Ostracode-based reconstruction from 23,300 to about 20,250 cal yr BP of climate, and paleohydrology of a groundwater-fed pond near St. Louis, Missouri   总被引：1，自引：0，他引：1  

Brandon Curry  Denis Delorme 《Journal of Paleolimnology》2003,29(2):199-207

The water chemistry of a groundwater-fed sinkhole-pond near St. Louis, Missouri, and its associated climate during the last glaciation are reconstructed by comparison with autecological data of modern ostracodes from about 5,500 sites in Canada. A 4.8-m succession of fossiliferous sediment yielded ostracode assemblages that collectively are generally found today in ponds in North America including the species Cyclocypris ampla, C. laevis, Cypridopsis vidua, Candona crogmaniana, C. distincta, and C. ohioensis. Fossils of Picea needles and the ostracode Cyclocypris sharpei imply that best analog sites for the succession are in central to south-central Canada. The pond formed 23,300 ± 400 cal yr BP when a sinkhole became plugged by a clay bed about 1 m thick. By about 20,250 cal yr BP, the pond had desiccated and was buried by loess. The sediment accumulation rate was about 0.18 cm/yr, and each sample interval (6 cm) represents a time slice of 33 years. Data from this record provides the first fairly high resolution proxy record of the glacial paleoclimate of the mid-latitude of North America. The analog data indicate the water in the hydrologically-open spring-fed pond was less than 1 m deep. The paleoclimatic reconstructions imply gradually drier conditions and uniform, cool temperatures. The shallow water depth indicates that the temperature reconstruction is robust with mean annual temperatures (MATs) that ranged between 0.8 and 3.9°C, and mean July temperatures that ranged from 16.8 and 18.1°C. Other estimated climatic parameters include mean annual precipitation (MAP; 430 to 840 mm/yr), and moisture balance (P-E; –111 to 298 mm/yr). Compared to values measured today at St. Louis, the MAP was about 400 mm less, MAT about 10°C cooler, and P-E, about the same. These values are consistent with other published reconstructions based on modern analog analysis of fossil beetles and pollen, and paleothermometry based on amino acid racemization. The total dissolved solids (TDS) progressively increased from about 87 to 431 mg/L. Changes in TDS reflect either the balance between the relative inputs of karst groundwater and overland flow, or changes in the duration of water-rock interaction associated with the groundwater. The postulated long-term 900 ± 200 year cyclicity of growing-season moisture and temperature, attributed to El Niño-Southern Oscillation cycles, is not expressed in the reconstructed hydrologic or climatic data. This is attributed, in part, to the mediating effect on temperature by monothermic groundwater input to this flow-through system.  相似文献   

Differential rotation of relativistic superfluid in neutron stars     

David Langlois  David M. Sedrakian  & Brandon Carter 《Monthly notices of the Royal Astronomical Society》1998,297(4):1189-1201

It is shown how to set up a mathematically elegant and fully relativistic superfluid model that can provide a realistic approximation (neglecting small anisotropies due to crust solidity, magnetic fields, etc., but allowing for the regions with vortex pinning) of the global structure of a rotating neutron star, in terms of just two independently moving constituents. One of these represents the differentially rotating neutron superfluid, while the other part represents the combination of all the other ingredients, including the degenerate electrons, the superfluid protons in the core, and the ions in the crust, the electromagnetic interactions of which will tend to keep them locked together in a state of approximately rigid rotation. Order of magnitude estimates are provided for relevant parameters such as the resistive drag coefficient.  相似文献   

Macrogeomorphic evolution of the post-Triassic Appalachian mountains determined by deconvolution of the offshore basin sedimentary record   总被引：1，自引：1，他引：1  

Frank J. Pazzaglia  & Mark T. Brandon 《Basin Research》1996,8(3):255-278

A perplexing macrogeomorphic problem is the persistence of topography in mountain ranges that were initially formed by orogenic events hundreds of millions of years old. In this paper, we deconvolve the post-Triassic macrogeomorphic history of a portion of one of these ranges, the central and northern Appalachians, using a well-documented offshore isopach sedimentary record of the US Atlantic margin. Topography is an important signature of tectonic, eustatic and/or geomorphic processes that produces changes in the erodible thickness of the crust (ETC). We define ETC as the total thickness of crust that would have to be consumed by erosion to reduce the mean elevation of a landscape to sea level. We use the term ‘source flux’, designated by ν˙, to describe the rate of change in ETC attributed to deep-seated geological processes such as crustal thickening, crustal extension, magmatic intrusions or dynamic flow in the mantle. In a mountain belt, the rate of change of mean elevation with respect to a base level, designated by ? ′, can be represented as ? ′ = c(ν˙ ? k _d z ′ ?; ? _c ) ?& hairsp;l˙ , where k _d is a proportionality constant relating the mean mechanical erosion rate to mean elevation, ? _c is the mean chemcial erosion rate, c  is a compensation ratio (held constant for Airy isostasy at 0.21) and l˙  is the rate of eustatic sea-level change. This equation describes the sum of constructive source terms, two destructive erosion terms and a eustatic sea-level term. We use this simple linear equation to develop a landscape evolution model based on the concept of a unit response function. The unit response function is analogous to a unit hydrograph which describes the relationship between input (rainfall) and output (discharge) in a hydrological system. In our case, we solve for the general relationship between the source flux into the mountain belt and the erosional flux out of the belt. Offshore sediment volumes are used to determine the erosional flux. Drainage basin area is treated as either a constant (pinned drainage divide) or as increasing through time (migrating drainage divide). We use a third-order polynomial fit to a global sea-level curve to account for long-term eustatically driven changes in ETC and in drainage basin area. Chemical erosion is treated as a constant fixed at 5 m Myr^?1. We consider two end-member models. The first is a ‘tectonic’ model in which the source flux is allowed to vary while k _d is assumed to be constant over geological time and equal to its mean Pleistocene value of about 0.07 Myr^?1. The second is an ‘erodibility’ model in which k _d is allowed to vary, reflecting changes in climatic, climatic variables and rock-type erodibility, while the source flux is held constant at zero. The ‘tectonic’ model reveals four important increases in the source flux, ranging from 200 to 2000 m Myr^?1 that occur over short (<10 Myr) time spans, followed by a protracted period (>25 Myr) where ν˙ drops below zero to values of ?1000 to ?6000 m Myr^?1. The ‘erodibility’ model produces a topography that decays in a step-like fashion from an initial mean elevation that ranges between ～1800 and 2300 m. These models cannot unequivocally distinguish the relative importance of tectonic vs. climatic processes in the macrogeomorphic evolution of the post-rift Appalachians, but they do provide some first-order quantitative prediction about these two end-member options. In light of existing stratigraphic, geological and thermochronological data, we favour the tectonic model because most of the events correlate well in time and form with known syn- and post-rift magmatic events. Nevertheless, the most recent episode of increased sediment flux to the offshore basins during the Miocene remains difficult to explain by either model. Limited evidence suggests that this event may reflect asthenospheric flow-driven uplift and the development of dynamically supported topography at a time when mechanical erosion rates were increasing in response to a cooling terrestrial climate.  相似文献   

Lateral thinking: 2-D interpretation of thermochronology in convergent orogenic settings   总被引：1，自引：0，他引：1  

Geoffrey E. Batt  Mark T. Brandon 《Tectonophysics》2002,349(1-4)

Lateral motion of material relative to the regional thermal and kinematic frameworks is important in the interpretation of thermochronology in convergent orogens. Although cooling ages in denuded settings are commonly linked to exhumation, such data are not related to instantaneous behavior but rather to an integration of the exhumation rates experienced between the thermochronological ‘closure’ at depth and subsequent exposure at the surface. The short spatial wavelength variation of thermal structure and denudation rate typical of orogenic regions thus renders thermochronometers sensitive to lateral motion during exhumation. The significance of this lateral motion varies in proportion with closure temperature, which controls the depth at which isotopic closure occurs, and hence, the range of time and length scales over which such data integrate sample histories. Different chronometers thus vary in the fundamental aspects of the orogenic character to which they are sensitive. Isotopic systems with high closure temperature are more sensitive to exhumation paths and the variation in denudation and thermal structure across a region, while those of lower closure temperature constrain shorter-term behaviour and more local conditions.Discounting lateral motion through an orogenic region and interpreting cooling ages purely in terms of vertical exhumation can produce ambiguous results because variation in the cooling rate can result from either change in kinematics over time or the translation of samples through spatially varying conditions. Resolving this ambiguity requires explicit consideration of the physical and thermal framework experienced by samples during their exhumation. This can be best achieved through numerical simulations coupling kinematic deformation to thermal evolution. Such an approach allows the thermochronological implications of different kinematic scenarios to be tested, and thus provides an important means of assessing the contribution of lateral motion to orogenic evolution.  相似文献   

The Nature of Uncertainty in Historical Geographic Information   总被引：8，自引：1，他引：8  

Brandon Plewe 《Transactions in GIS》2002,6(4):431-456

While the presence of uncertainty in the geometric and attribute aspects of geographic information is well known, it is also present in temporal information. In spatiotemporal GIS databases and other formal representations, uncertainty in all three aspects of geography (space, time, and theme) must often be modeled, but a good data model must first be based on a sound theoretical understanding of spatiotemporal uncertainty. The nature of both uncertainty inherent in a phenomenon (often termed indeterminacy) and uncertainty in assertions of that phenomenon can be better understood through the Uncertain Temporal Entity Model , which characterizes the cause, type, and form of uncertainties in the spatial, temporal, and attribute aspects of geographic information. These uncertainties are the result of complexities and problems in two processes: the process of conceptualization, by which humans make sense of an infinitely complex reality, and measurement, by which we create formal representations (e.g. GIS) of those conceptual models of reality. Based on this framework, the nature and form of uncertainty is remarkably consistent across various situations, and is approximately equivalent in the three aspects, which will enable consistent solutions for representation and processing of spatiotemporal data.  相似文献   

Density-induced settling of tephra through organic lakesediments     

Brandon Beierle  Jeffrey Bond 《Journal of Paleolimnology》2002,28(4):433-440

Tephras are often used in paleolimnology and other stratigraphicapplications as a chronostratigraphic marker. Where analytical errors inradiocarbon or other dating methods make precise comparison between sitesdifficult, tephras provide an absolute stratigraphic reference that can be usedto assess the relative ages of events across a region. Applications oftephrochronology typically make the assumption that a tephra is deposited atwhat was the top of the stratigraphic sequence at the time of deposition, andthat the contact between the tephra and underlying sediments is anisochron. This paper presents evidence from two lakes in western Canadawhich suggest that tephras may be very mobile within the sedimentary column,particularly in low-density organic lake sediments. Analysis of sedimentcores from Copper Lake, Alberta, suggest that the 6730 BP Mazama tephra moveddown-core by the equivalent of more than 3000 years. A vertical exposureof the Holocene sediments from Doal Lake, Yukon Territory, reveals that the1200 BP White River (WR) tephra, found throughout the southern Yukon, moveddown-sequence through organic lake sediments, to a stratum dating toapproximately 10,000 BP, creating conformable tephra beds at that level. Thistype of stratigraphic displacement has important consequences for the use oftephrochronology in lakes and other soft-sediment environments andunderscores the importance of properly identifying tephras and criticallyassessing their stratigraphic context within a lake core.  相似文献   

Os and Os enrichments and high-He/He sources in the Earth’s mantle: Evidence from Icelandic picrites     

Alan D. Brandon  David W. Graham  Bjarni Gautason 《Geochimica et cosmochimica acta》2007,71(18):4570-4591

Picrites from the neovolcanic zones in Iceland display a range in ¹⁸⁷Os/¹⁸⁸Os from 0.1297 to 0.1381 (γ_Os = + 2.1 to +8.7) and uniform ¹⁸⁶Os/¹⁸⁸Os of 0.1198375 ± 32 (2σ). The value for ¹⁸⁶Os/¹⁸⁸Os is within uncertainty of the present-day value for the primitive upper mantle of 0.1198398 ± 16. These Os isotope systematics are best explained by ancient recycled crust or melt enrichment in the mantle source region. If so, then the coupled enrichments displayed in ¹⁸⁶Os/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os from lavas of other plume systems must result from an independent process, the most viable candidate at present remains core-mantle interaction. While some plumes with high ³He/⁴He, such as Hawaii, appear to have been subjected to detectable addition of Os (and possibly He) from the outer core, others such as Iceland do not.A positive correlation between ¹⁸⁷Os/¹⁸⁸Os and ³He/⁴He from 9.6 to 19 Ra in Iceland picrites is best modeled as mixtures of 1 Ga or older ancient recycled crust mixed with primitive mantle or incompletely degassed depleted mantle isolated since 1-1.5 Ga, which preserves the high ³He/⁴He of the depleted mantle at the time. These mixtures create a hybrid source region that subsequently mixes with the present-day convecting MORB mantle during ascent and melting. This multistage mixing scenario requires convective isolation in the deep mantle for hundreds of million years or more to maintain these compositionally distinct hybrid sources. The ³He/⁴He of lavas derived from the Iceland plume changed over time, from a maximum of 50 Ra at 60 Ma, to approximately 25-27 Ra at present. The changes are coupled with distinct compositional gaps between the different aged lavas when ³He/⁴He is plotted versus various geochemical parameters such as ¹⁴³Nd/¹⁴⁴Nd and La/Sm. These relationships can be interpreted as an increase in the proportion of ancient recycled crust in the upwelling plume over this time period.The positive correlation between ¹⁸⁷Os/¹⁸⁸Os and ³He/⁴He demonstrates that the Iceland lava He isotopic compositions do not result from simple melt depletion histories and consequent removal of U and Th in their mantle sources. Instead their He isotopic compositions reflect mixtures of heterogeneous materials formed at different times with different U and Th concentrations. This hybridization is likely prevalent in all ocean island lavas derived from deep mantle sources.  相似文献   

A Rb-Sr and Sm-Nd isotope geochronology and trace element study of lunar meteorite LaPaz Icefield 02205     

K. Rankenburg  A.D. Brandon 《Geochimica et cosmochimica acta》2007,71(8):2120-2135

Rubidium-strontium and samarium-neodymium isotopes of lunar meteorite LaPaz Icefield (LAP) 02205 are consistent with derivation of the parent magma from a source region similar to that which produced the Apollo 12 low-Ti olivine basalts followed by mixing of the magma with small amounts (1-2 wt%) of trace element-enriched material similar to lunar KREEP-rich sample SaU 169. The crystallization age of LAP 02205 is most precisely dated by an internal Rb-Sr isochron of 2991 ± 14 Ma, with an initial ⁸⁷Sr/⁸⁸Sr at the time of crystallization of 0.699836 ± 0.000010. Leachable REE-rich phosphate phases of LAP 02205 do not plot on a Sm-Nd mineral isochron, indicating contamination or open system behavior of the phosphates. Excluding anomalous phases from the calculation of a Sm-Nd isochron yields a crystallization age of 2992 ± 85 (initial ε¹⁴³Nd = +2.9 ± 0.8) that is within error of the Rb-Sr age, and in agreement with other independent age determinations for LAP 02205 from Ar-Ar and U-Pb methods. The calculated ¹⁴⁷Sm/¹⁴⁴Nd source ratios for LAP 02205, various Apollo 12 and 15 basalts, and samples with strong affinities to KREEP (SaU 169, NWA 773, 15386) are uncorrelated with their crystallization ages. This finding does not support the involvement of a common KREEP component as a heat source for lunar melting events that occurred after crystallization of the lunar magma ocean.  相似文献   

The rayed crater Zunil and interpretations of small impact craters on Mars   总被引：1，自引：0，他引：1  

Alfred S. McEwen  Brandon S. Preblich  Natalia A. Artemieva  Michelle Hurst  Devon M. Burr 《Icarus》2005,176(2):351-381

A 10-km diameter crater named Zunil in the Cerberus Plains of Mars created ∼¹⁰⁷ secondary craters 10 to 200 m in diameter. Many of these secondary craters are concentrated in radial streaks that extend up to 1600 km from the primary crater, identical to lunar rays. Most of the larger Zunil secondaries are distinctive in both visible and thermal infrared imaging. MOC images of the secondary craters show sharp rims and bright ejecta and rays, but the craters are shallow and often noncircular, as expected for relatively low-velocity impacts. About 80% of the impact craters superimposed over the youngest surfaces in the Cerberus Plains, such as Athabasca Valles, have the distinctive characteristics of Zunil secondaries. We have not identified any other large (?10 km diameter) impact crater on Mars with such distinctive rays of young secondary craters, so the age of the crater may be less than a few Ma. Zunil formed in the apparently youngest (least cratered) large-scale lava plains on Mars, and may be an excellent example of how spallation of a competent surface layer can produce high-velocity ejecta (Melosh, 1984, Impact ejection, spallation, and the origin of meteorites, Icarus 59, 234-260). It could be the source crater for some of the basaltic shergottites, consistent with their crystallization and ejection ages, composition, and the fact that Zunil produced abundant high-velocity ejecta fragments. A 3D hydrodynamic simulation of the impact event produced 10¹⁰ rock fragments ?10 cm diameter, leading to up to 10⁹ secondary craters ?10 m diameter. Nearly all of the simulated secondary craters larger than 50 m are within 800 km of the impact site but the more abundant smaller (10-50 m) craters extend out to 3500 km. If Zunil is representative of large impact events on Mars, then secondaries should be more abundant than primaries at diameters a factor of ∼1000 smaller than that of the largest primary crater that contributed secondaries. As a result, most small craters on Mars could be secondaries. Depth/diameter ratios of 1300 small craters (10-500 m diameter) in Isidis Planitia and Gusev crater have a mean value of 0.08; the freshest of these craters give a ratio of 0.11, identical to that of fresh secondary craters on the Moon (Pike and Wilhelms, 1978, Secondary-impact craters on the Moon: topographic form and geologic process, Lunar Planet. Sci. IX, 907-909) and significantly less than the value of ∼0.2 or more expected for fresh primary craters of this size range. Several observations suggest that the production functions of Hartmann and Neukum (2001, Cratering chronology and the evolution of Mars, Space Sci. Rev. 96, 165-194) predict too many primary craters smaller than a few hundred meters in diameter. Fewer small, high-velocity impacts may explain why there appears to be little impact regolith over Amazonian terrains. Martian terrains dated by small craters could be older than reported in recent publications.  相似文献   

Simple infrared techniques for measuring beach surface moisture     

Brandon L. Edwards  Steven L. Namikas  Eurico J. D'Sa 《地球表面变化过程与地形》2013,38(2):192-197

Knowledge of surface moisture on beaches is vital for aeolian process studies because moisture increases transport thresholds and limits mass flux. A number of beach surface moisture measurement techniques have been employed in the field, including sample extraction, commercial soil moisture sensors, and remote sensing techniques. Each method has significant limitations in the context of aeolian process studies. This study was designed to test infrared optic techniques for measuring beach surface moisture. A simple infrared sensor (narrow‐band radiometer) was developed to measure beach surface moisture content. The accuracy and practical usability of the narrow‐band radiometer were assessed in comparison to a commercial handheld spectroradiometer. Field calibrations conducted at Cape San Blas, Florida and Padre Island, Texas indicated that the narrow‐band radiometer performed quite well. The R² values exceeded 0·98 in each case, and the standard error averaged about 1% moisture content compared with gravimetric moisture contents determined from 1·5 mm deep surface scrapes. The performance of the two instruments was found to be comparable, with the narrow‐band radiometer slightly outperforming the spectroradiometer. In practical applications, the narrow‐band radiometer also has logistical advantages and is better suited to measure large numbers of points. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献