The synthetic seismic expression of the Messinian salinity crisis from onshore records: Implications for shallow‐ to deep‐water correlations

The onshore–offshore correlation of sedimentary successions is a common problem in basin analysis, but it becomes critical for the full understanding of the Messinian salinity crisis (MSC), a complex array of palaeoenvironmental events which affected the Mediterranean basin at the end of the Miocene. The outcrop records show that the Messinian stratigraphic architectures may be highly complex as the deposits of the different MSC evolutionary stages can be lithologically similar and separated by erosional surfaces and/or morphostructural highs. The correct definition of the nature and stratigraphic position of Messinian deposits in offshore areas through seismic data may be almost impossible, especially where core data are sparse. To bridge the gap between onshore and offshore records, we have built synthetic seismic sections from well‐constrained outcrop successions. Our results provide useful insights and warnings for the interpretation of offshore data, pointing out that MSC units having different age, nature and depositional settings, may show similar seismic facies and geometries. Conversely, the same deposit may result in different seismic facies, either with parallel and high‐amplitude reflections or even transparent or chaotic due to interference patterns of seismic reflections related to dominant frequency. It follows that a correct interpretation of the nature and age of deep‐seated Messinian deposits can only be obtained through the integration of seismic and core data, and considering the onshore record. The application of our approach to the Balearic Promontory results in an alternative interpretation with respect to previous models. We show that this offshore area has good analogues in the onshore of the Betic Cordillera and includes both shallow and intermediate depth sub‐basins that underwent a strong post‐Messinian subsidence.     

Water status in winter wheat grown under salt stress

Properties of the soil surface layer, the temporal pattern of the microclimate variables as well as crop condition were combined to analyze the characteristics of the evapotranspiration from winter wheat fields in a saline soil area. In order to accomplish this analysis, evapotranspiration was divided into evaporation from the soil and transpiration from wheat. Moreover, the effect of soil salinity on evapotranspiration was evaluated through the relationship between actual evapotranspiration and potential evapotranspiration (Ea/Eo) and the total soil water potential (y) was divided into two components: matric potential (yM) and osmotic potential (yo). Two sites with different salinity levels were chosen for this study, located in Hebei Province, China. Measurements were conducted in April-May 1997 and May 1998. The Bowen ratio method was used to estimate the actual evapotranspiration (Ea), whereas potential evapotranspiration (Eo) was estimated using Penman’s equation. Measurements of soil evaporation (Es) were obtained with micro-lysimeters, and transpiration was calculated from the difference between Ea and Es. The results show that transpiration comprised on average almost 80 % of total evapotranspiration. Evaporation from the soil differed slightly between years, but this variation was dominated by the leaf area index (LAI), which ranged from 4 to 5 during the study period of 1997 and 1998. Soil electric conductivity (EC), which is directly related to osmotic potential, ranged from 1.9 to 3.5 mS cm-1 in 1997 and was negligible in 1998. Our results indicate that lower osmotic potential decreases the total soil water potential, thus affecting plant transpiration. Hence, it is possible to say that soil salinity actually decreases evapotranspiration from winter wheat fields.     

Biophysical properties and biomass production of elephant grass under saline conditions

Elephant grass (Pennisetum purpureum Schum.) is a new fast-growing alternative forage crop. However, salinity is a major concern for its production in the arid south-western United States. This study was conducted in the arid Imperial Valley of Southern California to evaluate salt tolerance of elephant grass. Salinity treatments were created in field plots irrigated with water possessing an electrical conductivity (EC_iw) of 1·5, 5, 10, 15, 20, and 25 dS m⁻¹, respectively. Canopy spectral reflectance, temperature, plant height, leaf area index (LAI), chlorophyll-SPAD meter readings, and dry weights were measured over time. Results indicated that canopy reflectance in the near-infrared spectral region was reduced incrementally with increasing levels of salt stress. Canopy temperature increased with increasing salinity, especially at longer times after salinity treatment. Plant height and LAI were reduced with increasing salinity. Biomass accumulation was reduced incrementally with increasing salinity. About 50% yield reduction was found when EC_iw increased from 5 to 25 dS m⁻¹. The study shows that elephant grass is sensitive to salt stress, and relatively low salinity must be maintained to achieve a high rate of growth and biomass production.     

Selectivity of various types of salt-resistant plants for K over Na

Selectivity by whole plants for K⁺ over Na⁺ in three types (salt excluding, salt secreting and salt diluting) of salt-resistant plants was investigated. An estimating formula of Selective Absorption (SA) capacity of root systems was derived; the Selective Transport (ST n) capacities between K⁺ and Na⁺ by various parts of the three types of plants were compared. The results showed that the SA value of salt-excluding plants were higher than that of salt-secreting and salt-diluting plants, the ST1 (root:stem) value was much higher, indicating that both the capacity of selective absorption and the capacity of selective transport by root systems were strong. In salt-secreting plants, the SA value lay between salt-diluting and salt-excluding plants, while the ST1 value was the lowest, indicating that the majority of Na⁺ uptake by root systems was transported up to their aerial parts and then the surplus salt was secreted in salt glands. In salt-diluting plants, the SA value was the lowest, indicating that the majority of Na⁺ taken up by the root systems entered into plant body perhaps to satisfy the requirements for osmotic adjustment and growth, and the ST1 value lay between salt-secreting and salt-excluding plants. These data strongly indicate that the SA and ST1 values reflect the K⁺ and Na⁺ selectivity characteristics of salt-resistant plants. Therefore, the various types of salt-resistant plants would be classified by using the values of SA and ST1. We suggest that this provides a way for distinguishing various types of salt-resistant plants. Comparing the ST n values of the six species in our paper, we concluded that the selectivity of transporting K⁺ over Na⁺ into the actively photosynthesizing organs and particularly into the developing ears is extremely high. Our conclusion is in accordance with previous results that demonstrate that the capacities of selective transport by different parts of the plant for K⁺ over Na⁺ are best quantified by ST n values. The necessity, feasibility and wide-ranging applicability of the formulas for estimating SA and ST n values have been discussed in detail.     

Seabird guano influences on desert islands: soil chemistry and herbaceous species richness and productivity

An understanding of the effects of guano deposition on arid soil chemistry and the consequences for plant communities is lacking. This study examined patterns of herbaceous species richness and productivity, soil chemistry, soil moisture and soil respiration on 11 islands in the Gulf of California, six of which receive seabird guano deposition. Species richness was significantly lower on islands with guano (“Bird” islands) than islands without guano (“Non-bird” islands), with very little overlap in species composition; however, productivity was significantly greater on Bird than on Non-bird islands. As expected, Bird island soils had higher concentrations of NO₃⁻, NH₄⁺ and total nitrogen (N) than Non-bird island soils; and, measurements of δ¹⁵N indicate that the higher concentrations of N were derived from guano. We also found that soil moisture and respiration were significantly higher, but pH was significantly lower, on Bird than Non-bird islands. These results suggest that guano deposition in deserts stimulates productivity—even in dry years—due to elevated N and, indirectly, soil moisture. Guano deposition also results in a decrease in species richness and a change in species composition probably due to elevated N, N toxicity, or low pH. However, we also found that pH varied more on Bird than on Non-bird islands; and that salinity—while not different between island types—was significantly patchier on Bird than on Non-Bird islands. These results suggest that guano deposition affects not only the general chemical composition of soils, but also results in greater spatial variation in soil chemical composition, which may ultimately affect species richness and composition. Therefore, understanding spatial patterning in soil chemistry as a result of guano deposition is critical for understanding guano effects on plant richness and productivity.     

The analysis of nitrate in highly saline waters

The analysis of nitrate in seawater and hypersaline waters should take account of a significant “salt effect”. Procedures developed for fresh water and marine waters must be applied cautiously to highly saline waters. The most widely used standard method for the determination of nitrate-nitrogen (NO₃-N) in fresh and marine waters involves the quantitative reduction of nitrate by the Cd column technique followed by colorimetric procedures. In our study, three approaches to estimate NO₃-N in highly saline waters were examined. The first involved dilution. This approach overcame the salt effect but dilution limited the detection of low concentrations of nitrate in highly saline waters. The second involved the use of standard nitrate solutions in saline water. This method is not recommended because of nitrate impurities in AR grade salts. The third- and preferred approach-involved the use of standard additions. “Spikes” of a known volume of NO₃-N standard solution were added to natural saline waters. Nitrate values estimated by the stadard addition method were used to calculate an equation for salt error correction at different salinities applicable to waters with the same relative ionic composition as seawater. This could then be used to correct nitrate determinations in highly saline waters where standards made in distilled water were used for calibration. Many previously published data for NO₃-N in saline water used methods of analysis which do not take account of salt error and are therefore probably in error.     

Variability in biomarkers of different saline basins in China

42 samples (sediments and crude oils) from 5 different saline/hypersaline basins of China were examined using variety of geochemical techniques. A pronounced even over odd distribution ofn-alkanes is observed for the Ejinur, Jianghan samples. Abundance of isoprenoid alkanes, dominated by phytane (C₂₀ up to 20 per cent in EOM), and C₂₅ and C₃₀ components is another outstanding feature of these studied lacustrine hypersaline settings, indicating important contributuions from archaebacteria. Gammacerane is a major component of some Eocene Jiangham samples and Cretaceous Taian sediments. Absolute concentration of phytane and gammacerane appears to be linearly related to chlorine and residual (reduced) sulfur contents of sediments from Jianghan basin, suggesting sulfur incoporation might have played an important role in the enhacement of these biomarkers in the anoxic hypersaline, alkaline settings. Wide range of organic sulfur compounds in immature samples of Jianghan Basin reflects a significant pathway for sulfur incorporation under hypersaline, extremely anoxic/reducing conditions, although for some structures a direct origin from some sulfur archaebacteria can not be excluded. Brackish salt marsh sediment and oil from Lenghu depression contain abundant lupane, strong OEP inn-alkane series, and predominance of C₂₉ steranes, suggesting a markedly input from vascular higher plants. The marginal marine evaporate sediments from the hypersaline Triassic Yangtze Platform is unique for its predominant, complete series of isoprenoid alkanes up to C₃₆ (abundant) and C₄₀ (trace). These long-chain isoprenoids are probably derived from phytoplanktons in addition to archaebacteria. The marked difference in biomarker distributions from the various suites of samples support that application of these biomarkers to help effectively characterize different saline basins. The molecular variation is, however, not only due to their discrepancy in biological sources, but also the extension of sulfate reduction and the availability of metal ions during early diagenesis.     

A Holocene paleosalinity diatom record from southwestern Saskatchewan, Canada: Harris Lake revisited

Fossil diatoms were analysed from a 10.3 m core from Harris Lake, Cypress Hills, Saskatchewan, and a diatom-salinity transfer function was used to construct a history of Holocene salinity changes for the lake. The diatom paleosalinity record indicates that Harris Lake remained fresh <0.5 g l-¹ throughout the Holocene, with only slight increases in salinity between approximately 6500 and 5200 years BP. This interval corresponds to the only period in the lake's history when planktonic diatoms were abundant; benthic Fragilaria taxa, mainly F. pinnata, F. construens and F. brevistriata were dominant throughout most of the Holocene. The shift from a benthic to a planktonic diatom flora between 6500 and 5200 years BP may be an indirect response to a warmer climate that reduced forest cover in the watershed and allowed greater rates of inorganic sedimentation. The small salinity increase that accompanies the floristic change is probably not the result of lower lake levels; in fact the lake was probably deeper at this point than in the later Holocene. This paleosalinity record indicates that Harris Lake did not experience episodes of hypersalinity during the mid-Holocene, as suggested by a previous study, and that the lake may have been fresh during the early Holocene as well.     

黑河下游地下水波动带土壤盐分空间变异特征分析

以黑河下游地下水波动带为研究区,根据64个样点的实验数据,采用地统计分析法对区域土壤盐分空间变异函数理论模型进行了选择,同时还利用GIS交叉验证(Cross-Validation)对目前较为薄弱的空间插值方法的适用性选择问题进行了分析.并在此基础上对整个区域土壤盐分空间分布进行了模拟,进而分析了其空间变异特征.结果表明:(1) 受地下水影响,黑河下游地下水波动带土壤盐分含量整体较高,且其空间分布具有明显的变异特征.在沿河流流向及垂直于河流流向方向上,各盐分要素在整体规律性变化基础上都存在局部性变异,且这种局部性变异在垂直于河流流向方向上比河流流向方向更显著;(2) 各盐分要素的变程为本研究今后合理布置采样点提供了参考依据,同时也为黑河下游地区水文及生态研究提供了有关研究尺度的参考信息;(3) 研究区土壤盐分空间分布模拟也为该区植被恢复提供了必要的土壤盐分信息,这对本区乃至整个黑河下游植被恢复及生态重建工作都具有重要指导意义.     

Effect of salinity on rheological and strength properties of cement-stabilized clay minerals

Abstract

This paper presents an experimental investigation into the effect of salinity on Atterberg limits, flowability, viscosity and strength properties of cement-stabilized clay minerals. Three groups of clay minerals (illite, kaolinite and montmorillonite) were obtained. Specimens with different porewater salinities were prepared by mixing the air-dried clays with sodium chloride (NaCl) at various salt concentrations (i.e., 0%, 2% 4%, 6% and 8%). Atterberg limits test results indicated that liquid limit and plasticity index decreased insignificantly with increasing salinity for Kaolinite and illite but significantly for montmorillonite. Flow test results indicated that of all specimens of three groups of clay minerals with or without adding cement consistently increase with increasing salinity. The flow value of montmorillonite increased more significantly than kaolinite and illite. Viscosity test results indicated that all the specimens tested behave as Bingham plastic. Flow value consistently decreased with increasing dynamic viscosity or yield stress, regardless of clay mineralogy, porewater salinity and cement amount. Strength test results indicated that all cement-stabilized specimens exhibit strain softening behavior. Unconfined compressive strength for three groups of clay minerals stabilized with cement consistently decreased with increasing salinity indicating that the presence of salt had an adverse effect on the development of strength.