Paleomagnetism of Middle Proterozoic mafic intrusions and Upper Proterozoic (Nankoweap) red beds from the Lower Grand Canyon Supergroup, Arizona     

Arlo B. Weil  John W. Geissman  Matt Heizler  Rob Van der Voo 《Tectonophysics》2003,375(1-4):199

Paleomagnetic data from lavas and dikes of the Unkar igneous suite (16 sites) and sedimentary rocks of the Nankoweap Formation (7 sites), Grand Canyon Supergroup (GCSG), Arizona, provide two primary paleomagnetic poles for Laurentia for the latest Middle Proterozoic (ca. 1090 Ma) at 32°N, 185°E (dp=6.8°, DM=9.3°) and early Late Proterozoic (ca. 850–900 Ma) at 10°S, 163°E (dp=3.5°, DM=7.0°). A new ⁴⁰Ar/³⁹Ar age determination from an Unkar dike gives an interpreted intrusion age of about 1090 Ma, similar to previously reported geochronologic data for the Cardenas Basalts and associated intrusions. The paleomagnetic data show no evidence of any younger, middle Late Proterozoic tectonothermal event such as has been revealed in previous geochronologic studies of the Unkar igneous suite. The pole position for the Unkar Group Cardenas Basalts and related intrusions is in good agreement with other ca. 1100 Ma paleomagnetic poles from the Keweenawan midcontinent rift deposits and other SW Laurentia diabase intrusions. The close agreement in age and position of the Unkar intrusion (UI) pole with poles derived from rift related rocks from elsewhere in Laurentia indicates that mafic magmatism was essentially synchronous and widespread throughout Laurentia at ca. 1100 Ma, suggesting a large-scale continental magmatic event. The pole position for the Nankoweap Formation, which plots south of the Unkar mafic rocks, is consistent with a younger age of deposition, at about 900 to 850 Ma, than had previously been proposed. Consequently, the inferred 200 Ma difference in age between the Cardenas Basalts and overlying Nankoweap Formation provides evidence for a third major unconformity within the Grand Canyon sequence.  相似文献   

Reconstruction of urban stormwater contamination of an estuary using catchment history and sediment profile dating     

Andrew Swales  R. Bruce Williamson  Laurence F. Van Dam  Morag J. Stroud  Matt S. McGlone 《Estuaries and Coasts》2002,25(1):43-56

Cores were collected from the length of Pakuranga estuary, a small urban estuary in Auckland, New Zealand, to determine sedimentation and contaminant history, and in particular the impact of urbanization. Catchment sediment loads for the most recent history (1953–1995), including urbanization since 1960, were reconstructed using the landcover history and soil erosion modeling. Pollen and¹⁴C dating and pre-urban landcover history were used to reconstruct early estuary sedimentation (i.e., post-3000 yr BP to 1960). Heavy metal concentrations, particle size,¹³⁷Cs, pollen, and catchment sediment loads were all needed to disentangle the complex estuarine response to urbanization.¹³⁷Cs profiles did not reflect the historical fallout pattern, but deposition of¹³⁷Cs-labelled eroded catchment soil, coinciding with peaks in urban construction. Temporal variations in stormwater¹³⁷Cs concentrations are likely due to varying contributions from¹³⁷Cs-rich topsoil and¹³⁷Cs-poor subsoils. A similar pattern was observed in heavy-metal concentrations and attributed to street runoff rather than topsoil being diluted by metal-poor subsoils. Dating of the sediment profiles showed that during urbanization sedimentation rates in the tidal creek and estuary were higher than sedimentation rates associated with past agricultural landuse and the original forest landcover. Urbanization has brought about substantial environmental changes in the upper estuary through continued infilling of shallow, intertidal areas, contamination by heavy metals to levels of ecological concern, sediment textural changes, and rapid mangrove colonization of formerly bare intertidal sediments.  相似文献   

Improving sub-salt imaging using 3D basin model derived velocities     

R. Stephan Petmecky  Martin L. Albertin  Nick Burke 《Marine and Petroleum Geology》2009

The quality of depth imaging is directly related to the accuracy of the underlying velocity model. In most sub-salt settings, lack of angular illumination severely degrades the resolution and accuracy of velocity information derived from the seismic data itself. A standard approach for building a starting velocity model uses more reliable velocity information outboard of salt which is subsequently extrapolated to populate the sub-salt regions. The shortcoming of this method lies in the assumption that the effective stress observed outboard of salt can be extrapolated beneath salt solely as a function of depth below mudline.  相似文献   

Basic hydrology,limnology, and meteorology of modern Lake El’gygytgyn,Siberia     

Matt Nolan  Julie Brigham-Grette 《Journal of Paleolimnology》2007,37(1):17-35

A survey of the modern physical setting of Lake El’gygytgyn, northeastern Siberia, is presented here to facilitate interpretation of a 250,000-year climate record derived from sediment cores from the lake bottom. The lake lies inside a meteorite impact crater that is approximately 18 km in diameter, with a total watershed area of 293 km², 110 km² of which is lake surface. The only surface water entering the lake comes from the approximately 50 streams draining from within the crater rim; a numbering system for these inlet streams is adopted to facilitate scientific discussion. We created a digital elevation model for the watershed and used it to create hypsometries, channel networks, and drainage area statistics for each of the inlet streams. Many of the streams enter shallow lagoons dammed by gravel berms at the lakeshore; these lagoons may play a significant role in the thermal and biological dynamics of the lake due to their higher water temperatures (>6°C). The lake itself is approximately 12 km wide and 175 m deep, with a volume of 14.1 km³. Water temperature within a column of water near the center of this oligotrophic, monomictic lake never exceeded 4°C over a 2.5 year record, though the shallow shelves (<10 m) surrounding the lake can reach 5°C in summer. Though thermally stratified in winter, the water appears completely mixed shortly after lake ice breakup in July. Mean annual air temperature measured about 200 m from the lake was −10.3°C in 2002, and an unshielded rain gage there recorded 70 mm of rain in summer of 2002. End of winter snow water equivalent on the lake was approximately 110 mm in May 2002. Analysis of NCEP reanalysis air temperatures (1948–2002) reveals that the 8 warmest years and 10 warmest winters have occurred since 1989, with the number of days below −30°C dropping from a pre-1989 mean of 35 to near 0 in recent years. The crater region is windy as well as cold, with hourly wind speeds exceeding 13.4 m s⁻¹ (30 mph) typically at least once each month and 17.8 m s⁻¹ (40 mph) in winter months, with only a few calm days per month; wind may also play an important role in controlling the modern shape of the lake. Numerous lines of evidence suggest that the physical hydrology and limnology of the lake has changed substantially over the past 3.6 million years, and some of the implications of these changes on paleoclimate reconstructions are discussed. This is the second in a series of eleven papers published in this special issue dedicated to initial studies of El'gygytgyn Crater Lake and its catchment in NE Russia. Julie Brigham-Grette, Martin Melles, Pavel Minyuk were guest editors of this special issue.  相似文献   

Attenuation effect on seasonal basin-scale water storage changes from GRACE time-variable gravity   总被引：3，自引：0，他引：3  

J. L. Chen  C. R. Wilson  J. S. Famiglietti  Matt Rodell 《Journal of Geodesy》2007,81(4):237-245

In order to effectively recover surface mass or geoid height changes from the gravity recovery and climate experiment (GRACE) time-variable gravity models, spatial smoothing is required to minimize errors from noise. Spatial smoothing, such as Gaussian smoothing, not only reduces the noise but also attenuates the real signals. Here we investigate possible amplitude attenuations and phase changes of seasonal water storage variations in four drainage basins (Amazon, Mississippi, Ganges and Zambezi) using an advanced global land data assimilation system. It appears that Gaussian smoothing significantly affects GRACE-estimated basin-scale seasonal water storage changes, e.g., in the case of 800 km smoothing, annual amplitudes are reduced by about 25–40%, while annual phases are shifted by up to 10°. With these effects restored, GRACE-estimated water storage changes are consistently larger than model estimates, indicating that the land surface model appears to underestimate terrestrial water storage change. Our analysis based on simulation suggests that normalized attenuation effects (from Gaussian smoothing) on seasonal water storage change are relatively insensitive to the magnitude of the true signal. This study provides a numerical approach that can be used to restore seasonal water storage change in the basins from spatially smoothed GRACE data.  相似文献   

Orogenic andesites and plate tectonics: James B. Gill. Minerals and Rocks,xiv + 390 pp., 109 figs., DM. 98.00, Vol. 16, Springer-Verlag,Berlin, 1981, v U.S. $44.60 (approx.)     

Kevin Burke 《Precambrian Research》1983,19(3):317-318

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Nutrient and carbon removal ratios and fluxes in the Ross Sea, Antarctica     

Colm Sweeney  Walker O. Smith  Burke Hales  Robert R. Bidigare  Craig A. Carlson  L. A. Codispoti  Louis I. Gordon  Dennis A. Hansell  Frank J. Millero  Mi-OK Park  Taro Takahashi 《Deep Sea Research Part II: Topical Studies in Oceanography》2000,47(15-16)

Net community production (NCP) and nutrient deficits (Def(X)) were calculated using decreases in dissolved CO₂ and nutrient concentrations due to biological removal in the upper 200 m of the water column during four cruises in the Ross Sea, Antarctica along 76°30′S in 1996 and 1997. A comparison to excess dissolved and particulate organic carbon showed close agreement between surplus total organic carbon (TOC) and NCP during bloom initiation and productivity maximum; however, when TOC values had returned to low wintertime values NCP was still significantly above zero. This seasonal NCP, 3.9±1 mol C m⁻², must be equivalent to the particle export to depths greater than 200 m over the whole productive season. We estimate that the annual export was 55±22% of the seasonal maximum in NCP. The fraction of the seasonal maximum NCP that is exported through 200 m is significantly higher than that measured by moored sediment traps at a depth of 206 m. The removal of carbon, nitrate and phosphate (based on nutrient disappearance since early spring) and their ratios showed significant differences between regions dominated by diatoms and regions dominated by the haptophyte Phaeocystis antarctica. While the ΔC/ΔN removal ratio was similar (7.8±0.2 for diatoms and 7.2±0.1 for P. antarctica), the ΔN/ΔP and ΔC/ΔP removal ratios for diatoms (10.1±0.3 and 80.5±2.3) were significantly smaller than those of P. antarctica (18.6±0.4 and 134.0±4.7). The similarity in ΔC/ΔN removal ratios of the two assemblages suggests that preferential uptake of phosphate by diatoms caused the dramatic differences in ΔC/ΔP and ΔN/ΔP removal ratios. In contrast to low ΔC/ΔP and ΔN/ΔP removal ratio in diatom-dominated areas early in the growing season, deficit N/P and C/P ratios in late autumn indicate that the elemental stochiometry of exported organic matter did not deviate significantly from traditional Redfield ratios. Changes in biologically utilized nutrient and carbon ratios over the course of the growing season indicated either a substantial remineralization of phosphate or a decrease in phosphate removal relative to carbon and total inorganic nitrogen over the bloom period. The species dependence in C/P ratios, and the relative constancy in the C/N ratios, makes N a better proxy of biological utilization of CO₂.  相似文献   

A Marine Isotope Stage 4 age for Pleistocene raised beach deposits near Fethard,southern Ireland          下载免费PDF全文

Colman Gallagher  Matt W. Telfer  Colm Cofaigh 《第四纪科学杂志》2015,30(8):754-763

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Multiple re‐advances of a Lake Vättern outlet glacier during Fennoscandian Ice Sheet retreat,south‐central Sweden          下载免费PDF全文

Sarah L. Greenwood  Matt O'Regan  Henrik Swärd  Tom Flodén  Roman Ananyev  Denis Chernykh  Martin Jakobsson 《Boreas: An International Journal of Quaternary Research》2015,44(4):619-637

Lake Vättern represents a critical region geographically and dynamically in the deglaciation of the Fennoscandian Ice Sheet. The outlet glacier that occupied the basin and its behaviour during ice‐sheet retreat were key to the development and drainage of the Baltic Ice Lake, dammed just west of the basin, yet its geometry, extent, thickness, margin dynamics, timing and sensitivity to regional retreat forcing are rather poorly known. The submerged sediment archives of Lake Vättern represent a missing component of the regional Swedish deglaciation history. Newly collected geophysical data, including high‐resolution multibeam bathymetry of the lake floor and seismic reflection profiles of southern Lake Vättern, are used here together with a unique 74‐m sediment record recently acquired by drill coring, and with onshore LiDAR‐based geomorphological analysis, to investigate the deglacial environments and dynamics in the basin and its terrestrial environs. Five stratigraphical units comprise a thick subglacial package attributed to the last glacial period (and probably earlier), and an overlying >120‐m deglacial sequence. Three distinct retreat–re‐advance episodes occurred in southern Lake Vättern between the initial deglaciation and the Younger Dryas. In the most recent of these, ice overrode proglacial lake sediments and re‐advanced from north of Visingsö to the southern reaches of the lake, where ice up to 400 m thick encroached on land in a lobate fashion, moulding crag‐and‐tail lineations and depositing till above earlier glacifluvial sediments. This event precedes the Younger Dryas, which our data reveal was probably restricted to north‐central sectors of the basin. These dynamics, and their position within the regional retreat chronology, indicate a highly active ice margin during deglaciation, with retreat rates on average 175 m a^?1. The pronounced topography of the Vättern basin and its deep proglacial‐dammed lake are likely to have encouraged the dynamic behaviour of this major Fennoscandian outlet glacier.  相似文献   

The relationship between temperature and assault in New Zealand     

Matt N. Williams  Stephen R. Hill  John Spicer 《Climatic change》2015,131(4):559-573

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