Generation of mid-ocean ridge basalts at pressures from 1 to 7 GPa

We propose a model for the generation of average MORBs based on phase relations in the CaO-MgO-Al₂O₃-SiO₂-CO₂ system at pressures from 3 to 7 GPa and in the CaO-MgO-Al₂O₃-SiO₂-Na₂O-FeO (CMASNF) system at pressures from ∼0.9 to 1.5 GPa. The MELT seismic tomography (Forsyth et al., 2000) across the East Pacific Rise shows the largest amount of melt centered at ∼30-km depth and lesser amounts at greater depths. An average mantle adiabat with a model-system potential temperature (T_p) of 1310°C is used that is consistent with this result. In the mantle, additional minor components would lower solidus temperatures ∼50°C, which would lower T_p of the adiabat for average MORBs to ∼1260°C. The model involves generation of carbonatitic melts and melts that are transitional between carbonatite and kimberlite at very small melt fractions (<0.2%) in the low-velocity zone at pressures of ∼2.6 to 7 GPa in the CMAS-CO₂ system, roughly the pressure range of the PREM low-velocity zone. These small-volume, low-viscosity melts are mixed with much larger volumes of basaltic melt generated at the plagioclase-spinel lherzolite transition in the pressure range of ∼0.9 to 1.5 GPa.In this model, solidus phase relations in the pressure range of the plagioclase-spinel lherzolite transition strongly, but not totally, control the major-element characteristics of MORBs. Although the plagioclase-spinel lherzolite transition suppresses isentropic decompression melting in the CMAS system, this effect does not occur in the topologically different and petrologically more realistic CMASNF system. On the basis of the absence of plagioclase from most abyssal peridotites, which are the presumed residues of MORB generation, we calculate melt productivity during polybaric fractional melting in the plagioclase-spinel lherzolite transition interval at exhaustion of plagioclase in the residue. In the CMASN system, these calculations indicate that the total melt productivity is ∼24%, which is adequate to produce the oceanic crust. The residual mineral proportions from this calculation closely match those of average abyssal peridotites.Melts generated in the plagioclase-spinel lherzolite transition are compositionally distinct from all MORB glasses, but do not have a significant fractional crystallization trend controlled by olivine alone. They reach the composition field of erupted MORBs mainly by crystallization of both plagioclase and olivine, with initial crystallization of either one of these phases rapidly joined by the other. This is consistent with phenocryst assemblages and experimental studies of the most primitive MORBs, which do not show an olivine-controlled fractionation trend. The model is most robust for the eastern Pacific, where an adiabat with a T_p of ∼1260°C is supported by the MELT seismic data and where the global inverse correlation of (FeO)₈ with (Na₂O)₈ is weak. Average MORBs worldwide also are well modeled. A heterogeneous mantle consisting of peridotite of varying degrees of major-element depletion combined with phase-equilibrium controls in the plagioclase-spinel lherzolite transition interval would produce the form of the global correlations at a constant T_p, which suggests a modest range of T_p along ridges. Phase-composition data for the CMASNF system are presently not adequate for quantitative calculation of (FeO)₈-(Na₂O)₈-(CaO/Al₂O₃)₈ systematics in terms of this model. The near absence of basalts in the central portion of the Gakkel Ridge suggests a lower bound for T_p along ridges of ∼1240°C, a potential temperature just low enough to miss the solidus for basalt production at ∼0.9 GPa. An upper bound for T_p is poorly constrained, but the complete absence of picritic glasses in Iceland and the global ridge system suggests an upper bound of ∼1400°C. In contrast to some previous models for MORB generation that emphasize large potential temperature variations in a relatively homogeneous peridotitic mantle, our model emphasizes modest potential temperature variations in a peridotitic mantle that shows varying degrees of heterogeneity. Calculations indicate that melt productivity changes from 0 to 24% for a change in T_p from 1240 to 1260°C, effectively producing a rapid increase to full crustal thickness or decrease to none as ridges appear and disappear.     

A decade of change in the Skidaway River estuary I. Hydrography and nutrients

The Skidaway River estuary is a tidally-dominated subtropical estuary in the southeastern USA surrounded by extensiveSpartina salt marshes. Weekly smapling at high and low tide began in 1986 for hydrography, nutrients, chlorophylla, particulate matter, and microbial and plankton biomass and composition; hydrographic and nutrient data during 1986–1996 are reported here. Salinity varied inversely with river discharge and exhibited variability at all time scales but with no long-term trend. Water temperature typically ranged over 25°C and was without apparent long-term frend. Seasonal cycles in concentrations of NO₃, NH₄, PO₄, Si(OH)₄, and DON were observed, with annual maxima generally occurring in late summer. Superimposed on seasonal cycles, all five nutrients exhibited steady increases in minimum, mean, and maximum concentrations; mean concentrations increased c. 50–150% during the decade. Nutrient concentrations were highly correlated with water temperature over the ten-year period, but weakly related to salinity and discharge. Nutrients were strongly correlated with one another, and the relative ratios among inorganic nutrients showed little long-term trend. Correlations among temperature and nutrient concentrations exhibited considerable inter-annual variability. Major spikes in organic and inorganic nutrient concentrations coincided with significant rainfall events; concentrations increased hyperbolically with rainfall. Although pristine compared to more heavily impacted waterways primarily outside the region, residential development and population density have been increasing rapidly during the past 15–20 years. Land use is apparently altering nutrient loading over the long-term (months-years), and superimposed on this are stochastic meteorological events that accelerate these changes over the short term (days-weeks).     

Water resources and ecological conditions in the Tarim Basin

Temporal sequential analyses of the hydrological observational data in the Tarim Basin over the last forty years revealed an annual increase of 2× 10⁷m³ in the water quantities at the three headstreams of the upper courses and an annual decrease of 3 × 10⁷m³ in the water flow from Alaer, which is on the upper main stream. A prediction of the trends indicates that there can be severe situations under which intermittent water interceptions occur. By means of approximate estimations on vegetative water consumption through phreatic evaporation combined with a quota assessment, the ecological water demands required to maintain the ecological environment in the mainstream area over the three different targeted years of 2005, 2010 and 2030 are defined as standing at 31.86 × 10⁸m³, 36.27 × 10⁸m³ and 41.04 × 10⁸m³ respectively. Ecological fragility indexes are established on the basis of the selection of environmental sensitivity factors. Rational evaluations give proof that the lower reaches of the mainstream have already turned into zones where their ecological environments are gravely damaged. Multi-objective optimization should be conducted and protective schemes be framed within the threshold limits of the bearing capacities of water resources and the environment     

Sandy desertification in the north of China

With the economic and social development, desertification exerted increasingly profound influences on natural environment and social development and attracted widespread attention of international communities. China, as one of the countries facing severe desertification problems, has witnessed some progress in understanding and combating the process of desertification through many years of hard work. Based on the experiences and research achievements, this paper briefly discusses the causes, developmental processes, damage assessment and control mechanism of desertification in the north of China so as to provide some basic experiences for the further study of desertification.     

Remote sensing parameterization of the processes of energy and water cycle over desertification areas

In order to understand the processes of land surface-atmosphere interaction over desertification area, it is indispensable to utilize of satellite remote sensing. Two scenes of Landsat TM were used to produce a set of maps of surface reflectance, MSAVI, vegetation coverage, surface temperature, net radiation, soil heat flux, sensible heat flux and latent heat flux. Statistical analysis based on these maps revealed some quantitative significant land surface characteristics. Future developments of the method are also discussed.     

Remote sensing analysis of desert vegetation and its landscape changes: The case in middle reaches of Tarim River Basin,Xinjiang, China

Based on remote sensing information sources including B/W aerial photos of 1983, pseudo-color aerial photos of 1992 and JERS-1/OPS VNIR image of 1996, vegetation types of Yingbazha, in the middle reaches of the Tarim River Basin in Xinjiang, China are mapped using ARC/INFO and related software. The changes in vegetation areas and distribution conditions are analyzed. As a result of natural and human influences, vegetation changes have temporal and spatial characteristics. According to the principles of landscape ecology and geographical information science, the landscape changes are indicated. Moreover, the remote sensing and GIS techniques are integrated to study vegetation and its landscape.     

Kosa (yellow sand) components in precipitation collected at central Japan

In east Asia, acidic gases derived from fossil fuel combustion have increased in the past decades. On the other hand, the Asian dust, also called Kosa (yellow sand) is transported following windstorms from arid lands in the Asian continent. Many researchers have been interested in the reaction between acidic aerosols and Kosa aerosols as well as the long-range transport of these emissions. To investigate the characteristics of chemical components in precipitation on a long-term basis over Japan, precipitation was sequentially collected from April 1984 to March 1997 at Kanazawa located near the coast of the Sea of Japan. Precipitation samples were collected at 1 mm intervals for the first 5 mm rainfall and all volume of rainwater after 6 mm for all precipitation events with an automatic wet only precipitation collector. According to the analyses of precipitation including Kosa aerosols during Kosa periods, the reaction in the air between Kosa and acidic components during the long-range transport was discussed.     

Damage and recovery of the Gurbantunggut Desert vegetation following engineering activities

The fragile ecological environment of the Gurbantunggut Desert is damaged/disturbed by human activities relating to the development of oil-gas resources and the constructions of desert road and great engineering in the Jungger Basin. It was mainly represented: soil compaction, vegetation cleaning, burial of vegetation, oil polluting, and soil disturbance. With investigation and experiment, we found that when the way and intensity of engineering activities disturbing the eco-environment does not make its ecological stability disintegrated, the desert vegetation has a capacity of natural recovery. To speed and strengthen the process of vegetation recovery efficient assistant measurements, including stabilizing mobile sands promptly and sowing seeds of shrub and herb plants in good time will be needed.     

Expansion and contraction of Chinese deserts during the Quaternary

Episodic dune formations during the Quaternary are found in many deserts of China. The causes of desert expansions on different time scales are not the same. Desert extension at about 1.1 and 0.9 Ma ago were the response to the active tectonic movements, whereas the desert evolutions on the ten-thousand years time scale were the response to the orbital scale climatic changes. Spatial scale studies on desert evolution indicate that desert margins shifted greatly during the last glacial maximum (LGM) and the Holocene optimum, its changing from 125°E of the LGM to 105°E of the climatic optimum. Historical desertification in the semiarid China is not a response to climate drought but largely associated with the human impacts (mainly over-cultivation) since about 2300 years ago, which leads to the reworking of the underlying LGM sands.