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John W. Valley - One of the best experts on this subject based on the ideXlab platform.
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Primitive Oxygen-Isotope Ratio recorded in magmatic zircon from the Mid-Atlantic Ridge
American Mineralogist, 2009Co-Authors: Aaron J. Cavosie, Noriko T. Kita, John W. ValleyAbstract:The Oxygen-Isotope composition of the Earth’s upper mantle is an important reference for understanding mantle and crust geochemical cycles. Olivine is the most commonly used mineral for determining the influence of crustal processes on the Oxygen-Isotope Ratio (δ 18 O) of primitive rocks, however it is an uncommon mineral in continental crust and readily alters at or near Earth’s surface. Here we report the first measurements of Oxygen-Isotope Ratios in zircon from oceanic crust exposed at a mid-ocean ridge. Measurements of δ 18 O and trace elements were made by ion microprobe on zircon in polished rock chips of gabbro and veins in serpentinized peridotite drilled from the Mid-Atlantic Ridge. The zircon grains contain both oscillatory and sector growth zoning, features characteristic of magmatic zircon. Values of δ 18 O (zircon) = 5.3 ± 0.8‰ (2 st. dev., n = 68) for the population are consistent with the interpretation that these grains are igneous in origin and formed in high-temperature isotopic equilibrium with mantle Oxygen. The δ 18 O values demonstrate that zircon in oceanic crust preserves primitive δ 18 O in spite of sub-solidus alteRation of the whole rock. The fact that the primitive δ 18 O (zircon) values fall in a narrow range (5.3 ± 0.8‰) strengthens the use of Oxygen Isotopes in zircon as a tracer to identify processes of exchange in a wide range of modern and ancient crustal environments, including subducted oceanic crust (eclogite), and also in the oldest known pieces of Earth, >3900 million-year-old detrital zircon grains from Western Australia.
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crustal evolution and recycling in a juvenile continent Oxygen Isotope Ratio of zircon in the northern arabian nubian shield
Lithos, 2009Co-Authors: Yaron Katzir, Yaron Beerishlevin, John W. ValleyAbstract:Abstract Crustal recycling patterns during the evolution of the Neoproterozoic Arabian-Nubian Shield (ANS) were defined using the Oxygen Isotope Ratio of zircon [δ18O(Zrn)]. Evidence for early (~ 870–740 Ma) crustal recycling in the northernmost ANS (southern Israel and Sinai, Egypt) is given by laser fluorination analysis of bulk zircon separates, which yield higher than mantle δ18O(Zrn) values of several island arc complex (IAC) orthogneisses (6.9 to 8.2‰) and also from the average δ18O(Zrn) value of 6.4‰ determined for detrital zircons (~ 870–780 Ma) from the Elat-schist; the latter representing the oldest known rock sources in the region. These results indicate prolonged availability of surface-derived rocks for burial or subduction, melting, and assimilation at the very early stages of island arc formation in the ANS. Other IAC intrusions of ~ 800 Ma show mantle-like δ18O(Zrn) values, implying that not all magmas involved supracrustal contribution. Much younger (650–625 Ma) deformed syn-collisional calc-alkaline (CA1) intrusions are characterized by δ18O(Zrn) values of 5.0 to 7.9‰ indicating continued recycling of the felsic crust. The main sample set of this study comprises rocks from the mostly granitic, post-collisional calc-alkaline (CA2: ~ 635–590 Ma) and alkaline (AL: ~ 608–580 Ma) magmatic suites. Despite having distinct geochemical characteristics and petrogenetic paths and spans of magmatic activity, the two suites are indistinguishable by their average δ18O(Zrn) values of 5.7 and 5.8‰ pointing to the dominance of mantle-like δ18O sources in their formation. Nonetheless, grouping the two suites together reveals geographical zoning in δ18O(Zrn) where a large southeastern region of δ18O(Zrn) = 4.5 to 5.9‰ is separated from a northwestern belt with δ18O(Zrn) = 6 to 8‰ by a ‘6‰ line’. It is thus suggested that all CA2 and AL magmas of the northernmost ANS were derived from mantle-like δ18O reservoirs in the mafic lower-crust and the lithospheric-mantle, respectively. However, while in the northwestern belt these magmas intruded a thick crustal section and assimilated ~ 15–35%, high-δ18O IAC+CA1 material, magmas in the southeastern region intruded a thinner crust and little or no contamination occurred. The proposed NW–SE variance in crustal thickness during the late Neoproterozoic fits well with the geometry of the fan shaped rifting model proposed by Stern [Stern, R.J., 1985. The Najd Fault System, Saudi Arabia and Egypt: a late Precambrian rift related transform system. Tectonics 4, 497–511.] for this region. Deep parts of the lithosphere were beginning to rift at ~ 630 Ma, allowing the asthenospheric mantle to rise and transfer heat to the lithosphere. This resulted in vast melting of the mafic lower-crust to produce the batholithic CA2 magmas. Later (~ 610 Ma) percolation of lithospheric-mantle melts (possibly along deep seated lithospheric-scale faults) introduced AL magmas to shallow levels of the crust. Intrusion of CA2 and AL mantle-like δ18O parent magmas into the thinned southeastern crust did not involve assimilation of older crust whereas similar intrusion into the thicker northwestern crust resulted in mild assimilation of high-δ18O pre-635 Ma crust. An important implication from our results is that petrogenesis of some high-δ18O AL magmas of the northernmost ANS involved assimilation of supracrustal material. Felsic intrusions of the AL suite were previously described as A-type granites derived solely from mantle melts with no crustal components. Our results contribute to the “A-type petrogenesis debate” by showing that their formation can involve recycling of crustal material.
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optically continuous silcrete quartz cements of the st peter sandstone high precision Oxygen Isotope analysis by ion microprobe
Geochimica et Cosmochimica Acta, 2007Co-Authors: Jacque L Kelly, Bin Fu, N T Kita, John W. ValleyAbstract:A detailed Oxygen Isotope study of detrital quartz and authigenic quartz overgrowths from shallowly buried (<1 km) quartz arenites of the St. Peter Sandstone (in SW Wisconsin) constrains temperature and fluid sources during diagenesis. Quartz overgrowths are syntaxial (optically continuous) and show complex luminescent zonation by cathodoluminescence. Detrital quartz grains were separated from 53 rocks and analyzed for Oxygen Isotope Ratio by laser fluorination, resulting
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post caldera volcanism in situ measurement of u pb age and Oxygen Isotope Ratio in pleistocene zircons from yellowstone caldera
Earth and Planetary Science Letters, 2001Co-Authors: Ilya N Bindeman, John W. Valley, Joseph L Wooden, Harold M PersingAbstract:The Yellowstone Plateau volcanic field, the site of some of the largest known silicic volcanic eruptions, is the present location of NE-migrating hotspot volcanic activity. Most volcanic rocks in the Yellowstone caldera (0.6 Ma), which formed in response to the climactic eruption of 1000 km 3 of Lava Creek Tuff (LCT), have unusually low Oxygen Isotope Ratios. Ion microprobe analysis of both U^Pb age and N 18 O in zircons from these low-N 18 O lavas reveals evidence of complex inheritance and remelting. A majority of analyzed zircons from low-N 18 O lavas erupted inside the Yellowstone caldera have cores that range in age from 2.4 to 0.7 Ma, significantly older than their eruption ages (0.5^0.4 Ma). These ages and the high-N 18 O cores indicate that these lavas are largely derived from nearly total remelting of normal-N 18 O Huckleberry Ridge Tuff (HRT) and other pre-LCT volcanic rocks. A post-HRT low-N 18 O lava shows similar inheritance of HRT-age zircons. The recycling of volcanic rocks by shallow remelting can change the water content and eruptive potential of magma. This newly proposed mechanism of intracaldera volcanism is best studied by combining in situ analysis of Oxygen and U^Pb Isotope Ratios of individual crystals. fl 2001 Elsevier Science B.V. All rights reserved.
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Ion microprobe analysis of Oxygen Isotope Ratios in quartz from Skye granite: healed micro-cracks, fluid flow, and hydrothermal exchange
Contributions to Mineralogy and Petrology, 1996Co-Authors: John W. Valley, Colin M. GrahamAbstract:Quartz grains in hydrothermally altered granites from the Isle of Skye are highly heterogeneous and not equilibrated in Oxygen Isotope Ratio at the 20 μm scale. Ion microprobe analysis of one grain shows a gradient of 13‰ over 400 μm and a greater range in δ 18O than all quartz previously analyzed on the Isle of Skye. Other crystals from the same outcrop are homogeneous. Digitized cathodoluminescence images reveal patterns of magmatic zoning and brittle fracturing not otherwise detectable. The ion probe analysis correlates low δ 18O values on a micro-scale to one set of healed cracks. Thus, quartz exchanges Oxygen Isotopes primarily by solution and reprecipitation along fractures, in contrast to more reactive feldspar that appears to exchange from the grain boundary inward. Macroscopic models of Isotope exchange are not realistic for these rocks; the flow of hydrothermal fluids was heterogeneous, anisotropic and crack controlled.
James R. Ehleringer - One of the best experts on this subject based on the ideXlab platform.
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Oxygen Isotope RatioS OF WATERS AND RESPIRED CO2 IN AMAZONIAN FOREST AND PASTURE ECOSYSTEMS
Ecological Applications, 2005Co-Authors: Jean Pierre Henry Balbaud Ometto, Lawrence B. Flanagan, Luiz Antonio Martinelli, James R. EhleringerAbstract:The Oxygen Isotope Ratio (6180, SMOW) of atmospheric CO2 is a powerful indicator of large-scale CO2 exchange on land. Oxygen isotopic exchange between CO2 and water in leaves and soils controls the 8180 of atmospheric CO2. Currently there is little empirical information on the spatial and temporal variation in the s'80 of leaf and stem water in tropical ecosystems. We measured the seasonal dynamics of 6880 in atmospheric CO2 and water in different ecosystem compartments in both primary forest and pasture ecosystems in three different regions of the Amazonian Basin of Brazil (Ji-Parand, Manaus, and Santarem). Within regions, the source (stem) water 8180 values for primary forests and pastures were similar; neither vegetation type exhibited distinct wet-dry season patterns. Daytime leaf water Isotope Ratios were strongly correlated with predictions of the Craig- Gordon model. The s880 value of leaf water was positively correlated with leaf height above ground because of associated variation in vapor pressure deficit and the 8180 of atmospheric water vapor within forest canopies. Consistent with these observations, the s'1O value of leaf cellulose was positively correlated with forest height. Leaf water from pasture grasses was more 180 enriched than leaf water from forest vegetation. There was a tendency for daytime leaf water to be more enriched in 180 during the dry season, reflecting generally lower humidity conditions during the dry season. Nighttime measurements of the Oxygen Isotope Ratio of ecosystem respired CO2 in both forest and pasture vegetation were not consistent with values expected for CO2 in equilibrium with stem (soil) water, despite nighttime vapor pressure deficits close to zero. Apparently, the 6'1O of leaf water lagged and did not attain isotopic equilibrium at night. Thus, the deviation of nighttime 8180 values of ecosystem respiRation from that expected from a CO2 efflux in equilibrium with soil (stem) water increased as 6180 values of ecosystem respiRation became 180 enriched. Dis- crimination against CO2 containing 180 (AC'8OO) during photosynthesis was calculated based on measured leaf water 6180 values. Forests had consistently higher modeled AC'800 values than pastures. The daytime Isotope effects we calculate for photosynthesis and respiRation were consistent with previous model predictions of a strong depletion of 180 in atmospheric CO2 over the Amazon Basin of Brazil.
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expressing leaf water and cellulose Oxygen Isotope Ratios as enrichment above source water reveals evidence of a peclet effect
Oecologia, 2004Co-Authors: Margaret M Barbour, Graham D Farquhar, John S Roden, James R. EhleringerAbstract:There is an increasing ecological interest in understanding the gradients in H218O enrichment in leaf water (i.e. a Peclet effect), because an appreciation of the significance of the Peclet effect is important for improving our understanding of the mechanistic processes affecting the 18O composition of leaf water and plant organic material. In data sets where both source water and leaf water 18O data are available, we can evaluate the potential contribution of a Peclet effect. As an example, we recalculate data published earlier by Roden and Ehleringer (1999, Oecologia 121:467–477) as enrichments in leaf water (ΔL) and cellulose (Δcell) above source water. Based on these recalculations, we present support for the relevance of a Peclet effect in leaves. Further, we demonstrate that the subtle variations in ΔL and Δcell caused by a Peclet effect may be masked in experimental systems in which variation in the source water Oxygen Isotope Ratio is considerable.
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a rapid and precise method for sampling and determining the Oxygen Isotope Ratio of atmospheric water vapor
Rapid Communications in Mass Spectrometry, 2002Co-Authors: John S Roden, Brent R. Helliker, Craig S Cook, James R. EhleringerAbstract:A quantitative method for cryogenically sampling atmospheric water vapor on the temporal scale of 10 to 15 min in the field or laboratory is described. The sample apparatus is lightweight, affordable, and easy to assemble. The method allows for H2O:CO2 equilibRation within the same sampling tubes and hence increases turnaround time for δ18O analysis. Quantitative analysis in the laboratory showed recovery of a vaporized, known, 18O water standard to 0.2‰ precision. Copyright © 2002 John Wiley & Sons, Ltd.
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Grass blades as tree rings: Environmentally induced changes in the Oxygen Isotope Ratio of cellulose along the length of grass blades
New Phytologist, 2002Co-Authors: Brent R. Helliker, James R. EhleringerAbstract:Summary • In this study, we tested the hypothesis that environmentally induced changes in the Oxygen Isotope Ratio of leaf water are recorded in grass blade cellulose during leaf-blade expansion. • Grasses were grown hydroponically in chambers that allowed for control of relative humidity while keeping isotopic inputs (namely source water) constant. • In experiments where relative humidity was changed from 35% to 93% during grass blade expansion, a 10‰ shift in cellulose δ 18 O was observed along single grass blades of Lolium multiflorum . However, statistically significant changes were not detectable with relative humidity of 93% to 70%. • It is shown that grass blades, analogously to tree rings, record environmental change on an interseasonal basis. In light of this, care must be taken to compare leaves of the same developmental stage to avoid confusion of environmental effects with physiological effects in interpretations of leaf organic material δ 18 O. The results presented here confirm the ability of the current empirical models to predict the Oxygen Isotope Ratio of cellulose in both grass blades that expanded in constant and variable growth conditions.
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spatial and temporal variation in the carbon and Oxygen stable Isotope Ratio of respired co2 in a boreal forest ecosystem
Tellus B, 1999Co-Authors: Lawrence B. Flanagan, David S Kubien, James R. EhleringerAbstract:We measured the stable Isotope Ratio of respired carbon dioxide at two spatial scales in a black spruce forest in northern Canada: CO 2 released from the forest floor and CO 2 released from the entire ecosystem at night. Despite wide variation in the δ 13 C values of organic matter among above-ground plant species, and along a continuum from moss through to the mineral soil, the carbon Isotope Ratio of respired CO 2 was quite similar to the δ 13 C value for the dominant black spruce foliage. The CO 2 released from the forest floor during the fall was slightly enriched in 13 O compared to CO 2 respired by the entire ecosystem, perhaps because soil respiRation contributes a larger percentage to total ecosystem respiRation later in the year as the soil warms. Short-term changes in the Oxygen Isotope Ratio of precipitation and variation in enrichment of 18 O during evapoRation and transpiRation had significant effects on the δ 18 O value of respired CO 2 . Changes in the Oxygen Isotope Ratio of water in moss tissue can have a large effect on total ecosystem respired CO 2 both because a large surface area is covered by moss tissue in this ecosystem and because the equilibRation between CO 2 diffusing through the moss and water in moss tissue is very rapid. During the summer we observed that the δ 18 O value of CO 2 respired from the forest floor was relatively depleted in 18 O compared to CO 2 respired from the entire ecosystem. This was because water in black spruce foliage had higher δ 18 O values than moss and soil water, even at night when transpiRation had stopped. DOI: 10.1034/j.1600-0889.1999.00018.x
Rolf T W Siegwolf - One of the best experts on this subject based on the ideXlab platform.
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the effect of 18o labelled water vapour on the Oxygen Isotope Ratio of water and assimilates in plants at high humidity
New Phytologist, 2018Co-Authors: Marco M Lehmann, Gregory R Goldsmith, Lola Schmid, Arthur Gessler, Matthias Saurer, Rolf T W SiegwolfAbstract:Summary Our understanding of how temporal variations of atmospheric water vapour and its isotopic composition (δ18OV) influence water and assimilates in plants remains limited, restricting our ability to use δ18O as a tracer of ecophysiological processes. We exposed oak (Quercus robur) saplings under wet and dry soil moisture conditions to 18O-depleted water vapour (c. − 200‰) at high relative humidity (c. 93%) for 5 h, simulating a fog event. We then traced the step change in δ18OV into water and assimilates (e.g. sucrose, hexoses, quercitol and starch) in the leaf lamina, main veins and twigs over 24 h. The immediate δ18OV effect was highest for δ18O of leaf lamina water, but 40% lower on δ18O of main vein water. To a smaller extent, we also observed changes in δ18O of twig xylem water. Depending on the individual assimilation rate of each plant, the 18O-label was partitioned among different assimilates, with highest changes in δ18O of starch/sucrose and lowest in δ18O of quercitol. Additionally, 18O-label partitioning and allocation towards leaf starch and twig phloem sugars was influenced by the plant water status. Our results have important implications for water Isotope heterogeneity in plants and for our understanding of how the δ18O signal is incorporated into biomarkers.
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Inferring foliar water uptake using stable Isotopes of water
Oecologia, 2017Co-Authors: Gregory R Goldsmith, Marco M Lehmann, Lucas A. Cernusak, Matthias Arend, Rolf T W SiegwolfAbstract:A growing number of studies have described the direct absorption of water into leaves, a phenomenon known as foliar water uptake. The resultant increase in the amount of water in the leaf can be important for plant function. Exposing leaves to isotopically enriched or depleted water sources has become a common method for establishing whether or not a plant is capable of carrying out foliar water uptake. However, a careful inspection of our understanding of the fluxes of water Isotopes between leaves and the atmosphere under high humidity conditions shows that there can clearly be isotopic exchange between the two pools even in the absence of a change in the mass of water in the leaf. We provide experimental evidence that while leaf water Isotope Ratios may change following exposure to a fog event using water with a depleted Oxygen Isotope Ratio, leaf mass only changes when leaves are experiencing a water deficit that creates a driving gradient for the uptake of water by the leaf. Studies that rely on stable Isotopes of water as a means of studying plant water use, particularly with respect to foliar water uptake, must consider the effects of these isotopic exchange processes.
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Influence of atmospheric circulation patterns on the Oxygen Isotope Ratio of tree rings in the Alpine region
Journal of Geophysical Research: Atmospheres, 2012Co-Authors: Matthias Saurer, Anne Kress, Markus Leuenberger, Katja T. Rinne, Kerstin Treydte, Rolf T W SiegwolfAbstract:[1] The Oxygen Isotope Ratio of precipitation and tree rings is a complex function of climate variables and atmospheric dynamics, which often makes the interpretation of δ18O for palaeoclimate research challenging. Here we analyzed monthly precipitation δ18O series for 1973–2004 and annually resolved tree ring δ18O chronologies for 1945–2004 for three sites in Switzerland: one north of the Alps, one at high-elevation within the Alps, and one south of the Alps. The goal of the study was to improve the understanding of the tree ring archive by a systematic analysis of nonlocal parameters related to atmospheric circulation, in particular, geopotential height field anomalies and the frequency of synoptic weather situations, in addition to the usual local climate parameters like temperature, sunshine duRation, and relative humidity. We observed that on average high-pressure situations during summer were associated with relatively high δ18O and low-pressure situations were associated with relatively low δ18O, for both the Isotope Ratio in precipitation and tree rings. However, correlations to the frequency of weather types were not higher than simple correlations to local temperature. Accordingly, we constructed a combined index from temperature and air pressure that proved to be a good predictor of δ18O in precipitation and used this as the source water term in a tree ring Isotope fractionation model. This enabled us to use the model beyond the period where Isotope values for precipitation are available, opening new perspectives in the interpretation of long tree ring δ18O chronologies.
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Temperature versus species-specific influences on the stable Oxygen Isotope Ratio of tree rings
Trees, 2009Co-Authors: Christina E. Reynolds-henne, Matthias Saurer, Rolf T W SiegwolfAbstract:Stable isotopic Ratios integrate ecosystem variability while reflecting change in both environmental and biological processes. At sites, where climate does not strongly limit tree growth, co-occurring trees may display large discrepancies in stable Oxygen isotopic Ratios (δ18O) due to the interplay between biological processes (competition for light and nutrients, individual tree physiology, etc.) and climate. For a better quantification of the Isotope variability within and among trees, the climatic and/or individual tree effects on seasonal δ18O variations in precipitation, soil water, leaf water and leaf organic material (whole leaf, cellulose and starch) and annual δ18O variations in tree-ring cellulose for Fagus sylvatica (Fs), Quercus robur (Qr), Carpinus betulus (Cb) and Pinus sylvestris (Ps) were studied in a mature temperate forest in Switzerland, using a mixed linear regression model technique. Furthermore, the influence of environmental factors on δ18O was assessed by means of three common Isotope fractionation models. Our statistical analysis showed that except for Ps, a greater portion of δ18O variance in leaf compounds can be explained by individual tree effects, compared to temperature. Concerning tree-ring cellulose, only Fs and Ps show a significant temperature signal (maximum 12% of the variance explained), while the individual tree effect significantly explains δ18O for all species for a period of 38 years. Large species differences resulted in a limited ability of the Isotope fractionation models to predict measured values. Overall, we conclude that in a diverse mixed forest stand, individual tree responses reduce the potential extraction of a temperature signal from δ18O.
Libby A. Stern - One of the best experts on this subject based on the ideXlab platform.
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forensic utility of a nitrogen and Oxygen Isotope Ratio time series of ammonium nitrate and its isolated ions
Talanta, 2018Co-Authors: Brittany L Grimm, Libby A. Stern, Alexander J LoweAbstract:Abstract Ammonium nitrate (AN) based fertilizers are inexpensive and easily obtained, characteristics that often lead to their use in homemade explosive devices. The stable nitrogen and Oxygen Isotope Ratios ( 15 N/ 14 N and 18 O/ 16 O, expressed as δ 15 N and δ 18 O) of AN have the potential to aid in forensic investigations by providing supplemental properties for sample-to-sample comparison in materials which are otherwise chemically identical. The forensic utility of stable Isotope analyses depends on demonstrated variation between different sources and minimal variation within a source. To test the variability within a single manufacturer (here considered a source), a total of 26 samples representing two production time periods and two product lines were analyzed for bulk δ 15 N and δ 18 O. Additionally, because AN is known to have a modest isotopic range, a potassium nitrate precipitation method was developed to separate the component ions (NO 3 - and NH 4 + ) for individual δ 15 N analysis and increased discriminatory power. The average δ 15 N and δ 18 O of bulk AN (− 0.10‰ and + 22.8‰, respectively) is similar to the isotopic signature of atmospheric N 2 and O 2 , the starting reactants in AN production. The bulk δ 15 N, δ 18 O, and NO 3 - δ 15 N show average values from both product lines that differ by 1.5‰, 2.0‰, and 2.6‰, respectively, between the production periods of June and November 2015. Conversely, the NH 4 + δ 15 N remained relatively consistent over time. Furthermore, whereas samples in the two product lines produced on the same day in June are isotopically similar, there are isotopic differences between samples in the two product lines manufactured within 6 h of each other in November. The observed variability could be useful in comparing AN from two or more bombs, or a bomb and a stash of AN in a suspect's possession, but the observed lot-to-lot differences within one manufacturer could complicate attribution efforts. In contrast, the NH 4 + δ 15 N values, which appear to be the most consistent over time within this factory, need to be further explored as a potentially reliable signal.
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influence of soils on Oxygen Isotope Ratio of atmospheric co2
Global Biogeochemical Cycles, 2001Co-Authors: Libby A. Stern, Ronald Amundson, Troy W BaisdenAbstract:The soil component of atmospheric CO 2 Oxygen Isotope budget is evaluated in light of the recent recognition of abiotic Oxygen Isotope exchange of atmospheric CO 2 and soil water in excess of soil respiratory CO 2 flux. By using variations of published analytical models, we estimate that the amount of atmospheric CO 2 that undergoes Oxygen Isotope exchange with soil water, exclusive of soil-respired CO 2 , to be approximately 0.2-0.7 μmol m -2 s -1 for representative unsaturated soils from a range of biomes. Globally, the amount of atmospheric CO 2 that undergoes Oxygen Isotope exchange with soil water through purely nonbiological processes is probably significantly larger than the current annual fossil fuel combustion, yet this process has been neglected in all recent global 18 O-CO 2 budgets. Furthermore, abiotic Oxygen Isotope exchange with soils will occur roughly equally in soils with low and high respiRation rates, suggesting that soils with low respiRation rates are currently under-represented disproportionately in the existing global CO 2 Oxygen Isotope budgets. Because soils with low respiRation rates tend to have the most extreme soil water δ 18 O values, their underrepresentation may have a large and heretofore unsuspected impact on the global atmospheric C 18 O 16 O budget. Finally, soil carbon is no longer in steady state due to land use practices, and this additional source of CO 2 to the atmosphere may contribute to the decreasing trend in atmospheric CO 2 -δ 18 O values through time.
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Processes controlling the Oxygen Isotope Ratio of soil CO2: analytic and numerical modeling
Geochimica et Cosmochimica Acta, 1999Co-Authors: Libby A. Stern, W. Troy Baisden, Ronald AmundsonAbstract:The {sup 18}O/{sup 16}O Ratio of soil CO{sub 2} is important for both global and ecosystem scale budgets of the {sup 18}O/{sup 16}O Ratio of atmospheric CO{sub 2}, as well as for using this Ratio as a proxy for the isotopic Ratio of soil water. The {sup 18}O/{sup 16}O Ratio of soil CO{sub 2} reflects that of soil water due to the Isotope exchange which occurs during the CO{sub 2} hydRation reaction. The rate of this Isotope exchange reaction strongly influences the Oxygen Isotope Ratio of soil CO{sub 2}, and may be about two orders of magnitude slower than the Isotope exchange reaction between CO{sub 2}(aq) and water (effective first order rate constant, k{sub eff}, of 10{sup {minus}4} versus 10{sup {minus}2} s{sup {minus}1} at 25 C). This difference in rate reflects transport across the soil water/soil air interface, which limits the overall rate of reaction. A diffusion-production-reaction model (Hesterberg and Siegenthaler, 1991) suggests that the {sup 18}O/{sup 16}O Ratio of soil CO{sub 2} at depth deviates from the equilibrium value by {approximately}1--2{per_thousand} if k{sub eff} = 10{sup {minus}4} s{sup {minus}1}, negating the assumption that soil CO{sub 2} is in isotopic equilibrium with soil water. A sensitivity analysis of this modelmore » indicates the following factors affect {sup 18}O/{sup 16}O Ratio of soil CO{sub 2} (in order of decreasing importance): (1) the Isotope Ratio of soil water; (2) the rate constant of the Isotope exchange; (3) soil air-filled pore space and tortuosity. Notably, soil respiRation rate has only a minor effect. Advection of soil gas due to wind pumping has a negligible effect on the {delta}{sup 18}O value of soil CO{sub 2} except in most extreme cases. Likewise, transient effects on the Oxygen Isotope Ratio of soil CO{sub 2} due to changes in soil respiRation rate are small.« less
Lawrence B. Flanagan - One of the best experts on this subject based on the ideXlab platform.
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Oxygen Isotope RatioS OF WATERS AND RESPIRED CO2 IN AMAZONIAN FOREST AND PASTURE ECOSYSTEMS
Ecological Applications, 2005Co-Authors: Jean Pierre Henry Balbaud Ometto, Lawrence B. Flanagan, Luiz Antonio Martinelli, James R. EhleringerAbstract:The Oxygen Isotope Ratio (6180, SMOW) of atmospheric CO2 is a powerful indicator of large-scale CO2 exchange on land. Oxygen isotopic exchange between CO2 and water in leaves and soils controls the 8180 of atmospheric CO2. Currently there is little empirical information on the spatial and temporal variation in the s'80 of leaf and stem water in tropical ecosystems. We measured the seasonal dynamics of 6880 in atmospheric CO2 and water in different ecosystem compartments in both primary forest and pasture ecosystems in three different regions of the Amazonian Basin of Brazil (Ji-Parand, Manaus, and Santarem). Within regions, the source (stem) water 8180 values for primary forests and pastures were similar; neither vegetation type exhibited distinct wet-dry season patterns. Daytime leaf water Isotope Ratios were strongly correlated with predictions of the Craig- Gordon model. The s880 value of leaf water was positively correlated with leaf height above ground because of associated variation in vapor pressure deficit and the 8180 of atmospheric water vapor within forest canopies. Consistent with these observations, the s'1O value of leaf cellulose was positively correlated with forest height. Leaf water from pasture grasses was more 180 enriched than leaf water from forest vegetation. There was a tendency for daytime leaf water to be more enriched in 180 during the dry season, reflecting generally lower humidity conditions during the dry season. Nighttime measurements of the Oxygen Isotope Ratio of ecosystem respired CO2 in both forest and pasture vegetation were not consistent with values expected for CO2 in equilibrium with stem (soil) water, despite nighttime vapor pressure deficits close to zero. Apparently, the 6'1O of leaf water lagged and did not attain isotopic equilibrium at night. Thus, the deviation of nighttime 8180 values of ecosystem respiRation from that expected from a CO2 efflux in equilibrium with soil (stem) water increased as 6180 values of ecosystem respiRation became 180 enriched. Dis- crimination against CO2 containing 180 (AC'8OO) during photosynthesis was calculated based on measured leaf water 6180 values. Forests had consistently higher modeled AC'800 values than pastures. The daytime Isotope effects we calculate for photosynthesis and respiRation were consistent with previous model predictions of a strong depletion of 180 in atmospheric CO2 over the Amazon Basin of Brazil.
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spatial and temporal variation in the carbon and Oxygen stable Isotope Ratio of respired co2 in a boreal forest ecosystem
Tellus B, 1999Co-Authors: Lawrence B. Flanagan, David S Kubien, James R. EhleringerAbstract:We measured the stable Isotope Ratio of respired carbon dioxide at two spatial scales in a black spruce forest in northern Canada: CO 2 released from the forest floor and CO 2 released from the entire ecosystem at night. Despite wide variation in the δ 13 C values of organic matter among above-ground plant species, and along a continuum from moss through to the mineral soil, the carbon Isotope Ratio of respired CO 2 was quite similar to the δ 13 C value for the dominant black spruce foliage. The CO 2 released from the forest floor during the fall was slightly enriched in 13 O compared to CO 2 respired by the entire ecosystem, perhaps because soil respiRation contributes a larger percentage to total ecosystem respiRation later in the year as the soil warms. Short-term changes in the Oxygen Isotope Ratio of precipitation and variation in enrichment of 18 O during evapoRation and transpiRation had significant effects on the δ 18 O value of respired CO 2 . Changes in the Oxygen Isotope Ratio of water in moss tissue can have a large effect on total ecosystem respired CO 2 both because a large surface area is covered by moss tissue in this ecosystem and because the equilibRation between CO 2 diffusing through the moss and water in moss tissue is very rapid. During the summer we observed that the δ 18 O value of CO 2 respired from the forest floor was relatively depleted in 18 O compared to CO 2 respired from the entire ecosystem. This was because water in black spruce foliage had higher δ 18 O values than moss and soil water, even at night when transpiRation had stopped. DOI: 10.1034/j.1600-0889.1999.00018.x
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Discrimination against C18O16O during photosynthesis and the Oxygen Isotope Ratio of respired CO2 in boreal forest ecosystems
Global Biogeochemical Cycles, 1997Co-Authors: Lawrence B. Flanagan, J. Renee Brooks, Gregory T. Varney, James R. EhleringerAbstract:Our objective was to analyze factors that influence changes in the Oxygen Isotope Ratio (δ18O) of atmospheric CO2 within boreal forest ecosystems. We made measurements in the three major forest types (black spruce, jack pine, and aspen) at the southern and northern ends of the boreal forest in central Canada. This research was part of a larger study, the Boreal Ecosystem-Atmosphere Study (BOREAS). In terrestrial ecosystems the δ18O value of atmospheric CO2 is strongly influenced by Isotope effects that occur during photosynthesis and respiRation. Of primary importance is an equilibrium Isotope effect that occurs between Oxygen in CO2 and Oxygen in soil water and plant chloroplast water. During the equilibrium reaction the Oxygen Isotope Ratio of CO2 becomes enriched in 18O relative to that of water. We measured seasonal changes in the Oxygen Isotope Ratio of (1) water input to the ecosystems (precipitation), (2) water taken up by the major plant species from the soil (plant stem water), and (3) water in plant leaves. We used this information in calculations of Isotope discrimination during photosynthesis and soil respiRation. Discrimination against C18O16O during photosynthetic gas exchange (ΔA) (influenced by equilibRation with chloroplast water) averaged approximately 21‰ at midday and was similar for all forest types. In contrast, CO2 released during plant and soil respiRation had an average δ18O value of −14.4‰ but was less depleted in 18O than would be expected for respired CO2 in isotopic equilibrium with soil water. This effect was most pronounced in black spruce sites because of the extensive coverage of moss on the ground surface and the observation that water in the upper moss layers can have an Oxygen Isotope Ratio substantially different from water in deeper soil layers.
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influence of vegetation and soil co2 exchange on the concentRation and stable Oxygen Isotope Ratio of atmospheric co2 within a pinus resinosa canopy
Oecologia, 1995Co-Authors: Lawrence B. Flanagan, Gregory T. VarneyAbstract:Measurements were made of the concentRation and stable Oxygen isotopic Ratio of carbon dioxide in air samples collected on a diurnal basis at two heights within a Pinus resinosa canopy. Large changes in CO2 concentRation and isotopic composition were observed during diurnal time courses on all three symple dates. In addition, there was strong vertical stratification in the forest canopy, with higher CO2 concentRations and more negative δ18O values observed closer to the soil surface. The observed daily increases in δ18O values of forest CO2 were dependent on relative humidity consistent with the modelled predictions of isotopic fractionation during photosynthetic gas exchange. During photosynthetic gas exchange, a portion of the CO2 that enters the leaf and equilibrates with leaf water is not fixed and diffuses back out of the leaf with an altered Oxygen isotopic Ratio. The Oxygen Isotope Ratio of CO2 diffusing out of a leaf depends primarily on the 18O content of leaf water which changes in response to relative humidity. In contrast, soil respiRation caused a decline in the δ18O values of forest CO2 at night, because CO2 released from the soil has equilibrated with soil water which has a lower 18O content than leaf water. The observed relationship between diurnal changes in CO2 concentRation and Oxygen isotopic composition in the forest environment were consistent with a gas mixing model that considered the relative magnitudes of CO2 fluxes associated with photosynthesis, respiRation and turbulent exchange between the forest and the bulk atmosphere.
