Remarks on potential vorticity mixing over topography and momentum conservation     

《Deep Sea Research Part I: Oceanographic Research Papers》2000,47(4):737-743

Recent works have shown that parameterizing eddy fluxes as a downgradient diffusion of potential vorticity (PV) generates circulation over topography. This result is examined in the context of a simple barotropic quasi-geostrophic model. It is shown that, unless a constraint on the eddy diffusivity is maintained, the PV mixing parameterization creates a fictitious torque which generates angular momentum. These considerations indicate that momentum conservation is not maintained in recent models that purport to show generation of net alongslope flow associated with PV mixing over topography.  相似文献   

Discrete wavelet transform-based investigation into the variability of standardized precipitation index in Northwest China during 1960–2014     

Peng?Yang  Jun?XiaEmail author  Chesheng?Zhan  Yongyong?Zhang  Sheng?Hu 《Theoretical and Applied Climatology》2018,131(1-2):167-179

This study analyses spatio-temporal trends in precipitation, temperature, and river discharge in the northeast of Iran during recent decades (1953–2013). The Pettitt, SNHT, Buishand, Box-Pierce, Ljung-Box, and McLeod-Li methods were applied to examine homogeneity in time series studied. The nonparametric Mann-Kendall and Sen’s slope estimator tests were used to detect possible significant (p < 0.05) temporal trends in hydrometeorological time series and their magnitude, respectively. For time series with autocorrelation, the trend-free pre-whitening (TFPW) method was used to determine significant trends. To explore spatial distributions of trends, their magnitudes were interpolated by the inverse distance whitening (IDW) method. Trend analysis shows that for daily, monthly, and annual precipitation time series, 12.5, 19, and 12.5 % of the stations revealed significant increasing trends, respectively. For mean temperature, warming trends were found at 38, 23, and 31 % of the stations on daily, monthly, and annual timescales, in turn. Daily and monthly river discharge decreased at 80 and 40 % of the stations. Overall, these results indicate significant increases in precipitation and temperature but decreases in river discharge during recent decades. Hence, it can be concluded that decreasing trends in river discharge time series over the northeast of Iran during 1953–2013 are in response to warming temperatures, which increase the rate of evapotranspiration. Differences between the results of our comprehensive large-scale study and those of previous researches confirm the necessity for more model-based local studies on climatic and environmental changes across the northeast of Iran.  相似文献   

Microbial ammonia oxidation and enhanced nitrogen cycling in the Endeavour hydrothermal plume   总被引：1，自引：0，他引：1  

Phyllis Lam  James P. Cowen  Ronald D. Jones 《Geochimica et cosmochimica acta》2008,72(9):2268-2286

Ammonium was injected from the subseafloor hydrothermal system at the Endeavour Segment, Juan de Fuca Ridge, into the deep-sea water column resulting in an -rich (?177 nM) neutrally buoyant hydrothermal plume. This was quickly removed by both autotrophic ammonia oxidation and assimilation. The former accounted for at least 93% of total net removal, with its maximum rate in the neutrally buoyant plume (?53 nM d⁻¹) up to 10-fold that in background deep water. Ammonia oxidation in this plume potentially added 26-130 mg into the deep-sea water column. This oxidation process was heavily influenced by the presence of organic-rich particles, with which ammonia-oxidizing bacteria (AOB) were often associated (40-68%). AOB contributed up to 10.8% of the total microbial communities within the plume, and might constitute a novel lineage of β-proteobacterial AOB based on 16S rRNA and amoA phylogenetic analyses. Meanwhile, assimilation rates were also substantially enhanced within the neutrally buoyant plume (?26.4 nM d⁻¹) and accounted for at least 47% of total net removal rates. The combined oxidation and assimilation rates always exceeded total net removal rates, suggesting active in situregeneration rates of at least an order of magnitude greater than the particulate nitrogen flux from the euphotic zone. Ammonia oxidation is responsible for turnover of 0.7-13 days and is probably the predominant in situ organic carbon production process (0.6-13 mg C m⁻² d⁻¹) at early stages of Endeavour neutrally buoyant plumes.  相似文献   

Choosing the Right Policy Tools to Encourage Watershed Stewardship through the Study of Attitude     

Matt DeAngelo  Max Nielsen-Pincus 《社会与自然资源》2017,30(11):1328-1342

Recently, a number of “Payment for Watershed Services” programs have aimed to engage private landowners in watershed stewardship initiatives by offering financial incentives for adopting watershed best management practices. However, a growing field of research suggests that financial incentives alone may be of limited utility to encourage widespread and long-standing behavior change, and other policy tools may be required. This research examines how attitudes may influence enrollment in watershed stewardship programs to shed light the application of incentive, capacity building, and symbolic policy tools. We distributed a questionnaire to rural landowners in the Clackamas River watershed, OR, and received 281 valid responses (29% response rate). We found that attitudes associated with trust, ecological understanding, and technical capacity played more fundamental roles compared with financial considerations. We interpret these findings by evaluating the likely efficacy of various policy tools, and stress the importance of engaging landowners by building capacity and trust.  相似文献   

Complementary Investigative Techniques for Site Assessment with Low-Level Contaminants     

Mark E. Byrnes  R. Wayne Nelson  Robert G. La  Poe  Dennis E. Lundquist  William McNeill 《Ground Water Monitoring & Remediation》1990,10(4):90-95

A remedial investigation (RI) was performed in an area downgradient from an abandoned missile silo at Vandenberg Air Force Base, California, as part of the United States Air Force Installation Restoration Program (IRP). A number of complementary investigative techniques were used to assure a reliable assessment of site contamination. These included the review of aerial photographs, the use of an organic vapor analyzer (OVA) and carbon adsorption/mass spectrometer (MS) method to conduct a soil-gas survey; magnetic and electromagnetic geophysical surveys; bedrock permeability testing; and the chemical analysis of soil, sediment, surface water, and ground water samples. The results from this investigation revealed the presence of an undocumented landfill and a small trichloroethylene plume in ground water at concentrations ranging from 6.7 ppb to 31 ppb. The investigation also identified local ground water flow direction, provided strong evidence of the location of potential sources of contamination, and defined the downgradient extent of ground water contamination. Because the identified contaminants have not as yet reached the environmentally sensitive wetland at the base of the slope below this facility, there is still time to propose remedial alternatives that would serve to protect this environmentally sensitive area.  相似文献   

A natural cement analogue study to understand the long-term behaviour of cements in nuclear waste repositories: Maqarin (Jordan)     

《Applied Geochemistry》2016

The geological storage of nuclear waste includes multibarrier engineered systems where a large amount of cement-based material is used. Predicting the long term behaviour of cement is approached by reactive transport modelling, where some of the boundary conditions can be defined through studying natural cement analogues (e.g. at the Maqarin natural analogue site). At Maqarin, pyrometamorphism of clay biomicrites and siliceous chalks, caused by the in-situ combustion of organic matter, produced various clinker minerals. The interaction of infiltrating groundwater with these clinker phases resulted in a portlandite-buffered hyperalkaline leachate plume, which migrated into the adjacent biomicrite host rock, resulting in the precipitation of hydrated cement minerals.In this study, rock samples with different degrees of interaction with the hyperalkaline plume were investigated by various methods (mostly SEM-EDS). The observations have identified a paragenetic sequence of hydrous cement minerals, and reveal how the fractures and porosity in the biomicrite have become sequentially filled. In the alkaline disturbed zone, C-A-S-H (an unstoichiometric gel of Ca, Al, Si and OH) is observed to fill the pores of the biomicrite wallrock, as a consequence of reaction with a high pH Ca-rich fluid circulating in fractures. Porosity profiles indicate that in some cases the pores of the rock adjacent to the fractures became tightly sealed, whereas in the veins some porosity is preserved. Later pulses of sulphate-rich groundwater precipitated ettringite and occasionally thaumasite in the veins, whereas downstream in the lower pH distal regions of the hyperalkaline plume, zeolite was precipitated.Comparing our observations with the reactive transport modelling results reveals two major discrepancies: firstly, the models predict that ettringite is precipitated before C-A-S-H, whereas the C-A-S-H is observed as the earlier phase in Maqarin; and, secondly, the models predict that ettringite acts as the principal pore-filling phase in contrast to the C-A-S-H observed in the natural system. These discrepancies are related to the fact that our data were not available at the time the modelling studies were performed. However, all models succeeded in reproducing the porosity reduction observed at the fracture–rock interface in the natural analogue system.  相似文献   

The impact of weather variations on maize yields and household income: Income diversification as adaptation in rural China     

《Global Environmental Change》2017

Climate change is threatening global food production and could potentially exacerbate food insecurity in many parts of the world. China is the second largest maize producer. Variations in maize yields in China are likely to have major implications for food security in the world. Based on longitudinal data of 4861 households collected annually between 2004 and 2010, we assess the impact of weather variations on maize yields in the two main producing regions in China, the Northern spring maize zone and the Yellow-Huai Valley summer maize zone. We also explore the role of adaptation, by estimating the response of Chinese farmers in both regions, in particular in terms of income diversification. With the use of household and time fixed effects, our estimates relate within-household variations in household outcomes (maize yields, net income, land and input use) to within-location variations in weather conditions. Temperature, drought, wet conditions, and precipitations have detrimental effects on maize yields in the two maize zones. The impact is stronger in the Northern spring maize zone where one standard deviation in temperature and drought conditions decreases maize yields by 1.4% and 2.5%, respectively. Nonetheless, such impact does not seem to translate into a significant fall in total net income. Adaptation seems to be key in explaining such a contrast in the Northern spring maize zone where the largest impact is estimated. On the contrary, we find a lower impact in the other region, the Yellow-Huai Valley summer maize zone but such impact is likely to intensify. The lack of adaptation observed in that region results into detrimental impacts on net farm and total income. Enhancing adaptative behaviors among Chinese farmers even further is likely to be key to future food security in China and in the rest of the world.  相似文献   

Orogenic gold in the Egyptian Eastern Desert: Widespread gold mineralization in the late stages of Neoproterozoic orogeny     

《Gondwana Research》2019

The Arabian-Nubian Shield (ANS) is a rapidly emerging world-class province for gold resources mainly in structurally-controlled quartz ± carbonate veins that are best classified as late Neoproterozoic orogenic gold deposits. Gold has been mined in the Eastern Desert of Egypt, in the northwestern part of the ANS, for >6000 years, that is since the times of the Pharaohs, but production prior to the 1900s was likely only about 25 t and mainly from alluvial workings. In the first half of the 1900s, about 7 t Au was recovered from the El Sid orogenic gold deposit. Today, Sukari is the single major producing mine in the Eastern Desert but many other significant gold occurrences are being actively explored.Formation of the ANS took place during closure of the Mozambique Ocean between the East and West Gondwana continental blocks. Ocean closure led to amalgamation of numerous ca. 870–625 Ma juvenile arc and back-arc igneous and sedimentary rock sequences, with many resulting terrane sutures marked by mafic-ultramafic ophiolitic assemblages and fragments. The 100 m.y. of orogeny beginning at ca. 650 Ma included crustal shortening, lithospheric reworking, escape tectonics, and eventual orogenic collapse. Peak metamorphism was reached in different parts and depths of the orogen diachronously between 620 and 585 Ma, magmatism was widespread during 650–580 Ma, and rapid exhumation of the metamorphosed rocks and mid-crustal intrusions took place from ca. 600 to 580 Ma. Regional fault sets that controlled much of the gold occurrences were related to initial transpression by oblique convergence between the arcs and associated with subsequent sinistral shearing reported as overlapping the exhumation. Because existing geological data are not adequate to fully evaluate the overall terrane history, we have subdivided the Eastern Desert into nine structural blocks, rather than arc terranes, based commonly on bounding shear zones and major faults.The greatest abundance of gold deposits is associated with the NW-trending Najd Fault System that comprises many splays throughout the blocks in the Central Eastern Desert that underwent episodes of shearing at ca. 640–570 Ma. Important deposits are also notably widespread along reactivated east-west thrust faults in the Allaqi-Sol Hamed block of the South Eastern Desert, with significant shearing at 610–580 Ma. Sulfide mineralogy of the Eastern Desert gold-bearing veins is dominated by pyrite, arsenopyrite, and (or) pyrrhotite, in addition to subordinate chalcopyrite, sphalerite, galena and tetrahedrite as well as alteration minerals that include white mica, chlorite, and carbonate, are those typical of orogenic gold deposits. Many gold occurrences are located along sheared margins to granitic intrusions or along contacts between different lithologies; sheared silica- and carbonate-altered ultramafic rocks along many fault zones are particularly widely associated with many of the gold occurrences. Ore-forming fluids were generally low-salinity aqueous-carbonic with most measured δ¹⁸O of mineralized quartz ranging from 8‰ to 15‰, δ¹³C for associated carbonate from −10‰ to −1‰, and δ³⁴S for ore-related sulfides varying between −5 and +10‰, but much more consistent for individual occurrences or deposits. Gold was deposited at temperatures of generally between 250° and 370 °C, depending on location, and largely variable pressures. The few obtained absolute dates on ore formation, from the Fawakhir and Atalla deposits in the Central Eastern Desert, indicate that at least some of the mineralization was formed at ca. 600 Ma.Orogenic gold formed after the cessation of arc-terrane accretion in the ANS and during a period in which most of the shield became established with a 30–40 km-thick continental crust and underwent a transition from compressional/transpressional terrane accretion to post-amalgamation transtensional shearing. This also was marked by a petrogenetic transition from pre- to syn-accretionary, arc-related calc-alkaline I-type magmatism to late- or post-tectonic A-type magmatism within the newly formed shield. Concurrently, the Eastern Desert was affected by widespread crustal flow of aqueous-carbonic fluids, particularly on the through-going, extensive strike-slip shears of the Najd Fault System. Gold and sulfide minerals were deposited in the Eastern Desert shallow and middle crust coevally with rapid terrane exhumation, during changes in fluid chemistry associated with pressure cycling and multiple vein quartz precipitation.  相似文献   

Transverse structure of subtidal flow in a weakly stratified subtropical tidal inlet     

Amy F. Waterhouse  Arnoldo Valle-Levinson 《Continental Shelf Research》2010,30(3-4):281-292

The transverse structure of exchange flows and lateral flows as well as their relationship to the subtidal variability are investigated in a subtropical inlet, Ponce de Leon Inlet, Florida. Two surveys were executed during different phases of the tidal month to determine the spatial structure of subtidal exchange flows. Data from fixed moorings were used to depict the temporal variability of the spatial structure established in the surveys. The data suggested a tidally rectified pattern of net outflow in the channel and inflow over shoals with a negligible influence of streamwise baroclinic pressure gradients on the dynamics and slight modifications due to the wind. Onshore winds strengthened net inflows but weakened net outflows, rarely reversing them, while offshore winds increased net outflows and weakened net inflows. Curvature effects were found to be important in modifying secondary circulations. Slight modifications to the secondary flows were also caused by stream-normal baroclinicity during one survey. Most important, the intensity of the exchange flows was modulated by tides, with the largest exchange flows developing in response to the strongest tidal rectification of spring tides.  相似文献   

Active tectonics,fault patterns,and stress field of Deception Island: A response to oblique convergence between the Pacific and Antarctic plates     

《Journal of South American Earth Sciences》2007,23(2-3):256-268

Palaeostress results derived from brittle mesoscopic structures on Deception Island (Bransfield Trough, Western Antarctica) show a recent stress field characterized by an extensional regime, with local compressional stress states. The maximum horizontal stress (σ_y) shows NW–SE and NNE–SSW to NE–SW orientations and horizontal extension (σ₃) in NE–SW and WNW–ESE to NW–SE directions. Alignments of mesofractures show a maximum of NNE–SSW orientation and several relative maxima striking N030-050E, N060-080E, N110-120E, and N160-170E. Subaerial and submarine macrofaults of Deception Island show six main systems controlling the morphology of the island: N–S, NNE–SSW, NE–SW, ENE–WSW to E–W, WNW–ESE, and NNW–SSE. Geochemical patterns related to submarine hydrothermally influenced fault and fissure pathways also share the same trends. The orientation of these fault systems is compared to Riedel shear fractures. Following this model, we propose two evolutionary stages from geometrical relationships between the location and orientation of joints and faults. These stages imply a counter-clockwise rotation of Deception Island, which may be linked to a regional left-lateral strike-slip. In addition, the simple shear zone could be a response to oblique convergence between the Antarctic and Pacific plates. This stress direction is consistent with the present-day movements between the Antarctic, Scotia, and Pacific plates. Nevertheless, present basalt-andesitic volcanism and deep earthquake focal mechanisms may indicate rollback of the former Phoenix subducted slab, which is presently amalgamated with the Pacific plate. We postulate that both mechanisms could occur simultaneously.  相似文献