Low molecular weight α-hydroxy carboxylic and dicarboxylic acids in reducing marine sediments

The occurrence and distribution of low molecular weight α-hydroxy carboxylic and dicarboxylic acids was studied in reducing marine sediments collected in the Santa Barbara Basin and the Cariaco Trench. Four compounds were found to occur in both basin sediments: glycolic, lactic, oxalic and succinic acids. In general concentrations were low (? 1 μmol/g for the hydroxy acids and ? 100μmol/g for the dicarboxylic acids), and generally decreased with depth. Subsurface maxima were observed for lactate and succinate in the Santa Barbara Basin. Both the vertical profiles and lactate enantiomer ratios suggested microbial origin and control for the distribution of these compounds. A preliminary model for the microbial mediation of the early diagenesis of organic compounds in reducing marine sediments is presented.     

The Luna 20 lithic fragments,and the composition and origin of the lunar highlands

Luna 20 soil 22003,1 (250–500 μ) is similar to Apollo 16 soil 61501,47 (250–500 μ) in terms of the percentage of different types of particles. However, among the lithic fragments, the Apollo 16 sample contains a greater percentage of fragments with more than 70 wt. % modal plagioclase and a significantly greater proportion of KREEP-rich particles. Modal analyses of non-mare lithic fragments in Luna 20 and Apollo 11, 14, 15 and 16 indicate that the KREEP-poor highland regions (the bulk of the lunar terrae), though relatively feldspathic, are compositionally inhomogeneous, ranging in plagioclase content from approximately 35 to 100 wt. %. The average plagioclase content lies in the range 45–70 wt.%. Luna 20 pyroxene analyses cluster in two groups, one more magnesian than the other. The groups persist when pyroxene analyses from KREEP-poor noritic, troctolitic and anorthositic lithic fragments from Apollo 11, 14, 15 and 16 and Luna 20 are included. Olivine compositions mimic these pyroxene groups.Within each pyroxene group Cr₂O₃ and TiO₂ decrease as $\text{Fe}\text{(Fe + Mg)}$ increases, suggesting a relationship by fractional crystallization. The two groups suggest that at least two magma compositions were involved. To account for these observations we envisage a Moon-wide magma system in which initial accretionary heterogeneities were imperfectly erased by diffusion and convection. During the cooling of this magma system fractional crystallization was effected by the flotation of plagioclase and sinking of pyroxene, olivine and perhaps ilmenite. The endproduct was an upper layer enriched in plagioclase and a lower layer enriched in mafic silicates. KREEP-rich rocks, which are predominantly noritic in major element composition, may be mechanical mixtures of KREEP-poor norite and material residual after fractional crystallization of the surface magma system.     

Alteration of the upper oceanic crust,DSDP site 417: mineralogy and chemistry

Basalts from DSDP Site 417 (109 Ma) exhibit the effects of several stages of alteration reflecting the evolution of seawater-derived solution compositions and control by the structure and permeability of the crust. Characteristic secondary mineral assemblages occur in often superimposed alteration zones within individual basalt fragments. By combining bulk rock and single phase chemical analyses with detailed mineralogic and petrographic studies, chemical changes have been determined for most of the alteration stages identified in the basalts.

1. Minor amounts of saponite, chlorite, and pyrite formed locally in coarse grained portions of massive units, possibly at high temperatures during initial cooling of the basalts. No chemical changes could be determined for this stage.
2. Possible mixing of cooled hydrothermal fluids with seawater resulted in the formation of celadonite-nontronite and Fe-hydroxide-rich black halos around cracks and pillow rims. Gains of K, Rb, H₂O, increase of Fe³⁺/Fe^T, and possibly some losses of Ca and Mg occurred during this stage.
3. Extensive circulation of oxygenated seawater resulted in the formation of various smectites, K-feldspar, and Fe-hydroxides in brown and light grey alteration zones around formerly exposed surfaces. K, Rb, H₂O, and occasionally P were added to the rocks, Fe³⁺/Fe^T increased, and Ca, Mg, Si and occasionally Al and Na were lost.
4. Anoxic alteration occurred during reaction of basalt with seawater at low water-rock ratios, or with seawater that had previously reacted with basalt. Saponite-rich dark grey alteration zones formed which exhibit very little chemical change: generally only slight increases in Fe³⁺/Fe^T and H₂O occurred.
5. Zeolites and calcite formed from seawater-derived fluids modified by previous reactions with basalt. Chemical changes involved increases of Ca, Na, H₂O, and CO₂ in the rocks.
6. A late stage of anoxic conditions resulted in the formation of minor amounts of Mn-calcites and secondary sulfides in previously oxidized rocks. No chemical changes were determined for this stage.

Recognition of such alteration sequences is important in understanding the evolution of submarine hydrothermal systems and in interpreting chemical exchange due to seawater-basalt reactions.     

Airborne Lidar Observations of the Transition Zone Between the Convective Boundary Layer and Free Atmosphere During the International H<Subscript>2</Subscript>O Project (IHOP) in 2002

Airborne, light detection and ranging (lidar) backscatter observations of the convective boundary layer from the International H₂O Project (IHOP) in 2002 are analysed to study the structure of the transition zone; the backscatter gradient between the convective boundary layer and free atmosphere. A new mathematical algorithm is developed and used to extract high-resolution (15 m) transition-zone boundaries from 6,500 km (flight legs) of airborne observations. The cospectra of transition-zone boundaries and its thickness indicate that thickness changes occur from boundaries moving in opposite directions (vertically) at small wavelengths (<1 km), while at longer wavelengths (>1 km) both boundaries move coherently, with the lower boundary changing altitude more rapidly. Daily probability distributions of the transition-zone thickness are positively skewed with a mode of 60 m. The structure of the transition zone shows no dependence on the “overall” Richardson number, unlike the entrainment zone. This study provides the first quantitative characterization of the structure of the instantaneous transition zone, a contribution towards an improved understanding of convective boundary-layer entrainment.     

Bivariate conditional sampling of buoyancy flux during an intense cold-air outbreak

Buoyancy fluxes in the marine atmospheric boundary layer (MABL) for the cloud street regime, observed during the Genesis of Atlantic Lows Experiment (GALE), have been analyzed using the technique of joint frequency distribution. For the lower half of the MABL, the results suggest that the buoyancy flux is mainly generated by the rising thermals and the sinking compensating ambient air, and is mainly consumed by the entrainment and detrainment of thermals, penetrative convection, and the entrainment from the MABL top.The results are compared to those from previous studies of mesoscale cellular convection (Air-Mass Transformation Experiment, AMTEX), the dry convective boundary layer, and the trade-wind MABL. For the lower MABL, the quadrant buoyancy fluxes, fractional coverages, and flux intensities are in good agreement with those of mesoscale cellular convection (AMTEX) and the dry convective boundary layer. The results suggest that, if the buoyancy flux is primarily driven by the temperature flux, the physical processes for generating buoyancy flux mentioned above are about the same for the lower boundary layers over land and ocean, even with different convective regimes. For the trade-wind MABL, the buoyancy flux is mainly driven by the moisture flux; the quadrant flux intensities are stronger than those of the other three studies except for the buoyant updrafts (thermals). These results suggest that the entrainment and detrainment of thermals are more effective in the trade-wind MABL than in the boundary layers driven by the temperature flux.Scale analysis of the buoyancy flux is in good agreement with that of AMTEX. For the lower half of the MABL, the buoyancy flux is mainly generated by the intermediate scale (200 m to 2 km), which includes the dominant convective thermals in the surface layer and the mixed layer. The scale smaller than 200 m is important only in the surface layer. The scale larger than 2 km, which includes the roll vortices, increases its significance upward. While most of the positive and negative fluxes are associated with the updrafts for the intermediate scale, the downdrafts are as important as updrafts for the larger scale.ST Systems Corporation, Lanham, MD, 20706, U.S.A.     

Possible Impacts of Global Warming on the Hydrology of the Ogallala Aquifer Region

The Ogallala or High Plains aquifer provides water for about 20% of the irrigated land in the United States. About 20 km³ (16.6 million acre-feet) of water are withdrawn annually from this aquifer. In general, recharge has not compensated for withdrawals since major irrigation development began in this region in the 1940s. The mining of the Ogallala has been pictured as an analogue to climate change in that many GCMs predict a warmer and drier future for this region. In this paper we attempt to anticipate the possible impacts of climate change on the sustainability of the aquifer as a source of water for irrigation and other purposes in the region. We have applied HUMUS, the Hydrologic Unit Model of the U.S. to the Missouri and Arkansas-White-Red water resource regions that overlie the Ogallala. We have imposed three general circulation model (GISS, UKTR and BMRC) projections of future climate change on this region and simulated the changes that may be induced in water yields (runoff plus lateral flow) and ground water recharge. Each GCM was applied to HUMUS at three levels of global mean temperature (GMT) to represent increasing severity of climate change (a surrogate for time). HUMUS was also run at three levels of atmospheric CO₂ concentration (hereafter denoted by [CO₂]) in order to estimate the impacts of direct CO₂ effects on photosynthesis and evapotranspiration. Since the UKTR and GISS GCMs project increased precipitation in the Missouri basin, water yields increase there. The BMRC GCM predicts sharply decreased precipitation and, hence, reduced water yields. Precipitation reductions are even greater in the Arkansas basin under BMRC as are the consequent water yield losses. GISS and UKTR climates lead to only moderate yield losses in the Arkansas. CO₂-fertilization reverses these losses and yields increase slightly. CO₂ fertilization increases recharge in the base (no climate change) case in both basins. Recharge is reduced under all three GCMs and severities of climate change.     

Impacts of Land Degradation on Historical Temperature Records from the Sonoran Desert

Previous research revealed that severe overgrazing and resultant land degradation in the semiarid areas of northern Mexico created significantly higher temperatures in the border area. In this investigation, the temperature and precipitation records from ten homogeneous stations are identified in the arid and hyperarid areas of northwest Sonora and are compared with the records from ten stations in southwestern Arizona. Our data show that the Mexican stations are again consistently warmer than the Arizona stations when statistical controls are applied to correct for the linear or non-linear effects of latitude and/or elevation. The stations in Sonora warm at a statistically significantly faster pace than the stations in Arizona during the study period. Furthermore, and consistent with other dryland areas undergoing land degradation, the stations in Sonora reveal a significant increase in the diurnal temperature range during the summer season. Local precipitation reduces the temperature differential between nations on the time scale of days, but enhances the differential on the time scale of months and seasons. Among other findings, the results show how land degradation in dryland areas appears to influence local historical temperature records.     

How much physical complexity is needed to model flood inundation?

Two‐dimensional flood inundation models are widely used tools for flood hazard mapping and an essential component of statutory flood risk management guidelines in many countries. Yet, we still do not know how much physical complexity a flood inundation model needs for a given problem. Here, three two‐dimensional explicit hydraulic models, which can be broadly defined as simulating diffusive, inertial or shallow water waves, have been benchmarked using test cases from a recent Environment Agency for England and Wales study, where results from industry models are also available. To ensure consistency, the three models were written in the same code and share subroutines for all but the momentum (flow) and time‐stepping calculations. The diffusive type model required much longer simulation times than the other models, whilst the inertia model was the quickest. For flows that vary gradually in time, differences in simulated velocities and depths due to physical complexity were within 10% of the simulations from a range of industry models. Therefore, for flows that vary gradually in time, it appears unnecessary to solve the full two‐dimensional shallow water equations. As expected, however, the simpler models were unable to simulate supercritical flows accurately. Finally, implications of the results for future model benchmarking studies are discussed in light of a number of subtle factors that were found to cause significant differences in simulations relative to the choice of model. Copyright © 2011 John Wiley & Sons, Ltd.     

Raman spectroscopic carbonaceous material thermometry of low-grade metamorphic rocks: Calibration and application to tectonic exhumation in Crete, Greece

We present new Raman spectra data of carbonaceous material (CM) to extend the range of the Raman spectra of CM thermometer (RSCM) to temperatures as low as 100 °C. Previous work has demonstrated that Raman spectroscopy is an excellent tool to describe the degree of graphitization of CM, a process that is independent of pressure but strongly dependent on metamorphic temperature. A linear relationship between temperature and the Raman parameter R2 (derived from the area of the defect band relative to the ordered graphite band) forms the basis of a previous thermometer. Because R2 shows little variability in low-temperature samples, 330 °C serves as a lower limit on the existing thermometer. Herein, we present Raman spectra from a suite of low-temperature (100 to 300 °C) samples from the Olympics Mountains and describe other aspects of the Raman spectra of CM that vary over this range. In particular, the Raman parameter R1 (the ratio of heights of the disordered peak to ordered peak) varies regularly between 100 and 350 °C. These data, together with published results from higher-temperature rocks, are used to calibrate a modified RSCM thermometer, applicable from 100 to 700 °C. Application to low-grade metasediments in the Otago region in the South Island of New Zealand gives temperatures consistent with previous estimates, demonstrating the reliability of the modified RSCM thermometer.We apply the modified RSCM thermometer to 53 samples from Crete to evaluate the role of the Cretan detachment fault in exhuming Miocene high pressure/low-temperature metamorphic rocks exposed there. The metamorphic rocks below the detachment (the Plattenkalk and Phyllite-Quartzite units) give metamorphic temperatures that range from 250 to 400 °C, consistent with previous petrologic estimates. We also demonstrate that the Tripolitza unit, which lies directly above the detachment, gives an average metamorphic temperature of about 260 °C. The modest break in metamorphic temperature in central Crete indicates that the Cretan detachment accounts for only 5 to 7 km of exhumation of the underlying HP-LT metamorphic rocks, which were initially accreted at ∼ 35 km. We argue that the bulk of the exhumation (∼ 28 km out of 35 km total) occurred by pervasive brittle stretching and erosion of structural units above the detachment.