Water-source contributions to barrier lakes and water-rock interactions in the Wudalianchi volcanic area, Northeast China

Wudalianchi is a typical continental Cenozoic volcanic group rich in potassic volcanic rocks (Northeast China). Five hydrologically connected barrier lakes (Lakes 5 to 1) and upwelling cold mineral springs occur, forming a complex lake-groundwater system. Clarifying the water-source contributions and the role of water-rock interactions in the hydrological cycling for barrier lakes remains a challenge from scientific and engineering perspectives. In this study, seasonal variations of multiple isotopes were analyzed. δ18O and δD data indicate that the Wudalianchi lakes were mainly fed by mineral springs. The values, however, were greatly influenced by precipitation (rain and snow) and varying evaporation intensities. In contrast, 87Sr/86Sr ratios varied little between seasons (0.70701–0.7079), suggesting similar water-rock interactions through time. Nonetheless, Sr isotopic mixing models suggested that shallow mineral springs generally contributed >50% of the water to lower reaches. In contrast, the upstream wetland contributed >50% to Lake 5 and decreased down-valley (10.3–53.6%). Calculations based on the δ18O and δD Rayleigh fractionation equation suggest that evaporation in upper reaches were higher than the lower reaches. The evaporation in July were generally higher than in October. This study demonstrates the homogenous water-rock interactions and the associated water mixing effects on the terrestrial volcanic area.

Field-based oxygen isotope fractionation for the conservation of imperilled fishes: an application with the threatened silver shiner Notropis photogenis

Identifying the realized thermal habitat of animals is important for understanding life history and population processes, yet methods to estimate realized thermal use are lacking for many small-bodied organisms, including imperilled fishes. Analysis of oxygen isotopes provides one solution, but requires the development of species-specific fractionation equations. To date, such equations have generally been limited to commercial or game fish species. Here, we developed a field-based fractionation equation for the threatened silver shiner Notropis photogenis to better understand the thermal ecology of the species in an urban watershed. Archived otoliths were analyzed for oxygen isotope values (δ18O). There was a significant linear relationship between otolith isotope fractionation and water temperature, described by δ18Ootolith(VPBD) - δ18Owater(VPBD) = 32.03 - 0.21(°C). Results indicate that otolith isotope techniques can be used to identify the average relative temperature occupied by silver shiner, representing the first investigation of oxygen isotopes to understand thermal occupancy of the species. This field-based equation provides an opportunity to understand how silver shiner may respond to alterations in stream temperatures resulting from urbanization and climate effects and may be useful in identifying thermal refugia for the species. Field-based, species-specific fractionation equations can provide insights into the thermal ecology of many small-bodied fishes, which are increasingly imperilled due to thermal stressors.

Major element and oxygen isotope geochemistry of vapour-phase garnet from the Topopah Spring Tuff at Yucca Mountain, Nevada, USA

Twenty vapour-phase garnets were studied in two samples of the Topopah Spring Tuff of the Paintbrush Group from Yucca Mountain, in southern Nevada. The Miocene-age Topopah Spring Tuff is a 350 m thick, devitrified, moderately to densely welded ash-flow tuff that is zoned compositionally from high-silica rhyolite to latite. During cooling of the tuff, escaping vapour produced lithophysae (former gas cavities) lined with an assemblage of tridymite (commonly inverted to cristobalite or quartz), sanidine and locally, hematite and/or garnet. Vapour-phase topaz and economic deposits associated commonly with topaz-bearing rhyolites (characteristically enriched in F) were not found in the Topopah Spring Tuff at Yucca Mountain. Based on their occurrence only in lithophysae, the garnets are not primary igneous phenocrysts, but rather crystals that grew from a F-poor magmaderived vapour trapped during and after emplacement of the tuff. The garnets are euhedral, vitreous, reddish brown, trapezohedral, as large as 2 mm in diameter and fractured. The garnets also contain inclusions of tridymite. Electron microprobe analyses of the garnets reveal that they are almandinespessartine (48.0 and 47.9 mol.%, respectively), have an average composition of (Fe1.46Mn1.45Mg0.03Ca0.10)(Al1.93Ti0.02)Si3.01O12and are comparatively homogeneous in Fe and Mn concentrations from core to rim. Composited garnets from each sample site have δ18O values of 7.2 and 7.4%. The associated quartz (after tridymite) has δ18O values of 17.4 and 17.6%, values indicative of reaction with later, low-temperature water. Unaltered tridymite from higher in the stratigraphic section has a δ18O of 11.1% which, when coupled with the garnet δ18O values in a quartz-garnet fractionation equation, indicates isotopic equilibration (vapour-phase crystallization) at temperatures of ∼600°C. This high-temperature mineralization, formed during cooling of the tuffs, is distinct from the later and commonly recognized low-temperature stage (generally 50−70°C) of calcite, quartz and opal secondary mineralization, formed from downward-percolating meteoric water, that locally coats fracture footwalls and lithophysal floors.

Oxygen isotope compositions of phosphate from arvicoline teeth and Quaternary climatic changes, Gigny, French Jura

Oxygen isotope compositions of biogenic phosphates from mammals are widely used as proxies of the isotopic compositions of meteoric waters that are roughly linearly related to the air temperature at high- and mid-latitudes. An oxygen isotope fractionation equation was determined by using present-day European arvicoline (rodents) tooth phosphate: δ18Op = 20.98(±0.59) + 0.572(±0.065) δ18Ow. This fractionation equation was applied to the Late Pleistocene karstic sequence of Gigny, French Jura. Comparison between the oxygen isotope compositions of arvicoline tooth phosphate and Greenland ice core records suggests to reconsider the previously established hypothetical chronology of the sequence. According to the δ18O value of meteoric water–mean air temperature relationships, the δ18O value of arvicoline teeth records variations in mean air temperatures that range from 0° to 15°C.