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91.
Stephanie Kastner Christian Ohlendorf Torsten Haberzettl Andreas Lücke Christoph Mayr Nora I. Maidana Frank Schäbitz Bernd Zolitschka 《Journal of Paleolimnology》2010,44(4):887-902
We studied the internal lake processes that control the spatial distribution and characteristics of modern sediments at the
ICDP (International Continental Scientific Drilling Program) deep drilling site in Laguna Potrok Aike, southern Patagonia,
Argentina. Sediment distribution patterns were investigated using a dense grid of 63 gravity cores taken throughout the lake
basin and 40 additional shoreline samples. Analysis of the surficial sediment distribution points to distinct internal depositional
dynamics induced by wind-driven lake internal currents. Distribution maps illustrate that the spatial characteristics of analysed
variables are linked to high erosional wave activity. Persistent wave action and littoral erosion along all shores, especially
the eastern shore, is caused by prevailing Southern Hemispheric Westerlies. Several sediment variables (grain size, benthic
diatoms, total inorganic carbon and calcium) indicate re-suspension of littoral sediment followed by re-distribution to profundal
accumulation areas near the eastern shore. Variations within the catchment influence sediment characteristics in the north-eastern
bay. That area is characterized by different mineralogical and sedimentological conditions as well as greater accumulation
of pollen, inorganic carbon and diatoms. These findings are related to the influence of episodic inflow into this bay. Spatial
differences in stable isotope values throughout the lake suggest that ephemeral tributaries around the lake basin may also
contribute to the detected spatial sediment variations. 相似文献
92.
The salinization of rivers, as indicated by salinity increases in the downstream direction, is characteristic of arid and semiarid regions throughout the world. Historically, salinity increases have been attributed to various mechanisms, including (1) evaporation and concentration during reservoir storage, irrigation, and subsequent reuse; (2) displacement of shallow saline ground water during irrigation; (3) erosion and dissolution of natural deposits; and/or (4) inflow of deep saline and/or geothermal ground water (ground water with elevated water temperature). In this study, investigation of salinity issues focused on identification of relative salinity contributions from anthropogenic and natural sources in the Lower Rio Grande in the New Mexico-Texas border region. Based on the conceptual model of the system, the various sources of water and, therefore, salinity to the Lower Rio Grande were identified, and a sampling plan was designed to characterize these sources. Analysis results for boron (delta(11)B), sulfur (delta(34)S), oxygen (delta(18)O), hydrogen (delta(2)H), and strontium ((87)Sr/(86)Sr) isotopes, as well as basic chemical data, confirmed the hypothesis that the dominant salinity contributions are from deep ground water inflow to the Rio Grande. The stable isotopic ratios identified the deep ground water inflow as distinctive, with characteristic isotopic signatures. These analyses indicate that it is not possible to reproduce the observed salinization by evapotranspiration and agricultural processes alone. This investigation further confirms that proper application of multiple isotopic and geochemical tracers can be used to identify and constrain multiple sources of solutes in complex river systems. 相似文献
93.
Caitlin M. Mayernik Stephanie A. Ewing W. Adam Sigler Kelsey G. Jencso Robert A. Payn 《水文研究》2024,38(2):e15069
Changes in seasonality and form of precipitation alter the structure and function of grassland and steppe ecosystems and pose challenges for land management and crop production in regions like the Northern Great Plains, North America. This research uses isotopic composition of water (δ18O and δ2H) to explore the sources and fate of soil water in lower-elevation agricultural areas of the Judith River watershed, in the headwaters of the Missouri River, USA. Extensive non-irrigated cereal crop production in this area occurs on well-drained soils and depends on careful water management. Our observations indicate that colder precipitation contributes isotopically distinct water to cultivated terrace soils relative to downgradient groundwaters and streams. Riparian waters also exhibit a higher fraction of contributions from colder precipitation relative to terrace groundwaters and streams. Apparent contributions from colder precipitation in terrace and riparian soil waters suggest that snowmelt is a key component of the water supply to these systems. Riparian waters also show evidence of evaporation suggesting that water spends sufficient time in some ponds and open channels in the riparian corridor to reflect fractionation by evaporation. The evolution of water isotopic composition from soils to shallow aquifers to stream corridors indicates source water partitioning as precipitation moves through this semi-arid agricultural landscape. The apparent mixing processes evident in this evolution reveal source water dynamics that are necessary to understand plant transpiration, solute processing, and contaminant leaching processes. 相似文献
94.
95.
Stephanie S. Kienast Ingrid L. Hendy John Crusius Tom F. Pedersen Stephen E. Calvert 《Journal of Oceanography》2004,60(1):189-203
The subarctic North Pacific is a high nitrate-low chlorophyll (HNLC) region, where phytoplankton growth rates, especially
those of diatoms, are enhanced when micronutrient Fe is added. Accordingly, it has been suggested that glacial Fe-laden dust
might have increased primary production in this region. This paper reviews published palaeoceanographic records of export
production over the last 800 kyrs from the open North Pacific (north of ∼35°N). We find different patterns of export production
change over time in the various domains of the North Pacific (NW and NE subarctic gyres, the marginal seas and the transition
zone). However, there is no compelling evidence for an overall increase in productivity during glacials in the subarctic region,
challenging the paradigm that dust-born Fe fertilization of this region has contributed to the glacial draw down of atmospheric
CO2. Potential reasons for the lack of increased glacial export production include the possibility that Fe-fertilization rapidly
drives the ecosystem towards limitation by another nutrient. This effect would have been exacerbated by an even more stable
mixed layer compared to today.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
96.
97.
Stephanie A. Ewing Wenbo Yang Greg Michalski Brian W. Stewart Ronald Amundson 《Geochimica et cosmochimica acta》2008,72(4):1096-1110
We measured Ca stable isotope ratios (δ44/40Ca) in an ancient (2 My), hyperarid soil where the primary source of mobile Ca is atmospheric deposition. Most of the Ca in the upper meter of this soil (3.5 kmol m−2) is present as sulfates (2.5 kmol m−2), and to a lesser extent carbonates (0.4 kmol m−2). In aqueous extracts of variably hydrated calcium sulfate minerals, δ44/40CaE values (vs. bulk Earth) increase with depth (1.4 m) from a minimum of −1.91‰ to a maximum of +0.59‰. The trend in carbonate-δ44/40Ca in the top six horizons resembles that of sulfate-δ44/40Ca, but with values 0.1-0.6‰ higher. The range of observed Ca isotope values in this soil is about half that of δ44/40Ca values observed on Earth. Linear correlation among δ44/40Ca, δ34S and δ18O values indicates either (a) a simultaneous change in atmospheric input values for all three elements over time, or (b) isotopic fractionation of all three elements during downward transport. We present evidence that the latter is the primary cause of the isotopic variation that we observe. Sulfate-δ34S values are positively correlated with sulfate-δ18O values (R2 = 0.78) and negatively correlated with sulfate δ44/40CaE values (R2 = 0.70). If constant fractionation and conservation of mass with downward transport are assumed, these relationships indicate a δ44/40Ca fractionation factor of −0.4‰ in CaSO4. The overall depth trend in Ca isotopes is reproduced by a model of isotopic fractionation during downward Ca transport that considers small and infrequent but regularly recurring rainfall events. Near surface low Ca isotope values are reproduced by a Rayleigh model derived from measured Ca concentrations and the Ca fractionation factor predicted by the relationship with S isotopes. This indicates that the primary mechanism of stable isotope fractionation in CaSO4 is incremental and effectively irreversible removal of an isotopically enriched dissolved phase by downward transport during small rainfall events. 相似文献
98.
99.
Fluid Inclusion and Stable Isotope Study at the Southeastern Martabe Deposit: Purnama,Barani and Horas Ore Bodies,North Sumatra,Indonesia
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The Martabe Au–Ag deposit, North Sumatra Province, Indonesia, is a high sulfidation epithermal deposit, which is hosted by Neogene sandstone, siltstone, volcanic breccia, and andesite to basaltic andesite of Angkola Formation. The deposit consists of six ore bodies that occurred as silicified massive ore (enargite–luzonite–pyrite–tetrahedrite–tellurides), quartz veins (tetrahedrite–galena–sphalerite–chalcopyrite), banded sulfide veins (pyrite–tetrahedrite–sphalerite–galena) and cavity filling. All ore bodies are controlled by N–S and NW–SE trending faults. The Barani and Horas ore bodies are located in the southeast of the Purnama ore body. Fluid inclusion microthermometry, and alunite‐pyrite and barite‐pyrite pairs sulfur isotopic geothermometry show slightly different formation temperatures among the ore bodies. Formation temperature and salinity of fluid inclusions of the Purnama ore body range from 200 to 260 C and from 6 to 8 wt.% NaCl equivalent, respectively. Formation temperature and salinity of fluid inclusions of the Barani ore body range from 200 to 220 °C and from 0 to 2.5 wt.% NaCl equivalent and those of the Horas ore body range from 240 to 275 °C and from 2 to 3 wt.% NaCl equivalent, respectively. The Barani and Horas ore bodies are less silicified and sulfides are less abundant than the Purnama ore body. A relationship between enthalpy and chloride content indicates mixing of hot saline fluids with cooler dilute fluids during the mineralization of each of the ore bodies. The δ18O values of quartz samples from the southeast ore bodies exhibit a wide range from +4.2 to +12.9‰ with an average value of +7.0‰. The δ18O values of H2O estimated from δ18O values of quartz, barite and calcite confirm the oxygen isotopic shift to near meteoric water trend, which support the incorporation of meteoric water. Salinity of the fluid inclusions decrease from >5 wt.% NaCl equivalent in the Purnama ore body to <3 wt.% NaCl equivalent in the Barani ore body, indicating different fluid systems during mineralization. The δ34S values of sulfide and sulfate in Purnama range from ? 4.2 to +5.5‰ and from +1.2 to +26.7‰, those in the Barani range from ? 4.3 to +26.4‰ and from +3.9 to +18.5‰ and those in the Horas ore body range from ? 11.8 to +3.5‰ and from +1.4 to +25.7‰, respectively. The δ34S of total bulk sulfur in southeastern ore bodies (Σδ34S) was estimated to be approximately +6‰. The estimated sulfur fugacity during formation of the Purnama and Horas ore bodies is relatively high. It was between 10?4.8 and 10?10.8 atm at 220 to 260 °C. Tellurium fugacity was between 10?7.8 and 10?9.5 atm at 260 °C and between 10?9 and 10?10.6 atm at 220 °C in the Purnama ore body. The Barani ore body was formed at lower fS2, lower than about 10?14 atm at 200 to 220 °C based on the presence of arsenopyrite and pyrrhotite in the early stage, and between 10?14 and 10?12 atm based on the existence of enargite and tennantite in the last stage. © 2016 The Society of Resource Geology 相似文献
100.
Alan E. Kehew Stephanie K. Ewald John M. Esch Andrew L. Kozlowski 《Boreas: An International Journal of Quaternary Research》2013,42(2):442-462
Tunnel valleys are common throughout the terrain of the Saginaw Lobe of the Laurentide Ice Sheet in southern Michigan. The set of valleys described in this paper is regularly spaced in a radial pattern behind the Kalamazoo Moraine, an ice‐marginal position formed during retreat from the Last Glacial Maximum. These valleys are divided into proximal and distal groups lying north and south, respectively, of a major river valley that cross‐cuts the tunnel valleys at right angles. Based on a series of rotasonic borings and core analysis, the proximal valleys are shallow, contain minimal sediment fill, and overlie fine‐grained diamicton and glaciolacustrine sediment, whereas the distal valleys are deeply incised into the substrate and are partially filled with coarse sediment. The distal valleys terminate within a broad zone of high‐relief, hummocky topography representing stagnation and collapse behind the Kalamazoo ice margin. The proximal valleys occur within a more subdued landscape located farther from the ice margin. Although some elements of existing genetic models are consistent with these valleys, none appears to be completely compatible with their stratigraphy and morphology. Initial incision of the valleys could have involved short‐lived moderate‐ to high‐discharge flows, followed by deposition during or after the events. The deep incision and thick, coarse sediment in distal valleys in the stagnant marginal zone probably involved supraglacial meltwater draining to the bed as the margin downwasted. Fining‐upward eskers inset into the valleys were formed by flows of declining energy in small late‐stage conduits. 相似文献