Comparison of Mineral Weathering and Biomass Macronutrient Uptake in Two Small Forested Watersheds Underlain by Quartzite Bedrock,Catoctin Mountain,Maryland, USA

To quantify chemical weathering and biological uptake, mass-balance calculations were performed on two small forested watersheds located in the Blue Ridge Physiographic Province in north-central Maryland, USA. Both watersheds, Bear Branch (BB) and Fishing Creek Tributary (FCT), are underlain by relatively unreactive quartzite bedrock. Such unreactive bedrock and associated low chemical-weathering rates offer the opportunity to quantify biological processes operating within the watershed. Hydrologic and stream-water chemistry data were collected from the two watersheds for the 9-year period from June 1, 1990 to May 31, 1999. Of the two watersheds, FCT exhibited both higher chemical-weathering rates and biomass nutrient uptake rates, suggesting that forest biomass aggradation was limited by the rate of chemical weathering of the bedrock. Although the chemical-weathering rate in the FCT watershed was low relative to the global average, it masked the influence of biomass base-cation uptake on stream-water chemistry. Any differences in bedrock mineralogy between the two watersheds did not exert a significant influence on the overall weathering stoichiometry. The difference in chemical-weathering rates between the two watersheds is best explained by a larger proportion of reactive phyllitic layers within the bedrock of the FCT watershed. Although the stream gradient of BB is about two-times greater than that of FCT, its influence on chemical weathering appears to be negligible. The findings of this study support the biomass nutrient uptake stoichiometry of K_1.0Mg_1.1Ca_0.97 previously determined for the study site. Investigations of the chemical weathering of relatively unreactive quartzite bedrock may provide insight into critical zone processes.     

天然实验流域降雨径流现象发生的悖论

作为滁州水文实验系统一部分的1号天然实验流域,面积7897m2,以安山岩为基底,上覆平均厚度2.46m的第四系沉积物。实测了包括地面径流和地面下径流的各种降雨径流响应,后者包括来自非饱和带的壤中流和饱和带的地下水径流。表明这些径流成分有着复杂的组合类型,主要是以地面径流为主的SR型和以地面下径流为主的SSR型,以及中间的和演化的类型。SR型实例中的地面径流量可占总径流的65%,而SSR实例中的地面下径流量可有90%。主要降水的7月,地面下贡献占54.5%,其中地下水径流即占33%。大部分地面径流与降水的18O组成有不小的差异,比较了同时进行测验的3个实验流域,在1400min的降水径流过程中,降水的平均δ18O为-1.210%,而同期地面径流的平均δ18O,2号水文山流域(512m2)为-1.132%,1号南大洼天然流域为-1.065%,3号只有薄层风化碎屑的牵牛花流域(4573m2)为-0.801%。这质疑了现行同位素流量过程线划分方法8个假定中的两个:地面径流的同位素组成不同于降水,天然流域尤其如此;各种水源在汇集过程中的同位素分馏影响并非都可忽略不计。实验流域因降水而产生径流,但所产生的径流却有着非本次降水的组分,这一降水径流悖论发生于各个实验流域。SR型和SSR型总的非本次降水组分分别达16%和64%。     

Pre-landfall evacuee perception of the meteorological hazards associated with Hurricane Gustav

In this study, evacuees from the path of Hurricane Gustav were surveyed to determine which meteorological hazards most influenced their decision to leave. Surveys were conducted along two major evacuation routes on August 30 and 31, 2008, to collect time-sensitive data on individual evacuation decisions related to the meteorological hazards from Hurricane Gustav. The regions of New Orleans, Houma, and Lafayette were represented most frequently, as determined by zip code data collected from the surveys. Responses were evaluated first by meteorological hazard for the entire dataset and then by three-digit zip code region. Overall, storm surge was the most important meteorological variable, followed by the size of the storm, wind, rain, and tornadoes. When separated into three-digit zip code regions, analyses revealed evacuees from in and around New Orleans were driven to evacuate as a result of the perceived threat from storm surge and storm size; residents in the Houma, Louisiana region were motivated to leave due to the threat from storm surge; and Lafayette and the surrounding areas were most-concerned with the threats posed by hurricane-force winds. Given the forecast track and intensity, survey respondents understood the meteorological hazards from Gustav and were motivated to leave based on personal evaluations of risk associated with the storm.     

Precise U–Pb dating of Paleoproterozoic mafic dyke swarms of the Dharwar craton,India: Implications for the existence of the Neoarchean supercraton Sclavia

We report seven high precision U–Pb age determinations for mafic dykes from a number of major Precambrian swarms located in the Dharwar craton, south India. These new age results define two previously unrecognized widespread Paleoproterozoic dyking events at 2221–2209 and 2181–2177 Ma, and confirm a third at 2369–2365 Ma. Three parallel E–W trending mafic dykes from the petrographically and geochemically variable Bangalore dyke swarm, the most prominent swarm in the Dharwar craton, yield indistinguishable U–Pb baddeleyite ages of 2365.4 ± 1.0, 2365.9 ± 1.5 and 2368.6 ± 1.3 Ma, indicating rapid emplacement in less than five million years. A compilation of Paleoproterozoic U–Pb ages for mafic magmatic events worldwide indicates that the 2369–2365 Ma Bangalore dyke swarm represents a previously unrecognized pulse of mafic magmatism on Earth.     

Extending electromagnetic methods to map coastal pore water salinities

The feasibility of mapping pore water salinity based on surface electromagnetic (EM) methods over land and shallow marine water is examined in a coastal wetland on Tampa Bay, Florida. Forward models predict that useful information on seabed conductivity can be obtained through <1.5 m of saline water, using floating EM-31 and EM-34 instruments from Geonics Ltd. The EM-31 functioned as predicted when compared against resistivity soundings and pore water samples and proved valuable for profiling in otherwise inaccessible terrain due to its relatively small size. Experiments with the EM-34 in marine water, however, did not reproduce the theoretical instrument response. The most effective technique for predicting pore water conductivities based on EM data entailed (1) computing formation factors from resistivity surveys and pore water samples at representative sites and (2) combining these formation factors with onshore and offshore EM-31 readings for broader spatial coverage. This method proved successful for imaging zones of elevated pore water conductivities/salinities associated with mangrove forests, presumably caused by salt water exclusion by mangrove roots. These zones extend 5 to 10 m seaward from mangrove trunks fringing Tampa Bay. Modeling indicates that EM-31 measurements lack the resolution necessary to image the subtle pore water conductivity variations expected in association with diffuse submarine ground water discharge of fresher water in the marine water of Tampa Bay. The technique has potential for locating high-contrast zones and other pore water salinity anomalies in areas not accessible to conventional marine- or land-based resistivity arrays and hence may be useful for studies of coastal-wetland ecosystems.     

Multiple spacecraft rendezvous maneuvers by differential drag and low thrust engines

A novel two-phase hybrid controller is proposed to optimize propellant consumption during multiple spacecraft rendezvous maneuvers in Low Earth Orbit. This controller exploits generated differentials in aerodynamic drag on each involved chaser spacecraft to effect a propellant-free trajectory near to the target spacecraft during the first phase of the maneuver, and then uses a fuel optimal control strategy via continuous low-thrust engines to effect a precision dock during the second phase. In particular, by varying the imparted aerodynamic drag force on each of the chaser spacecraft, relative differential accelerations are generated between each chaser and the target spacecraft along two of the three translational degrees of freedom. In order to generate this required differential, each chaser spacecraft is assumed to include a system of rotating flat panels. Additionally, each chaser spacecraft is assumed to have continuous low-thrust capability along the three translational degrees of freedom and full-axis attitude control. Sample simulations are presented to support the validity and robustness of the proposed hybrid controller to variations in the atmospheric density along with different spacecraft masses and ballistic coefficients. Furthermore, the proposed hybrid controller is validated against a complete nonlinear orbital model to include relative navigation errors typical of carrier-phase differential GPS (CDGPS). Limitations of the proposed controller appear relative to the target spacecraft’s orbit eccentricity and a general characterization of the atmospheric density. Bounds on these variables are included to provide a framework within which the proposed hybrid controller can effect an extremely low propellant rendezvous of multiple chaser spacecraft to a desired target spacecraft.     

Current challenges using models to forecast seawater intrusion: lessons from the Eastern Shore of Virginia, USA

A three-dimensional model of the aquifer system of the Eastern Shore of Virginia, USA was calibrated to reproduce historical water levels and forecast the potential for saltwater intrusion. Future scenarios were simulated with two pumping schemes to predict potential areas of saltwater intrusion. Simulations suggest that only a few wells would be threatened with detectable salinity increases before 2050. The objective was to examine whether salinity increases can be accurately forecast for individual wells with such a model, and to address what the challenges are in making such model forecasts given current (2009) simulation capabilities. The analysis suggests that even with current computer capabilities, accurate simulations of concentrations within a regional-scale (many km) transition zone are computationally prohibitive. The relative paucity of data that is typical for such regions relative to what is needed for accurate transport simulations suggests that even with an infinitely powerful computer, accurate forecasting for a single well would still be elusive. Useful approaches may include local-grid refinement near wells and geophysical surveys, but it is important to keep expectations for simulated forecasts at wells in line with chloride concentration and other data that can be obtained at that local scale.     

Projected changes in vertical temperature profiles for Australasia

The vertical temperature profile in the atmosphere reflects a balance between radiative and convective processes and interactions with the oceanic and land surfaces. Changes in vertical temperature profiles can affect atmospheric stability, which in turn can impact various aspects of weather systems. In this study, we analyzed recent-past trends of temperature over the Australian region using a homogenized monthly upper-air temperature dataset and four reanalysis datasets (NCEP, ERA-Interim, JRA-55 and MERRA). We also used outputs of 12 historical and future regional climate model (RCM) simulations from the NSW/ACT (New South Wales/Australian Capital Territory) Regional Climate Modelling (NARCliM) project and 6 RCM simulations from the CORDEX (Coordinated Regional Downscaling Experiment) Australasian project to investigate projected changes in vertical temperature profiles. The results show that the currently observed positive trend in the troposphere and negative trend in the lower stratosphere will continue in the future with significant warming over the whole troposphere and largest over the middle to upper troposphere. The increasing temperatures are found to be latitude-dependent with clear seasonal variations, and a strong diurnal variation for the near surface layers and upper levels in tropical regions. Changes in the diurnal variability indicate that near surface layers will be less stable in the afternoon leading to conditions favoring convective systems and more stable in the early morning which is favorable for temperature inversions. The largest differences of future changes in temperature between the simulations are associated with the driving GCMs, suggesting that large-scale circulation plays a dominant role in regional atmospheric temperature change.

     

Indigenous aliphatic amines in the aqueously altered Orgueil meteorite

The CI1 Orgueil meteorite is a highly aqueously altered carbonaceous chondrite. It has been extensively studied, and despite its extensive degree of aqueous alteration and some documented instances of contamination, several indigenous organic compounds including amino acids, carboxylic acids, and nucleobases have been detected in its carbon‐rich matrix. We recently developed a novel gas chromatographic method for the enantiomeric and compound‐specific isotopic analyses of meteoritic aliphatic monoamines in extracts and have now applied this method to investigate the monoamine content in Orgueil. We detected 12 amines in Orgueil, with concentrations ranging from 1.1 to 332 nmol g^?1 of meteorite and compared this amine content in Orgueil with that of the CM2 Murchison meteorite, which experienced less parent‐body aqueous alteration. Methylamine is four times more abundant in Orgueil than in Murchison. As with other species, the amine content in Orgueil extracts shows less structural diversity than that in Murchison extracts. We measured the compound‐specific stable carbon isotopic ratios (δ¹³C) for 5 of the 12 monoamines detected in Orgueil and found a range of δ¹³C values from –20 to +59‰. These δ¹³C values fall into the range of other meteoritic organic compounds, although they are ¹³C‐depleted relative to their counterparts extracted from the Murchison meteorite. In addition, we measured the enantiomeric composition for the chiral monoamines (R)‐ and (S)‐sec‐butylamine in Orgueil, and found it was racemic within experimental error, in contrast with the l ‐enantiomeric excess found for its amino acid structural analog isovaline. The racemic nature of sec‐butylamine in Orgueil was comparable to that previously observed in Murchison, and to other CM2 and CR2 carbonaceous chondrites measured in this work (ALH 83100 [CM1/2], LON 94101 [CM2], LEW 90500 [CM2], LAP 02342 [CR2], and GRA 95229 [CR2]). These results allow us to place some constraints on the effects of aqueous alteration observed over the monoamine concentrations in Orgueil and Murchison, and to evaluate the primordial synthetic relationships between meteoritic monoamines and amino acids.