bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal history of comets during residence in the Oort Cloud: Effect of radiogenic heating in combination with the very low thermal conductivity of amorphous ice
Authors: Haruyama Jun'ichi , Yamamoto Tetsuo , Mizutani Hitoshi , Greenberg J. Mayo
Published: February 2008

The thermal history of cometary nuclei during residence in the Oort cloud is studied with the use of the very low thermal conductivity of amorphous ice recently obtained by Kouchi et al. [1992a]. The heat sources included are (1) radioactive nuclides 40K, 232Th, 235U, and 238U with their chondritic abundances, and (2) latent heat released in transition from amorphous ice to crystalline ice. We model the cometary nucleus as a porous aggregate of grains with each individual grain being composed of a refractory core and an icy mantle. It is assumed that the ice is initially amorphous. The bulk thermal conductivity of a cometary nucleus is assumed to be expressed by the product of the thermal conductivity of individual grains and a reduction factor resulting from the porous structure of the nucleus. Numerical results of the thermal history are presented for various conditions including one case which includes heating by 26Al decay. It is shown that the thermal histories are clearly classified into two distinct types depending mainly on the nucleus thermal conductivity κ. (1) Comets with small κ experience a runaway increase in the internal temperature to higher than 120 K during residence in the Oort cloud, in which case most of the ice in the nucleus crystallizes. (2) Comets with a sufficiently large κ on the other hand, do not exhibit a runaway heating and the temperature is limited to < 100 K so that the initial amorphous ice is almost completely preserved. A criterion of nuclear ice crystallization is presented in an analytic expression derived from the analysis of the physical processes of the crystallization. A brief discussion is given on the implications of the results for the sources of volatile molecules observed in the coma.
Assumptions about the presence of natural glasses on Mars
Authors: Bouška Vladimír , Bell James F.
Published: February 2008

Natural glasses occur not only on Earth but also in lunar rocks and in some meteorites. On geological grounds, natural glasses could be expected on the surface of Mars and their presence would have important implications for the weathering and alteration of the near‐surface layer. Ultramafic and mafic magmatism rich in iron and some volatiles appears to have been important on Mars as has been impact cratering, volcanism, and possibly hydrothermal alteration. Available data indicate that we can presume the past or present existence of volcanic basaltic glasses, impact glasses formed by thermal fusion of meteorite‐impact targets, and diaplectic (mainly maskelynite) glasses. Conditions appear unsuitable for the formation of tektites on the Martian surface, and the production of measurable amounts of fulgurite or combustion glass seems improbable. Prospects for the remote and in situ identification of Martian glasses and their subsequent weathering products are discussed and include orbital reflectance and thermal emission spectroscopic measurements and Mössbauer spectroscopy from a surface lander.
Microfaunal Evidence for Elevated Pliocene Temperatures in the Arctic Ocean
Authors: Cronin Thomas M. , Whatley Robin , Wood Adrian , Tsukagoshi Akira , Ikeya Noriyuki , Brouwers E. M. , Briggs W. M.
Published: February 2008

The migration of thermophilic marine Ostracoda into the Arctic Ocean during the Pliocene indicates that winter and summer ocean temperatures around Arctic margins were ≥ 0 °C and > 3 °C, respectively, and that ice‐free conditions existed for most or all of the Arctic. By at least 3.5–3.0 Ma, probably earlier, the opening of the Bering Strait allowed marine organisms to migrate through the Arctic Ocean, mostly from the Pacific Ocean. Migrant taxa such as Cythere, Hemicythere, and Neomonoceratina are known from Pliocene deposits of Alaska and Canada as well as Neogene deposits of the North Pacific and Atlantic oceans. On the basis of ecological and Zoogeographic information on ostracode species from more than 800 modern “core top” samples for the North Atlantic, North Pacific, and Arctic Oceans, we determined winter and summer temperature tolerances for migrant taxa to be at or above about 0 °C and 3 °C. This suggests ice‐free summers, and probably, a perennially ice‐free Arctic Ocean in some regions. Elevated water temperatures in the Arctic Ocean between 3.5 and 2.0 Ma is supported by evidence for late Pliocene increased meridional heat transport in the North Atlantic Ocean.
Integrated Late Eocene‐Oligocene Stratigraphy of the Alabama Coastal Plain: Correlation of Hiatuses and Stratal Surfaces to Glacioeustatic Lowerings
Authors: Miller Kenneth G. , Thompson Peter R. , Kent Dennis V.
Published: February 2008

We integrated strontium and oxygen isotopic, biostratigraphic, and magnetostratigraphic studies of two upper Eocene‐Oligocene boreholes drilled near Bay Minette and St. Stephens Quarry (SSQ), Alabama. Continuous coring provided fresh, unweathered material for magnetostratigraphic studies, minimizing problems reported from nearby outcrops. Difficulties with each technique were encountered because of diagenesis, absence of marker fossils, and the presence of unconformities; however, by integrating results from isotopic stratigraphy, biostratigraphy, and magnetostratigraphy, we correlated these relatively shallow‐water deposits to the geomagnetic polarity time scale (GPTS). At the SSQ borehole, the upper Eocene to lower Oligocene section is apparently complete within our stratigraphic resolution (0.2–0.5 m.y.), allowing us to estimate the ages of several stratal surfaces. Late Eocene Sr isotope age estimates are as expected at the SSQ borehole, but Oligocene ages are ∼1 m.y. older than expected due to diagenesis. At the Bay Minette borehole, a latest Eocene‐earliest Oligocene and a late early Oligocene hiatus were detected. We correlate these two hiatuses and stratal surfaces at SSQ with global δ18O increases inferred to represent glacioeustatic lowerings and with evidence for hiatuses on other continental margins: (1) a distinct disconformity at the base of the Chickasawhay Limestone at both boreholes and a hiatus at Bay Minette correlates with a global δ18O increase; we revise the age of this surface (equivalent to the TB 1.1 sequence boundary) making it ∼2 m.y. older than previously reported; and (2) a surface at the top of the Shubuta Member (lowermost Oligocene) has been interpreted both as a condensed section and a disconformity; this surface at SSQ and a hiatus at Bay Minette correlate with a sharp global δ18O increase and with hiatuses on the New Jersey and Irish margins. The timing of the hiatuses and stratal surfaces correlates with the inflection of the δ18O increases and not with the maximum values, supporting models that indicate that unconformities form during the maximum rates of sea level fall.
Seasonal Trends and Preservation Biases of Polycystine Radiolaria in the Northern California Current System
Authors: Welling Leigh A. , Pisias Nicklas G.
Published: February 2008

We analyzed a 2‐year sediment trap record of 76 species of Radiolaria from three locations across the northern California Current System. Q‐mode factor analysis identifies the fundamental trends in the radiolarian data with a five‐factor model that explains 86% of the trap data. The factor assemblages that emerge from this analysis have temporal and spatial aspects that reflect fundamental oceanographic features of the eastern North Pacific. The most dominant process revealed by this analysis is related to the seasonality of the California Current. Two factors illustrate this: the California Current factor, which has highest loadings during summer and fall, and the Winter factor, which is most important in the winter and spring. Together they account for 75% of the information in the trap samples from the two moorings located in the coastal region extending to ∼300 km offshore. The oceanic environment sampled by the mooring 650 km offshore is strongly influenced by transitional waters and those from the Central and Subarctic Gyres. Loadings of the three factors related to these oceanographic regions show marked differences between years, which illustrates the latitudinal variability in the divergence of the North Pacific Current. Spatial distributions of the trap factors in surface sediments beneath the California Current System generally agree with the temporal and spatial patterns they exhibit in the traps. Nonetheless, the trap factor model can only describe up to 50% of the radiolarian sediment data, which means the species ratios in the sediment traps are significantly different than those in the sediments. A factor model using only robust species reveals the same basic trends as the first analysis, but the culled model is able to describe a larger fraction of the sediment data (63%–91% in sediments directly beneath the mooring sites). However, the increase in the amount of sediment data explained by the culled model is not distributed equally across all factors but is due to very high estimates of one factor in the coastal sediments, the California Current factor. At the Nearshore mooring site (130 km offshore) this factor explains 69% of the sediment data versus only 32% of the data in the traps where it was derived. This discrepancy suggests that either these two years of trap data are anomalous and have underestimated the predominance of the California Current assemblage over long time periods or the culled factor model, rather than eliminating the dissolution bias, actually overemphasizes the importance of well‐preserved species in the coastal environment.
Seasonal salinity changes during Mediterranean sapropel deposition 9000 years B.P.: Evidence from isotopic analyses of individual planktonic foraminifera
Authors: Tang Carol M. , Stott Lowell D.
Published: February 2008

Using stable isotopic analyses of individual tests of planktonic foraminifera with different seasonal and depth habitats, we have investigated temporal variations in surface water salinities in the eastern Mediterranean during the past 13,000 years. Low oxygen isotopic values document the presence of anomalously low salinity conditions in the eastern Mediterranean at about 13,000 years B.P., 4000 years before the deposition of the youngest sapropel. By 11,000 years B.P., during the deposition of presapropel sediments, low‐salinity surface waters were more pervasive but were restricted to summer months and only affected the surface of the water column. At the onset of sapropel deposition about 9000 years B.P., surface salinities in the eastern Mediterranean had decreased by almost 4‰ and had extended to depths of at least 75 m. The low‐salinity surface lens was not, however, a permanent feature of the eastern Mediterranean during sapropel deposition. Low‐salinity surface conditions occurred in the fall and winter seasons, but normal sea surface salinities returned during spring months on a seasonal and/or interannual basis. Thus, a low‐salinity surface lens could have existed continuously for months or decades at a time but would have been interrupted intermittently by a return to normal conditions. Any triggering mechanism proposed for Mediterranean sapropels must account for the seasonal fluctuations and ephemeral nature of low‐salinity conditions in the eastern Mediterranean. Despite normal salinities returning occasionally during spring months, a low‐salinity surface lens may have greatly inhibited the production of Levantine intermediate water, prevented the formation of Adriatic deep water, and consequently, reduced ventilation of the eastern Mediterranean.
Latest Paleocene lithologic and biotic events in neritic deposits of southwestern New Jersey
Authors: Gibson Thomas G. , Bybell Laurel M. , Owens James P.
Published: February 2008

In the southwestern New Jersey Coastal Plain, four drill holes contain continuous neritic sedimentation across the Paleocene/Eocene boundary (calcareous nannofossil Zone NP 9/NP 10 boundary). Significant lithologic and biotic changes occur in these strata near the top of the Paleocene. Global warming, increased precipitation, and other oceanographic and climatic events that have been recognized in high‐latitude, deep‐oceanic deposits of the latest Paleocene also influenced mid‐latitude, shallow‐marine, and terrestrial environments of the western North Atlantic. The diverse, well‐preserved calcareous nannofossil flora that is present throughout the entire New Jersey boundary section accurately places these events within the uppermost part of the upper Paleocene Zone NP 9. Several rapid but gradational changes occur within a 1.1‐m interval near the top of Zone NP 9. The changes include (1) a change in lithology from glauconitic quartz sand to clay, (2) a change in clay mineral suites from illite/smectite‐dominated to kaolinite‐dominated, (3) a change in benthic foraminiferal assemblages to a lower diversity fauna suggestive of low‐oxygen environments, (4) a significant increase in planktonic foraminiferal abundance, and (5) an increased species turnover rate in marine calcareous nannofossils. Pollen was sparse in the New Jersey drill holes, but terrestrial sporomorph species in Virginia exhibit increased turnover rates at a correlative level. Foraminiferal assemblages and lithology indicate that relative sea level rose in New Jersey at the same time as these late Paleocene events occurred in late Biochron NP 9. The higher sea levels influenced sediment type and absolute abundance of planktonic foraminifers in the deposits. Above the initial increase of kaolinite in the upper part of Zone NP 9, the kaolinite percentage continues to increase, and the maximum kaolinite value occurs in the uppermost part of Zone NP 9. There are few changes in either the sediments or the biota precisely at the Zone NP 9/NP 10 boundary in New Jersey. The clay‐rich deposits with a high kaolinite clay mineral suite, the lowered diversity benthic foraminiferal assemblages, the abundant planktonic foraminiferal specimens, and the calcareous nannofossil assemblages continued essentially unchanged into the earliest Eocene Zone NP 10. Within the lower part of Zone NP 10, the kaolinite percentage decreased to very low values.
Middle Miocene ocean‐climate transition: High‐resolution oxygen and carbon isotopic records from Deep Sea Drilling Project Site 588A, southwest Pacific
Authors: Flower B. P. , Kennett J. P.
Published: February 2008

High‐resolution stable isotopic records are presented for the epi‐benthic foraminifer Cibicidoides, the inferred shallow‐dwelling planktonic Globigerinoides quadrilobatus, and the inferred deep‐dwelling planktonic Globoquadrina dehiscens from the middle Miocene (∼16–12 Ma) of Deep Sea Drilling Project site 588A, Lord Howe Rise, southwest Pacific. High‐resolution, multiple species oxygen and carbon isotopic data define the timing and character of the well‐known middle Miocene climatic‐oceanographic transition with a resolution comparable to Quaternary records. The benthic foraminiferal δ18O record is marked by several large fluctuations from ∼16 to 14.8 Ma, followed by a series of rapid (<50 kyr) δ18O increases that suggest a new state of the ocean‐climate system after 14.8 Ma. The total middle Miocene benthic oxygen isotopic increase of 1.2‰ is largely incorporated in two steps, an increase of 0.8‰ from 14.5 to 14.0 Ma and a second increase of 0.7‰ from 13.45 to 12.45 Ma. Each step is comprised of a series of marked δ18O increases, indicative of rapid East Antarctic ice sheet growth and contemporaneous deepwater cooling. A strong covariance of 0.7‰ between the benthic and deep‐dwelling planktonic species from 14.5 to 14.0 Ma (including a rapid increase from 14.1 to 14.05 Ma) suggests a 0.7‰ increase in the δ18O composition of seawater (δ18Osw) because of East Antarctic ice sheet growth. Comparison of the δ18O record of Gs. quadrilobatus suggests that surface waters warmed at this site by ∼3°C from 14.1 to 13.6 Ma. Carbon isotopic time series for each species generally covary throughout the early to middle Miocene interval (∼16–12 Ma), confirming that δ13C variations in this interval largely represent reservoir changes. High‐resolution δ13C data allow improved resolution of the latter five of six δ13C maxima within the well‐known early to middle Miocene carbon isotopic excursion (the Monterey Carbon Isotopic Excursion from 17.0 to 13.5 Ma). This is useful for global correlation. The last of these maxima ends with a 1‰ decrease centered from 13.9 to 13.7 Ma, ∼300 kyr after the δ18O increase considered to reflect East Antarctic ice growth. Covariance between benthic δ18O and δ13C from ∼16 to 13.8 Ma suggests a sensitive relation between global carbon cycling and the ocean‐climate system prior to 13.8 Ma. Episodic increases in organic carbon burial may have contributed to deep‐sea benthic δ13C maxima and synchronous global cooling. The positive relationship ended at ∼13.8 Ma, indicative of changing relations between global carbon cycling and the ocean‐climate system brought on by the increased stability of the East Antarctic ice sheet after a major growth phase from 14.5 to 14.0 Ma.
A direction finding ionosonde for ionospheric propagation research
Authors: Black Q. R. , Wood J. F. , Sonsteby A. G. , Sherrill W. M.
Published: February 2008

A direction finding ionosonde consisting of a seven‐element, 50‐m crossed baseline interferometer has been developed to provide group path delay, azimuth, and elevation of each resolved mode over the sounding frequency range. Direction finding (df) ionograms have been acquired automatically, using various noncooperative compatible sounder transmissions. A diurnal sequence of df ionograms obtained over a 2344‐km mid‐latitude path is reported, including plots of group delay, azimuth, and elevation versus frequency, as well as an azimuth/elevation scatterplot showing the distribution of angular occupancy for the path. The results show modes arriving closest to the great circle bearing (that is, with lowest azimuth error) arrive via the lowest elevation angles and with least group delay. Greatest bearing accuracy was observed on daytime sporadic E modes; measured at 0.85 MOF, the azimuth standard deviation was 0.3°. One‐hop F region low‐ray azimuth measured at 0.85 MOF showed standard deviations between 0.4° and 0.98°. Fractional degrees per megahertz linear azimuth gradients with frequency were also observed at various times of day. One‐hop F region high rays showed smoothly varying elevation as a function of frequency, increasing with group path delay as expected. Variable lateral deviation from the great circle plane was observed for high rays up to 5° in azimuth equatorward at various times of day; azimuth standard deviation at 0.85 MOF varied from 0.91° to 1.7°.
Planar bianisotropic waveguides
Authors: Yang Hung‐Yu , Uslenghi Piergiorgio L. E.
Published: February 2008

This paper presents an analysis of several planar bianisotropic waveguides including grounded slabs and strip lines. The bianisotropic properties of the materials are prescribed in a special but practical form so that the propagating waves are either transverse electric or transverse magnetic modes. Certain unique characteristics of the waves are discussed including mode elimination, unidirectional propagation, and the existence of complex modes. Both analytical and numerical results of guided wave properties are discussed.
Evolution of the Turfan Basin, Chinese central Asia
Authors: Allen M. B. , Windley B. F. , Zhang Chi , Guo Jinghui
Published: February 2008

The Turfan Basin extends approximately 550 km east‐west by 100 km north‐south within northwest China and is located between branches of the intracontinental Tien Shan orogenic belt. At present it is an intermontane foreland basin loaded by southward transported thrust sheets at its northern margin. This deformation is a long‐distance manifestation of the India‐Asia collision, but the orientation and location of Cenozoic structures is strongly influenced by the pre‐Cenozoic history of the region. The exposed basement marginal to the Turfan Basin consists of volcanics, granitoids, volcaniclastics, and deepwater sedimentary rocks associated with the North Tien Shan island arc and subduction‐accretion complexes north of the North Tien Shan fault. This arc had accreted to the Central Tien Shan by late Carboniferous to early Permian times. The North Tien Shan fault represents a major boundary between Paleozoic arcs and accretionary complexes to the north and Proterozoic basement of the Central Tien Shan and Tarim Block to the south. Postcollisional thrusting appears to have provided a tectonic load for Lower Permian subsidence and sedimentation. Early postcollisional strata are turbidites, but there was a shoaling of facies until the deposition of Permo‐Triassic conglomerates. No marine sediments have been deposited in the region since the Paleozoic. At least two Mesozoic episodes of compressional deformation are recorded at the northern margin of the basin, which was an intermontane foreland basin during this era. Lower Jurassic fluvial and lacustrine clastic rocks lie above a regional unconformity. An Oligocene unconformity marks the onset of coarse clastic sedimentation in the Cenozoic and probably indicates the start of major compressional deformation related to the India‐Asia collision. Sedimentation rates have increased through the Cenozoic. The neotectonics of the Turfan Basin are dominated by a southward directed thrust system, emergent within the basin at the Fire Mountains range. Southward directed thrusting appears to have been a feature of Mesozoic age tectonics, and possibly the late Paleozoic tectonics of the basin; the most important neotectonic structures in the Turfan Basin follow older structural trends.
High cooling and denudation rates at Kongur Shan, Eastern Pamir (Xinjiang, China) revealed by 40Ar/39Ar alkali feldspar thermochronology
Authors: Arnaud N. O. , Brunel M. , Cantagrel J. M. , Tapponnier P.
Published: February 2008

Orthogneiss samples taken from the Kongur antiform show ages varying from 2 Ma to 1 Ma for 40Ar/39Ar ages of biotites and muscovites and fission tracks on apatites, leading to cooling rates of 150°C/m.y. Modeling of K‐feldspars highlights the effect of a range of diffusion domains with contrasting diffusion characteristics, yielding closure temperatures from 400° to 150°C. The feldspar data document the cooling history since 5 Ma and indicate a sudden change in cooling rates of the antiform at 2 Ma. At that time, cooling increases by a factor of 5, from an average of 20°C/m.y. to a minimum of 150°C/m.y. Consideration of the regional thermal history, ongoing uplift, and erosional history of the antiform during the Quaternary suggests that denudation rates have been of the order of 5–7 km/m.y. since 2 m.y. ago and could be associated with significant upward surface movement triggered by major normal faulting. The antiform is interpreted to have formed during thrusting at the Pamir front as a result of the development of thrust ramps and normal faulting at the crustal scale. Ramp stacking is an important process of mountain building, and normal faulting in this context must be regarded as a very efficient way of building high relief.
Distribution, age, and deformation of Late Pleistocene marine terraces at Mahia Peninsula, Hikurangi Subduction Margin, New Zealand
Authors: Berryman Kelvin R.
Published: February 2008

A sequence of seven marine terraces of late Pleistocene age constitutes approximately 40% of the area of Mahia Peninsula in eastern North Island. The peninsula is one of the closest land areas to the Hikurangi Trough (approximately 80 km further east), where the Pacific plate is being subducted beneath the Australian plate. Terrace ages of 40, 59, 81, 106, 124, 176, and 212 ka are assigned by correlation with the marine terrace sequence of Huon Peninsula, Papua New Guinea. Paleontological data and amino acid dating of marine components of the 124 ka terrace at Mahia Peninsula provide a basis for correlation to the Huon Peninsula sequence. Loess and tephra stratigraphy constrain the ages of older and younger terraces. The marine terrace sequence is tilted to the WNW on the flank of the NNE trending Lachlan anticline. Uplift of at least 130 m of the circa 124‐kyr‐old shoreline indicates uplift rates of at least 1 mm/yr at the axis of the anticline. Younger marine terraces have been uplifted more rapidly (about 3 mm/yr), and there is a progressive increase in rate to the youngest Holocene shoreline. Structural contours normalized to the circa 124 ka terrace illustrate the tilt on the flank of the Lachlan anticline and on a secondary structure, the Aurora Point fold. Seismic profiling has confirmed the presence of a reverse fault (the Lachlan fault) at the eastern margin of the Lachlan anticline. The Lachlan fault is inferred to be responsible for growth of the anticline and deformation of the marine terraces. The relationship of the reverse fault to the subduction thrust, which separates the two crustal plates at about 20 km depth below Mahia Peninsula and dips about 6°–12° to the NW, is uncertain. However, the late Pleistocene and Holocene growth of the Lachlan anticline indicates substantial coupling across the plate interface, deforming the inner part of the frontal wedge of the Australian plate.
Transit Times for Water in a Small Till Catchment from a Step Shift in the Oxygen 18 Content of the Water Input
Authors: Rodhe A. , Nyberg L. , Bishop K.
Published: February 2008

A 6,300‐m2 catchment in SW Sweden was covered by a roof and irrigated by deacidified lake water. The δ18O of lake water differed from that of soil water and groundwater in the catchment, making it possible to follow the replacement of old (preirrigation) water by new (irrigation) water. After 7.5 months of irrigation the old water in the catchment had been replaced by new water. The breakthrough curve for new water gave a nearly exponential distribution of flow corrected transit times, with a mean transit time corresponding to a runoff of 54 mm. The estimated outflow of old water by runoff and evapotranspiration, 186 mm, compared well with independent estimates of the water storage at the onset of irrigation, 205 mm. The spatial pattern of the δ18O of soil water and groundwater within the catchment demonstrated the strong influence of the topography on the water flow.
Macroscopic Percolation Model of Immiscible Displacement: Effects of Buoyancy and Spatial Structure
Authors: Ioannidis Marios A. , Chatzis Ioannis , Dullien Francis A. L.
Published: February 2008

Percolation theory, commonly used to study quasi‐static immiscible displacement at the microscopic scale, is here extended to simulations of gravity‐stable drainage and imbibition in three‐dimensional porous media with spatially correlated macroscopic properties. The result of this extension is a macroscopic percolation on a regular lattice of sites, where lattice sites represent regions in a porous medium characterized by different macroscopic properties (e.g., absolute permeability, capillary pressure, and relative permeability curves). These properties are assigned to lattice sites by virtue of parametrizations in terms of local permeability. We present a general formulation of macroscopic percolation that accounts for gravitational effects, which can be important in large‐scale immiscible displacements with nonzero density difference. In such cases, we find that the local saturation distribution is markedly different from the distribution of saturation under conditions of negligible buoyancy. Displacements with nonzero density difference proceed with the formation of a transition zone of length inversely proportional to a macroscopic Bond number which characterizes the relative importance of capillary and buoyancy forces at the macroscopic scale. Several important features of percolation at the microscopic scale are also manifested at the macroscopic scale. These include the effects of lattice dimensionality and spatial correlation on the macroscopic percolation threshold and accessibility characteristics. In the absence of buoyancy forces, the large‐scale capillary pressure and relative permeability behavior of a heterogeneous system is dictated mainly by the structure of the permeability field and can be explained in terms of macroscopic accessibility. Spatial correlation of permeability is found to have pronounced effects on the large‐scale drainage relative permeability curves.
Thermal evolution and slip history of the Renbu Zedong Thrust, southeastern Tibet
Authors: Quidelleur Xavier , Grove Marty , Lovera Oscar M. , Harrison T. Mark , Yin An , Ryerson F. J.
Published: February 2008

Cretaceous granitoids of the Gangdese batholith, southeastern Tibet, were overthrust by upper greenschist to epidote‐amphibolite facies Tethyan rocks derived from the Indian shelf along the north directed (∼30° dip) Renbu Zedong Thrust (RZT). Thermochronological results obtained from a NE‐SW transect near Lian Xian show evidence of thermal effects related to thrusting. Granitoids immediately beneath the RZT exhibit considerable recrystallization to greenschist facies assemblages. Biotite and K‐feldspar 40Ar/39Ar ages measured along the traverse into the footwall increase systematically away from the RZT. The timing of initial upward displacement of the RZT hanging wall is constrained to have occurred at ∼18 Ma from the hornblende ages and the two K‐feldspar samples closest to the fault. K‐feldspars up to 15 km from the thrust yield 9–12 Ma ages for the initial ∼20% of 39Ar release. Distal samples yield 40Ar/39Ar ages nearly as old as the 70–110 Ma ion microprobe 206Pb*/238U ages determined for coexisting zircons. We have integrated our thermal history results with numerical heat flow models and found that while reheating to 320–280°C at shallow (∼7 km) levels due to rapid (>15 mm/yr) slip along the RZT at ∼10 Ma is capable of explaining the initial portion of the K‐feldspar age spectra, a prior common thermal history experienced by all samples cannot satisfactorily account for all the 40Ar/39Ar results. Instead, we find our thermal history constraints to be more completely explained by a numerical model in which (1) rocks currently at the surface originated from different depths,(2) footwall samples in close proximity to the RZT experienced fault drag from 19 to 15 Ma and (3) postthrusting denudation of the region involving localized tilting occurred at ∼10 Ma. The minimum average slip rate and displacement along the ramp during this period are 2 mm/yr and 12 km, respectively, but are likely to have been greater. The cooling episode recorded in all the K‐feldspar age spectra beginning at ∼10 Ma may either reflect denudation following regional uplift due to displacement along the ramp of the Main Himalayan Thrust or topographic collapse following cessation of RZT thrusting.
A methane flux time series for tundra environments
Authors: Whalen Stephen C. , Reeburgh William S.
Published: February 2008

Seasonal measurements of net methane flux were made at permanent sites representing important components of arctic tundra. The sites include Eriophorum tussocks, intertussock depressions, moss‐covered areas, and Carex stands. Methane fluxes showed high diel, seasonal, intra site, and between site variability. Eriophorum tussocks and Carex dominated methane release to the atmosphere, with mean annual (± 1σ) net methane fluxes of 8.05 ± 2.50 g CH4 m−2 and 4.88 ± 0.73 g CH4 m−2, respectively. Methane fluxes from the moss sites and intertussock depressions were much lower (0.47 ± 0.16 and 0.62 ± 0.28 g CH4 m−2 yr−1. Over 90% of the mean annual methane flux from the Eriophorum, intertussock depressions, and Carex sites occurred between thaw and freeze‐up. Some 40% of the mean annual methane flux from the moss sites occurred during winter. Composite methane fluxes for tussock tundra and Carex‐dominated wet meadow tundra environments were produced by weighting measured component fluxes according to areal coverage. Tussock and wet meadow tundra account for an estimated global methane emission of 19–33 Tg yr−1.
Diffusive flux of methane from warm wetlands
Authors: Barber Timothy R. , Burke Roger A. , Sackett William M.
Published: February 2008

Diffusion of methane across the air‐water interface from several wetland environments in south Florida was estimated from measured surface water concentrations using an empirically derived gas exchange model. The flux from the Everglades sawgrass marsh system varied widely, ranging from 0.18 ± 0.21 mol CH4/m2/yr for densely vegetated regions to 2.01 ± 0.88 for sparsely vegetated, calcitic mud areas. Despite brackish salinities, a strong methane flux, 1.87 ± 0.63 mol CH4/m2/yr, was estimated for an organic‐rich mangrove pond near Florida Bay. The diffusive flux accounted for 23, 36, and 13% of the total amount of CH4 emitted to the atmosphere from these environments, respectively. The average dissolved methane concentration for an organic‐rich forested swamp was the highest of any site at 12.6 μM; however, the calculated diffusive flux from this location, 2.57 ± 1.88 mol CH4/m2/yr, was diminished by an extensive plant canopy that sheltered the air‐water interface from the wind. The mean diffusive flux from four freshwater lakes, 0.77 ± 0.73 mol CH4/m2/yr, demonstrated little temperature dependence. The mean diffusive flux for an urbanized, subtropical estuary was 0.06 ± 0.05 mol CH4/m2/yr.
Nitrous oxide flux following tropical land clearing
Authors: Luizão Flávio , Matson Pamela , Livingston Gerald , Luizão Regina , Vitousek Peter
Published: February 2008

The importance of seasonal cycles of N2O flux from tropical ecosystems and the possibility that tropical deforestation could contribute to the ongoing global increase in N2O concentrations were assessed by measuring N2O flux from forest, cleared land, and pasture over an annual cycle in the central Amazon. A pasture that had been converted from tropical forest had threefold greater annual N2O flux than a paired forest site; similar results were obtained in spot measurements in other pastures. If these results are general, such tropical pastures represent a globally significant source of increased N2O.
The CH4 ‐ CO ‐ OH conundrum: A simple analytic approach
Authors: Guthrie Paul D.
Published: February 2008

The interdependence of tropospheric CH4, CO and OH is explored using a simple analytic three‐component box model. The steady state solution for this model places constraints on the relative strengths of the source fluxes of CH4 and CO and the production rate for OH. It is demonstrated that if these quantities vary independently the system can only remain stable for a narrow range of conditions. In order for the system to remain stable and well‐behaved for a wide range of fluxes, the fluxes of CO and CH4 and the production rate for OH must be covariant, either due to internal feedback processes or to coordinated response to external conditions. The observed steady state abundances of methane in ice records representing preindustrial and glacial times, although sparse, suggest covariant fluxes and OH production. Otherwise the global flux of methane must have varied by less than about 20% during deglaciation and emergence of the boreal wetlands, a period of major change in global hydrology, climate, and biome distributions.
A theoretical relation between Dst and the solar wind merging electric field
Authors: Siscoe G. , Crooker N.
Published: February 2008

Using a model in which magnetic merging near the nose of the magnetosphere results in energy transfer from the solar wind by means of a tangential force on the geomagnetic tail, we develop a linear relation between the time rate of change of Dst, representing energy transfer to the magnetosphere, and the merging electric field, with the proportionality factor an explicit function of magnetospheric parameters. The linear relation is in agreement with a recent empirical result; and using the empirical numerical coefficient, we find the distance from the earth to the neutral line in the tail during substorms to be in the range 10 Re to 30 Re.
The reaction of OH with CH4
Authors: Margitan J. J. , Kaufman F. , Anderson J. G.
Published: February 2008

The reaction of OH radicals with CH4 has been investigated over the temperature range 290‐440°K in a flow system with UV resonance fluorescence detection of OH. Its rate constant can be represented in Arrhenius form as k1 = 3.83 × 10−12 exp(−3660(cal/mole)/RT) cm³ sec−1. This result is compared with the results of other experimenters and is used to calculate stratospheric destruction rates of methane.
Nitric oxide gamma and delta band emission at twilight
Authors: Feldman Paul D. , Takacs Peter Z.
Published: February 2008

Nitric oxide twilight emission above 140 km in the γ‐ and δ‐bands was observed with a rocket‐borne spectrophotometer. The relative intensity of the two band systems indicates that the emission is produced predominantly by the chemiluminescent pre‐association of oxygen and nitrogen atoms.
High temperature electrical conductivity of albite
Authors: Piwinskii A. J. , Duba A.
Published: February 2008

The electrical conductivity (σ) of natural albite, measured to 1384 K, is a function of time. After 3200 hours at temperatures between 1353 and 1384 K, the σ increases about four orders of magnitude. These results indicate that melting need not be invoked to produce large changes in σ.
Mg isotopic anomalies in the Allende Meteorite and correlation with O and Sr effects
Authors: Lee Typhoon , Papanastassiou D. A.
Published: February 2008

Mg isotopic anomalies were determined on inclusions in the Allende meteorite. Measured Mg isotopic ratios were corrected for mass fractionation by normalizing to the terrestrial (normal) 25Mg/24Mg. Relative to normal Mg, Allende samples show distinct isotopic anomalies which correspond to enrichment of 24Mg or 26Mg of up to 3 per mil or depletion of 25Mg of up to 1.5 per mil. One sample shows a depletion of 24Mg or 26Mg by 1.7 per mil or an enrichment of 25Mg by 0.85 per mil. These anomalies are smaller by a factor of ten than found for the oxygen isotopes. Samples with large oxygen effects (up to 5%) yield normal Mg indicating a lack of correlation between the O and Mg effects. A Ca‐Al rich chondrule with primitive 87Sr/86Sr and normal 84Sr/86Sr has a significant Mg anomaly. The Mg anomalies do not correlate with Al/Mg and do not appear to be due to 26Al decay. The anomaly in the only sample depleted in 26Mg cannot be due to 26Al decay. The isotopic composition of Mg in a sample of the total meteorite is normal. These results accentuate the important character of Allende inclusions but do not uniquely identify the nuclear processes responsible for the Mg and O effects.
Satellite photography of eddies in the Gulf Loop Current
Authors: Maul George A. , Norris Dean R. , Johnson William R.
Published: February 2008

Cyclonic ocean eddies, approximately 12‐32 kilometers in diameter, have been photographed in the eastern Gulf of Mexico by SKYLAB. Apparently caused by horizontal velocity shear, these features are associated with the Gulf Loop Current, whose position was known a fortnight before and after the observation. The eddies were discovered in sunglint‐enhanced patterns of streamlines on the surface, and appear to be embedded in the flow.
Argon in the Martian atmosphere
Authors: Levine Joel S. , Riegler Guenter R.
Published: February 2008

Heretofore undetected argon may be a significant constituent of the Martian atmosphere with a partial pressure of 1.8 mbar. The addition of argon to the 5.5±0.8 mbar of CO2, deduced from ground‐based spectroscopy, is consistent with the values of total pressure deduced from the Mariner radio occulation measurements. The presence of argon in the Martian atmosphere contributes to the cooling of the upper thermosphere and exosphere and may explain the difference between predicted and observed temperatures. The argon concentrations predicted here may be related to the recent observation of several tens of percent of an inert gas, deduced from Mars‐6 data.
Studies of winds in the upper troposphere with a sensitive VHF radar
Authors: Green J. L. , Warnock J. M. , Winkler R. H. , VanZandt T. E.
Published: February 2008

A large 40.475 MHz Doppler radar has been constructed near Sunset, Colorado, 16 km west of Boulder, in order to study winds, the troposphere, stratosphere, and mesosphere. Early results from the troposphere are presented here. It is concluded that: (1) With a sensitive VHF radar it is possible to detect echoes from throughout the upper troposphere almost all the time, under both clear and cloudy conditions; (2) Horizontal winds derived from the Doppler shifts of the echoes are consistent at heights more than 2 km above the local terrain with rawinsonde winds measured over 55 km away; (3) The reflectivity of the scatterers in the troposphere is not strongly aspect sensitive, up to an angle of 30° from the zenith.
Estimate of the contribution of the space shuttle effluent to the natural stratospheric aerosol
Authors: Hofmann D. J. , Carroll D. E. , Rosen J. M.
Published: February 2008

During the period 1979–1990, the National Aeronautics and Space Administration is planning some 580 (or approximately one per week) space shuttle flights. Each of these flight configurations will include two large solid fuel rocket boosters (SRB) which will emit aluminum oxide (Al2O3) particles as exhaust products. We estimate the equilibrium distribution of such particles in the stratosphere and suggest that the shuttle contribution of very small particles, r∼0.01 µm may be nonnegligible.
Heat flow near the South Atlantic Triple Junction, 55°S, 0°E
Authors: Lee Tien‐Chang , Von Herzen Richard P.
Published: February 2008

Fifteen heat‐flow values, representing the southernmost measurements made in the Atlantic, show a pattern of heat flow with tectonic features around the ridge‐fracture‐fracture triple junction which are explained by effects of hydrothermal circulation. High values (>4 μcal/cm²sec) on ridges which flank the axial valleys may either reflect a topographically controlled ascending limb of a hydrothermal circulation system, or result from conductive cooling of crust subsequent to the sealing of cracks generated at the spreading center. Intermediate to low values (0.4 to 1.8) in the depressions of fracture zones and axial valleys are consistent with a model of thermal cracking and subsequent hydrothermal circulation in the newly formed rocks. Low values (<1.3) over the ridge flanks may also be a consequence of secondary circulation in cracks re‐opened during dehydration of rocks previously hydrated near the spreading centers. If such secondary circulation is widespread, it would imply at least two, and perhaps more, minima in profiles of oceanic heat‐flow vs. age.
Variation in the number of meteoroid impacts on the Moon with lunar phase
Authors: Dainty A. M. , Stein S. , Toksöz M. N.
Published: February 2008

Data from the ALSEP 12 and 14 seismometers of the Apollo Passive Seismic Experiment Network are used to determine the statistics of the mass distribution of meteoroids and their direction of approach in near earth space. The cumulative number of meteoroid impacts detected is analyzed as a function of phase of the moon, taking into account the sensitivity of the seismometer and the seismic wave propagation characteristics of the moon. We conclude that, in the mass range studied (>5 kg), (a) most meteoroid orbits lie near the plane of the ecliptic, (b) the orbits have aphelia between 2 and 5 a.u., and (c) the mass distribution statistics are described by the relation log10 n = B + G log10 m, where n = number of meteoroids of mass m grams or greater, B ∼ 0.9 and G = −1.5.
Green's Functions of a diffusion equation
Authors: Yeh Gour‐Tsyh
Published: February 2008

In diffusion modeling, it is often imperative to solve partial differential equations describing the concentration distribution of a substance released into a fluid subject to various kinds of initial and boundary conditions. The use of Green's theorems to solve such problems is one of the most powerful and promising methods because there are almost no limitations on the type of source conditions and functions depicting boundary values once the corresponding Green's function is obtained. Green's functions for a steady state, three‐dimensional turbulent diffusion equation with variable coefficients are, therefore, systematically presented for Dirichlet and Neumann boundary value problems in a number of important regions. Within the framework of power law approximations for velocity and diffusivities, solutions for most of the diffusion problems can be obtained by the proper choice of Green's functions given in this letter and the adroit manipulation of an integral.
Interpretation of apparent phase motion in micropulsation signals
Authors: Southwood D. J.
Published: February 2008

It is shown that phase differences in geomagnetic micropulsation signals seen at different locations should bear a direct relationship to energy flux in the magnetospheric wave structure.
Three dimensional model of seismic velocity variation in the Earth's mantle
Authors: Sengupta M. K. , Toksöz M. N.
Published: February 2008

Three dimensional velocity models for the earth's mantle were obtained satisfying P, S and some PcP and ScS travel time anomalies from deep focus earthquakes. A method of successive approximation was used to compute the uniform relative velocity perturbation over three dimensional blocks of size 10°×10° in longitude and latitude, and 500 km in thickness. Features of these velocity perturbations indicated that lateral heterogeneity is the most pronounced in the upper mantle and near the core‐mantle boundary. The upper mantle anomalies are correlated with surface tectonic features, but these are not correlated with deep mantle anomalies.
Seismic faulting in the presence of a large compressive stress
Authors: Dahlen F. A.
Published: February 2008

The dynamical laws which guarantee conservation of momentum and continuity of traction across a tangential slip surface are the same in an infinite, homogeneous, elastic medium which is subject to a large, uniform, compressive isotropic stress as in one which is not. It follows that many of the results from the theory of seismic faulting are applicable in this case. Any consideration which explicitly involves the concept of energy is different in the two instances, because in the former there are additional second order terms which arise in calculating the work done against the initial stress. The relative contribution of these terms to the net elastic strain energy released by a typical seismic fault appears to be no more than a few percent.
Magnetic merging in collisionless plasmas
Authors: Francfort Ph. , Pellat R.
Published: February 2008

We study the quasi‐neutral sheet model of Alfven [1968] as modified by Hill [1974]. If the particle motion is adiabatic the model implies a trapped particle population. In the opposite limit of Speiser [1970] and Eastwod [1972] we compute the current distribution and the magnetic field reversal compatible with a new invariant of the particle motion.
Solar magnetic sector effects on the vertical atmospheric electric field at Vostok, Antarctica
Authors: Park C. G.
Published: February 2008

The vertical atmospheric electric field was measured at Vostok, Antarctica (78°S, 107°E) during the period March‐November 1974 using a rotating dipole antenna. It is found that the electric field is depressed by ˜15% 1‐3 days following the passage of solar magnetic sector boundaries. This effect is more clearly evident in the austral winter when Vostok is in continuous darkness. No significant difference is found between the away‐to‐toward boundaries and the toward‐to‐away boundaries. These results suggest the possibility that the global fair‐weather atmospheric electric field is an important factor in the solar activity‐weather relationships.