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191.
In deltas, subsidence leads to a relative sea-level rise (RSLR) that is often much greater than eustatic rise alone. Because of high RSLR, deltaic wetlands will be affected early by an acceleration of eustatic sea-level rise. If there is sufficient vertical accretion, wetlands can continue to exist with RSLR; however, lack of sediment input eventually leads to excessive water logging and plant death. Areas with low tidal range, such as the Mediterranean and Gulf of Mexico, are especially vulnerable to rising water levels because the elevational growth range of coastal vegetation is related to tide range. Reduction of suspended sediments in rivers and prevention of wetland flooding by river dikes and impoundments have reduced sediment input to Mediterranean and Gulf of Mexico deltaic wetlands. This sediment deficit will become more important with an acceleration in sea-level rise from global warming. Most sediment input occurs during strong pulsing events such as river floods and storms, and management policies and decisions are especially designed to protect against such events. Management approaches must be reoriented to take advantage of pulsing events to nourish marsh surfaces with sediments. We hypothesize that deltas can be managed to withstand significant rates of sea-level rise by taking advantage of pulsing events leading to high sediment input, and that this type of management approach will enhance ecosystem functioning. 相似文献
192.
Darren W. Schnare James M.D. Day Marc D. Norman Yang Liu Lawrence A. Taylor 《Geochimica et cosmochimica acta》2008,72(10):2556-2572
Apollo 15 low-Ti mare basalts have traditionally been subdivided into olivine- and quartz-normative basalt types, based on their different SiO2, FeO, and TiO2 whole-rock compositions. Previous studies have reconciled this compositional diversity by considering the olivine- and quartz-normative basalts as originating from different lunar mantle source regions. To provide new information on the compositions of Apollo 15 low-Ti mare basalt parental magmas, we report a study of major and trace-element compositions of whole rocks, pyroxenes, and other phases in the olivine-normative basalts 15016 and 15555 and quartz-normative basalts 15475 and 15499. Results show similar rare-earth-element patterns in pyroxenes from all four basalts. The estimated equilibrium parental-melt compositions from the trace-element compositions of pyroxenes are similar for 15016, 15555 and 15499. Additionally, an independent set of trace-element distribution coefficients has been determined from measured pyroxene and mesostasis compositions in sample 15499. These data suggest that fractional crystallization may be a viable alternative to compositional differences in the mantle source to explain the 25% difference in whole-rock TiO2, and corresponding differences in SiO2 and FeO between the Apollo 15 olivine- and quartz-normative basalts. In this model, the older (3.35 Ga) quartz-normative basalts, with lower TiO2 experienced olivine, chromite, and Cr-ulvöspinel fractionation at ‘crustal levels’ in magma chambers or dikes, followed by limited near-surface mineral fractionation, within the lava flows. In contrast, the younger (3.25 Ga) olivine-normative basalts experienced only limited magmatic differentiation at ‘crustal-levels’, but extensive near-surface mineral fractionation to produce their evolved mineral compositions. A two-stage mineral-fractionation model is consistent with textural and mineralogical observations, as well as the mineral trace-element constraints developed by this study. 相似文献
193.
194.
Chris E. Ostrander Margaret A. McManus Eric H. DeCarlo Fred T. Mackenzie 《Estuaries and Coasts》2008,31(1):192-203
While the physical forcing mechanisms that govern the outflows of major rivers throughout the world are well documented in the literature, comparably less research has been done to examine the mechanisms that govern the contributions of small rivers and streams to coastal ocean systems. These rivers and streams provide a direct means for the transport of anthropogenic and terrigenous materials from watersheds to coastal oceans. This study describes the temporal and spatial variability of freshwater plumes from Kaneohe Stream, Hawaii, USA, after storm events in the Kaneohe Bay watershed. Freshwater plumes were examined using a combination of fixed moorings, synoptic shipboard surveys, and lagrangian surface drifters. Data sets were collected over the course of 19 months from August 2005 to March 2007 with particular attention paid to storms during the boreal winters. Stream discharge and duration were found to exert a primary control on plume persistence in the southern Kaneohe Bay system. Time series data show a strong coherence between wind forcing and surface currents, which, in combination with data derived from shipboard and aerial surveys, indicate that the spatial variability of freshwater plumes is primarily determined by atmospheric forcing. 相似文献
195.
F. Heuze R. Archuleta F. Bonilla S. Day M. Doroudian A. Elgamal S. Gonzales M. Hoehler T. Lai D. Lavallee B. Lawrence P. -C. Liu A. Martin L. Matesic B. Minster R. Mellors D. Oglesby S. Park M. Riemer J. Steidl F. Vernon M. Vucetic J. Wagoner Z. Yang 《Soil Dynamics and Earthquake Engineering》2004,24(3):199-223
The Campus Earthquake Program (CEP) of the University of California (UC) started in March 1996, and involved a partnership among seven campuses of the UC—Berkeley, Davis, Los Angeles, Riverside, San Diego, Santa Barbara, Santa Cruz—and the Lawrence Livermore National Laboratory (LLNL). The aim of the CEP was to provide University campuses with site-specific assessments of their earthquake strong motion exposure, to complement estimates they obtain from consultants according to the state-of-the-practice (SOP), i.e. Building Codes (UBC 97, IBC 2000), and Probabilistic Seismic Hazard Analysis (PSHA). The Building Codes are highly simplified tools, while the more sophisticated PSHA is still somewhat generic in its approach because it usually draws from many earthquakes not necessarily related to the faults threatening the site under study.Between 1996 and 2001, the site-specific studies focused on three campuses: Riverside, San Diego, and Santa Barbara. Each campus selected 1–3 sites to demonstrate the methods and procedures used by the CEP: Rivera Library and Parking Lots (PL) 13 and 16 at UCR, Thornton Hospital, the Cancer Center, and PL 601 at UCSD, and Engineering I building at UCSB. The project provided an estimate of strong ground motions at each selected site, for selected earthquake scenarios. These estimates were obtained by using an integrated geological, seismological, geophysical, and geotechnical approach, that brings together the capabilities of campus and laboratory personnel. Most of the site-specific results are also applicable to risk evaluation of other sites on the respective campuses.The CEP studies have provided a critical assessment of whether existing campus seismic design bases are appropriate. Generally speaking, the current assumptions are not acknowledging the severity of the majority of expected motions. Eventually, both the results from the SOP and from the CEP should be analyzed, to arrive at decisions concerning the design-basis for buildings on UC campuses. 相似文献
196.
Experiments were conducted at 6–30 kb and 875–1200°C on two garnet pyroxenite xenoliths from the Bullenmerri and Gnotuk Maars of western Victoria, Australia. The (garnet + clinopyroxene + plagioclase + spinel) assemblage of DR9734 was stable between 10 and 12.5 kb, and 950 and 1,050°C. The compositions of its natural mineral phases were most closely approximated in experiments at 12.5 kb and 1,000–1,050°C. The (garnet + spinel + clinopyroxene + orthopyroxene + amphibole) assemblage of DR10165 was stable at pressures > 8 kb and temperatures > 950°C. However, differences between natural and experimental mineral compositions indicate that the mineral assemblage of this xenolith persisted metastably after cooling below 950°C with chemical exchange continuing down to approximately 850–900°C. When the experimental data for DR9734 and DR10165 are applied to mineralogical data for other mafic and ultramafic xenoliths from the Bullenmerri and Gnotuk Maars, they indicate that previous pressure and temperature estimates for individual xenoliths are 2–3 kb and 50°C too high. These corrections increase average temperatures for the geotherm beneath western Victoria by about 50°C over a depth range of 30–45 km and confirm its perturbed (high-temperature) character.This paper is a contribution to IGCP Project 304 (Lower Crustal Processes) 相似文献
197.
Brian C. Perez John W. Day Jr. Dubravko Justic Robert R. Twilley 《Estuarine, Coastal and Shelf Science》2003,57(5-6):1065-1078
Nutrient fluxes were measured between Fourleague Bay, a shallow Louisiana estuary, and the Gulf of Mexico every 3 h between February 1 and April 30, 1994 to determine how high velocity winds associated with cold fronts and peak Atchafalaya River discharge influenced transport. Net water fluxes were ebb-dominated throughout the study because of wind forcing and high volumes of water entering the northern Bay from the Atchafalaya River. Flushing time of the Bay averaged <8 days; however, more rapid flushing occurred in response to northerly winds with approximately 56% of the volume of the Bay exported to the Gulf in 1 day during the strongest flushing event. Higher nitrate+nitrite (NO2+NO3), total nitrogen (TN), and total phosphorus (TP) concentrations were indicative of Atchafalaya River input and fluxes were greater when influenced by high velocity northerly winds associated with frontal passage. Net exports of NO2+NO3, TN, and TP were 43.5, 98.5, and 13.6 g s−1, respectively, for the 89-day study. An average of 10.6 g s−1 of ammonium (NH4) was exported to the Gulf over the study; however, concentrations were lower when associated with riverine influence and wind-driven exports suggesting the importance of biological processes. Phosphate (PO4) fluxes were nearly balanced over the study with fairly stable concentrations indicating a well-buffered system. The results indicate that the high energy subsidy provided by natural pulsing events such as atmospheric cold fronts and seasonal river discharge are efficient mechanisms of nutrient delivery to adjacent wetlands and nearshore coastal ecosystems and are important in maintaining coastal sustainability. 相似文献
198.
S. E. Davis III D. L. Childers J. W. Day Jr D. T. Rudnick F. H. Sklar 《Estuarine, Coastal and Shelf Science》2001,52(6):753
Flow-through flumes were used to quantify net areal fluxes of nutrients in the fringe mangrove zone of lower Taylor River in the southern Everglades National Park. We also quantified net areal fluxes along the open water portion of the channel to determine the relative importance of either zone (vegetated vs. unvegetated) in the regulation of nutrient exchange in this system. Taylor River's hydrology is driven mainly by precipitation and wind, as there is little influence of tide. Therefore, quarterly samplings of the vegetated and unvegetated flumes were slated to include typical wet season and dry season periods, as well as between seasons, over a duration of two years. Concentrations of dissolved and total organic carbon (DOC and TOC) were highest during the wet season and similar to one another throughout the study, reflecting the low particulate loads in this creek. Dissolved inorganic nitrogen (nitrate+nitrite+ammonium) was 10–15% of the total nitrogen (TN) content, with NO−x and NH+4 showing similar concentration ranges over the 2-year study. Soluble reactive phosphorus (SRP) was usually <0·05μM, while total phosphorus (TP) was typically an order of magnitude higher. Net areal fluxes were calculated from nutrient concentration change over the length of the flumes. Most flux occurred in the vegetated zone. Dissolved inorganic nitrogen and DOC were usually taken up from the water column; however, we saw no seasonal pattern for any constituent over the course of this study. Total nutrients (TOC, TN, and TP) showed little net exchange and, like SRP, had fluxes that shifted irregularly throughout the study. Despite the lack of a clear seasonal pattern, there was a great deal of consistency between vegetated flumes, especially for NO−x and NH+4, and fluxes in the vegetated flumes were generally in the same direction (import, export, or no net flux) during a given sampling. These findings suggest that the fringe mangrove zone is of considerable importance in regulating nutrient dynamics in lower Taylor River. Furthermore, the influence of this zone may at times extend into northeast Florida Bay, as the bay is the primary recipient of water and nutrients during the wet season. 相似文献
199.
Harmful algae and mariculture in New Zealand 总被引:3,自引:0,他引:3
Rhodes L. L.; Mackenzie A. L.; Kaspar H. F.; Todd K. E. 《ICES Journal of Marine Science》2001,58(2):398-403
200.