Scattering of rossby waves by partial barriers     

W. D. McKee 《地球物理与天体物理流体动力学》2013,107(1):83-89

Abstract

The scattering of Rossby waves by partial barriers is studied. It is found that only a small fraction of the wave energy will penetrate the gap between South America and Antarctica if the wave period is less than a month.  相似文献   

Use of a vertical δ18O profile to constrain hydraulic properties and recharge rates across a glacio-lacustrine unit, Nantucket Island, Massachusetts, USA     

Mark Person  Andee Marksamer  Brandon Dugan  Peter E. Sauer  Kyle Brown  David Bish  Kathy J. Licht  Mark Willett 《Hydrogeology Journal》2012,20(2):325-336

  相似文献   

High-precision osmium isotopes in enstatite and Rumuruti chondrites     

David van Acken  Alan D. Brandon 《Geochimica et cosmochimica acta》2011,75(14):4020-4036

Isotopic heterogeneity within the solar nebula has been a long-standing issue. Studies on primitive chondrites and chondrite components for Ba, Sm, Nd, Mo, Ru, Hf, Ti, and Os yielded conflicting results, with some studies suggesting large-scale heterogeneity. Low-grade enstatite and Rumuruti chondrites represent the most extreme ends of the chondrite meteorites in terms of oxidation state, and might thus also present extremes if there is significant isotopic heterogeneity across the region of chondrite formation. Osmium is an ideal tracer because of its multiple isotopes generated by a combination of p-, r-, and s-process and, as a refractory element; it records the earliest stages of condensation.Some grade 3-4 enstatite and Rumuruti chondrites show similar deficits of s-process components as revealed by high-precision Os isotope studies in some low-grade carbonaceous and ordinary chondrites. Enstatite chondrites of grades 5-6 have Os isotopic composition identical within error to terrestrial and solar composition. This supports the view of digestion-resistant presolar grains, most likely SiC, as the major carrier of these anomalies. Destruction of presolar grains during parent body processing, which all high-grade enstatite chondrites, but also some low-grade chondrites seemingly underwent, makes the isotopically anomalous Os accessible for analysis. The magnitude of the anomalies is consistent with the presence of a few ppm of presolar SiC with a highly unusual isotopic composition, produced in a different stellar environment like asymptotic giant branch stars (AGB) and injected into the solar nebula. The presence of similar Os isotopic anomalies throughout all major chondrite groups implies that carriers of Os isotopic anomalies were homogeneously distributed in the solar nebula, at least across the formation region of chondrites.  相似文献   

Triaxial shear testing of polished slickensided surfaces   总被引：1，自引：1，他引：0  

Christopher L. Meehan  Binod Tiwari  Thomas L. Brandon  J. Michael Duncan 《Landslides》2011,8(4):449-458

A series of consolidated-drained triaxial tests were performed on precut and polished clay specimens to measure their drained residual strength. Two soils were tested during this research program: Rancho Solano Clay and San Francisco Bay Mud. Specimens were tested using a specially modified triaxial compression device which was developed to minimize the effects of end-platen restraint on the measured strengths. Special attention was paid to the influence of changing specimen area and membrane effects during the test. Triaxial test results were compared with baseline measurements of drained residual shear strength that were made for each of the clay soils using Bromhead ring shear tests and polished-specimen direct shear tests. Residual strength values measured in the triaxial device were higher than the drained residual strengths measured in the Bromhead ring shear apparatus and the direct shear device, indicating that this test approach is more challenging than the use of direct shear tests conducted on polished slickensided surfaces. Comparison of single stage and multistage triaxial test data indicates that multistage triaxial testing may work well for specimens that fail along a well-defined plane, provided that careful attention is given to the effects of end platen restraint, membrane restraint, and changes in specimen area during shear.  相似文献   

Burial and degradation of organic carbon in Louisiana shelf/slope sediments     

Troy P. Sampere  Thomas S. Bianchi  Mead A. Allison  Brent A. McKee 《Estuarine, Coastal and Shelf Science》2011

The primary focus of this paper is to better understand carbon burial on the Louisiana continental margin using spatial scales that covered various shelf depositional areas far-field and near-field (sediment and organic carbon inputs relative to river mouth proximity) and covering a variety of sedimentation rates. Box-cores samples were collected in July 2003; cores were collected along two depositional transects extending westward and southward from the Southwest Pass (SW Pass). A key difference between the two transects sampled in this study was the greater occurrence of mobile muds derived from spillover from shallower regions along the westward 50 m isobaths. The dominant mechanism for mixing in the surface active zone (SAZ) on the inner Louisiana shelf was due to physical, not biological, forces. Burial efficiencies for organic carbon (57.2–91.5%) and total nitrogen (44.2–86.9%) ranged widely across all shelf stations. Lower burial efficiencies for bulk organic carbon, total nitrogen, and pigment biomarkers were associated with mobile muds west of Southwest Pass. Chlorophyll a concentrations were significantly higher than pheopigments at depth at the Mississippi River and Southwest Pass stations, making up 40.4 and 77.4% of total pigment concentrations in the (SAZ) and 46.2 and 63.2% in the accumulation zone (AZ), respectively. These results are in agreement with earlier plant pigment studies which showed that a large fraction of the phytodetritus delivered to the inner shelf was derived from coastal and river diatoms. The amount of lignin preserved with depth decreased with increasing residence time in the SAZ and diagenetic zone (DZ) along the canyon transect but not along the western transect. Trends for lignin concentration followed previously identified surface sediment trends indicating overall lower burial of refractory terrestrial material at depth with greater distance offshore.  相似文献   

Quantifying the influence of channel sinuosity on the depositional mechanics of channelized turbidity currents: A laboratory study     

Kyle M. Straub  David MohrigJames Buttles  Brandon McElroyCarlos Pirmez 《Marine and Petroleum Geology》2011,28(3):744-760

Here we present results from a suite of laboratory experiments that highlight the influence of channel sinuosity on the depositional mechanics of channelized turbidity currents. We released turbidity currents into three channels in an experimental basin filled with water and monitored current properties and the evolution of topography via sedimentation. The three channels were similar in cross-sectional geometry but varied in sinuosity. Results from these experiments are used to constrain the run-up of channelized turbidity currents on the outer banks of moderate to high curvature channel bends. We find that a current is unlikely to remain contained within a channel when the kinetic energy of a flow exceeds the potential energy associated with an elevation gain equal to the channel relief; setting an effective upper limit for current velocity. Next we show that flow through bends induces a vertical mixing that redistributes suspended sediment back into the interiors of depositional turbidity currents. This mixing counteracts the natural tendency for suspended sediment concentration and grain size to stratify vertically, thereby reducing the rate at which sediment is lost from a current via deposition. Finally, the laboratory experiments suggest that turbidity currents might commonly separate from channel sidewalls along the inner banks of bends. In some cases, sedimentation rates and patterns within the resulting separation zones are sufficient to construct bar forms that are attached to the channel sidewalls and represent an important mechanism of submarine channel filling. These bar forms have inclined strata that might be mistaken for the deposits of point bars and internal levees, even though the formation mechanism and its implications to channel history are different.  相似文献   

Biophysical controls on accretion and elevation change in Caribbean mangrove ecosystems     

Karen L. McKee   《Estuarine, Coastal and Shelf Science》2011,91(4):259

  相似文献   

Martian mantle mineralogy investigated by the Lu–Hf and Sm–Nd systematics of shergottites     

V. Debaille  Q.-Z. Yin  A.D. Brandon  B. Jacobsen 《Earth and Planetary Science Letters》2008,269(1-2):186-199

Chemical heterogeneities in the Martian mantle are believed to result from the crystallization of a magma ocean in the first 100 million years of its history. Shergottite meteorites from Mars are thought to retain a compositional record of such early differentiation and the resulting mineralogy at different depths. The coupled ¹⁷⁶Lu–¹⁷⁶Hf and ¹⁴⁷Sm–¹⁴³Nd isotope systematics in 9 shergottites are used here to investigate these issues. Three compositional groups in the shergottites display distinct isotope systematics. One group, commonly termed as depleted, is characterized by positive ¹⁷⁶Hf_i from + 46.2 to + 50.4 and ¹⁴³Nd_i from + 36.2 to + 39.1. Another, termed as enriched, has negative ¹⁷⁶Hf_i = − 16.5 to − 13.2 and ¹⁴³Nd_i = − 7.0 to − 6.5. The third group is intermediate between the depleted and enriched groups with positive ¹⁷⁶Hf_i = + 30.0 to + 33.4 and ¹⁴³Nd_i = + 16.9. Together, they describe mixing curves between ¹⁷⁶Hf/¹⁷⁷Hf, ¹⁴³Nd/¹⁴⁴Nd, Lu/Hf, and Sm/Nd, implying that they sample two distinct sources in the Martian mantle. All shergottites are characterized by (Sm/Nd)_source < (Sm/Nd)_sample, but (Lu/Hf)_source > (Lu/Hf)_sample. This decoupling can be explained by two successive partial melting episodes in the depleted shergottite source and localized in the Martian upper mantle. The genesis of shergottites can be modeled using non-modal equilibrium partial melting in a source initially composed of 60% olivine, 21% clinopyroxene, 9% orthopyroxene, and 10% garnet, with degrees of partial melting of 8.8% and 3.9%, respectively, for the two successive events. The enriched end-member of the shergottite mixing curve is best modeled by late-stage quenched residual melt resulting from the crystallization of a magma ocean. The depleted shergottite source may be modeled as a mixture of cumulates and residual melt, as convection in the Martian magma ocean is expected to reduce the incompatible trace element heterogeneity in the final solidified layers. Consequently, equilibrium crystallization is preferred to model the crystallization of the Martian magma ocean. The models that best explain the shergottite data are those where the magma ocean is at a depth of at least 1350 km in Mars.  相似文献   

Fates of dissolved and particulate materials from the Mississippi river immediately after discharge into the northern Gulf of Mexico,USA, during a period of low wind stress     

M.J. Dagg  T. Bianchi  B. McKee  R. Powell 《Continental Shelf Research》2008

In June 2003, we conducted a two-part field exercise to examine biogeochemical characteristics of water in the lower Mississippi river during the 4 days prior to discharge and in the Mississippi river plume over 2 days after discharge. Here we describe the fates of materials immediately after their discharge through Southwest Pass of the Mississippi delta into the northern Gulf of Mexico. Changes in surface water properties immediately after discharge were much larger and more rapid than changes prior to discharge. Total suspended matter (TSM) declined, probably due to sinking, dissolved macronutrients were rapidly diminished by mixing and biological uptake, and phytoplankton populations increased dramatically, and then declined. This decline appeared to begin at salinities of approximately 10 and was nearly complete by 15. A large increase in dissolved organic carbon (DOC) occurred over approximately the same salinity range. Weak winds (<2 m s⁻¹) during and preceding this cruise apparently led to the formation of an extensive but thin freshwater lens from the river. This lens spread widely without much mixing, and the bloom of phytoplankton that occurred between discharge and a salinity of 10 was probably a freshwater community seeded from the lower river. Phytoplankton bloomed for a period of about 1–2 days, then declined dramatically, apparently releasing large amounts of DOC. Macronutrients from the river were utilized by the river phytoplankton community in the extensive freshwater lens. This contrasted with the more typical situation in which river nutrients stimulate a marine phytoplankton bloom at salinities in the mid-20s. We concluded that the direct effects of dissolved and particulate bio-reactive materials discharged by the Mississippi river were spatially restricted at this time to low-salinity water, at least as surface phenomena. After being transported through the lower river essentially unaltered, these materials were biogeochemically processed within days and tens of km. More generally, the mixing rate of plume water with receiving oceanic water has profound effects on the food web structure and biogeochemical cycling in the plume.  相似文献   

Comparison of hypoxia among four river-dominated ocean margins: The Changjiang (Yangtze), Mississippi,Pearl, and Rhône rivers     

C. Rabouille  D.J. Conley  M.H. Dai  W.-J. Cai  C.T.A. Chen  B. Lansard  R. Green  K. Yin  P.J. Harrison  M. Dagg  B. McKee 《Continental Shelf Research》2008

We examined the occurrence of seasonal hypoxia (O₂<2 mg l⁻¹) in the bottom waters of four river-dominated ocean margins (off the Changjiang, Mississippi, Pearl and Rhône Rivers) and compared the processes leading to the depletion of oxygen. Consumption of oxygen in bottom waters is linked to biological oxygen demand fueled by organic matter from primary production in the nutrient-rich river plume and perhaps terrigenous inputs. Hypoxia occurs when this consumption exceeds replenishment by diffusion, turbulent mixing or lateral advection of oxygenated water. The margins off the Mississippi and Changjiang are affected the most by summer hypoxia, while the margins off the Rhône and the Pearl rivers systems are less affected, although nutrient concentrations in the river water are very similar in the four systems. Spring and summer primary production is high overall for the shelves adjacent to the Mississippi, Changjiang and Pearl (1–10 g C m⁻² d⁻¹), and lower off the Rhône River (<1 g C m⁻² d⁻¹), which could be one of the reasons of the absence of hypoxia on the Rhône shelf. The residence time of the bottom water is also related to the occurrence of hypoxia, with the Mississippi margin showing a long residence time and frequent occurrences of hypoxia during summer over very large spatial scales, whereas the East China Sea (ECS)/Changjiang displays hypoxia less regularly due to a shorter residence time of the bottom water. Physical stratification plays an important role with both the Changjiang and Mississippi shelf showing strong thermohaline stratification during summer over extended periods of time, whereas summer stratification is less prominent for the Pearl and Rhône partly due to the wind effect on mixing. The shape of the shelf is the last important factor since hypoxia occurs at intermediate depths (between 5 and 50 m) on broad shelves (Gulf of Mexico and ECS). Shallow estuaries with low residence time such as the Pearl River estuary during the summer wet season when mixing and flushing are dominant features, or deeper shelves, such as the Gulf of Lion off the Rhône show little or no hypoxia.  相似文献