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1.
It has been established that idealized western boundary currents, which encounter a gap in their supporting boundary, will assume one of two dominant steady states: a loop current state and a gap leaping state, and that transitions between these states display hysteresis. However, a question of whether the idealized geometries considered to date apply to the Gulf of Mexico Loop Current (LC) remained. Here, the nonlinear potential vorticity advection-diffusions equations are solved, for Gulf of Mexico topography, using Newton’s method. We demonstrate that, in application to the LC in the Gulf of Mexico, the original conclusions do hold and additionally describe peculiarities of the more realistic steady states. The existence of our numerically calculated steady LC states in the actual Gulf of Mexico are supported through analysis of historical sea surface height data, and implications of our results for LC modeling and forecasting are discussed.
相似文献2.
Sedimentary furrows in fine-grained sediments have been observed in a variety of settings ranging from the deep ocean and
deep lake bottoms to shallow estuaries and are commonly described as persistent, long-term features of the seabed. A series
of 12 sidescan sonar surveys over the course of three years reveal that transient, longitudinal sedimentary furrows regularly
form and then occasionally dissipate within the middle portion of the York River. Varying furrow morphologies were observed
depending on current conditions, ranging from large regularly space (0.7–7 m) linear furrows during low current conditions
to large patches of meandering furrows as the mean current increases or no bed forms during the higher current conditions.
Based on210Pb and137Cs profiles of kasten cores, differences in physical mixing depths of ∼25 cm between cores collected <2 m apart indicate a
high degree of small-scale spatial heterogeneity within the seabed. By documenting the position of kasten cores using a digital
sidescan sonar system, we showed that a core taken within a furrow had a mixing depth 15 cm shallower than an adjacent core
taken between furrows. A time-series of mixing depths over the 35 mo of the study reveals that, along with the ∼25 cm scale
differences in mixing depths due to the formation and destruction of furrows, there is a longer temporal signal of mixing
producing 100-cm-scale changes in mixing depths on the annual to interannual time frame. Although the formation and destruction
of the furrows appear to be a significant process contributing to decimeter-scale seabed mixing, there is a longer-term unknown
process which is controlling the meter-scale seabed mixing. 相似文献
3.
Assessment of the historical trace metal contamination of sediments in the Elizabeth River, Virginia
Two sediment cores (Southern Branch, PC-1, and Western Branch, WB-2) were taken from the highly industrialized Elizabeth River, Virginia. The concentrations of trace metals cadmium, cobalt, chromium, copper, nickel, lead and zinc, major elements iron, manganese and aluminum, organic carbon content and the specific surface area of the sediments were determined in each of the cores. Down-core variations in metals varied significantly in each core with maximum contamination events occurring at different times in different portions of the river. In PC-1, maximum metal concentrations were seen after the appearance of (137)Cs. In contrast, the highest levels in WB-2 occurred well before the appearance of (137)Cs. Although stricter environmental regulations have caused a decrease in metal concentrations since the 1980s, the concentrations in the surface sediments of many trace metals were elevated to levels 2-5 times higher than the levels at the bottom of the cores in both the Southern and Western Branches of the river. 相似文献
4.
Clifford E. Singer T. S. Gopi Rethinaraj Samuel Addy David Durham Murat Isik Madhu Khanna Brandon Kuehl Jianding Luo Wilma Quimio Kothavari Rajendran Donna Ramirez Ji Qiang Jürgen Scheffran T. Nedjla Tiouririne Junli Zhang 《Climatic change》2008,88(3-4):309-342
Probability distributions for carbon burning, atmospheric CO2, and global average temperature are produced by time series calibration of models of utility optimization and carbon and heat balance using log-linear production functions. Population growth is used to calibrate a logistically evolving index of development that influences production efficiency. Energy production efficiency also includes a coefficient that decreases linearly with decreasing carbon intensity of energy production. This carbon intensity is a piecewise linear function of fossil carbon depletion. That function is calibrated against historical data and extrapolated by sampling a set of hypotheses about the impact on the carbon intensity of energy production of depleting fluid fossil fuel resources and increasing cumulative carbon emissions. Atmospheric carbon balance is determined by a first order differential equation with carbon use rates and cumulative carbon use as drivers. Atmospheric CO2 is a driver in a similar heat balance. Periodic corrections are included where required to make residuals between data and model results indistinguishable from independently and identically distributed normal distributions according to statistical tests on finite Fourier power spectrum amplitudes and nearest neighbor correlations. Asymptotic approach to a sustainable non-fossil energy production is followed for a global disaggregation into a tropical/developing and temperate/more-developed region. The increase in the uncertainty of global average temperature increases nearly quadratically with the increase in the temperature from the present through the next two centuries. 相似文献
5.
Elizabeth A. Canuel Elizabeth J. Lerberg Rebecca M. Dickhut Steven A. Kuehl Thomas S. Bianchi Stuart G. Wakeham 《Marine pollution bulletin》2009,59(4-7):154
We used the Sacramento-San Joaquin River Delta CA (Delta, hereafter) as a model system for understanding how human activities influence the delivery of sediment and total organic carbon (TOC) over the past 50–60 years. Sediment cores were collected from sites within the Delta representing the Sacramento River (SAC), the San Joaquin River (SJR), and Franks Tract (FT), a flooded agricultural tract. A variety of anthropogenic tracers including 137Cs, total DDE (∑DDE) and brominated diphenyl ether (BDE) congeners were used to quantify sediment accumulation rates. This information was combined with total organic carbon (TOC) profiles to quantify rates of TOC accumulation. Across the three sites, sediment and TOC accumulation rates were four to eight-fold higher prior to 1972. Changes in sediment and TOC accumulation were coincident with completion of several large reservoirs and increased agriculture and urbanization in the Delta watershed. Radiocarbon content of TOC indicated that much of the carbon delivered to the Delta is “pre-aged” reflecting processing in the Delta watershed or during transport to the sites rather than an input of predominantly contemporary carbon (e.g., 900–1400 years BP in surface sediments and 2200 yrs BP and 3610 yrs BP at the base of the SJR and FT cores, respectively). Together, these data suggest that human activities have altered the amount and age of TOC accumulating in the Delta since the 1940s. 相似文献
6.
Nicholas Castle Ethan Kuehl John Jones Allan Treiman 《Meteoritics & planetary science》2020,55(1):3-19
Petrographic examination of the xenolith and xenocryst populations in the olivine‐phyric shergottite Elephant Moraine 79001 Lithology A shows more chemical heterogeneity than previously documented. Analyses of olivine grains in 18 megacrysts and in 4 lithic fragments show that these two populations either do not have the same source or that this source is heterogeneous in terms of its time–temperature history. Additionally, among the four lithic fragments analyzed, two are distinctive (1) one contains a major‐element‐equilibrated, euhedral olivine grain and (2) the second contains high‐magnesium pyroxene cores. Furthermore, two populations of ferroan olivine were identified (1) one more ferroan than any other reported in EET 79001 and (2) a slightly less ferroan, unequilibrated olivine, but with a restricted range in Mg#. We have also observed an equilibrated pyroxene grain associated with a zoned olivine megacryst. As a result, we recognize that the xenolith/xenocryst population does not represent the incorporation of a single xeno‐lithology into Lithology A, and propose that it be subdivided into a suite of seven identified lithologies, with the understanding that more are likely to be identified with further study. The abundance of Ca in olivine in the xeno‐lithologies suggests a set of crustal, rather than deep mantle, lithologies. Diffusion rates in olivine suggest that the lithologies were incorporated shortly before rapid cooling of the host magma, preserving preexisting mineral chemical zoning. These mineral chemical zones could have been preserved at lower crustal temperatures for up to 10s of Ka. Trace‐element studies of these distinct populations would be required to test whether they are related by igneous processes from a common source magma. 相似文献
7.
Biologically dominated lower Chesapeake Bay and the physically dominated York River subestuary are contrasted in terms of the dynamics of sediment mixing, strata formation and sea-bed particle residence times. Two lower bay sites were examined; both are located within the bay stem plains and are characterized by muddy sand and an abundance of large, deep-dwelling organisms. X-radiographs indicate extensive biological reworking of sediments, with no long-term preservation of physical stratification.210Pb profiles reveal low sediment accumulation rates at both lower bay sites (<0·1 cm year−1), but significant differences in biological mixing depths (25vs40 cm) and biodiffusivity (>80vs6–30 cm2year−1). In contrast, the York River site, located within a partially-filled palaeochannel, is predominantly mud with a depauperate benthic community dominated by small, short-lived, shallow-dwelling organisms. Although210Pb accumulation rates at the York River site (<0·2 cm year−1) are similar to those measured in the lower bay, there is little bioturbation. In addition, transient bed forms at the York River site form laterally persistent, linear ridges and furrows sub-parallel to the channel, spaced 10–20 m apart. These observations, coupled with evidence of episodic erosion and deposition from radioisotope and porosity profiles, and X-radiographs, suggest that the upper 60–120 cm of the sea-bed are dominated by physical mixing. Deep mixing and low accumulation rates result in long residence times of particles in the mixed upper portion of the sea-bed (102year) at both locations, despite different mixing controls [i.e. biological (diffusive)vsphysical (advective)]. 相似文献
8.
Steven A. Kuehl Gregg J. Brunskill Kathy Burns David Fugate Tara Kniskern Linda Meneghini 《Continental Shelf Research》2004,24(19):2417-2429
A sediment budget is constructed for the slope and narrow continental shelf off the Sepik River in order to estimate the relative importance of turbid plumes versus bottom gravity transport through a near-shore submarine canyon in the dispersal of sediment across this collision margin. 210Pb geochronology and inventories of Kasten cores are consistent with the northwestward dispersal of sediment from the river mouth via hypopycnal and possible isopycnal plumes. Sediment accumulation rates are 5 cm yr−1 on the upper slope just off of the Sepik mouth, decreasing gradually to 1 cm yr−1 toward the northwest, and decreasing abruptly offshore (<0.2 cm yr−1 at 1200 m water depth). A sediment budget indicates that only about 7–15% of the Sepik River sediment discharge accumulates on the adjacent open shelf and slope. The remainder presumably escapes offshore via gravity flows through a submarine canyon, the head of which extends into the river mouth. The divergent sediment pathways observed off the Sepik River (i.e., surface and subsurface plumes versus sediment gravity flows through a canyon) may be common along high-yield collision margins of the Indo–Pacific archipelago, and perhaps are analogous to most margins during Late Quaternary low sea-level conditions. 相似文献
9.
Christina R. Pondell Aaron J. Beck Steven A. Kuehl Elizabeth A. Canuel 《Aquatic Geochemistry》2016,22(2):97-115
The determination of sediment accumulation rates in environments with temporal variations in texture is challenging using traditional radioisotope methods, largely due to low activities associated with coarse sediments. This study used Englebright Lake, an impoundment in northern California, as a model system to examine the application of plutonium isotopes in lacustrine environments where the interlayering of coarse and fine sediments complicates the geochronology. Inductively coupled plasma mass spectrometry was used to quantify plutonium isotopes and low limits of detection allowed for the measurement of plutonium in sand, clay, and silt fractions. Although measurable levels of plutonium were found in sand fractions, over 75 % of the total plutonium activity was found in fine-grain-size fractions (<63 μm). Correlations between cesium-137 and plutonium activities in fine-grained sediments (r = 0.81–0.98, p < 0.005) suggest that plutonium isotopes may be substituted for cesium isotopes in coarse-grained sediments where cesium is typically below detectable levels. Sediment accumulation rates calculated from grain-size normalized plutonium activity profiles ranged from 6 to 145 cm year?1 in Englebright Lake and identified a sediment depocenter at the delta front upstream of Englebright Dam. Progradation of the delta front reflected changes in sediment supply from the watershed in response to flood events, whereas average annual accumulation responded to human impacts. This study extends the application of plutonium isotopes for sediment geochronology to aquatic environments dominated by coarse sediments and provides new information that contributes to a better understanding of the processes influencing sediment deposition in Englebright Lake. 相似文献
10.
C. A. Nittrouer D. J. DeMaster S. A. Kuehl B. A. McKee K. W. Thorbjarnarson 《Geo-Marine Letters》1984,4(3-4):211-213
Accumulation rates (100-yr time scale) in proximal regions of allochthonous dispersal systems range from centimeters to millimeters per year, depending primarily on fluvial discharge of sediment. A general decrease in accumulation rate occurs from proximal to distal regions, and the across-shelf maximum rate is in the mid-shelf region. Sedimentary structure generally changes from stratified in proximal deposits to homogeneous in distal deposits. Although the content of sand and coarse silt commonly decreases along dispersal systems, progressive fining of clay and fine silt is not as obvious. Deposition rates (100-day time scale) in proximal deposits can be significantly more rapid (centimeters per month) than longer-term accumulation rates. Rapid sedimentation does not necessarily produce mass movement. 相似文献