratios, increased susceptibility to secondary stresses, altered fire regime, shifts in competitive interactions and community composition, changes in species richness and other measures of biodiversity, and increases in invasive species. The range of critical loads of nutrient N reported for U.S. ecoregions, inland surface waters, and freshwater wetlands is 1–39 kg N ha−1 yr−1, spanning the range of N deposition observed over most of the country. The empirical critical loads of N tend to increase in the following sequence: diatoms, lichens and bryophytes, mycorrhizal fungi, herbaceous plants and shrubs, trees.

Book chapter↗
USGS1994

Analysis of nitrogen saturation potential in Rocky Mountain tundra and forest: implications for aquatic systems

We employed grass and forest versions of the CENTURY model under a range of N deposition values (0.02–1.60 g N m −2 y −1 ) to explore the possibility that high observed lake and stream N was due to terrestrial N saturation of alpine tundra and subalpine forest in Loch Vale Watershed, Rocky Mountain National Park, Colorado. Model results suggest that N is limiting to subalpine forest productivity, but that excess leachate from alpine tundra is sufficient to account for the current observed stream N. Tundra leachate, combined with N leached from exposed rock surfaces, produce high N loads in aquatic ecosystems above treeline in the Colorado Front Range. A combination of terrestrial leaching, large N inputs from snowmelt, high watershed gradients, rapid hydrologic flushing and lake turnover times, and possibly other nutrient limitations of aquatic organisms constrain high elevation lakes and streams from assimilating even small increases in atmospheric N. CENTURY model simulations further suggest that, while increased N deposition will worsen the situation, nitrogen saturation is an ongoing phenomenon.

Biogeochemistry↗
USGS1986

137Cs as a tracer of recent sedimentary processes in Lake Michigan

To determine recent sediment movement, we measured the levels of 137Cs (an artificial radionuclide produced during nuclear weapons testing) of 118 southern Lake Michigan samples and 27 in Green Bay. These samples, taken from 286 grab samples of the upper 3 cm of sediment, were collected in 1975 as part of a systematic study of Lake Michigan sediment. 137Cs levels correlated well with concentrations of organic carbon, lead, and other anthropogenic trace metals in the sediment. 137Cs had a higher correlation with silt-sized than with clay-sized sediment (0.55 and 0.46, respectively). Atmospherically derived 137Cs and trace metals are being redistributed by sedimentary processes in Lake Michigan after being incorporated in suspended sediment. We determined a distribution pattern of 137Cs that represents areas of southern Lake Michigan where sediment deposition is occurring. ?? 1986 Dr W. Junk Publishers.

Hydrobiologia↗
USGS1991

Simulation of lake ice and its effect on the late-Pleistocene evaporation rate of Lake Lahontan

A model of lake ice was coupled with a model of lake temperature and evaporation to assess the possible effect of ice cover on the late-Pleistocene evaporation rate of Lake Lahontan. The simulations were done using a data set based on proxy temperature indicators and features of the simulated late-Pleistocene atmospheric circulation over western North America. When a data set based on a mean-annual air temperature of 3?? C (7?? C colder than present) and reduced solar radiation from jet-stream induced cloud cover was used as input to the model, ice cover lasting ??? 4 months was simulated. Simulated evaporation rates (490-527 mm a-1) were ??? 60% lower than the present-day evaporation rate (1300 mm a-1) of Pyramid Lake. With this reduced rate of evaporation, water inputs similar to the 1983 historical maxima that occurred in the Lahontan basin would have been sufficient to maintain the 13.5 ka BP high stand of Lake Lahontan. ?? 1991 Springer-Verlag.

Climate Dynamics↗
USGS1995

Thermodynamics of gas and steam-blast eruptions

Eruptions of gas or steam and non-juvenile debris are common in volcanic and hydrothermal areas. From reports of non-juvenile eruptions or eruptive sequences world-wide, at least three types (or end-members) can be identified: (1) those involving rock and liquid water initially at boiling-point temperatures (‘boiling-point eruptions’); (2) those powered by gas (primarily water vapor) at initial temperatures approaching magmatic (‘gas eruptions’); and (3) those caused by rapid mixing of hot rock and ground- or surface water (‘mixing eruptions’). For these eruption types, the mechanical energy released, final temperatures, liquid water contents and maximum theoretical velocities are compared by assuming that the erupting mixtures of rock and fluid thermally equilibrate, then decompress isentropically from initial, near-surface pressure (≤10 MPa) to atmospheric pressure. Maximum mechanical energy release is by far greatest for gas eruptions (≤∼1.3 MJ/kg of fluid-rock mixture)-about one-half that of an equivalent mass of gunpowder and one-fourth that of TNT. It is somewhat less for mixing eruptions (≤∼0.4 MJ/kg), and least for boiling-point eruptions (≤∼0.25 MJ/kg). The final water contents of crupted boiling-point mixtures are usually high, producing wet, sloppy deposits. Final erupted mixtures from gas eruptions are nearly always dry, whereas those from mixing eruptions vary from wet to dry. If all the enthalpy released in the eruptions were converted to kinetic energy, the final velocity ( v max ) of these mixtures could range up to 670 m/s for boiling-point eruptions and 1820 m/s for gas eruptions (highest for high initial pressure and mass fractions of rock ( m r ) near zero). For mixing eruptions, v max ranges up to 1150 m/s. All observed eruption velocities are less than 400 m/s, largely because (1) most solid material is expelled when m r is high, hence v max is low; (2) observations are made of large blocks the velocities of which may be less than the average for the mixture; (3) heat from solid particles is not efficiently transferred to the fluid during the eruptions; and (4) maximum velocities are reduced by choked flow or friction in the conduit.

Bulletin of Volcanology↗
USGS1994

Eruptive activity at Mount St Helens, Washington, USA, 1984-1988: a gas geochemistry perspective

The results from two different types of gas measurement, telemetered in situ monitoring of reducing gases on the dome and airborne measurements of sulfur dioxide emission rates in the plume by correlation spectrometry, suggest that the combination of these two methods is particularly effective in detecting periods of enhanced degassing that intermittently punctuate the normal background leakage of gaseous effluent from Mount St Helens to the atmosphere. Gas events were recorded before lava extrusion for each of the four dome-building episodes at Mount St Helens since mid-1984. For two of the episodes, precursory reducing gas peaks were detected, whereas during three of the episodes, COSPEC measurements recorded precursory degassing of sulfur dioxide. During one episode (October 1986), both reducing gas monitoring and SO2 emission rate measurements simultaneously detected a large gas release several hours before lava extrusion. Had both types of gas measurements been operational during each of the dome-building episodes, it is thought that both would have recorded precursory signals for all four episodes. Evidence from the data presented herein suggests that increased degassing at Mount St Helens becomes detectable when fresh upward-moving magma is between 2 km and a few hundred meters below the base of the dome and between about 60 and 12 hours before the surface extrusion of lava. ?? 1994 Springer-Verlag.

Bulletin of Volcanology↗
USGS1969

Experimental introduction of excess Ar40 into a granitic melt

Samples of a Precambrian granite were melted in sealed capsules to produce a radiogenic Ar 40 atmosphere over the melt. The amount of Ar 40 incorporated in the quenched charge was then determined. Under these experimental conditions the amount of argon dissolved in the quenched melt was appreciable and could be an important source of error in potassiumargon dating.

Contributions to Mineralogy and Petrology↗
USGS1992

The plinian eruptions of 1912 at Novarupta, Katmai National Park, Alaska

The three-day eruption at Novarupta in 1912 consisted of three discrete episodes. Episode I began with plinian dispersal of rhyolitic fallout (Layer A) and contemporaneous emplacement of rhyolitic ignimbrites and associated proximal veneers. The plinian column was sustained throughout most of the interval of ash flow generation, in spite of progressive increases in the proportions of dacitic and andesitic ejecta at the expense of rhyolite. Accordingly, plinian Layer B, which fell in unbroken continuity with purely rhyolitic Layer A, is zoned from >99% to ???15% rhyolite and accumulated synchronously with emplacement of the correspondingly zoned ash flow sequence in Mageik Creek and the Valley of Ten Thousand Smokes (VTTS). Only the andesiterichest flow units that cap the flow sequence lack a widespread fallout equivalent, indicating that ignimbrite emplacement barely outlasted the plinian phase. On near-vent ridges, the passing ash flows left proximal ignimbrite veneers that share the compositional zonation of their valley-filling equivalents but exhibit evidence for turbulent deposition and recurrent scour. Episode II began after a break of a few hours and was dominated by plinian dispersal of dacitic Layers C and D, punctuated by minor proximal intraplinian flows and surges. After another break, dacitic Layers F and G resulted from a third plinian episode (III); intercalated with these proximally are thin intraplinian ignimbrites and several andesite-rich fall/flow layers. Both CD and FG were ejected from an inner vent <400 m wide (nested within that of Episode I), into which the rhyolitic lava dome (Novarupta) was still later extruded. Two finer-grained ash layers settled from composite regional dust clouds: Layer E, which accumulated during the D-F hiatus, includes a contribution from small contemporaneous ash flows; and Layer H settled after the main eruption was over. Both are distinct layers in and near the VTTS, but distally they merge with CD and FG, respectively; they are largely dacitic but include rhyolitic shards that erupted during Episode I and were kept aloft by atmospheric turbulence. Published models yield column heights of 23-26 km for A, 22-25 km for CD, and 17-23 km for FG; and peak mass eruption rates of 0.7-1x108, 0.6-2x108, and 0.2-0.4x108 kg s-1, respectively. Fallout volumes, adjusted to reflect calculated redistribution of rhyolitic glass shards, are 8.8 km3, 4.8 km3, and 3.4 km3 for Episodes I, II, and III. Microprobe analyses of glass show that as much as 0.4 km3 of rhyolitic glass shards from eruptive Episode I fell with CDE and 1.1 km3 with FGH. Most of the rhyolitic ash in the dacitic fallout layers fell far downwind (SE of the vent); near the rhyolite-dominated ignimbrite, however, nearly all of Layers E and H are dacitic, showing that the downwind rhyolitic ash is of 'co-plinian' rather than co-ignimbrite origin. ?? 1992 Springer-Verlag.

Bulletin of Volcanology↗
USGS1970

Tholeiitic basalt magmatism of Kilauea and Mauna Loa volcanoes of Hawaii

The primitive magmas of Kilauca and Mauna Loa are generated by partial melting of mantle peridotite at depths of −60 km or more. Results of high-pressure melting experiments indicate that the primitive melt must contain at least 20% MgO in order to have olivine as a liquidus mineral. The least fractionated lavas of both volcanoes have olivine (Fa 13 ) on the liquidus at 1 atmosphere, suggesting that the only substance lost from the primitive melt, during a rather rapid ascent to the surface, is olivine. This relation allows the primitive composition to be computed by adding olivine to the composition of an erupted lava until total MgO is at least 20 percent. Although roughly similar, historic lavas of the two volcanoes show a consistent difference in composition. The primitive melt of Mauna Loa contains 20% more dissolved orthopyroxene, a high-temperature melting phase in the mantle, and is deficient in elements such as potassium, uranium, and niobium, which presumably occur in minor low-melting phases. Mauna Loa appears to be the older volcano, deriving its magma at higher temperature and greater depth from a more depleted source rock.

Hawaii↗
USGS1995

Ostracode δ18O and δ13C evidence of Holocene environmental changes in the sediments of two Minnesota lakes

Stable oxygen and carbon isotope geochemistry of ostracode valves, abundance and assemblages of ostracode species, and sedimentological parameters from cores taken in Williams and Shingobee Lakes in north-central Minnesota show changes in climatic and hydrologic history during the Holocene. Isotopic records are consistent with the following scenario: Before 9800 yr B.P. the two lakes were connected. Increasing evaporation through the jack/red pine period (9800-7700 yr B.P.) led to lower lake levels, leaving small separated basins. The prairie period (7700-4000 yr B.P.) reflects high aridity, and lake levels reached low stands shortly before 6500 yr B.P. Low lake levels are associated with groundwater discharge between 6500 and 6000 yr B.P. The hardwood period (4000-3200 yr B.P.) corresponds to long cold winters and warm to cool summers with lower evaporation rates and slower sedimentation. During the white pine period (<3200 yr B.P.) evaporation increased and/or precipitation shifted to the summer months. These changes can be related to shifting atmospheric circulation patterns. Zonal flow was probably dominant during the early Holocene until the end of the prairie period ( c. 4000 yr B.P.). During the hardwood period a combination of zonal and meridional flow patterns caused long and cold winters and wetter summers. During the white pine period wintners were shorter and the meridional flow pattern more significant. Today meridional flow dominates the circulation pattern.

Minnesota↗
USGS1995

Radiocarbon ages of pre-bomb clams and the hard-water effect in Lakes Michigan and Huron

Five radiocarbon ages, all determined by accelerator mass spectrometry, have been obtained for two pre-bomb bivalves from Lake Michigan and one from Lake Huron. After correcting those ages for the fractionation of 14 C in calcite and for the radioactively inert CO 2 in the atmosphere, we find residual ages, caused by the hard water effect, of about 250 years for Lake Michigan and 440 years for Lake Huron.

Lake Huron, Lake Michigan↗