C51B-1026 0800h
Seasonality of precipitation and its effect on amount weighted annual average isotopic values: a sensitivity study
Many studies have highlighted temperature as the primary parameter controlling year-to-year variability in the concentration of stable isotopes of precipitation. However, for some regions, interpreting an isotope record solely with an isotope thermometer is shortsighted and possibly improper. The timing of precipitation throughout the year is a potentially potent influencing factor on annual isotopic values. For 63 sites within the United States we calculated time-series amount weighted annual isotopic values, based on 21 to 31 years of monthly precipitation data and site-specific sine curves idealizing seasonal isotope trends. The idealized isotopic trends were derived from four years of empirical isotopic data archived within the United States Network for Isotopes in Precipitation database and the precipitation data was sourced from the National Climate Data Center database. The derived time-series isolate the potential affect that the timing of precipitation, varying from one year to the next, may have on isotopic archives of annual resolution or greater.
http://www.nrel.colostate.edu/projects/usnip/
C51B-1027 0800h
Continuous and high resolution time series of water vapor 18O/16O ratio in surface air in New England
In this paper we report the results of the analysis of a high time resolution (1hr) continuous time series of water vapor 18O/16O ratio near the ground from December 2003 to December 2004. The observation is made with tunable diode laser analyzer and is calibrated with a dripping device that drips water of known isotope ratio in an evaporating flask. It is shown that during precipitation events the water vapor isotope ratio is in equilibrium with raindrops but is much higher than the equilibrium value during snow storms. The temporal variations in the vapor isotope ratio are influenced by regional airmass movement, land/sea breeze circulation, boundary layer entrainment and evaporation/transpiration cycle. A frontal hysteresis phenomenon is also observed.
http://pantheon.yale.edu/~xhlee/pubs.htm
C51B-1028 0800h
Deuterium Excess in Bolivian Precipitation: From Short-term Station Data to ice Core Records
We present here deuterium excess (d) variations in current Bolivian precipitation collected at both monthly and daily resolutions from September 1999 in the Zongo Valley. This valley is located on the northeast side of the Bolivian Cordillera Real, 40km northwesterly from the Illimani summit where an ice core has been drilled in 1999. Interestingly, monthly deuterium excess shows a clear seasonal cycle correlated with isotopes whereas at daily resolution this correlation is much less visible, suggesting different controls on d depending on time resolution. We interpret d variations in terms of possible recycling over Amazonia, different moisture sources between wet and dry season and re-evaporation of droplets during fall. We also examine if an "altitude effect" is shown over the andean region by comparing stations at different elevations. Then, we examine how deuterium excess varies in snow and ice at Illimani summit and we discuss the possible post-depositional processes influencing d. At last, we compare our results with simulations from 1- Rayleigh-based simple models and 2- the REMO meso-scale model focused on tropical South America.
C51B-1029 0800h
Stable Isotopes in Precipitation and Groundwater as Tracers for Climate Change in Central Australia
The IAEA GNIP listings of stable isotopes in precipitation reflect the cumulative effects of isotopic shifts accompanying the evaporative and condensation history of the sample, and are increasingly being used in the evaluation of General Circulation Models. At the same time, the properties of age-dated groundwater reflect the isotopic composition of precipitation at recharge and are contributing to paleoclimate studies in warm arid zones. In high latitudes, the isotope ratios in groundwater respond principally to paleo-temperature changes. However, in warm arid zones rainfall distribution patterns become increasingly important. Under these conditions, it is assumed that effective recharge occurs when rainfall intensities (mm/month) exceed a threshold value, and that the stable isotope ratios of the groundwater samples predominantly reflect the weighted mean of rainfall exceeding this threshold. Experimental evidence is presented to support this hypothesis, derived from a detailed comparison of GNIP data from Melbourne and Alice Springs and from a general survey of data from a range of inland stations around the globe. Results from the observational analysis indicate that: 1) the variance of the stable isotope ratio distribution in rainfall increases with aridity; 2) the more intense the rainfall (mm/month), the more depleted are the isotopes; and 3) the magnitude of this intensity effect increases with aridity. From a detailed comparison of the stable isotope ratios in rainfall and in age-dated groundwater from the Alice Springs region, it is concluded that the rainfall intensity threshold (mm/month) leading to effective recharge is about 80 mm/month but has been decreasing over the past 4000 years or so. This is consistent with independent paleoclimate evidence that conditions around Lake Eyre in Central Australia were more benign in the past. The use of stable isotopes to link rainfall distribution patterns with effective recharge in warm arid zones provides a basis for evaluating isotope enabled GCMs, and for assessing their potential in predicting the impact of climate change scenarios on groundwater sustainability in these critical regions.
C51B-1030 0800h
Stable-Isotope Signals Of Paleo-Winter Temperatures In Permafrost Ice Wedges Of Central Yakutia, Northeastern Siberia
In the scope of a joint German-Russian research project, dealing with the Late Quaternary development of climate and environment in the Verkhoyansk Mountains and the lowlands of central Yakutia in eastern Siberia, the stable isotope composition of ice wedges was studied. Ice wedges are hosted in ice-rich soils and sediments of the permafrost landscape, and mainly form during spring time, when meteoric waters released by snow melt fill frost cracks that opened during the winter. Repeated frost cracking through the years leads to lateral and vertical ice-wedge growth. The stable isotope signal of meteoric waters stored in the ice wedges can therefore be used as a paleoclimatic indicator of winter temperatures. From our collected data, coldest winter temperatures occurred between approximately 40 and 30 ka BP, as indicated by very light isotope composition of oxygen around -31 permill VSMOW and hydrogen around -235 permill VSMOW in a late Pleistocene ice wedge. The isotopic composition of lower Holocene ice wedges reflect relatively warm winter temperatures around 8.5 ka BP with values of around -26 permill VSMOW for oxygen and around -199 permill VSMOW for hydrogen. Young ground ice, formed between 2.3 and 0.7 ka BP, documents climate deterioration during the late Holocene with light isotope composition of around -28 permill for oxygen and around -215 permill VSMOW for hydrogen. Using the relationship between isotope signals in Recent ground ice and modern climate data of central Yakutia, a semiquantitative estimate of paleotemperatures can be inferred. Thus, the mean winter temperature during the late Pleistocene was 2 to 5 degrees Celsius colder than today, while the lower Holocene was probably up to 3 degrees Celsius warmer than today. During the upper Holocene, the mean winter temperature fluctuated between 3 degrees Celsius colder than today and 1.5 degree Celsius warmer than today. However, the inference of absolute paleotemperatures suffers from uncertainties in regard to kinetic fractionation processes that are hardly to assess.
C51B-1031 0800h
Relationship Between Atmospheric Circulation and Winter Precipitation d18O in Central New York State
Oxygen isotope values of meteoric precipitation (d18O) are strongly influenced by water vapor source and can therefore be used as a tool for the reconstruction of atmospheric circulation. However, this approach requires an understanding of how differing patterns of atmospheric circulation influence precipitation d18O values. This study examines the relationship between atmospheric circulation and winter precipitation d18O values in central New York State. Circulation back trajectories and d18O values for 132 precipitation samples were calculated and combined with an analysis of weather maps to determine the circulation type for each event. Lake effect precipitation generated the lowest d18O values, with low pressure over New England and northwesterly flow over the Great Lakes. Events with the highest d18O values were associated with low pressure north of New York and strong southerly flow within the warm sector. Less frequent were the Atlantic coastal and warm frontal overrunning events, both of which yielded relatively depleted precipitation.
C51B-1032 0800h
Isotopic composition and origin of snow over Siberia
The spatial isotopic distribution of snowpack over Siberia, where winter temperatures are as cold as those of the polar regions, was observed by the Trans-Siberian Snow Survey (TSSS) and Trans-Verkhoyansk Snow Survey Expedition (TVSSE) in March 2000 and March 2001. The results show inland dD depletion and a constant deuterium value d in snowpack over Siberia. To explore the relationship between source region variability and the isotopic composition of snow, a model simulation was performed that reproduced the observed isotopic composition of snow. Moisture sources for Siberian precipitation were estimated using the CCSR/NIES atmospheric general circulation model (AGCM). A simple isotopic model was used to evaluate the total isotopic changes during transport from the designated source region to the region of precipitation. The results showed that the variability of the contribution of each source to the snow results in the large isotopic variability, and the fact that the model reproduced the observed inland depletion of dD in snowpack suggests that GCM-predicted source contributions were verified by observed values. However, the modeled d values did not match observed values over Siberia. Observations of d values in precipitation show an increase during autumn towards a maximum in late autumn and then a decrease during winter; however the modeled d value reached a maximum in early autumn and decreased toward a minimum in winter. The simple isotope model does not consider additional moisture evaporation joining an airmass moving from a source region. Thus, the disagreement between the modeled and observed d values of snow suggest that moisture supplied from the land surface during transportation significantly contributes to autumn snow. The increased d values of Siberian snow show that evaporation from open water or from the soil surface, which are accompanied by isotopic fractionation,is more important than transpiration flux, which does not change the isotopic content. Contributions of land- derived moisture that has evaporated from open water play an important role in eastern Siberian snow.
C51B-1033 0800h
Tritium in Australian Precipitation: a 40 Year Record
Tritium, the radioisotope of hydrogen, directly incorporated into water molecules in the global hydrological system, is the most commonly used radioisotope indicator of groundwater recharge. Tritium in precipitation has been measured in Australia over the past 40 years, as an essential research tool in hydro-climate studies and to contribute to the Global Network for Isotopes in Precipitation (GNIP). Tritium, which as tritiated water (3H 1H O) is very mobile in the environment, delivers the benefit of tracing groundwater systems in a 10 - 20 year timeframe as a result of last century's atmospheric thermonuclear testing. The concentration of tritium in Australian precipitation reached a maximum level of 160 TU in 1963, during one of the most intense periods of nuclear testing. Our data reveal Australia experienced a `minor' bomb pulse compared to the Northern Hemisphere eg. in Ottawa, Canada a value of 6000 TU was recorded in 1963 for tritium in precipitation. From 1963 to 1980 we observe a rapid drop in the concentration of tritium, more than expected from natural decay, mainly due to the wash out of tritium into the oceans and groundwater. Since 1990 the levels of tritium have stabilised globally and regionally. Currently the levels of tritium in Australia have stabilised to 2 to 3 TU latitudinally across the continent, a factor of 10 lower than values observed at stations in the Northern Hemisphere. At present, levels of tritium in Australia appear to have ceased declining and our analyses suggest that today the tritium in precipitation is predominantly natural. We believe that it may be possible that the increased levels observed in the Northern Hemisphere, due to nuclear power generation [1] could `leak' into the Southern Hemisphere. This is important for research in Australia because it could hinder the exploitation of tritium in providing information on the origin and mechanism of recharge of shallow groundwaters and rivers [2]. 1. J.D. Happell, et al. A history of atmospheric tritium gas (HT) 1950-2002. Tellus(2004) 56B, 183-193. 2. D.J.M. Stone, et al. Investigation of Groundwater-Streamflow interactions in the Bega alluvial aquifer using tritium and stable isotopic ratios. ANA 2001, 4th Conference on Nuclear Science and Engineering in Australasia, pp 191-197, Sydney, NSW.
C51B-1034 0800h
SWING - The Stable Water Isotope Intercomparison Group
For more than four decades the isotopic composition of water stored in various archives (e.g. ice cores, ground water) has been used to study changes in the hydrological cycle on timescales from glacial-interglacial to short term variations. However, the interpretation of isotopic variations in terms of climate change is often handicapped by a lack of other relevant observational climate parameters (e.g. temperature, relative humidity, precipitation) both in space and time. Modeling the isotopic composition of water within the hydrological cycle of general circulation models (GCM) may help to overcome this deficit on available climate data. Isotope GCMs simulate the 18O/16O (and/or 2H/1H) relation as an independent quantity within a closed "model world" where all other relevant climate parameters are known, too. This enables an improved analysis of (simulated) isotope variability in terms of climate change and during the last decade several state-of-the-art GCM models (GISS, ECHAM, LMD, GENESIS and others) have been enhanced by the capability to simulate the atmospheric and/or ocean cycle of H218O and 1H2H16O. To ensure isotope GCM reliability, any isotope GCM simulation must pass a rigorous comparison of simulated versus observed isotope data (model-data-intercomparison). In addition, artifacts in isotope simulations based on a specific GCM model parameterization or set-up have to be identified by comparing simulation results of several different isotope GCMs (model-model intercomparison). In our presentation we will give an overview and first results of the SWING project, founded in 2004. This initiative serves as a community platform for experts from the various isotope research groups and has the following objectives: (1) enable an overview about ongoing isotope GCM modeling capabilities; (2) define and perform future common GCM isotope simulation experiments of the various research groups; (3) identify the most important need of new observational isotope data in space, time and the various aggregate forms of water; (4) strengthen the linkage between the modeling community and the key contributors of observational water isotope data; (5) serve as an interface to other model intercomparison studies.
http://www.bgc-jena.mpg.de/projects/SWING
C51B-1035 0800h
Estimation and Validation of \delta$^{18}$O Global Distribution with Rayleigh-type two Dimensional Isotope Circulation Model
A simple water isotope circulation model on a global scale that includes a Rayleigh equation and the use of _grealistic_h external meteorological forcings estimates short-term variability of precipitation $^{18}$O. The results are validated by Global Network of Isotopes in Precipitation (GNIP) monthly observations and by daily observations at three sites in Thailand. This good agreement highlights the importance of large scale transport and mixing of vapor masses as a control factor for spatial and temporal variability of precipitation isotopes, rather than in-cloud micro processes. It also indicates the usefulness of the model and the isotopes observation databases for evaluation of two-dimensional atmospheric water circulation fields in forcing datasets. In this regard, two offline simulations for 1978-1993 with major reanalyses, i.e. NCEP and ERA15, were implemented, and the results show that, over Europe ERA15 better matched observations at both monthly and interannual time scales, mainly owing to better precipitation fields in ERA15, while in the tropics both produced similarly accurate isotopic fields. The isotope analyses diagnose accuracy of two-dimensional water circulation fields in datasets with a particular focus on precipitation processes.
C51B-1036 0800h
Stable isotope values of rainfall and surface waters from Panama and Costa Rica
Our understanding of stable isotope values of tropical rainfall is limited. In physiographically-diverse areas like Panama and Costa Rica, isotope data from a few GNIP stations at sea level is not directly applicable to sites at different elevations and locations. Thus, physical and climatic variables that control rainfall isotope values in areas with significant relief such as Central America are poorly known. Because of this problem, we utilize surface water samples as a rainfall proxy to investigate the climatic controls on the stable isotope composition of waters from Panama and Costa Rica. We measured $\delta^{18}$O and $\delta$D on ~270 river and lake waters collected from Lake Nicaragua in the west to the Darien of Panama in the east, on both the Caribbean and Pacific slopes and intervening Cordilleras. We statistically analyzed the isotope data from Panama using correlation and multiple stepwise regression against environmental and geographic variables. GNIP Isotope data from Panama City were analyzed for temporal variability. Temporally, rainfall $\delta^{18}$O values are negatively correlated with precipitation amount. Spatially, surface water $\delta^{18}$O values decrease with distance from the Caribbean, an indication of progressive rainout as air masses traverse the isthmus. The Panama surface water line is defined as $\delta$D = (7.6 $\pm$ 0.09) x $\delta^{18}$O + (10.1 $\pm$ 0.6), statistically identical to that from Costa Rica. Respective Panama meteoric water lines based on monthly and annual data are $\delta$D = (7.4 $\pm$ 0.07) x $\delta^{18}$O + (5.5 $\pm$ 0.3), and $\delta$D = (8.2 $\pm$ 0.60) x $\delta^{18}$O + (10.6 $\pm$ 3.3). We conclude that the dominant control on rain and surface water $\delta^{18}$O values associated with the Central American Monsoon are temporal and spatial amount effects. Surface water $\delta^{18}$O values also show significant correlations with drainage basin parameters such as median and stream head elevation, and latitude and longitude. An equation derived from stepwise multiple regression relates $\delta^{18}$O values to various physical parameters and explains 74% of the observed isotopic variation. Weaker correlations were found between deuterium excess and physiographic variables. Our results support the use of spatial sampling of surface waters for use as a rainfall $\delta^{18}$O proxy, and will benefit ongoing paleoclimatic research in humid tropical regions.
C51B-1037 0800h
A new automated system for the rapid analysis of atmospheric water vapor samples for stable isotope composition.
An automated technique for measuring the isotopic composition ($\delta$D) of atmospheric water vapor is being developed at the University of New Mexico. Air is sampled using 12 mL glass vials with screw-on caps. Our analytical system flushes the sample vial with He, isolates the water vapor in a cold trap, and then reduces the water with carbon at $1300\deg$C to form H$_{2}$ and CO. Isotopic ratios are then measured in continuous flow using a Finnigan Delta plus XL mass spectrometer. With Albuquerque-area air, sample size is approximately 100 nanomoles of H$_{2}$O. Our system is robotic and interfaces with a commercially available autosampler. This enables us to analyze ~100 air samples per day with little supervision. Standards are prepared by sampling water vapor in equilibrium with waters of known isotopic composition within a glovebox. The advantage of such a system is that it will allow us to analyze atmospheric water vapor at temporal and spatial resolutions not practical with more time-consuming traditional techniques. Sample vials are compact, inexpensive, and easy to use, enabling air samples to be acquired with little effort. One potential application for this technique includes problems requiring high temporal resolution (and a high number of samples) such as diurnal cycles of evapotranspiration. Other applications include studies of the three-dimensional spatial distribution of $\delta$D in water vapor to assess water vapor transport pathways.
C51B-1038 0800h
Oxygen Isotope Ratios of Recently Fallen Snow Over the Wasatch Mountains of Northern Utah
Storm-to-storm variability of oxygen isotope ratios in recently fallen snow over the Wasatch Mountains of northern Utah are examined based on snow cores collected from the windward base (1650 m AMSL) to near the crest (3200 m AMSL) roughly 15 km away. Such observations provide insight into the origins and microphysical processing of hydrometeors falling along the windward slopes of this narrow, steeply sloped range. Oxygen isotope data is combined with local meteorological observations and trajectory analyses for each storm. No relationship is found between isotope ratio and crest level wind direction. The observations obtained are evaluated for their usefulness in determining the predominant source for storm moisture, whether marine or continental. Examples of the elevation dependence of oxygen isotope ratios, and their variability from storm to storm, will be presented and discussed.
C51B-1039 0800h
Intra-storm Variation in Oxygen and Hydrogen Isotope Ratios of Precipitation from Dallas, TX: 2001-2003
We report the results of precipitation sampling for $\delta$D and $\delta$$^1^8$O covering the years 2001-2003. Dallas, north central Texas, is located such that it samples moisture from the Gulf of Mexico and from fronts emanating from the polar region or the Pacific. Previous isotope hydrographs of White Rock Creek, Dallas, indicated departures from simple mixing along the local meteoric water line (MWL) consistent with variability in the slope and D excess of the precipitation in $\delta$D - $\delta$$^1^8$O space. In addition to sampling the precipitation, we measured amounts, surface temperature and water temperature for some samples. Over 1000 D-$^1^8$O pairs have been measured with up to 40 samples of a single storm to get variability on the 10-20 minute scale. Key results are: 1) On surface temperature-isotope plot, the total variability exhibited by Dallas precipitation captures a large portion the global mean ranges as reported in the IAEA data set. 2) The isotopic ratios measured in intra-storm events exhibit very little correlation with surface temperature. 3) The maximum decrease in the $\delta$$^1^8$O value within a single storm event is usually less than 10 per mil with no obvious large Rayleigh-type depletion events; with fraction of vapor remaining values of about 0.5. 4) This observation is confirmed by measurements of $\delta$D in water vapor versus 1/concentration that show a similar magnitude depletion of the heavy isotope, i.e. about 8x the oxygen isotope effect. 5) Within single storms, distinct cloud bands also exhibit distinct isotope ranges with different MWL slopes and intercepts suggestive of advective transport of water vapor. 6) A significant subset of the water vapor data also shows a 70 per mil variation in $\delta$D at nearly constant water vapor concentration also suggestive of advective transport of water associated with changes in wind direction. On a long term basis (here monthly means), there is a tendency for MWL slopes and deuterium excess to increase with decreasing surface temperature with MWL slopes $>$9 yielding D excesses as high as 20. The local meteoric water line for Dallas is $\delta$D=7 $\delta$$^1^8$O+8. Distribution diagrams for each of the 3 years have distinctive shapes with the wettest year exhibiting the narrowest distribution with Gulf moisture more dominant. The driest year produced the distribution with the largest spread in isotope ratios. These new results present a much more complex data set than what would be predicted by simple closed or open system models of precipitation.
C51B-1040 0800h
Variations of d18O in Precipitation Along Different Vapor Transport Paths of Asia
Three sampling cross-sections along the south path starting from the tropics through the vapor passage in Yunnan-Guizhou Plateau to the middle-low reaches of Yangtze River, the north path from west China, via north China, to Japan under the westerlies and the plateau path from South Asia over the Himalayas to the northern Tibetan Plateau, are set up, based on the IAEA/WMO global survey network and sampling sites on the Tibetan Plateau. The variations, and the relationship with precipitation and temperature, of the d18O in precipitation along the three cross-sections are analyzed and compared. Along the south path, the seasonal differences of mean d18O in precipitation are small at the stations located in tropics, but increase markedly from Bangkok towards the north, with the d18O in the rainy season smaller than that in the dry season. The d18O values in precipitation fluctuate on the whole, which shows that the different vapor sources. Along the north path, the seasonal differences of the mean d18O in precipitation for the stations in the west of Zhengzhou are all greater than that in the east of Zhengzhou. During the cold half-year, the mean d18O in precipitation reaches its minimum in Urumqi with the lowest temperature owing to the wide Cold-High Pressure over the Mongolia, then increases gradually with longitude, and keeps at a roughly same level at the stations eastward from Zhengzhou. During the warm half-year, the d18O values in precipitation are lower in the east than in the west, influencing by the summer monsoon over East Asia markedly. Along the plateau path, the mean d18O values in precipitation in the rainy season are correspondingly high in the south parts of the India Subcontinent, and then decrease gradually with latitude. A sharp depletion of the stable isotopic compositions in precipitation takes place due to the very strong rainout of the stable isotopic compositions in vapor in the process of the lifting on the southern slope of the Himalayas. The low level of the d18O in precipitation is from Nyalam to the Tanggula Mountains during the rainy season all the while, but d18O increases with increasing latitude persistently from the Tanggula Mountains to the northern Tibetan Plateau because of the replenishment of vapor with the relative heavy stable isotopic compositions originating from the inner Plateau. During the dry season, the mean d18O values in precipitation decrease basically along the path from the south to the north. Generally, the mean d18O in precipitation during the rainy season are lower than those in the dry season for the regions controlled by the monsoons over South Asia or the Plateau, on the contrary for the regions without monsoon or by weak monsoon. Key words: stable isotope; vapor transport path; temperature; precipitation
C51B-1041 0800h
Simulation of stable isotopic pools and fluxes by a land-surface scheme forced with observed isotopic ratios in precipitation and atmospheric water vapour.
Stable isotopes provide independent tools for evaluating key components of the hydrological and carbon cycles as simulated by land-surface schemes (LSS). The Project for Intercomparison of Land-surface Parameterisation Schemes (PILPS http://www.pilps.mq.edu.au) is initiating a new type of experiment (IPILPS) to assess the ability of LSSs to reproduce isotopic components of water and mass (carbon) budgets. The project aims to intercompare LSS simulations of diurnal and annual cycles of isotopic pools and fluxes, and to evaluate the performance of isotope-enabled LSSs under varying environmental conditions. The need for evaluation data is driving a new experimental effort concentrating on the measurement of stable water isotopes (SWI), in precipitation, atmospheric and canopy water vapour, soil water and leaf/stem water, on annual and diurnal time scales at three sites in the GEWEX CSE Amazon, Murray-Darling and Baltic Sea basins. We present diurnal and annual cycles of stable isotopes in the ecosystem as simulated by an isotope enabled LSS (ISOLSM)$^1$ over an agricultural pasture in Wagga Wagga (SE Australia). Climatological values of SWI in precipitation and water vapour, as well as continuous in situ D/H ratios of atmospheric water vapour obtained during a three-week field campaign, are used to force the LSS. The D/H ratio was measured using a fully automated and mobile Fourier Transform Infrared (FTIR)$^2$ spectrometer. The sensitivity of simulated isotopes (in soil water, plants and canopy air space, as well as isotopic exchanges between the land surface and the atmosphere) to the atmospheric forcing is analysed. The results highlight the importance of intensive field campaigns for measuring SWI in the environment as both forcing and evaluation data for land surface simulations. 1. A. Henderson-Sellers et al., 2004, Using stable water isotopes to evaluate basin-scale simulations of surface water budgets, in press, J. Hydrometeorol. 2. D.W.T. Griffith et al., 2002, Air-Land exchanges of CO2, CH4 and N2O measured by FTIR spectrometry and micrometeorological techniques, Atmos. Environ., 36, 1833-42.
C51B-1042 0800h
Isotopic composition of atmospheric water vapor observed at Taiga forest in Eastern Siberia
Recycling of water is important for the summer precipitation process in the inland region such as Eastern Siberia. The isotopic composition of summer precipitation at Eastern Siberia is much higher than that expected from its latitude and distance from the ocean. The authors clarified the sources of water for plants and water movement in the soil at Taiga forest using stable isotopes of water. The isotopic composition of the atmospheric water vapor was obtained in tower and aircraft observations. Aircraft and tower samplings were carried out in the field campaign during intensive observation period in 2000 at Yakutsk. Sampling using tethered balloon was also made several times during summer in 1998 to 2000. The isotopic composition of water vapor changed seasonally. The d18O of water vapor increased after leaf opening of the forest and the pattern of its variation after leaf opening was similar to that of precipitation. Vertical profile of water vapor d18O also changed before and after leaf opening. These results suggest importance of transpired water vapor through forest vegetation.
C51B-1043 0800h
The isotopic composition of water vapor and the concurrent Atmospheric circulations around Qilian Mountains in the northeast part of the Tibetan Plateau
The stable isotopic ratio of water vapor (d18O and dD) was measured near July-1st (Qiyi) Glacier (39.25N, 97.75E, 4250 m) in Northwest China. From 11 to 17 August 2003, we collected and then analyzed atmospheric samples at July-1st Glacier and a base camp (BC), that was 4km from Qiyi. Then the temporal change of the isotopic composition was compared with the atmospheric circulation fields using and European Centre for Medium-range Weather Forecasts (ECMWF) objective analysis dataset. After a snowfall in the early afternoon of the 14th, a strong northerly wind was observed in the evening. The isotopic analysis revealed a clear difference in the evening of 14th, which showed lighter dD (and d18O ) and much higher d-excess compared with those of the previous period. The snowfall and the strong northerly wind that followed were identified with the passing of a deep trough with a cold front that was observed at 500 hPa and dry air from the northwest. A back trajectory analysis revealed that the dry air from the northwest came from the upper troposphere/lower stratosphere. We also found a diurnal change in the isotopic composition. These results suggest that air from the Tibetan Plateau has relatively high d18O and dD values and relatively low d-excess values, while, the air from the northern desert has relatively small d18O, relatively small dD values and relatively high d-excess values.
C51B-1044 0800h
ISOSTORM: A High-Resolution Volunteer Network To Characterize the Isotopic Composition of Event-Based Precipitation Across North America
The oxygen and hydrogen isotopic composition of precipitation in the mid and high latitudes is very much related to the temperature of the air within which the precipitation forms. Consequently, time-averaged isotopic compositions correlate strongly with concurrent mean atmospheric temperatures. Substantial deviations from the expected temperature-isotope relationship are largely determined by the length of the averaging period and by the trajectory and history of the precipitating air mass (continental effects and orographic rainout). Consequently, detailed attribution of the deviations is difficult, especially when using the standard monthly precipitation samples, including those used by the Global Network for Isotopes in Precipitation (GNIP). We propose to establish a dense network of volunteers associated with automated weather stations at universities, colleges or schools across North America to collect daily, synchronized samples of precipitation for isotopic analysis. With the onset of a significant synoptic-scale storm (e.g., a large Colorado Low), the volunteers would be asked to collect daily samples for a week, or so, perhaps several times per year. Spatial and temporal analysis of the isotopic signatures in the precipitation will produce valuable insight into the effects of the atmospheric circulation on isotopic compositions, which in turn will provide better isotope-based palaeoclimate reconstructions (from tree-rings, for example).
C51B-1045 0800h
Using Stable Oxygen Isotopes to Partition Seasonal Precipitation Inputs in the Kolyma River.
The Kolyma River in northeastern Siberia is one of the five largest in Russia, with an annual discharge into the Arctic Ocean of 123 km$^{3}$, of which 90% occurs in the summer. It is unique among Eurasian rivers because the entire watershed area is underlain by continuous permafrost. Also, it has experienced the smallest increase of discharge over the last several decades. On average, the Kolyma river basin receives 300 to 500 mm of precipitation, of which, approximately 50% occurs in the summer. We analyzed Kolyma river water collected near the Northeast Science Station in Cherskii, Russia, from August 2002 through October 2003 for stable oxygen isotopic composition ($\delta^{18}$O) in order to partition the river flow into summer and winter precipitation inputs. The $\delta^{18}$O of the summer precipitation end-member was characterized by sampling 31 rain events at Cherskii during 2002 and 2003. Winter precipitation $\delta^{18}$O was estimated by a spatial survey of snow samples from 25 locations (within 300 km of Cherskii), collected from the top and bottom of the snowpack, prior to snowmelt on April 8-19, 2003. Inputs and standard deviations to a two end-member mixing model were -16 $\pm$ 5 $\permil$ (VSMOW) for summer and -26 $\pm$ 5 $\permil$ for winter. Throughout the ice-free season of river flow, the contribution from snow inputs to the Kolyma was approximately 55% with a standard error of 8%. (Error does not include bias based on restricted spatial sampling of precipitation in the watershed region.) During spring snowmelt (at the peak of the discharge pulse), snow inputs reached 84%. Weighting the fractions of rain/snow by discharge data from the Kolymskoye gauging station 160 km upstream of the collection site, for an entire year from October 2003 to 2004, yielded percent seasonal contributions to the total annual discharge of 38% summer rain and 63% winter snow with a standard error of 8%.
C51B-1046 0800h
Isolating the tropical Pacific and Atlantic influences upon simulated Amazonian hydrology and water isotopes
The ECHAM4 GCM outfitted with water isotope physics is used to assess the impacts of SST variations in the Pacific and Atlantic oceans upon isotopic variability of water vapor in the Amazon, the principal moisture source for Andean snowfall. A 128-year control experiment which imposes observed SSTs over the period 1870-1997 is compared to runs in which SST climatology prevails everywhere but in the tropical Pacific (TP) or in the tropical Atlantic (TA). As a result of variance reduction relative to the control, the TP run exhibits an enhancement of the interannual correlation of ENSO with tropical South American rainfall, and hence with the latter's isotopic composition via the amount effect. Analogously, a weak but significant positive correlation between Amazon rainfall and equatorial Atlantic SSTs is amplified in the TA run. Analysis of two rainfall datasets supports an Atlantic influence of this sign; possible mechanisms are discussed. Multidecadal variability in the Amazon is stronger in the TA than in the TP simulation, in contrast to the case for shorter timescales.
C51B-1047 0800h
Field measurements of isotope fractionation during evaporation
The isotopic fractionation associated with evaporation from bodies of water is an important process for the interpretation of isotope data from the atmosphere and in the development of ice core climate records. Existing theories include the resistance model of Craig and Gordon [1965] summarized by Gat [1996], and the exchange model of Merlivat and Jouzel [1979, MJ79]. The MJ79 model was recently extended by He and Smith [1999, HS99] to include a limited exchange between the liquid at depth and the air-liquid interface. These theories are relatively complex as they must include not only equilibrium fractionation but also the kinetic effects of diffusion and the influence of the "back-flux" of water vapor to the surface. Laboratory experiments by Craig et al. (1963), Merlivat and Coantic [1975], Merlivat [1978] and Cappa et al. (2003) have added new insight and generated proposals for parameterizations. Field testing of these theories and parametrizations has received little attention however. The primary difficulty to be overcome is that water vapor collected near the liquid interface is not representative of the evaporative flux, as it contains a significant proportion of vapor from the free atmosphere above. We overcome this problem using a two-level sampling technique developed by Keeling [1960] for carbon isotopes and applied to water vapor by Yakir and Wang [1996], Gat [2000], and He and Smith [2003] among others. Vapor collections were carried out over the Hudson River in the summer 2003 at altitudes of 12 and 190 centimeters. Samples from the two levels showed systematic differences in specific humidity and isotope ratio, allowing the isotope ratio in the net flux to be determined. In general, flux isotope ratios were significantly lighter than the collected vapor. Furthermore, the flux ratios are significantly lighter than that in equilibrium with river water samples. This small data set provides an opportunity to evaluate existing theories of isotope fractionation and to guide future ocean surveys of isotope flux ratios.
C51B-1048 0800h
Geochemical Characteristics And Zones Of Surface Snow On East Antarctic Ice sheet
Geochemical characteristics and zones of surface snow on east Antarctic Ice Sheet Jiancheng KANG1,4, Leibao LIU1, Dahe QIN2, Dali WANG1, Jiahong WEN1, Dejun TAN1, Zhongqin LI2, Jun LI3 & Xiaowei ZHANG1,4 1 Polar Research Institute of China, Shanghai 200129, China; 2 Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China; 3 Australian Antarctic Division and Antarctic Climate and Ecosystems CRC, Private Bag 80 Hobart, Tasmania, 7001, Australia; 4 Geography Department of Lanzhou University, Lanzhou 730000, China Correspondence should be addressed to Jiancheng KANG (email: kangjc@sh163.net, kangjc@126.com ) Abstract The surface-snow geochemical characteristics are discussed on the East Antarctic Ice Sheet, depending on the stable isotopes ratios of oxygen and hydrogen, concentration of impurities (soluble-ions and insoluble micro-particle) in surface snow collected on the ice sheet. The purpose is to study geochemical zones on the East Antarctic Ice Sheet and to research sources and transportation route of the water vapor and the impurities in surface snow. It has been found that the ratio coefficients, as S1, d1 in the equation OD = S1 O18O + d1, are changed near the elevation 2000m on the ice sheet. The weight ratio of Cl-/Na+ at the area below the elevation of 2000m is close to the ratio in the sea salt; but it is about 2 times that of the sea salt, at the inland area up to the elevation of 2000m. The concentrations of non-sea-salt Ca2+ ion (nssCa2+) and fine-particle increase at the interior up to the elevation 2000m. At the region below the elevation of 2000m, the impurity concentration is decreasing with the elevation increasing. Near coastal region, the surface snow has a high concentration of impurity, where the elevation is below 800m. Combining the translating processes of water-vapor and impurities, it suggests that the region up to the elevation 2000m is affected by large-scale circulation with longitude-direction, and that water-vapor and impurities in surface snow come from long sources. The region below the elevation 2000m is affected by some strong cyclones acting at peripheral region of the ice sheet, the sources of water and impurities could be at high latitude sea and coast. The area below elevation 800m is affected by local coastal cyclones. Keywords: Antarctic Ice Sheet, Snow, Geochemical Zones