Anthropogenic impacts on nitrogen fixation rates between restored and natural Mediterranean salt marshes

To test the effects of site and successional stage on nitrogen fixation rates in salt marshes of the Venice Lagoon, Italy, acetylene reduction assays were performed with Salicornia veneta‐ and Spartina townsendii‐vegetated sediments from three restored (6–14 years) and two natural marshes. Average nitrogen fixation (acetylene reduction) rates ranged from 31 to 343 μmol C₂H₄·m^?2·h^?1 among all marshes, with the greatest average rates being from one natural marsh (Tezze Fonde). These high rates are up to six times greater than those reported from Southern California Spartina marshes of similar Mediterranean climate, but substantially lower than those found in moister climates of the Atlantic US coast. Nitrogen fixation rates did not consistently vary between natural and restored marshes within a site (Fossei Est, Tezze Fonde, Cenesa) but were negatively related to assayed plant biomass within the acetylene reduction samples collected among all marshes. Highest nitrogen fixation rates were found at Tezze Fonde, the location closest to the city of Venice, in both natural and restored marshes, suggesting possible site‐specific impacts of anthropogenic stress on marsh succession.     

Hydrothermal Fluid Sources of the Fengjia Barite–fluorite Deposit in Southeast Sichuan,China: Evidence from Fluid Inclusions and Hydrogen and Oxygen Isotopes

The Fengjia barite–fluorite deposit in southeast Sichuan is a stratabound ore deposit which occurs mainly in Lower Ordovician carbonate rocks. Here we present results from fluid inclusion and oxygen and hydrogen isotope studies to determine the nature and origin of the hydrothermal fluids that generated the deposit. The temperature of the ore‐forming fluid shows a range of 86 to 302 °C. Our detailed microthermometric data show that the temperature during mineralization of the fluorite and barite in the early ore‐forming stage was higher than that during the formation of the calcite in the late ore‐forming stage. The salinity varied substantially from 0.18% to 21.19% NaCl eqv., whereas the density was around 1.00 g/cm³. The fluid composition was mainly H₂O (>91.33%), followed by CO₂, CH₄ and traces of C₂H₆, CO, Ar, and H₂S. The dominant cation was Na⁺ and the dominant anion Cl^‐, followed by Ca²⁺, SO₄^2‐, K⁺, and Mg²⁺, indicating a mid–low‐temperature, mid‐low‐salinity, low‐density NaCl–H₂O system. Our results demonstrate that the temperature decreased during the ore‐forming process and the fluid system changed from a closed reducing environment to an open oxidizing environment. The hydrogen and oxygen isotope data demonstrate that the hydrothermal fluids in the study area had multiple sources, primarily formation water, as well as meteoric water and metamorphic water. Combined with the geological setting and mineralization features we infer that the stratabound barite–fluorite deposits originated from mid–low‐temperature hydrothermal fluids and formed vein filling in the fault zone.     

Molybdenum Mass Fractions and Isotopic Compositions of International Geological Reference Materials

The double‐spike method with multi‐collector inductively coupled plasma‐mass spectrometry was used to measure the Mo mass fractions and isotopic compositions of a set of geological reference materials including the mineral molybdenite, seawater, coral, as well as igneous and sedimentary rocks. The long‐term reproducibility of the Mo isotopic measurements, based on two‐year analyses of NIST SRM 3134 reference solutions and seawater samples, was ≤ 0.07‰ (two standard deviations, 2s, n = 167) for δ^98/95Mo. Accuracy was evaluated by analyses of Atlantic seawater, which yielded a mean δ^98/95Mo of 2.03 ± 0.06‰ (2s, n = 30, relative to NIST SRM 3134 = 0‰) and mass fraction of 0.0104 ± 0.0006 μg g^?1 (2s, n = 30), which is indistinguishable from seawater samples taken world‐wide and measured in other laboratories. The comprehensive data set presented in this study serves as a reference for quality assurance and interlaboratory comparison of high‐precision Mo mass fractions and isotopic compositions.     

MORPHOMETRIC ANALYSIS OF KARST DEPRESSIONS ON A MEDITERRANEAN KARST MASSIF

Dolines are the most typical landform in karst landscapes. They play an important role in karst hydrogeology; they are effective sediment traps that provide information about past climates and karst evolution, and can also be indicators of tectonic activity. The morphometic analysis of those depressions can help in their study and interpretation. However, detailed identification and delineation of dolines becomes increasingly difficult when the study area is very large, of limited access or covered by vegetation. Here, we have used a method developed by the authors for automatic identification and delineation of karst depressions based on the use of a pit removal algorithm. Next a complete morphometric analysis of the karst depressions has been carried out. The main advantage of the methodology is that by delineating and identifying the depth of each doline from its rim, many morphometric statistics can be obtained, including the relation of doline depth and area with respect to altitude, the main doline orientations, doline density, the fractal dimension of doline area and frequency, and point field analysis, among others. The results are interpreted from a geomorphological point of view and the mapped dolines can be easily integrated into geomorphological mapping. A case study is presented from the large Sierra Gorda karst massif in southern Spain, where 3100 depressions have been identified. A strong structural control of doline shape and orientation is demonstrated; the main areas of doline density are highlighted and doline size is shown to follow a fractal law.     

Effect of loading duration on uncertainty in creep analysis of clay

Some studies suggest that creep parameters should be determined using a greater quantity of creep test data to provide more reliable prediction regarding the deformation of soft soils. This study aims to investigate the effect of loading duration on model updating. One‐dimensional consolidation data of intact Vanttila clay under different loading durations collected from the literature is used for demonstration. The Bayesian probabilistic method is used to identify all unknown parameters based on the consolidation data during the entire consolidation process, and their uncertainty can be quantified through the obtained posterior probability density functions. Additionally, the optimal models are also determined from among 9 model candidates. The analyses indicate that the optimal models can describe the creep behavior of intact soft soils under different loading durations, and the adopted method can evaluate the effect of loading duration on uncertainty in the creep analysis. The uncertainty of a specific model and its model parameters decreases as more creep data are involved in the updating process, and the updated models that use more creep data can better capture the deformation behavior of an intact sample. The proposed method can provide quantified uncertainty in the process of model updating and assist engineers to decide whether the creep test data are sufficient for the creep analysis.     

Geographic information science and the analysis of place and health

The representation of place is a key theoretical advancement that geographic information science can offer to improve the understanding of environmental determinants of health, but developing robust computational representations of place requires a substantial departure from conventional notions of geographic representation in GIS. Unlike conventional GIS representations based on either objects or locations, we suggest that place representation should incorporate dynamic subjective, experiential, and relational aspects of place, as the influence of place on health behavior concerns not only the features that can be objectively observed at a particular location but also the environmental perceptions of the individual, as molded by biological, social, and experiential characteristics. In addition, assessments of environmental exposures on health outcomes should focus on individuals' time–activity patterns and microenvironment profiles, which form a potentially unique personalized exposure environment for each individual. Addressing these representational challenges via collaborative research has the potential to advance both geographic information science and health research.     

Chemical oceanographic influences on sediment accumulations of a carbonate ramp: Holocene Yucatan Shelf,Mexico

Global controls on the oceanographic influences on the nature of carbonate factories are broadly understood. The details of the influences of changes in temperature and nutrients across individual carbonate shelves are less well constrained, however. This study explores spatial and temporal variations in chemical oceanography along and across the Yucatan Shelf, a modern carbonate ramp, and how these factors relate to variable bottom character, sediment and sediment geochemistry. In‐situ sensors and remote‐sensing data indicate the sporadic presence of cool, upwelled water with low dissolved oxygen and elevated Chlorophyll‐a. This current‐driven, westward flow of upwelled water is most evident in a zone just offshore of the northern peninsular shoreline, but its influence wanes ca 75 km offshore and as the shore turns southward. The impacts of this water mass include a transitional photozoan–heterozoan assemblage with biosiliceous components, relict grains and common thin Holocene sediment accumulations nearshore; further offshore are coralgal reefs and expansive sand plains. Geochemical proxies of bulk sediment, including high δ¹⁸O and elevated HREE/LREE (heavy rare‐earth element/light rare‐earth element) ratios near, and downcurrent of, the upwelling source, are interpreted to represent the signal of nearshore, westward movement of the cool and nutrient‐rich, upwelled water. Collectively, these data emphasize how local processes such as upwelling and longshore transport can variably influence carbonate sediment accumulations and their geochemical signatures, both along and across individual shelves. These data and insights provide an analogue for the influences of spatial variability of water masses in the geological record, and for accurate interpretation of stratigraphic changes of sedimentary and geochemical proxy data in carbonate archives.     

Effect of the Dry Ashing Method on Cadmium Isotope Measurements in Soil and Plant Samples

A dry ashing method is commonly used to remove organic material from samples prior to geochemical analysis. In the course of this study, the Cd isotope ratios of a series of soil and plant reference materials and samples were studied to evaluate the effect of the dry ashing method on measurement results of Cd isotope ratios. The samples were pre‐treated using the dry ashing method and high‐pressure bomb for comparison. The results show that the digestion using high‐pressure bombs did not lead to Cd loss, but using the dry ashing method would cause different proportions of Cd loss. The whole range of Cd isotope difference between two methods was from ?0.07‰ to 3.01‰. There was also an obvious difference in measured Cd isotope ratios from the same leaf sample pre‐treated independently by the dry ashing method, indicating that the amount of Cd loss and the effect on Cd isotope measurement during dry ashing is related to the properties of the samples. Therefore, dry ashing may not be appropriate for the removal of organic material in Cd isotope ratio measurement, especially for samples with high organic contents. The δ^114/110Cd values of reference materials NIST SRM 1573a and GSD‐30 are reported for the first time in this study.     

Early solar system aqueous activity: K isotope evidence from Allende

The alkali element K is moderately volatile and fluid mobile; thus, it can be influenced by both primary processes (evaporation and recondensation) in the solar nebula and secondary processes (thermal and aqueous alteration) in the parent body. Since these primary and secondary processes would induce different isotopic fractionations, K isotopes could become a potential tracer to distinguish them. Using recently developed methods with improved precision (0.05‰, 95% confidence interval), we systematically measured the K isotopic compositions and major/trace elemental compositions of chondritic components (18 chondrules, 3 CAIs, 2 matrices, and 5 bulks) in the carbonaceous chondrite fall Allende. Among all the components analyzed in this study, CAIs, which formed initially under high‐temperature conditions in the solar nebula and were dominated by nominally K‐free refractory minerals, have the highest K₂O content (average 0.53 wt%) and have K isotope compositions most enriched in heavy isotopes (δ⁴¹K: ?0.30 to ?0.25‰). Such an observation is consistent with previous petrologic studies that show CAIs in Allende have undergone alkali enrichment during metasomatism. In contrast, chondrules contain lower K₂O content (0.003–0.17 wt%) and generally lighter K isotope compositions (δ⁴¹K: ?0.87‰ to ?0.24‰). The matrix and bulks are nearly identical in K₂O content and K isotope compositions (0.02–0.05 wt%; δ⁴¹K: ?0.62 to ? 0.46‰), which are, as expected, right in the middle of CAIs and chondrules. This strongly indicates that most of the chondritic components of Allende suffered aqueous alteration and their K isotopic compositions are the ramification of Allende parent‐body processing instead of primary nebular signatures. Nevertheless, we propose the small K isotope fractionations observed (< 1‰) among Allende components are likely similar to the overall range of K isotopic fractionation that occurred in nebular environment. Furthermore, the K isotope compositions seen in the components of Allende in this study are consistent with MC‐ICP‐MS analyses of the components in ordinary chondrites, which also show an absence of large (10‰) isotope fractionations. This is not expected as evaporation experiments in nebular conditions suggest there should be large K isotopic fractionations. Nevertheless, possible nebular processes such as chondrules back exchanging with ambient gas when they formed could explain this lack of large K isotopic variation.