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1.
Katrina D. van Drongelen Douglas Rumble III Kimberly T. Tait 《Meteoritics & planetary science》2016,51(6):1184-1200
Northwest Africa (NWA) 5232, an 18.5 kg polymict eucrite, comprises eucritic and exogenic CM carbonaceous chondrite clasts within a clastic matrix. Basaltic clasts are the most abundant eucritic clast type and show a range of textures and grain size, from subophitic to granoblastic. Other eucritic clast types present include cumulate (high‐En pyroxene), pyroxene‐lath, olivine rich with symplectite intergrowths as a break‐down product of a quickly cooled Fe‐rich metastable pyroxferroite, and breccia (fragments of a previously consolidated breccia) clasts. A variable cooling rate and degree of thermal metamorphism, followed by a complex brecciation history, can be inferred for the clasts based on clast rounding, crystallization (and recrystallization) textures, pyroxene major and minor element compositions, and pyroxene exsolution. The range in δ18O of clasts and matrix of NWA 5232 reflects its origin as a breccia of mixed clasts dominated by eucritic lithologies. The oxygen isotopic compositions of the carbonaceous chondrite clasts identify them as belonging to CM group and indicate that these clasts experienced a low degree of aqueous alteration while part of their parent body. The complex evolutionary history of NWA 5232 implies that large‐scale impact excavation and mixing was an active process on the surface of the HED parent body, likely 4 Vesta. 相似文献
2.
Katrina J. Butkas Yvonne Vadeboncoeur M. Jake Vander Zanden 《Aquatic Sciences - Research Across Boundaries》2011,73(1):153-169
Studies of aquatic invertebrate production have been primarily conducted at the level of individual taxa or populations. Advancing
our understanding of the functioning and energy flow in aquatic ecosystems necessitates scaling-up to community and whole-lake
levels, as well as integrating across benthic and pelagic habitats and across multiple trophic levels. In this paper, we compare
a suite of non-cohort based methods for estimating benthic invertebrate production at subpopulation, habitat, and whole-lake
levels for Sparkling Lake, WI, USA. Estimates of the overall mean benthic invertebrate production (i.e. whole-lake level)
ranged from 1.9 to 5.0 g DM m−2 y−1, depending on the method. Production estimates varied widely among depths and habitats, and there was general qualitative
agreement among methods with regards to differences in production among habitats. However, there were also consistent and
systematic differences among methods. The size-frequency method gave the highest, while the regression model of Banse and
Mosher (Ecol Monogr 50:355–379, 1980) gave the lowest production estimates. The regression model of Plante and Downing (Can J Fish Aquat Sci 46:1489–1498, 1989) had the lowest average coefficients of variation at habitat (CV = 0.17) and whole-lake (CV = 0.08) levels. At the habitat
level, variance in production estimates decreased with sampling effort, with little improvement after 10–15 samples. Our study
shows how different production estimates can be generated from the same field data, though aggregating estimates up to the
whole-lake level does produce an averaging effect that tends to reduce variance. 相似文献
3.
Katrina Ann Macintosh David Griffiths 《Aquatic Sciences - Research Across Boundaries》2014,76(3):331-338
The density and composition of stream bed metal deposits are affected by physical, chemical and biological processes. In this paper we investigate the importance of these processes and their relation to algal and non-photosynthetic detrital (NPD) biomass in a set of upland streams in Northern Ireland. Deposit density and Fe, Mn, Al and P concentrations varied with stream pH across sites but not seasonally. No effects of stream bed erosion or photoreduction were detected on deposit densities. Seasonal variation in stream water metal concentrations was correlated with rainfall. NPD biomass was a significant predictor of both spatial and seasonal variation in deposit concentrations. There were strong, non-linear, relations between NPD biomass and deposit metal concentrations, with Fe and Mn becoming relatively more important and algal biomass declining above threshold deposit/NPD densities. The results suggest that NPD biomass influences deposit density and reduces the biomass of photosynthetic autotrophs above a threshold deposit density. 相似文献
4.
Diffusive equilibrium in thin films provides evidence of suppression of hyporheic exchange and large‐scale nitrate transformation in a groundwater‐fed river 
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下载免费PDF全文 P. Byrne H. Zhang S. Ullah A. Binley A.L. Heathwaite C.M. Heppell K. Lansdown M. Trimmer 《水文研究》2015,29(6):1385-1396
The hyporheic zone of riverbed sediments has the potential to attenuate nitrate from upwelling, polluted groundwater. However, the coarse‐scale (5–10 cm) measurement of nitrogen biogeochemistry in the hyporheic zone can often mask fine‐scale (<1 cm) biogeochemical patterns, especially in near‐surface sediments, leading to incomplete or inaccurate representation of the capacity of the hyporheic zone to transform upwelling NO3?. In this study, we utilised diffusive equilibrium in thin‐films samplers to capture high resolution (cm‐scale) vertical concentration profiles of NO3?, SO42?, Fe and Mn in the upper 15 cm of armoured and permeable riverbed sediments. The goal was to test whether nitrate attenuation was occurring in a sub‐reach characterised by strong vertical (upwelling) water fluxes. The vertical concentration profiles obtained from diffusive equilibrium in thin‐films samplers indicate considerable cm‐scale variability in NO3? (4.4 ± 2.9 mg N/L), SO42? (9.9 ± 3.1 mg/l) and dissolved Fe (1.6 ± 2.1 mg/l) and Mn (0.2 ± 0.2 mg/l). However, the overall trend suggests the absence of substantial net chemical transformations and surface‐subsurface water mixing in the shallow sediments of our sub‐reach under baseflow conditions. The significance of this is that upwelling NO3?‐rich groundwater does not appear to be attenuated in the riverbed sediments at <15 cm depth as might occur where hyporheic exchange flows deliver organic matter to the sediments for metabolic processes. It would appear that the chemical patterns observed in the shallow sediments of our sub‐reach are not controlled exclusively by redox processes and/or hyporheic exchange flows. Deeper‐seated groundwater fluxes and hydro‐stratigraphy may be additional important drivers of chemical patterns in the shallow sediments of our study sub‐reach. © 2015 The Authors. Hydrological Processes Published by John Wiley & Sons Ltd. 相似文献
5.
Olivier Rouxel Edward Sholkovitz Katrina J. Edwards 《Geochimica et cosmochimica acta》2008,72(14):3413-3430
Dissolved Fe concentrations in subterranean estuaries, like their river-seawater counterparts, are strongly controlled by non-conservative behavior during mixing of groundwater and seawater in coastal aquifers. Previous studies at a subterranean estuary of Waquoit Bay on Cape Cod, USA demonstrate extensive precipitation of groundwater-borne dissolved ferrous iron and subsequent accumulation of iron oxides onto subsurface sands. Waquoit Bay is thus an excellent natural laboratory to assess the mechanisms of Fe-isotope fractionation in redox-stratified environments and determine potential Fe-isotope signatures of groundwater sources to coastal seawater. Here, we report Fe isotope compositions of iron-coated sands and porewaters beneath the intertidal zone of Waquoit Bay. The distribution of pore water Fe shows two distinct sources of Fe: one residing in the upward rising plume of Fe-rich groundwater and the second in the salt-wedge zone of pore water. The groundwater source has high Fe(II) concentration consistent with anoxic conditions and yield δ56Fe values between 0.3 and −1.3‰. In contrast, sediment porewaters occurring in the mixing zone of the subterranean estuary have very low δ56Fe values down to −5‰. These low δ56Fe values reflect Fe-redox cycling and result from the preferential retention of heavy Fe-isotopes onto newly formed Fe-oxyhydroxides. Analysis of Fe-oxides precipitated onto subsurface sands in two cores from the subterranean estuary revealed strong δ56Fe and Fe concentration gradients over less than 2m, yielding an overall range of δ56Fe values between −2 and 1.5‰. The relationship between Fe concentration and δ56Fe of Fe-rich sands can be modeled by the progressive precipitation of Fe-oxides along fluid flow through the subterranean estuary. These results demonstrate that large-scale Fe isotope fractionation (up to 5‰) can occur in subterranean estuaries, which could lead to coastal seawater characterized by very low δ56Fe values relative to river values. 相似文献
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7.
The United Nations Framework on Climate Change (UNFCCC), at its thirteenth meeting in 2005 (COP-11), agreed to start a work program to explore a range of policy approaches and positive incentives for Reducing Emissions from Deforestation and Degradation (REDD). This process was further encouraged in the 2007 COP-13 with the explicit consideration of REDD activities as a means to enhance mitigation action by developing countries in the future. This paper outlines the context of this ongoing political process by reviewing the science indicating that land-use change is a key contributor of greenhouse emissions globally and the assumptions that REDD activities may be competitive—in terms of cost effectiveness—in comparison to other mitigation options. The paper then examines REDD proposals submitted by Parties before COP-13 and identifies key economic, technological, methodological and institutional challenges associated with their implementation. These proposals are discussed in the light of major drivers of deforestation and ongoing efforts to address deforestation. This reveals another set of challenges which, if not taken into account, may undermine REDD effectiveness. The paper aims to aid the policy process and contribute to the best possible design of a REDD framework under the future climate regime. 相似文献
8.
Katrina Allen Kennedy 《Journal of Paleolimnology》1994,10(2):69-84
Medicine Lake is a highly saline, meromictic, magnesium sulfate, closed-basin lake in northeastern South Dakota. The geochemical, mineralogical, and magnetic stratigraphies of sediments deposited from about 10.8 to 4.5 ka B.P. document the evolution of the saline brine in response to climatic change in the early to mid-Holocene. During the spruce occupation of the Medicine Lake catchment (10.8–10.0 ka B.P.), dark-grey massive basal sediments with low total-sulfur and carbonate content, upwardly increasing organic-carbon content, and high magnetic susceptibility were deposited in a deep freshwater lake. As the vegetation in the area changed from spruce to birch to oak and elm and finally to prairie between 10.0 and 9.2 ka B.P., and as the lake became shallow and salinity increased from <2 to >10%, light-and dark-grey calcareous and organic-carbon-rich banded sediments with low total-sulfur content and low magnetic susceptibility were deposited. Previous studies have shown that during the forest/prairie transition the lake then changed abruptly from fresh to saline as it lost a substantial portion of its volume. During the early prairie period (9.2–5.5 ka B.P.), alternating sections of aragonite-rich laminae and grey massive sediments with high total-sulfur content and multiple gypsum layers were deposited in a meromictic environment under conditions of fluctuating lake levels and salinity. Continued aridity during the mid-Holocene (5.5–4.5 ka B.P.) probably maintained the lake at relatively low levels and high salinity as dark-grey generally massive sediments with moderate total-sulfur, carbonate, and organic-carbon content and no measurable magnetic susceptibility were deposited. 相似文献
9.
Andreas Pack Katrina Kremer Nina Albrecht Klaus Simon Andreas Kronz 《Geochemical transactions》2010,11(1):4
Background
In aerodynamic levitation, solids and liquids are floated in a vertical gas stream. In combination with CO2-laser heating, containerless melting at high temperature of oxides and silicates is possible. We apply aerodynamic levitation to bulk rocks in preparation for microchemical analyses, and for evaporation and reduction experiments. 相似文献10.
Ann Louise Heathwaite Catherine Heppell Andrew Binley Patrick Byrne Katrina Lansdown Mark Trimmer Sami Ullah Hao Zhang 《水文研究》2021,35(4):e14135
We report the complex spatial and temporal dynamics of hyporheic exchange flows (HEFs) and nitrogen exchange in an upwelling reach of a 200 m groundwater-fed river. We show how research combining hydrological measurement, geophysics and isotopes, together with nutrient speciation techniques provides insight on nitrogen pathways and transformations that could not have been captured otherwise, including a zone of vertical preferential discharge of nitrate from deeper groundwater, and a zone of rapid denitrification linking the floodplain with the riverbed. Nitrate attenuation in the reach is dominated by denitrification but is spatially highly variable. This variability is driven by groundwater flow pathways and landscape setting, which influences hyporheic flow, residence time and nitrate removal. We observed the spatial connectivity of the river to the riparian zone is important because zones of horizontal preferential discharge supply organic matter from the floodplain and create anoxic riverbed conditions with overlapping zones of nitrification potential and denitrification activity that peaked 10–20 cm below the riverbed. Our data also show that temporal variability in water pathways in the reach is driven by changes in stage of the order of tens of centimetres and by strength of water flux, which may influence the depth of delivery of dissolved organic carbon. The temporal variability is sensitive to changes to river flows under UK climate projections that anticipate a 14%–15% increase in regional median winter rainfall and a 14%–19% reduction in summer rainfall. Superimposed on seasonal projections is more intensive storm activity that will likely lead to a more dynamic and inherently complex (hydrologically and biogeochemically) hyporheic zone. We recorded direct evidence of suppression of upwelling groundwater (flow reversal) during rainfall events. Such flow reversal may fuel riverbed sediments whereby delivery of organic carbon to depth, and higher denitrification rates in HEFs might act in concert to make nitrate removal in the riverbed more efficient. 相似文献