New constraints on the sources and behavior of neodymium and hafnium in seawater from Pacific Ocean ferromanganese crusts     

Tina van de Flierdt  Martin Frank  Alex N. Halliday  James R. Hein 《Geochimica et cosmochimica acta》2004,68(19):3827-3843

The behavior of dissolved Hf in the marine environment is not well understood due to the lack of direct seawater measurements of Hf isotopes and the limited number of Hf isotope time-series obtained from ferromanganese crusts. In order to place better constraints on input sources and develop further applications, a combined Nd-Hf isotope time-series study of five Pacific ferromanganese crusts was carried out. The samples cover the past 38 Myr and their locations range from sites at the margin of the ocean to remote areas, sites from previously unstudied North and South Pacific areas, and water depths corresponding to deep and bottom waters.For most of the samples a broad coupling of Nd and Hf isotopes is observed. In the Equatorial Pacific ε_Nd and ε_Hf both decrease with water depth. Similarly, ε_Nd and ε_Hf both increase from the South to the North Pacific. These data indicate that the Hf isotopic composition is, in general terms, a suitable tracer for ocean circulation, since inflow and progressive admixture of bottom water is clearly identifiable.The time-series data indicate that inputs and outputs have been balanced throughout much of the late Cenozoic. A simple box model can constrain the relative importance of potential input sources to the North Pacific. Assuming steady state, the model implies significant contributions of radiogenic Nd and Hf from young circum-Pacific arcs and a subordinate role of dust inputs from the Asian continent for the dissolved Nd and Hf budget of the North Pacific.Some changes in ocean circulation that are clearly recognizable in Nd isotopes do not appear to be reflected by Hf isotopic compositions. At two locations within the Pacific Ocean a decoupling of Nd and Hf isotopes is found, indicating limited potential for Hf isotopes as a stand-alone oceanographic tracer and providing evidence of additional local processes that govern the Hf isotopic composition of deep water masses. In the case of the Southwest Pacific there is evidence that decoupling may have been the result of changes in weathering style related to the buildup of Antarctic glaciation.  相似文献   

Late Quaternary deformation and slip rates in the northern San Andreas fault zone at Olema Valley, Marin County, California     

Karen Grove  Tina M. Niemi 《Tectonophysics》2005,401(3-4):231-250

Quaternary sedimentary deposits along the structural depression of the San Andreas fault (SAF) zone north of San Francisco in Marin County provide an excellent record of rates and styles of neotectonic deformation in a location near where the greatest amount of horizontal offset was measured after the great 1906 San Francisco earthquake. A high-resolution gravity survey in the Olema Valley was used to determine the depth to bedrock and the thickness of sediment fill along and across the SAF valley. In the gravity profile across the SAF zone, Quaternary deposits are offset across the 1906 fault trace and truncated by the Western and Eastern Boundary faults, whose youthful activity was previously unknown. The gravity profile parallel to the fault valley shows a basement surface that slopes northward toward an area of present-day subsidence near the head of Tomales Bay. Surface and subsurface investigations of the late Pleistocene Olema Creek Formation (Qoc) indicate that this area of subsidence was located further south during deposition of the Qoc and that it has migrated northward since then. Localized subsidence has been replaced by localized contraction that has produced folding and uplift of the Qoc. This apparent alternation between transtension and transpression may be the result of a northward-diverging fault geometry of fault strands that includes the valley-bounding faults as well as the 1906 SAF trace. The Vedanta marsh is a smaller example of localized subsidence in the fault zone, between the 1906 SAF trace and the Western Boundary fault. Analyses of Holocene marsh sediments in cores and a paleoseismic trench indicate thickening, and probably tilting, toward the 1906 trace, consistent with coseismic deformation observed at the site following the 1906 earthquake.New age data and offset sedimentary and geomorphic features were used to calculate four late Quaternary slip rate estimates for the SAF at this latitude. Luminescence dates of 112–186 ka for the middle part of the Olema Creek Formation (Qoc), the oldest Quaternary deposit in this part of the valley, suggest a late Pleistocene slip rate of 17–35 mm/year, which replaces the unit to a position adjacent to its sediment source area. A younger alluvial fan deposit (Qqf; basal age 30 ka) is exposed in a quarry along the medial ridge of the fault valley. This fan deposit has been truncated on its western side by dextral SAF movement, and west-side-down vertical movement that has created the Vedanta marsh. Paleocurrent measurements, clast compositions, sediment facies distributions, and soil characteristics show that the Bear Valley Creek drainage, now located northwest of the site, supplied sediment to the fan, which is now being eroded. Restoration of the drainage to its previous location provides an estimated slip rate of 25 mm/year. Furthermore, the Bear Valley Creek drainage probably created a water gap located north of the Qqf deposit during the last glacial maximum 18 ka. The amount of offset between the drainage and the water gap yields an average slip rate of 21–30 mm/year. Finally, displacement of a 1000-year-old debris lobe approximately 20 m from its hillside hollow along the medial ridge indicates a minimum late Holocene slip rate of 21–25 mm/year. Similarity of the late Pleistocene rates to the Holocene slip rate, and to previous rates obtained in paleoseismic trenches in the area, indicates that the rates may not have changed over the past 30 ka, and perhaps the past 200–400 ka. Stratigraphic and structural observations also indicate that valley-bounding faults were active in the late Pleistocene and suggest the need for further study to evaluate their continued seismic potential.  相似文献   

ModTech: a GIS-enabled Ground Water Modeling Program   总被引：1，自引：0，他引：1  

Tina Pint  Shu-Guang Li 《Ground water》2006,44(4):506-508

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2005年国家自然科学基金批准项目一览表(地球科学部分)     

Pavel Ya. Groisman  Richard W. Knight 《地球科学进展》2006,21(1):85-108

~~2005年国家自然科学基金批准项目一览表(地球科学部分)~~  相似文献   

Deformed graptolites, finite strain and volume loss during cleavage formation in rocks of the taconic slate belt, New York and Vermont, U.S.A.     

Arthur Goldstein  Jonathan Knight  Kari Kimball 《Journal of Structural Geology》1998,20(12):1769-1782

An analysis of graptolites in the Taconic slate belt of eastern New York and western Vermont (U.S.A.) shows that they are nearly ideal strain markers. For the three species used in this study, Orthograptus whitfieldii, Orthograptus calcaratus and Climacograptus bicornis, the spacing of thecae is constant except for the first five or so thecae in the proximal part of the fossil rhabdosome. Further, the thecal apertures are perpendicular to the long axis of the stipe. Observations of the thecal spacing in deformed rocks leads to a determination of extension (e=(l_f−l₀)/l₀) and a measure of the angle between thecal aperture and stipe axis yields a direct determination of angular shear strain. In practice, we find it is most straightforward to use length changes to determine the magnitude of principal strains. In Taconic slates, e₁ ranges from 1.0 to 0.24, e₂ ranges from 0.23 to −0.43 and e₃ ranges from −0.56 to −0.74. Thus, we find that the absolute finite strain in these slates is constrictional at three sites, plane strain at another and a true flattening at only one site. An examination of volume changes based on strain results in determinations of between 81% volume loss and 7% volume gain, with volume losses between 28% and 81% in 9 out of 10 calculations. These conclusions are in accord with previous determinations of volume loss based on reduction spot analyses and are consistent with the observation that pressure dissolution was a common grain scale deformation process in cleavage formation but that these slates lack abundant veins, fibrous overgrowths or other identifiable sites of reprecipitation.  相似文献   

Globalization and global change research     

C. Gregory Knight 《GeoJournal》1998,45(1-2):27-32

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Extending the local: the small town and globalization     

David B. Knight 《GeoJournal》1998,45(1-2):145-149

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The water resources of Bulgaria: an overview     

C. Gregory Knight  Marieta P. Staneva 《GeoJournal》1996,40(4):347-362

Bulgaria's indigenous water resources derive largely from mountain watersheds where orographic effects enhance precipitation. These watersheds provide resources upon which lowland communities, industry, and irrigation depend. On the basis of a comprehensive network of water management structures, Bulgaria has been able to harness a considerable proportion of internal water resources for human use, although the nation is, by European standards, dry. This paper explores the physical and human geography of Bulgarian water resources as a perspective from which the multiple dimensions of current Bulgarian water research and water issues can be seen.  相似文献   

Ground-penetrating radar investigation of active faults along the Dead Sea Transform and implications for seismic hazards within the city of Aqaba, Jordan   总被引：1，自引：0，他引：1  

Lee Slater  Tina M. Niemi   《Tectonophysics》2003,368(1-4):33

Ground-penetrating radar (GPR) was used in an effort to locate a major active fault that traverses Aqaba City, Jordan. Measurements over an exposed (trenched) cross fault outside of the city identify a radar signature consisting of linear events and horizontal offset/flexured reflectors both showing a geometric correlation with two known faults at a control site. The asymmetric linear events are consistent with dipping planar reflectors matching the known direction of dip of the faults. However, other observations regarding this radar signature render the mechanism generating these events more complex and uncertain.GPR measurements in Aqaba City were limited to vacant lots. Seven GPR profiles were conducted approximately perpendicular to the assumed strike of the fault zone, based on regional geological evidence. A radar response very similar to that obtained over the cross fault was observed on five of the profiles in Aqaba City, although the response is weaker than that obtained at the control site. The positions of the identified responses form a near straight line with a strike of 45°. Although subsurface verification of the fault by trenching within the city is needed, the geophysical evidence for fault zone location is strong. The location of the interpreted fault zone relative to emergency services, military bases, commercial properties, and residential areas is defined to within a few meters. This study has significant implications for seismic hazard analysis in this tectonically active and heavily populated region.  相似文献   

Assessment of temporal changes in coastal sand dune environments using the log-hyperbolic grain-size method     

Jasper Knight  Julian D. Orford†  Peter Wilson  Sharon M. Braley 《Sedimentology》2002,49(6):1229-1252

Abstract Three cores (<9 m long) recovered from late Holocene coastal sand dunes in Northumberland, north‐eastern England, were examined stratigraphically and dated by the infrared stimulated luminescence (IRSL) method to the time period between the Medieval Warm Period (MWP; ≈ 900–1200 AD) and Little Ice Age (LIA; ≈ 1550–1800 AD). Grain‐size variability and CaCO₃ content were determined at 10‐mm intervals throughout parts of the cores. The sahara computer program was used to plot the log‐hyperbolic distribution (LHD) of grain‐size spectra from different stratigraphic levels and estimate the LHD's statistical components. Core samples plot variously in the depositional and erosional domains of the hyperbolic chi (χ) vs. xi (ξ) shape triangle, which describes components of sediment sorting. Depositional and erosional phases, discriminated within the LHD's shape triangle, cannot readily be resolved from core stratigraphies or correlated between cores. IRSL dates from single cores show a clustering about the period 1430–1540. Between the dated horizons, dunes accreted at ≈ 0·02–0·05 m year^?1, enabling the timing of excursions in χ and ξ to be estimated. These excursions, which can be resolved to the subannual level between the dated horizons, may be related to deposition by episodic storms or storm surges in the North Sea region, which is supported by historical documentary evidence. This innovative use of the LHD method and its derived statistics can be used successfully to describe temporal trends in coastal depositional environments and identify likely storm events in dune sediment records.  相似文献