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981.
Steve E. J. Richardson Richard J. Davies Mark B. Allen Simon F. Grant 《Basin Research》2011,23(6):702-719
The Quaternary to late Pliocene sedimentary succession along the margin of the South Caspian Basin contains numerous kilometre‐scale submarine slope failures, which were sourced along the basin slope and from the inclined flanks of contemporaneous anticlines. This study uses three‐dimensional (3D) seismic reflection data to visualise the internal structure of 27 mass transport deposits and catalogues the syndepositional structures contained within them. These are used to interpret emplacement processes occurring during submarine slope failure. The deposits consist of three linked structural domains: extensional, translational and compressive, each containing characteristic structures. Novel features are present within the mass transport deposits: (1) a diverging retrogression of the headwall scarp; (2) the absence of a conventional headwall scarp around growth stratal pinch outs; (3) restraining bends in the lateral margin; (4) a downslope increase in the throw of thrust faults. The results of this study shed light on the deformation that occurred during submarine slope failure, and highlight an important geological process in the evolution of the South Caspian Basin margin. 相似文献
982.
Climate change in the Mojave Desert (USA) may result in a greater intensity of summer (monsoon) rain events and greater atmospheric N deposition. Patches of the dominant biological crust moss Syntrichia caninervis were exposed to field treatments of low and high nitrogen supplementation, added summer rain, and combinations thereof, for a period of five years. In the lab, shoots were subjected to a moderate (single rapid-dry treatment) and a severe (double rapid-dry treatment) desiccation stress. Shoots were then hydrated and allowed to regenerate for 6 weeks on sterile sand. Shoots exhibited lower aboveground biomass from the +monsoon treatment, indicating carbon balance deficits under conditions of recurring summer rainfall. The low N treatment and the severe desiccation stress promoted shoot burning (chlorosis) and negatively affected regeneration responses. High N treatment, however, only adversely affected the resumption of apical meristematic growth. Summer irrigation and N application interacted to mitigate the effects of later protonemal emergence time, lower protonemal cover, and lower shoot numbers. Increased monsoonal frequency and higher N deposition as forecast for the northern Mojave Desert may result in declines in shoot biomass and reduced regenerational vigor in S. caninervis. 相似文献
983.
Atmospheric deposition of reactive nitrogen (Nr) has enriched oligotrophic lakes with nitrogen (N) in many regions of the
world and elicited dramatic changes in diatom community structure. The lakewater concentrations of nitrate that cause these
community changes remain unclear, raising interest in the development of diatom-based transfer functions to infer nitrate.
We developed a diatom calibration set using surface sediment samples from 46 high-elevation lakes across the Rocky Mountains
of the western US, a region spanning an N deposition gradient from very low to moderate levels (<1 to 3.2 kg Nr ha−1 year−1 in wet deposition). Out of the fourteen measured environmental variables for these 46 lakes, ordination analysis identified
that nitrate, specific conductance, total phosphorus, and hypolimnetic water temperature were related to diatom distributions.
A transfer function was developed for nitrate and applied to a sedimentary diatom profile from Heart Lake in the central Rockies.
The model coefficient of determination (bootstrapping validation) of 0.61 suggested potential for diatom-inferred reconstructions
of lakewater nitrate concentrations over time, but a comparison of observed versus diatom-inferred nitrate values revealed
the poor performance of this model at low nitrate concentrations. Resource physiology experiments revealed that nitrogen requirements
of two key taxa were opposite to nitrate optima defined in the transfer function. Our data set reveals two underlying ecological
constraints that impede the development of nitrate transfer functions in oligotrophic lakes: (1) even in lakes with nitrate
concentrations below quantification (<1 μg L−1), diatom assemblages were already dominated by species indicative of moderate N enrichment; (2) N-limited oligotrophic lakes
switch to P limitation after receiving only modest inputs of reactive N, shifting the controls on diatom species changes along
the length of the nitrate gradient. These constraints suggest that quantitative inferences of nitrate from diatom assemblages
will likely require experimental approaches. 相似文献
984.
G. Wilson D. Damaske H.-D. Möller K. Tinto T. Jordan 《Geophysical Journal International》2007,170(1):93-100
Magnetic anomaly data are presented from a new helicopter-borne high-resolution aeromagnetic survey in southern McMurdo Sound. Anomaly data have been acquired at a common 305 m elevation above the McMurdo and southern McMurdo ice shelves and draped over the volcanic islands that pin them. The resulting anomaly patterns provide a significant advance in the understanding of the rift related geology beneath the floating ice shelves. More extensive Erebus Volcanic Province (McMurdo Volcanic Group) rocks are indicated along with a significant blanket of glaci-volcaniclastic sediment on the seafloor between the volcanic islands in southern McMurdo Sound. These glaci-volcaniclastic sediments are inferred to originate from former grounding of the southern McMurdo Ice Shelf as a marine ice sheet. A strong N–S fabric is also observed in the anomaly data suggesting that the rift structure observed in the Victoria Land basin persists to the south beneath the McMurdo and southern McMurdo ice shelves. W–N–W transfer faults identified within the Transantarctic Mountain rift flank to the west are not obvious in the aeromagnetic data set, implying that the 'Discovery Accommodation Zone' may be restricted to the region between a southward extension of the range bounding fault that marks the limit of the Victoria Land Basin and the right lateral offset in the Transantarctic Mountain front in southern Victoria Land. 相似文献
985.
986.
Brent B. Wolfe Matthew D. Falcone Ken P. Clogg-Wright Cherie L. Mongeon Yi Yi Bronwyn E. Brock Natalie A. St. Amour William A. Mark Thomas W. D. Edwards 《Journal of Paleolimnology》2007,37(2):221-231
Recent advances in sample preparation techniques and mass spectrometry have fostered more routine oxygen isotope analysis
of aquatic cellulose in lake sediment cores, a proxy for lake water oxygen isotope history. These methodological developments
have significantly increased the feasibility of incorporating this approach into high-resolution, multi-site, and multi-proxy
studies, which are frequently necessary to answer complex hydrological, hydroecological and hydroclimatic questions requiring
a paleoenvironmental perspective. Direct translation of lake sediment aquatic cellulose oxygen isotope composition into lake
water oxygen isotope composition offers appreciable opportunity for quantitative paleohydrological reconstructions, as evidenced
by studies conducted over the past 15 years that span Holocene and pre-historical timescales. 相似文献
987.
This study of fossils (pollen, plant macrofossils, stomata and fish) and sediments (lithostratigraphy and geochemistry) from
the Wendel site in North Dakota, USA, emphasizes the importance of considering ground-water hydrology when deciphering paleoclimate
signals from lakes in postglacial landscapes. The Wendel site was a paleolake from about 11,500 14C yr BP to 11,100 14C yr BP. Afterwards, the lake-level lowered until it became a prairie marsh by 9,300 14C yr BP and finally, at 8,500 14C yr BP, an ephemeral wetland as it is today. Meanwhile, the vegetation changed from a white spruce parkland (11,500 to 10,500
14C yr BP) to deciduous parkland, followed by grassland at 9,300 14C yr BP. The pattern and timing of these aquatic and terrestrial changes are similar to coeval kettle lake records from adjacent
uplands, providing a regional aridity signal. However, two local sources of ground water were identified from the fossil and
geochemical data, which mediated atmospheric inputs to the Wendel basin. First, the paleolake received water from the melting
of stagnant ice buried under local till for about 900 years after glacier recession. Later, Holocene droughts probably caused
the lower-elevation Wendel site to capture the ground water of up-gradient lakes. 相似文献
988.
Anna E. van Yperen John M. Holbrook Miquel Poyatos‐Mor Cody Myers Ivar Midtkandal 《Basin Research》2021,33(1):513-543
The adequate documentation and interpretation of regional‐scale stratigraphic surfaces is paramount to establish correlations between continental and shallow marine strata. However, this is often challenged by the amalgamated nature of low‐accommodation settings and control of backwater hydraulics on fluvio‐deltaic stratigraphy. Exhumed examples of full‐transect depositional profiles across river‐to‐delta systems are key to improve our understanding about interacting controlling factors and resultant stratigraphy. This study utilizes the ~400 km transect of the Cenomanian Mesa Rica Sandstone (Dakota Group, USA), which allows mapping of down‐dip changes in facies, thickness distribution, fluvial architecture and spatial extent of stratigraphic surfaces. The two sandstone units of the Mesa Rica Sandstone represent contemporaneous fluvio‐deltaic deposition in the Tucumcari sub‐basin (Western Interior Basin) during two regressive phases. Multivalley deposits pass down‐dip into single‐story channel sandstones and eventually into contemporaneous distributary channels and delta‐front strata. Down‐dip changes reflect accommodation decrease towards the paleoshoreline at the Tucumcari basin rim, and subsequent expansion into the basin. Additionally, multi‐storey channel deposits bound by erosional composite scours incise into underlying deltaic deposits. These represent incised‐valley fill deposits, based on their regional occurrence, estimated channel tops below the surrounding topographic surface and coeval downstepping delta‐front geometries. This opposes criteria offered to differentiate incised valleys from flood‐induced backwater scours. As the incised valleys evidence relative sea‐level fall and flood‐induced backwater scours do not, the interpretation of incised valleys impacts sequence stratigraphic interpretations. The erosional composite surface below fluvial strata in the continental realm represents a sequence boundary/regional composite scour (RCS). The RCS’ diachronous nature demonstrates that its down‐dip equivalent disperses into several surfaces in the marine part of the depositional system, which challenges the idea of a single, correlatable surface. Formation of a regional composite scour in the fluvial realm throughout a relative sea‐level cycle highlights that erosion and deposition occur virtually contemporaneously at any point along the depositional profile. This contradicts stratigraphic models that interpret low‐accommodation settings to dominantly promote bypass, especially during forced regressions. Source‐to‐sink analyses should account for this in order to adequately resolve timing and volume of sediment storage in the system throughout a complete relative sea‐level cycle. 相似文献
989.
Gold and palladium have been determined in 42 geological reference samples with graphite furnace AAS after aqua regia digestion and SnCl2 -Hg extraction. A brief discussion on the quality of these analyses also is presented. 相似文献
990.