Identifying the δO signature of precipitation in grass cellulose and phytoliths: Refining the paleoclimate model

Cellulose and silica phytoliths were extracted from the leaves and stems of Calamovilfa longifolia, a C₄ grass, grown under varying climatic conditions across the North American prairies. The oxygen-isotope compositions of both cellulose and silica record a complex signal of the isotopic composition of the soil water that feeds the plants and the relative humidity conditions that influence transpiration rates, stomatal conductance, and ultimately the ¹⁸O-enrichment of leaf water. As the initial stages of cellulose formation occur in the leaves, cellulose in both the leaves and stems forms primarily from leaf water and does not differ greatly in its oxygen-isotope composition between these locations. In contrast, the δ¹⁸O values of leaf phytoliths are significantly enriched in ¹⁸O relative to stem phytoliths, reflecting the varying isotopic composition of the water in these tissues. The oxygen-isotope compositions of leaf cellulose may be used as a proxy for the isotopic composition of water involved in leaf phytolith formation, while the δ¹⁸O values of stem phytoliths can be used to determine the δ¹⁸O values of stem water involved in partial exchange reactions during the transport of carbohydrates through the plant. A comparison of the isotopic compositions of phytoliths with cellulose allows for the deduction of soil and leaf water δ¹⁸O values as well as temperature and relative humidity conditions during plant growth. This approach has application in paleoclimate studies that traditionally have required estimations of one or more of these variables because direct measurements were unavailable.     

Separating soil and leaf water O isotopic signals in plant stem cellulose

The oxygen-18 signal of soil and leaf water are both recorded in heterotrophically synthesized plant stem cellulose. Presently, these signals can only be teased apart with modeling and assumptions on the nature of the isotopic enrichment of leaf water. A method by which these two signals are chemically separated and analyzed is tested here. Heterotrophically synthesized cellulose from germinating seeds having a mixture of isotopic signals from the reserve carbohydrate (starch) and that of the water during cellulose synthesis was hydrolyzed and the resulting glucose converted to glucose phenylosazone. The analysis of the ¹⁸O/¹⁶O ratios of cellulose and of glucose phenylosazone were used to calculate the oxygen isotope ratio of the oxygen attached to the second carbon of the glucose moieties of the cellulose molecule. The calculated δ¹⁸O value of this oxygen was highly correlated with that of the water available for cellulose synthesis showing a nearly one-to-one relationship (slope = 1.027) and leading to the conclusion that it completely exchanges with water during heterotrophic cellulose synthesis. Once this method is refined so as to increase precision, it will be possible to derive the δ¹⁸O values of soil water available to plants from the oxygen isotope analysis of stem cellulose and its derivative.     

Oxygen and Hydrogen Isotopes of Waters in the Ordos Basin,China: Implications for Recharge of Groundwater in the North of Cretaceous Groundwater Basin

Hundreds of precipitation samples collected from meteorological stations in the Ordos Basin from January 1988 to December 2005 were used to set up a local meteoric water line and to calculate weighted average isotopic compositions of modern precipitation. Oxygen and hydrogen isotopes, with averages of ?7.8‰ and ?53.0‰ for δ18O and δD, respectively, are depleted in winter and rich in spring, and gradually decrease in summer and fall, illustrating that the seasonal effect is considerable. They also show that the isotopic difference between south portion and north portion of the Ordos Basin are not obvious, and the isotope in the middle portion is normally depleted. The isotope compositions of 32 samples collected from shallow groundwater (less than a depth of 150 m) in desert plateau range from ?10.6‰ to ?6.0‰ with an average of ?8.4‰ for δ18O and from ?85‰ to ?46‰ with an average of ?63‰ for δD. Most of them are identical with modern precipitation. The isotope compositions of 22 middle and deep groundwaters (greater than a depth of 275 m) fall in ranges from ?11.6‰ to ?8.8‰ with an average of ?10.2‰ for δ18O and from ?89‰ to ?63‰ with an average of ?76‰ for δD. The average values are significantly less than those of modern precipitation, illustrating that the middle and deep groundwaters were recharged at comparatively lower air temperatures. Primary analysis of 14C shows that the recharge of the middle and deep groundwaters started at late Pleistocene. The isotopes of 13 lake water samples collected from eight lakes define a local evaporation trend, with a relatively flat slope of 3.77, and show that the lake waters were mainly fed by modern precipitation and shallow groundwater.     

Isotopic variations of oxygen and hydrogen in groundwater of carbonate aquifer in an arid environment

The stable isotopic characteristics were used together with the total chloride to assess changes in groundwater from recharge zones into the carbonate aquifer in an arid environment. The aquifer under study represents a major source of groundwater and thermal springs in Al-Ain city, which are located at the northern part of Jabal Hafit in the United Arab Emirates (UAE). The relationship between oxygen and hydrogen isotopic composition of groundwater is established and is described by δD?=?2.2δ¹⁸O???9.96. The lower slope and y-intercept of groundwater samples relative to the local meteoric waterline suggests that the isotopic enrichment is due to the evaporation of shallow groundwater after recharge occurs. The majority of the shallow groundwater samples have a negative deuterium excess (d-excess) which might be ascribed to high a degree of evaporation, while most of the groundwater samples from deep wells, have a positive value of d-excess which may be related to a low degree of evaporation. The δ¹⁸O values of the thermal waters suggest enrichment towards δ¹⁸O of the carbonate rocks because of the exchange with oxygen at higher temperatures. A possible mixing between thermal or hot water and shallow groundwater is evident in some samples as reflected by δD vs. Cl and d-excess vs. δ¹⁸O plots.     

Paleoclimatic implications of the spatial patterns of modern and LGM European land-snail shell δO

The oxygen isotopic composition of land-snail shells may provide insight into the source region and trajectory of precipitation. Last glacial maximum (LGM) gastropod shells were sampled from loess from Belgium to Serbia and modern land-snail shells both record δ¹⁸O values between 0‰ and − 5‰. There are significant differences in mean fossil shell δ¹⁸O between sites but not among genera at a single location. Therefore, we group δ¹⁸O values from different genera together to map the spatial distribution of δ¹⁸O in shell carbonate. Shell δ¹⁸O values reflect the spatial variation in the isotopic composition of precipitation and incorporate the snails' preferential sampling of precipitation during the warm season. Modern shell δ¹⁸O decreases in Europe along a N-S gradient from the North Sea inland toward the Alps. Modern observed data of isotopes in precipitation (GNIP) demonstrate a similar trend for low-altitude sites. LGM shell δ¹⁸O data show a different gradient with δ¹⁸O declining toward the ENE, implying a mid-Atlantic source due to increased sea ice and a possible southern displacement of the westerly jet stream. Balkan LGM samples show the influence of a Mediterranean source, with δ¹⁸O values decreasing northward.     

Deuterium and oxygen 18 in precipitation and atmospheric moisture in the upper Urumqi River Basin, eastern Tianshan Mountains

The contribution of stable isotopes in meteorological, climatological and hydrological research is well known. This study analyzed the deuterium and oxygen 18 contents (δD and δ¹⁸O) of precipitation in event-based samples at three stations (Glacier No. 1, Zongkong, Houxia) along the upper Urumqi River Basin from May 2006 to August 2007. The δ¹⁸O in precipitation revealed a wide range and a distinct seasonal variation at all three stations, with enriched values occurring in summer and depleted values in winter. A statistically significant positive correlation was observed between the δ¹⁸O and δD and local surface air temperature, and better linear relationship existed between δ¹⁸O and air temperature than that of δD. This suggests that paleoclimatic archives relating to precipitation δ¹⁸O and δD can be useful for qualitative temperature reconstruction. The d-excess in precipitation also exhibited a seasonal variability. Based on NCEP/NCAR reanalysis data, three-dimensional isentropic back-trajectories in HYSPLIT model were employed to determine the moisture source for each precipitation event. Results indicate a dominant effect of westerly air masses in summer and the integrated influence of westerly and polar air masses in winter, and d-excess can be used as a sensitive tracer of the moisture transport history.     

Leaf cellulose δD and δO trends with elevation differ in direction among co-occurring, semiarid plant species

The potential to reconstruct paleoclimate from analyses of stable isotopes in fossil leaf cellulose could be enhanced by adequate calibration. This potential is likely to be particularly great in mid-latitude deserts, where a rich store of fossil leaves is available from rodent middens. Trends in δD and δ¹⁸O of leaf cellulose were examined for three species growing across climatic gradients caused by elevation and slope aspect in southeastern Utah, USA. The species differed in morphology (Pinus edulis vs. Yucca glauca), photosynthetic pathway (C₃Y. glauca vs. CAM Yucca baccata) or both (P. edulis vs. Y. baccata). The δD_LCN (leaf cellulose nitrate) and δ¹⁸O_LC (leaf cellulose) values of P. edulis decreased with elevation. Stem water δD values either increased (in spring) or did not change with elevation (in summer). Needle water δD values usually decreased with elevation and differed greatly with leaf age. These results suggest that δ cellulose values of P. edulis record the effects of climate on the isotopic composition of leaf water but not climate effects on meteoric water. In contrast to P. edulis, δD_LCN values of Y. glauca increased with elevation. The δ¹⁸O_LC values of Y. glauca also increased with elevation but less significantly and only on south-facing slopes. The δ cellulose values in both P. edulis and Y. glauca were most significantly related to changes in temperature, although temperature and precipitation were negatively correlated in the study area. Where all three species co-occurred, their δD_LCN values differed but their δ¹⁸O_LC values were the same. The disparity in δD_LCN between Y. baccata and the other species corresponds to differences in biochemical fractionations associated with photosynthetic pathway. Biochemical fractionations may also contribute to differences between the two C₃ species. Knowledge of factors affecting responses of individual plant species to environment may be required to infer climate from δD_LCN and δ¹⁸O_LC.     

Spatio-temporal variation of stable isotopes of river waters, water source identification and water security in the Heishui Valley (China) during the dry-season

Spatial variations of δD and δ¹⁸O among seven tributaries and their water sources were investigated in the Heishui Valley of the Yangtze River, China during the dry-season in 2004. A one-way ANOVA (analysis of variation) test showed that both δD (p?18O (p?=?0.045) spatially varied among the seven tributaries. The plot of δ¹⁸O versus δD for the river water collected at different locations showed that isotopic fractionation occurred during the snow and glacial melting process. The depleted δ¹⁸O and δD in the tributary waters distributed above the local meteoric water line (LMWL) suggested that the glacial and early snowpack meltwater largely recharged these streams during the early spring. The meltwater was isotopically distinguishable from the precipitation and river water, which had been evaporated during warmer and drier times. If glaciers and snow accumulation diminish with future climate warming, the recharge of these tributaries’ baseflow will decline and the security of the water resource in this watershed will be threatened.     

Investigation on the hydrodynamics of Ganga Alluvial Plain using environmental isotopes: a case study of the Gomati River Basin, northern India

An investigation using environmental isotopes (δ¹⁸O and δD) was conducted to gain insight into the hydrological processes of the Ganga Alluvial Plain, northern India. River-water, shallow-groundwater and lake-water samples from the Gomati River Basin were analyzed. During the winter season, the δ¹⁸O and δD compositions of the Gomati River water ranged from ?1.67 to ?7.62 ‰ and ?25.08 to ?61.50 ‰, respectively. Deuterium excess values in the river water (+0.3 to ?13 ‰) and the lake water (?20 ‰) indicate the significance of evaporation processes. Monthly variation of δ¹⁸O and δD values of the Gomati River water and the shallow groundwater follows a similar trend, with isotope-depleted peaks for δ¹⁸O and δD synchronized during the monsoon season. The isotopically depleted peak values of the river water (δ¹⁸O?=??8.30 ‰ and δD?=??57.10 ‰) can be used as a proxy record for the isotopic signature of the monsoon precipitation in the Ganga Alluvial Plain.     

Seasonal variability of oxygen and hydrogen stable isotopes in precipitation and cave drip water at Guilin,southwest China

The interpretation of climatic information from stalagmites has traditionally been a complex research problem, with oxygen isotopes playing a particularly important role in global climate change studies. This study investigates the relationship between oxygen isotope composition of the atmospheric in precipitation and cave drip water at Panlong cave in southwest China on seasonal timescales of variability. Time series seasonal variability was derived from Panlong cave in Guilin by collecting daily precipitation samples for stable isotope analysis during 2012. Results indicate that δ¹⁸O of precipitation contains a clear seasonal variation whereby higher values are mainly distributed during winter and lower values during summer. Seasonal variations in water sources affect the precipitation δ¹⁸O values. Drip water δ¹⁸O also displayed a seasonal cycle which is attenuated relative to δ¹⁸O of precipitation. Drip water time series display seasonal cycle ranges from 1.5 to 3.5 ‰ relative to Vienna Standard Mean Ocean Water, which mainly follow the precipitation δ¹⁸O seasonal cycle. Seasonal variation in drip water δ¹⁸O supports interpretations of the stalagmite δ¹⁸O record as a paleoclimate proxy sensitive to the local environment. This monitoring experiment revealed that drip water must be transported through the epikarst in approximately 1.5 months during cold periods, and <0.5 months during warm periods. Different residence time percolation is mainly affected by the atmospheric precipitation amount, depending on whether soil moisture reaches saturation.     

Climatic influences on the oxygen isotopic composition of biogenic silica in prairie grass

Samples of Calamovilfa longifolia were collected from across the North American prairies to investigate the relationship between the oxygen-isotope composition of biogenic silica (phytoliths) deposited in this grass and relative humidity, temperature, and the oxygen-18 enrichment of soil water relative to local precipitation. The δ¹⁸O values of silica in nontranspiring tissues were controlled by soil-water composition and temperature, whereas the oxygen-18 content of silica formed in leaf and inflorescence tissues was enriched further by transpiration. Accurate calculation of growing temperature was possible only when the oxygen-isotope compositions of both stem silica and soil water were known. However, the oxygen-isotope values of stem phytoliths can be used to calculate the variation in the isotopic composition of soil water across a North American temperature gradient.As plant organic matter decays and phytoliths are transferred to the soil, the temperature and soil-water signals carried by the oxygen-isotope composition of silica from nontranspiring tissues can be masked by the oxygen-18 enrichment of phytoliths from transpiring tissues. However, the overall oxygen-isotope composition of a soil-phytolith assemblage can be related to temperature using an empirical relationship based on temperature and the difference between soil-phytolith and estimated soil-water oxygen-isotope compositions.     

The relationship between phytolith- and plant-water δO values in grasses

Information regarding climatic conditions during plant growth is preserved by the oxygen-isotope composition of biogenic silica (phytoliths) deposited in grasses. The O-isotope compositions of phytoliths and the plant water from which they precipitate are dependent on soil-water δ¹⁸O values, relative humidity, evapotranspiration rates, and temperature. Plant water and phytoliths from two grass species, Ammophila breviligulata (C₃) and Calamovilfa longifolia (C₄) at Pinery Provincial Park in southwestern Ontario, Canada, were examined to determine the variability in their δ¹⁸O values. Stem water was unfractionated from soil-water in oxygen isotopic composition and the δ¹⁸O values of stem silica provide a good proxy for the soil water available to roots during the growing season. Greater spatial and temporal variation in the δ¹⁸O values of water in the top 5 cm of the soil, and their enhanced sensitivity to evaporative ¹⁸O enrichment, are reflected in the generally higher δ¹⁸O values of water in the shallow roots and rhizomes of these grasses. Water within the sheath and lower and upper leaf tissues experiences continual evaporation, becoming progressively enriched in ¹⁸O as it moves towards the tip of the leaf. However, the water from which leaf silica precipitates has not acquired the extreme ¹⁸O enrichment predicted using steady-state models, or measured for midday or average daily leaf water. Possible explanations for this behaviour include preferential deposition of silica at night; the existence of a secluded water fraction within the leaf, which experiences smaller diurnal variations in isotopic composition than leaf water at sites of evaporation; kinetic isotope effects during rapid precipitation of leaf silica; and incomplete exchange between the oxygen in the silicic acid and the leaf water.     

Oxygen and hydrogen isotope ratios in freshwater chert as indicators of ancient climate and hydrologic regime

The oxygen and hydrogen isotopic composition of Eocene and Miocene freshwater cherts in the western United States records regional climatic variation in the Cenozoic. Here, we present isotopic measurements of 47 freshwater cherts of Eocene and Miocene age from the Great Basin of the western United States at two different sites and interpret them in light of regional climatic and tectonic history. The large range of δ¹⁸O of terrestrial cherts measured in this study, from 11.2‰ to 31.2‰ (SMOW: Standard Mean Ocean), is shown to be primarily the result of variations in δ¹⁸O of surface water. The following trends and patterns are recognized within this range of δ¹⁸O values. First, in Cenozoic rocks of northern Nevada, chert δ¹⁸O records the same shift observed in authigenic calcite between the Eocene and Miocene that has been attributed to regional surface uplift. The consistent covariation of proxies suggests that chert reliably records and retains a signal of ancient meteoric water isotopic composition, even though our analyses show that chert formed from warmer waters (40°C) than coexisting calcite (20°C). Second, there is a strong positive correlation between δ¹⁸O and δD in Eocene age chert from Elko, Nevada and Salina, Utah that suggests large changes in lake water isotopic composition due to evaporation. Evaporative effects on lake water isotopic composition, rather than surface temperature, exert the primary control on the isotopic composition of chert, accounting for 10‰ of the 16‰ range in δ¹⁸O measured in Eocene cherts. From authigenic mineral data, we calculate a range in isotopic composition of Eocene precipitation in the north-central Great Basin of −10 to −14‰ for δ¹⁸O and −70 to −100‰ for δD, which is in agreement with previous estimates for Eocene basins of the western United States. Due to its resistance to alteration and record of variations in both δ¹⁸O and δD of water, chert has the potential to corroborate and constrain the cause of variations in isotope stratigraphies.     

Oxygen isotope ratios in fossil wood cellulose: Isotopic composition of Eocene- to Holocene-aged cellulose

We measured the δ¹⁸O of cellulose (δ¹⁸O_cel) extracted from fossil wood collected at 9 sites in the northern and southern hemispheres as a potential source of information about precipitation δ¹⁸O (δ¹⁸O_ppt) in the past and paleotemperatures. The samples had been buried in fluvial sediments for periods of time ranging from ca. 45 million to 250 years. At the oldest localities (high latitude, Eocene- through Pliocene-age sites in Canada and Russia), mean annual temperature (MAT) estimates derived from the modern relationship between MAT and δ¹⁸O_cel are 6-16 °C lower than the MAT estimates derived from other biological proxies. Estimates of Pleistocene and Holocene mean annual temperatures are close to the modern values at those sites. These results are consistent with other recent findings that the MAT/δ¹⁸O_ppt relationship across North America was not constant throughout the Cenozoic. Paleo-δ¹⁸O_ppt estimates derived from fossil cellulose and the modern North American relationship between δ¹⁸O_cel and δ¹⁸O_ppt are within the current annual range of δ¹⁸O_ppt values at all locations. The middle Eocene δ¹⁸O_ppt we determined from arctic cellulose samples (−21.9‰) is consistent with river water δ¹⁸O determined in two other studies (−19.1‰ to −22‰). These findings provide some evidence that a precipitation δ¹⁸O signal may be retained in wood cellulose during millions of years of burial, and that latitudinal patterns in δ¹⁸O_ppt may not have changed much during the past 45 Ma. These interpretations depend, of course, on the assumption that the isotopic composition of the cellulose has not changed during burial, an assumption for which it is difficult to gather direct evidence. XRD analysis shows that the crystalline form of the fossil cellulose we used to estimate paleoprecipitation δ¹⁸O and paleo-MAT is the same as that of modern cellulose, and that the samples are free of quartz and iron oxide contaminants that result in negative errors in measured δ¹⁸O_cel.     

A reexamination of cellulose carbon-bound hydrogen δD measurements and some factors affecting plant-water D/H relationships

The method of Epstein et al. (1976) for analysis of D/H ratios of cellulose carbon-bound hydrogen has been modified. This modified “renitration” method yields δD values which are in agreement with those obtained by the sodium chlorite delignification method. Comparison of results obtained by the renitration method with the published results of Epstein et al. (1976) indicate some differences in the δD values of individual samples. However, the overall plant-water δD relationship determined by Epstein et al., is not greatly changed upon redetermination by the renitration method. Additional data from a variety of plants representing a wide geographical range reveal that relative humidity is an important variable in determining the δD value of cellulose C-H hydrogen on this inter-regional scale. The role of relative humidity can be reasonably explained by a leaf water model that assumes an isotopic steady-state during transpiration. These results reaffirm the conclusion of Epstein et al., that the δD variations of the source water are the dominant control of the δD variations of cellulose C-H hydrogen from naturally grown plants. Thus, there is an expectation that these cellulose δD variations can have climate significance.     

Isotopic composition of cellulose from aquatic organisms

The stable isotopic ratios of oxygen, carbon and the non-exchangeable carbon-bound hydrogen of cellulose from marine plants (algae and higher vascular forms) and animals (tunicates) collected in their natural habitats and from freshwater vascular plants grown in the laboratory under controlled conditions were determined.The δ¹⁸Ovalues of cellulose from all the plants and animals were 27 ±3% more positive than the δ¹⁸O values of the waters in which the organisms grew. Temperature had little or no influence on this relationship for three species of freshwater vascular plants that were analyzed. The relationship between the δ¹⁸O values of cellulose and the water used in its synthesis is probably established by the isotopic fractionation that occurs during the hydration of carbonyl groups of the intermediates involved in cellulose synthesis.The δD values of the non-exchangeable hydrogen of cellulose (determined by analyzing cellulose nitrate) from different organisms that grew in the same environment differed by large amounts. This difference ranged up to 200‰ for different species of algae collected at a single site: the corresponding difference for different species of tunicates and vascular plants was 60 and 20‰ respectively. The δD values of cellulose nitrate from different species of freshwater vascular plants grown in water of constant temperature and isotopic composition differed by as much as 60‰ The relationship between the δD values of the carbon-bound hydrogen of cellulose and the water used in its synthesis displayed a significant temperature dependence for four species of freshwater vascular plants that were analyzed. The δD values of cellulose nitrate prepared from different parts of one of the plants grown under constant conditions differed by 40‰ Hydrogen isotopic fractionation during cellulose synthesis appears to be more variable among different species and displays a larger temperature dependence than was suggested by previous studies.     

Flux balance models for the oxygen and carbon isotope compositions of land snail shells

A simple flux balance model with a diffusive, evaporative boundary layer indicates that the time constant (characteristic time) for approach to oxygen isotope steady state in the body fluid of land snails is ∼19 min or less. These comparatively short times support an assumption that the snail’s aragonitic shell is commonly precipitated from a body fluid that is at, or near, isotopic steady state. The model indicates that the steady-state δ¹⁸O value of snail shell carbonate depends upon the temperature, relative humidity, δ¹⁸O of the input liquid water, and δ¹⁸O of ambient water vapor. Model shell δ¹⁸O values were calculated for the warm, wet months corresponding to times of snail activity at some European sites. Linear regression of these predicted values against published, measured values yielded the expression: δ¹⁸O_calc = 0.93(±0.13) δ¹⁸O_meas −0.9(±0.2), with r² = 0.65. As indicated by the value of r², there is scatter in the relationship, but the slope and intercept are close to one and zero, respectively, which lends credence to the model. Therefore, temporal or spatial changes recorded in the δ¹⁸O values of land snail shells appear to be selectively seasonal—commonly the warm, wet months—and include the effects of relative humidity.For carbon, the time constant for approach to isotopic steady state in the bicarbonate dissolved in the body fluid of land snails is predicted to be ∼16 min or less. New and published δ¹³C measurements of aragonite shell and associated organic matter exhibit an overall correlation, but with considerable scatter. As noted by previous workers, ¹³C-rich dietary “limestone” may account for some of the scatter. Additional scatter, according to the model presented herein, could arise from changes in the proportion of total oxidized carbon that is expelled by the snail as bicarbonate dissolved in body fluid (i.e., effects of relative changes in metabolic rates). These results affirm the need for caution in the interpretation of δ¹³C values of land snail aragonite shells solely in terms of dietary proportions of C₃ and C₄ plants.     

Karst springs as “natural” pluviometers: Constraints on the isotopic composition of rainfall in the Apennines of central Italy

This paper describes an indirect method to calculate the isotopic composition of rainfall by using the isotopic composition of karst springs fed by waters circulating in the most important regional aquifer of central Italy, i.e. the Mesozoic limestone sequence that forms the backbone of the Apennines. By using δ¹⁸O and δD data and the δ¹⁸O (and/or δD) average gradient for elevation, evaluated through the use of literature rainfall data and new measurements from a typical Alpine valley in northern Italy, the altitude of precipitation of their parent water has been re-calculated. Vertical descents of more than 2000 m, from recharge to discharge, have been assessed in some high flow-rate cold springs in the morphologically steep Adriatic sector of central Italy. A clear correlation between the vertical descents and more negative isotopic compositions at their relative emergence elevations is highlighted. In contrast, in the Tyrrhenian sector lower karstic drops (generally lower than 500 m) correlate with less negative isotopic composition of recharge areas.The δ¹⁸O iso-contour map of the “recalculated” parent rainfall in central Italy is more detailed than any possible isotopic map of rainfall made using pluviometers, unless large number of rainfall collectors were deployed on mountaintops. The data also show that the isotopic composition of rainfall depends on the source of the storm water. In particular, precipitation is isotopically heavier when originating in the Mediterranean Sea, and lighter when formed in the Atlantic Ocean. Consequently, the collision between air masses with such a different isotopic signature results in a relatively small latitudinal fractionation effect. The peninsular part of central Italy is very narrow, with several mountains and massifs more that 2000 m high, and any latitudinal variation in the isotopic composition between rainfall sourced in the Atlantic Ocean and in the Mediterranean Sea is much lower than that due to the isotopic fractionation due to elevation.     

O/O and D/H in goethite from a North American Oxisol of the Early Eocene climatic optimum

An Early Eocene Oxisol in the Ione Formation of California formed in a coastal continental weathering environment at a paleolatitude of ∼38°N. The dominant minerals in the Oxisol are goethite, quartz, and kaolinite. Material balance calculations were applied to new measurements of chemical composition, D/H, and ¹⁸O/¹⁶O ratios of Oxisol samples to determine the δD (−150 ± 3‰) and δ¹⁸O (−2.4 ± 0.3‰) values of the goethite (α-FeOOH). These data, in combination with the global meteoric water line (MWL), yielded an isotopic temperature of 21(±4) °C. The nominal value of 21 °C contrasts with the modern mean annual temperature (MAT) of 16 °C in that area. The warmer temperature is consistent with formation of the goethite during the Early Eocene climatic optimum. The isotopic composition of the goethite and a temperature of 21 °C imply ancient water with a δD value of −61(±4)‰ and a δ¹⁸O value of −8.9(±0.5)‰. This Early Eocene δ¹⁸O (or δD) value is more negative than values in the range of isotopic scatter observed for modern global precipitation at sites with a MAT of 21 °C.At times of warm global climates, the location of a near-surface atmospheric isotherm would generally shift relative to its location under modern climatic conditions. A simple Rayleigh-type condensation model indicates that, if one “follows the isotherm”, the associated scatter in δD and δ¹⁸O of precipitation in very warm global climates should shift (for a given isotherm) to more negative values that may be detectable in proxy records. The isotopic results from the goethite of the Early Eocene Oxisol appear to add to evidence in support of this idea.     

Delineating volcanic aquifer recharge areas using geochemical and isotopic tools

Relative recharge areas are evaluated using geochemical and isotopic tools, and inverse modeling. Geochemistry and water quality in springs discharging from a volcanic aquifer system in Guatemala are related to relative recharge area elevations and land use. Plagioclase feldspar and olivine react with volcanically derived CO₂ to produce Ca-montmorillonite, chalcedony and goethite in the groundwater. Alkalinity, Mg, Ca, Na, and SiO_2(aq) are produced, along with minor increases in Cl and SO₄ concentrations. Variations in groundwater δD and δ¹⁸O values are attributed to recharge elevation and used in concert with geochemical evolution to distinguish local, intermediate, and regional flow systems. Springs with geochemically inferred short flow paths provided useful proxies to estimate an isotopic gradient for precipitation (??.67 δ¹⁸O/100?m). No correlation between spring discharge and relative flow-path length or interpreted recharge elevation was observed. The conceptual model was consistent with evidence of anthropogenic impacts (sewage and manure) in springs recharged in the lower watershed where livestock and humans reside. Spring sampling is a low-budget approach that can be used to develop a useful conceptual model of the relative scale of groundwater flow (and appropriate watershed protection areas), particularly in volcanic terrain where wells and boreholes are scarce.