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981.
David A. Osleger Alan C. Heyvaert Joseph S. Stoner Kenneth L. Verosub 《Journal of Paleolimnology》2009,42(1):103-122
Sediment cores from Lake Tahoe permit the discrimination of turbidites initiated by seismic-induced debris flows from those
generated by severe storms and associated hyperpycnal currents over the last 7000 years using integrated textural, magnetic,
and geochemical signatures. Relative to fine-grained ‘background’ sediments, the majority of Tahoe turbidites exhibit coincident
trends of increased mean grain size, increased magnetic susceptibility, decreased TOC, higher δ13Corg and variable C/N. We interpret these characteristics to record the rapid influx of terrigenous sediments within runoff from
the watershed triggered by high-intensity storms. Correlation of multiple, individual turbidites between cores suggests a
synchronicity of occurrence, supporting the model of extreme hydrologic events as the trigger for most turbidity currents
into Lake Tahoe. In contrast, turbidites generated by seismic collapse of steep lake margins would have textural, magnetic
and geochemical signatures that would reflect a homogenized mix of autochthonous biogenic debris and multiple older turbidites.
Only one of the turbidites in the cores appears to be seismically generated. A second component of this study tested the hypothesis
that turbidite clustering reflects phases of increased storminess, paleoprecipitation and lake level. We correlated broad
patterns of turbidite frequency in the Tahoe cores with climate proxies from (1) elsewhere in the Tahoe watershed, (2) the
western Great Basin (primarily Pyramid Lake) and (3) the San Francisco bay estuary. The reasonable degree of temporal overlap
suggests that apparent trends in severe storm frequency recorded by clusters of turbidites provides a measure of long-term
regional paleoprecipitation and lake level. A key finding is an extended phase of dryness and a near absence of major storms
between ~3000 and ~900 cal yr B.P. in the Tahoe watershed. 相似文献
982.
Matthew N. Waters Michael F. Piehler Antonio B. Rodriguez Joseph M. Smoak Thomas S. Bianchi 《Journal of Paleolimnology》2009,42(1):51-64
Lake Mattamuskeet, North Carolina, USA is a large (162 km2) and shallow (mean depth = 1 m) coastal lake, which was significantly modified to support agricultural activities following
European settlement in 1850. Paleolimnological proxies measured on a 400-cm sediment core collected from Lake Mattamuskeet
reveal shifts in organic matter input and primary producer community structure in response to climatic and human impacts on
the lake during the late Holocene. Stratigraphic changes in organic matter content, nutrients, metals, lignin phenols and
photosynthetic pigments were used to divide the sediment core into three intervals. Interval I includes sediment deposited
between A.D. 360–1584 and indicates a clear-water, sand-bottom state with low algal abundance. In addition, the lake catchment
area experienced two significant fires during this interval that were recorded as charcoal layers in the core around A.D.
360 and A.D. 1435 (calibrated 14C AMS dates). Trophic structure changed with the onset of Interval II (A.D. 1584–1860) when total algal abundance increased,
and the primary producer community was comprised primarily of diatoms, chrysophytes, cryptophytes and cyanobacteria. During
this interval there was also an increase in terrestrial organic material input into the lake as well as a shift in plant type
from woody gymnosperms to non-woody angiosperms as determined from lignin data. Sediment deposited in Lake Mattamuskeet following
European settlement (Interval III, A.D. 1860-present) suggests a dramatic increase in organic-matter deposition, metals, primary-producer
abundance and the onset of cyanobacterial dominance. Sedimentary evidence indicates that shallow-water primary producers can
respond rapidly to climate change and human development. 相似文献
983.
984.
Siddhartha Mitra Joseph J. Lalicata Mead A. Allison Timothy M. Dellapenna 《Marine pollution bulletin》2009,58(6):851-857
To assess the extent to which Hurricanes Katrina and Rita affected polycyclic aromatic hydrocarbons (PAH) in the Gulf of Mexico (GOM), sediment cores were analyzed in late 2005 from: a shallow shelf, a deeper shelf, and a marsh station. Sediment geochronology, fabric, and geochemistry show that the 2005 storms deposited ∼10 cm of sediment to the surface of a core at 5-12A. Bulk carbon geochemistry and PAH isomers in this top layer suggest that the source of sediment to the top portion of core 5-12A was from a relatively more marine area. Particulate PAHs in the marsh core (04 M) appeared unaffected by the storms while sediments in the core from Station 5-1B (deeper shelf) were affected minimally (some possible storm-derived deposition). Substantial amounts of PAH-laden particles may have been displaced from the seabed in shallow areas of the water column in the GOM by these 2005 storms. 相似文献
985.
Exploration in the basalt covered areas of the Faroes offshore has always suffered from poor seismic imaging below the basalt. Long offset 2D and 3D seismic data were acquired and a significant improvement in the seismic image below top basalt has been achieved. Deep towing of the source and receiver cables helped by extending the seismic bandwidth towards lower frequencies. Bubble‐tuned rather than conventional peak‐tuned source arrays gave little, if any, incremental benefit. The improvement in the imaging comes primarily from the approach to processing the data. High frequencies (dominantly noise) are filtered out of the data early in the processing to concentrate on the low frequency data. Careful multiple removal is important with several passes of demultiple being applied to the data using both Surface‐Related Multiple Elimination (SRME) and Radon techniques. Velocity analysis is performed as an iterative process taking into account the geological model. Reprocessing legacy 2D surveys, acquired with wide‐ranging parameters, using these processing techniques improved these datasets significantly, indicating that sub‐basalt imaging seems to be more sensitive to processing than to the choice of acquisition parameters. 相似文献
986.
Rachel M. Allen Julian A. Simeonov Joseph Calantoni Mark T. Stacey Evan A. Variano 《Ocean Dynamics》2018,68(4-5):627-644
Turbulence measurements were collected in the bottom boundary layer of the California inner shelf near Point Sal, CA, for 2 months during summer 2015. The water column at Point Sal is stratified by temperature, and internal bores propagate through the region regularly. We collected velocity, temperature, and turbulence data on the inner shelf at a 30-m deep site. We estimated the turbulent shear production (P), turbulent dissipation rate (ε), and vertical diffusive transport (T), to investigate the near-bed local turbulent kinetic energy (TKE) budget. We observed that the local TKE budget showed an approximate balance (P?≈?ε) during the observational period, and that buoyancy generally did not affect the TKE balance. On a finer resolution timescale, we explored the balance between dissipation and models for production and observed that internal waves did not affect the balance in TKE at this depth. 相似文献
987.
Xiaonan Li Deyu Zhong Y. Joseph Zhang Yanjun Wang Yongqiang Wang Hongwu Zhang 《国际泥沙研究》2018,33(3):271-284
The choice of a river training strategy is extremely important for the Lower Yellow River (LYR). Currently, the wide-river training strategy applies in the training of the LYR. However, remarkable changes in the hydrological processes in the Yellow River basin, as well as immediate pressure from socio-economic development in the Yellow River basin, make it necessary to consider if there is a possibility to change the river training strategy from wide-river training to narrow-river training. This research investigates the impacts of different river training strategies on the LYR through numerical simulations. A one-dimensional (1-D) model was used to simulate the fluvial processes for the future 50 years and a three-dimensional (3-D) model was applied to study typical floods. The study focused on river morphology, the results show that if the present decreasing trend in both water discharge and sediment load persists, the deposition rate in the LYR will further decrease no matter what strategy is applied. Especially, narrow-river training can achieve the aim to increase the sediment transport capacity in the LYR compared with wide-river training. However, if the incoming water and sediment load recovers to the mean level of the last century, main channel shrinkage due to sedimentation inevitably occurs for both wide-river and narrow-river training. Most importantly, this study shows that narrow-river training reduces the deposition amount over the whole LYR, but it provides little help in alleviating the development of the “suspended river”. Instead, narrow-river training can cause aggradation in the transitional reach where the river pattern changes from highly wandering to meandering, further worsening the “hump deposition” there. Because of uncertainty regarding future changes in hydrological processes in the Yellow River basin, and the lack of feasible engineering measures to mitigate “suspended river” and “hump deposition” problems in the LYR, caution should be exercised with respect to changes in the river training strategy for the LYR. 相似文献
988.
Konrad C. Hafen Joseph M. Wheaton Brett B. Roper Philip Bailey Nicolaas Bouwes 《地球表面变化过程与地形》2020,45(11):2664-2674
Stream ecosystems can be dramatically altered by dam-building activities of North American beaver (Castor canadensis). The extent to which beavers’ ecosystem engineering alters riverscapes is driven by the density, longevity, and size (i.e. height and length) of the dams constructed. In comparison to the relative ubiquity of beaver dams on the landscape, there is a scarcity of data describing dam heights. We collected data describing dam height and dam condition (i.e. damaged or intact) of 500 beaver dams via rapid field survey, differentiating between primary and secondary dams and associating each dam with a beaver dam complex. With these data, we examined the influence of beaver dam type (primary/secondary), drainage area, streamflow, stream power, valley bottom width, and HUC12 watershed on beaver dam height with linear regression and the probability that a beaver dam was damaged with logistic regression. On average, primary dams were 0.46 m taller than secondary dams; 15% of observed dams were primary and 85% secondary. Dam type accounted for 21% of dam height variation (p <0.0001). Slope (p = 0.0107), discharge (p = 0.0029), and drainage area (p = 0.0399) also affected dam height, but each accounted for less than 3% of dam height variation. The average number of dams in a dam complex was 6.1 (SD ± 4.5) and ranged from 1 to 21. The watershed a beaver dam was located in accounted for the most variability (17.8%) in the probability that a beaver dam was damaged, which was greater than the variability explained by any multiple logistic regression model. These results indicate that temporally dynamic variables are important influencers of dam longevity and that beaver dam ecology is a primary factor influencing beaver dam height. © 2020 John Wiley & Sons, Ltd. 相似文献
989.
990.
Nibedita Bisoyi Harish Gupta Narayan Prasad Padhy Govind Joseph Chakrapani 《国际泥沙研究》2019,34(2):125-135
Most of the studies on Artificial Neural Network (ANN) models remain restricted to smaller rivers and catchments. In this paper, an attempt has been made to correlate variability of sediment loads with rainfall and runoff through the application of the Back Propagation Neural Network (BPNN) algorithm for a large tropical river. The algorithm and simulation are done through MATLAB environment. The methodology comprised of a collection of data on rainfall, water discharge, and sediment discharge for the Narmada River at various locations (along with time variables) and application to develop a threelayer BPNN model for the prediction of sediment discharges. For training and validation purposes a set of 549 data points for the monsoon (16 June-15 November) period of three consecutive years (1996–1998) was used. For testing purposes, the BPNN model was further trained using a set of 732 data points of monsoon season of four years (2006–07 to 2009–10) at nine stations. The model was tested by predicting daily sediment load for the monsoon season of the year 2010–11. To evaluate the performance of the BPNN model, errors were calculated by comparing the actual and predicted loads. The validation and testing results obtained at all these locations are tabulated and discussed. Results obtained from the model application are robust and encouraging not only for the sub-basins but also for the entire basin. These results suggest that the proposed model is capable of predicting the daily sediment load even at downstream locations, which show nonlinearity in the transportation process. Overall, the proposed model with further training might be useful in the prediction of sediment discharges for large river basins. 相似文献