Early Wisconsinan (MIS 4) Arctic ground squirrel middens and a squirrel-eye-view of the mammoth-steppe     

Grant D. Zazula  Duane G. Froese  Scott A. Elias  Svetlana Kuzmina  Rolf W. Mathewes 《Quaternary Science Reviews》2011,30(17-18):2220-2237

Fossil arctic ground squirrel (Spermophilus parryii) middens were recovered from ice-rich loess sediments in association with Sheep Creek-Klondike and Dominion Creek tephras (ca 80 ka) exposed in west-central Yukon. These middens provide plant and insect macrofossil evidence for a steppe-tundra ecosystem during the Early Wisconsinan (MIS 4) glacial interval. Midden plant and insect macrofossil data are compared with those previously published for Late Wisconsinan middens dating to ～25–29¹⁴C ka BP (MIS 3/2) from the region. Although multivariate statistical comparisons suggest differences between the relative abundances of plant macrofossils, the co-occurrence of steppe-tundra plants and insects (e.g., Elymus trachycaulus, Kobresia myosuroides, Artemisia frigida, Phlox hoodii, Connatichela artemisiae) provides evidence for successive reestablishment of the zonal steppe-tundra habitats during cold stages of the Late Pleistocene. Arctic ground squirrels were well adapted to the cold, arid climates, steppe-tundra vegetation and well-drained loessal soils that characterize cold stages of Late Pleistocene Beringia. These glacial conditions enabled arctic ground squirrel populations to expand their range to the interior regions of Alaska and Yukon, including the Klondike, where they are absent today. Arctic ground squirrels have endured numerous Quaternary climate oscillations by retracting populations to disjunct “interglacial refugia” during warm interglacial periods (e.g., south-facing steppe slopes, well-drained arctic and alpine tundra areas) and expanding their distribution across the mammoth-steppe biome during cold, arid glacial intervals.  相似文献   

Atmospheric dust variability from Arabia and China over the last 500,000 years     

Andrew P. Roberts  Eelco J. Rohling  Katharine M. Grant  Juan C. Larrasoaña  Qingsong Liu 《Quaternary Science Reviews》2011,30(25-26):3537-3541

Atmospheric mineral dust aerosols affect Earth’s radiative balance and are an important climate forcing and feedback mechanism. Dust is argued to have played an important role in past natural climate changes through glacial cycles, yet temporal and spatial dust variability remain poorly constrained, with scientific understanding of uncertainties associated with radiative perturbations due to mineral dust classified as “very low”. To advance understanding of the dust cycle, we present a high-resolution dust record from the Red Sea, sourced principally from Arabia, with a precise chronology relative to global sea level/ice volume variability. Our record correlates well with a high-resolution Asian dust record from the Chinese Loess Plateau. Importing our age model from the Red Sea to the Chinese Loess Plateau provides a first detailed millennial-scale age model for the Chinese loess, which has been notoriously difficult to date at this resolution and provides a basis for inter-regional correlation of Chinese dust records. We observe a high baseline of dust emissions from Arabia and China, even through interglacials, with strong superimposed millennial-scale variability. Conversely, the distal EPICA Dome C Antarctic ice core record, which is widely used to calculate the radiative impact of dust variations, appears biased to sharply delineated glacial/interglacial contrasts. Calculations based on this Antarctic dust record will therefore overestimate the radiative contrast of atmospheric dust loadings on glacial/interglacial timescales. Additional differences between Arabian/Asian and circum-Saharan records reveal that climate models could be improved by avoiding ‘global mean’ dust considerations and instead including large-scale regions with different dust source variability.  相似文献   

Recent large-magnitude floods and their impact on valley-floor environments of northeastern Yellowstone   总被引：1，自引：0，他引：1  

Grant A. Meyer   《Geomorphology》2001,40(3-4)

The Lamar River watershed of northeastern Yellowstone contains some of the most diverse and important habitat in the national park. Broad glacial valley floors feature grassland winter range for ungulates, riparian vegetation that provides food and cover for a variety of species, and alluvial channels that are requisite habitat for native fish. Rapid Neogene uplift and Quaternary climatic change have created a dynamic modern environment in which catastrophic processes exert a major influence on riverine–riparian ecosystems. Uplift and glacial erosion have generated high local relief and extensive cliffs of friable volcaniclastic bedrock. As a result, steep tributary basins produce voluminous runoff and sediment during intense precipitation and rapid snowmelt. Recent major floods on trunk streams deposited extensive overbank gravels that replaced loamy soils on flood plains and allowed conifers to colonize valley-floor meadows. Tree-ring dating identifies major floods in 1918, ca. 1873, and possibly ca. 1790. In 1996 and 1997, discharge during snowmelt runoff on Soda Butte Creek approached the 100-year flood estimated by regional techniques, with substantial local bank erosion and channel widening. Indirect estimates show that peak discharges in 1918 were approximately three times greater than in 1996, with similar duration and much greater flood plain impact. Nonetheless, 1918 peak discharge reconstructions fall well within the range of maximum recorded discharges in relation to basin area in the upper Yellowstone region. The 1873 and 1918 floods produced lasting impacts on the channel form and flood plain of Soda Butte Creek. Channels may still be locally enlarged from flood erosion, and net downcutting has occurred in some reaches, leaving the pre-1790 flood plain abandoned as a terrace. Gravelly overbank deposits raise flood-plain surfaces above levels of frequent inundation and are well drained, therefore flood-plain soils are drier. Noncohesive gravels also reduce bank stability and may have persistent effects on channel form. Overall, floods are part of a suite of catastrophic geomorphic processes that exert a very strong influence on landscape patterns and valley-floor ecosystems in northeastern Yellowstone.  相似文献   

New England granites: Trace element evidence regarding their origin and differentiation     

Grant Buma  Fred A. Frey  David R. Wones 《Contributions to Mineralogy and Petrology》1971,31(4):300-320

The abundances of Sc, rare earths, Zr, Hf, Ta and Th were determined in five New England granitic plutons. Similar data are reported for separated minerals from three of the units. The granites can be divided into the Massachusetts alkaline group (Cape Ann, Peabody, Quincy) and the Rhode Island subalkaline group (Narragansett Pier, Westerly).Analyses for three samples from each pluton indicate that the Westerly granite is heterogeneous in both major and trace elements. Th abundances vary considerably (factor of 2) in both Rhode Island granites. Zr and Hf are heterogeneously distributed in all the granites while the rare earths are more homogeneous (< ± 20% deviation from mean).Eu depletions in all the granites imply that feldspar was involved either as a cumulate during fractional crystallization or as a residual phase during anatexis. Because the Massachusetts granites are associated with syenites and show evidence for low water fugacity and denser rocks at depth, these granites probably developed from basic magmas as a result of extensive feldspar crystallization.Depletion of Tb, Dy, Yb, Lu, Zr, Hf and Ta in the Rhode Island granites suggest participation of zircon or garnet as residues of partial melting or as crystal cumulates. These granites crystallized under nearly water saturated conditions and are characterized by abundant pegmatites. The presence of a water rich phase may also have been important in the depletion of elements which form stable complexes in aqueous solutions.  相似文献   

Some chemical controls on igneous amphibole compositions     

R.Grant Cawthorn 《Geochimica et cosmochimica acta》1976,40(11):1319-1328

The composition of igneous amphiboles is extremely sensitive to magma composition and the nature of the co-existing phases. The extent of solid solution in amphiboles may be depicted in the basalt tetrahedron. They are more silica-undersaturated than the liquid from which they have crystallized for liquids containing about 11% normative nepheline. For liquids containing more than 11% normative nepheline the reverse is true. The existence of this compositional divide may be demonstrated for both intrusive and extrusive suites. Amphiboles are generally nepheline-normative, except when crystallizing from granodioritic to granitic magmas.Fractionation factors between amphibole and magma composition for MgO and FeO appear to increase with decreasing temperature; those for TiO₂ and Na₂O decrease with increasing pressure. The CaO content of amphiboles appears to be insensitive to the CaO content of the magma.  相似文献   

The structure and sequence of geological events in the Basement Complex of the Ibadan area,Western Nigeria     

Kevin Burke  S.J. Freeth  Norman K. Grant 《Precambrian Research》1976,3(6):537-545

The Basement Complex in Western Nigeria in general, and in the Ibadan area in particular, is composed primarily of a banded gneiss in which hornblende-biotite rich bands alternate with quartz-oligoclase rich bands. The banded gneiss, which originated as part of a sedimentary sequence, contains large lenses of granite gneiss and thin intercolated layers of quartzite and amphibolite.Two distinct major structural events can be clearly identified in the early geological history of the Ibadan area. Detailed field studies suggest that the formation of the Ibadan Granite Gneiss, which had yielded an Eburnean Rb-Sr isochron age, was associated with the later of these events. Five phases of dyke or vein formation, two of which pre-date the formation of the Granite Gneiss, have also been identified giving an overall sequence of geological events the first of which may correspond to the beginning of the Liberian orogeny, around 3000 m.y. ago, and the last of which reflects the waning of the Pan-African thermo-tectonic event about 500 m.y. ago.  相似文献   

A multivariate analysis of planktonic molluscan distribution in the Middle Atlantic Bight     

Michael Vecchione  George C. Grant 《Continental Shelf Research》1983,1(4):405-424

Faunal structure is described for the planktonic molluscs of the Middle Atlantic Bight based on two years of seasonal data from the continental shelf. Collection and taxa groups are constructed using numerical classification and reciprocal averaging ordination. Discriminant analysis is used to relate surface collection groups to physical variables, then taxa group distribution among these collection groups is analyzed by nodal fidelity analysis. The areal distribution of dominant species is presented by season, as is their surface temperature-salinity distribution.Four communities are recognized on the continental shelf. A subarctic community, including Limacina retroversa, Paedoclione doliiformis, and Clione limacina, is advected down the central shelf region from the northeast. A Gulf Stream community of weak vertical migrators, including Limacina trochiformis, Cavolinia longirostris, Creseis conica, Atlanta peroni, and A. gaudichaudi, is introduced onto the shelf in occasional intrusions across the shelf-edge front. A depth-limited warm-water community of strong vertical migrators, including Limacina inflata, L. bulimoides, L. lesueuri, and Cavolinia inflexa is generally confined offshore of the 100-m isobath since the extent of their daily vertical migration is greater than the bottom depths on the continental shelf. A coastal community, including the larvae of Loligo pealei and of Ensis directus is found in coastal water of local origin and is generally confined within a coastal boundary layer.  相似文献   

On the formation of metal-rich deposits at ridge crests     

J.M. Edmond  C. Measures  B. Mangum  B. Grant  F.R. Sclater  R. Collier  A. Hudson  L.I. Gordon  J.B. Corliss 《Earth and Planetary Science Letters》1979,46(1):19-30

Data from the hot springs at the Galapagos spreading center (T = 3–13°C) show depletions of the exiting waters in Cu, Ni, Cd, Se, Cr and U relative to ambient seawater. Manganese is strongly enriched. Iron shows highly variable behavior between vent fields but is in general low. The data confirm the occurrence of extensive subsurface mixing between the primary high-temperature, acid, reducing hydrothermal fluids and “groundwater”. The composition of the latter is indistinguishable from that of the free water column adjacent to the ridge axis. The final solutions are on the boundary between those forming MnO₂ crusts and those producing iron-manganese rich sediments. The suite of metal rich deposits observed at ridge crests — Mn-O, Fe-Mn-O, Fe-S — can be explained as the manifestation of the degree of subsurface mixing, decreasing from 100 : <1 to <1 : 1 across the series (assuming an end-member temperature of 350°C).  相似文献   

Stratigraphy,paleocurrents and stromatolites of Hadrynian (Upper Precambrian) rocks of Victoria Island,Arctic Archipelago,Canada     

Grant M Young 《Precambrian Research》1974,1(1):13-41

The Hadrynian (Upper Precambrian) rocks of Victoria Island in the western part of the Canadian Arctic Archipelago comprise a sedimentary sequence some 12,000 ft. (4,000 m) thick, capped by a 1,000 ft. (330 m)-thick basic volcanic and pyroclastic unit. These rocks outcrop in an elongate, topographic high called the Minto Arch, and in smaller inliers in Lower Paleozoic rocks in the south part of the island (Duke of York and Wellington highs).Stratigraphic and sedimentological investigation of these rocks in the southern part of the Wellington high near Cambridge Bay suggest that they were deposited by rivers flowing from a land mass that lay to the east. Clasts in the Hadrynian sedimentary rocks attest to the existence of at least two earlier periods of sedimentation and one phase of tectonic compression in the basement rocks.Study of the lower part of the Hadrynian succession (Glenelg and Reynolds Point Formations) in the northeastern part of the Minto Arch reveals a general thickening to the west. Rock types present include dolomite, limestone, siltstone, sandstone and shale deposited in a dominantly shallow marine environment together with some marine deltaic units and possibly distal fluvial deposits. Cross bedding and ripple-mark orientations suggest a paleoslope to the northwest. However, the pattern of cross-bedding distribution is complex with modes not only in the northwest quadrant, but also in the northeast and southwest quadrants, the latter possibly reflecting longshore currents in a generally shallow marine environment.Stromatolites are abundant in both formations studied. They are almost exclusively laterally linked shallow-dipping domes in the cherty dolomites of the Glenelg Formation. However the capping stromatolitic dolomite of this formation is composed of columnar stromatolites, as also are the abundant stromatolitic banks and reefs of the overlying Reynolds Point Formation. Elongate domes in the lower part of the Glenelg Formation have a preferred orientation in a northwest-southeast direction, perhaps reflecting tidal currents. The topmost stromatolitic unit of the Glenelg Formation in more easterly areas has large elongate mounds on its upper surface. Elongation of these mounds has a preferred orientation in a northeast-southwest direction. Possibly this orientation is related to longshore currents flowing along the depositional strike.The Hadrynian sediments of Victoria Island and areas to the south and west may have been deposited in a shallow embayment of the Upper Precambrian Sea, open to the northwest and in continuity with a wedge of sediment that thickened westward and extended along the length of the North American continent. There is some evidence to support the existence of three such embayments in the southern part of the Canadian Arctic Archipelago, separated and defined by northerly salients of older rocks of the Canadian Shield. These salients reflect an east-west tectonic stress on a very large scale.  相似文献   

Assimilation of Paleoseismic Data for Earthquake Simulation     

Lisa B. Grant  Miryha M. Gould 《Pure and Applied Geophysics》2004,161(11-12):2295-2306

— Simulation of the complete earthquake generation process requires assimilation of observational data on long-term behavior of faults over multiple rupture cycles. Paleoseismology, the study of pre-instrumental earthquakes from geological field investigations, provides the only observations of long-term fault behavior. Paleoseismic data present challenges for assimilation into models because data sets are small, sparse, analog, and contain qualitative uncertainties. Observations can be categorized as primary measurements from field data, or non-primary extrapolations or interpretations of primary data. Assimilation of non-primary data could yield biased simulation results. We present a primary data set for the San Andreas and Imperial faults for comparison with non-primary data in existing databases, and propose standard formats and structure for paleoseismic data assimilation into numerical simulation models .  相似文献