Isotopic Composition Of Meteoric Water Research Articles

Isotopic exchange of oxygen, sulfur, hydrogen and copper between aqueous phase and the copper minerals brochantite, libethenite and olivenite

AbstractFractionation factors for the isotopes of O, H, S, or Cu (as appropriate) were determined for the minerals brochantite [Cu4(SO4)(OH)6], libethenite [Cu2(PO4)(OH)] and olivenite [Cu2(AsO4)(OH)] and corresponding aqueous solutions at temperatures between 30 and 70°C. All samples used for this determination were synthetic and the degree of fractionation was expressed as 1000 ln α = (A × 106/T2) + B, where A and B are empirical parameters. A few natural libethenite samples from its type locality Ľubietová-Podlipa were also analysed and compared to the prediction based on the isotopic composition of meteoric water and our fractionation factors. The hydrogen fractionation factors agreed with the prediction well, whereas those for oxygen did not. A possible explanation is the disequilibrium of aqueous phosphate (and also arsenate) species and the solution in our experiments or the interaction of meteoric fluids with the isotopically heavy (in terms of oxygen) country rocks. Because the effects of isotopic disequilibrium in our experiments cannot be ruled out, the oxygen fractionation factors should be used with caution. The determined fractionation factors can be used as an isotope geothermometer, given that it can be proven that the phases of interest precipitated from the same fluid in equilibrium. Libethenite is predicted to have slightly lower δ65Cu values than its parental solution, but brochantite slightly higher δ65Cu values than its parental solution. Simple forward models, simulating neutralisation or reduction of mine drainage, show that precipitation of these minerals and removal of the co-existing fluid, could cause isotopic variations (in δ65Cu) on the order of 1‰ or more.

Prolonged early to middle Holocene drought in the Pacific Northwest inferred from lacustrine carbonate oxygen isotope values and sedimentology

Sediment-based lake-level reconstructions and the oxygen isotopic composition of lacustrine carbonate (δ18OCO3) provide insight on hydroclimate conditions during the early Holocene in western North America and the Pacific Northwest. There are relatively few such records with decadal temporal resolution, and changes in the isotopic composition of meteoric water potentially obscure the hydroclimate signal (precipitation-evaporation) in lacustrine δ18OCO3 data. Here, we present a Holocene-length drought index developed by combining the δ18OCO3 record from Castor Lake, a closed-basin system located in north-central Washington (USA), with the δ18OCO3 record from Lime Lake, which has an open-basin configuration and is located in northeastern Washington. The lake sediment records are dated using primarily a combination of radiocarbon measurements on terrestrial macrofossils, Pb-210, Cs-137, and tephrochronology. The δ18OCO3 drought index, along with changes in Castor Lake sediment facies, suggest periods of prolonged drought in the Pacific Northwest between 11,200 and 6200 cal yr BP, a finding that is supported by other lacustrine proxy records of lake level and temperature change. Early Holocene drought likely resulted from higher than present summer isolation that promoted longer, hotter summers and greater evaporation. Lower than present winter insolation at this time likely altered the location and/or strength of the westerlies, reducing cold season precipitation/snowpack and intensifying the dry conditions. This study highlights the sensitivity of hydroclimate in an already drought- and fire-prone region to the magnitude and duration of summer warmth and precipitation and snowpack levels during the winter.

Regional meteoric water line of the Yucatan Peninsula, Mexico

AbstractThe data set represents the Regional Meteoric Water Line (RMWL) for the Yucatan Peninsula (Mexico), a region with karstic aquifers but little knowledge of the isotopic composition of meteoric water. This data paper comprises the isotopic composition of meteoric water collected from November 2018 to November 2019 as monthly composite samples expressed as weighted monthly mean precipitation from 16 locations across the Yucatan Peninsula. The RMWL was δ2H = 7.803 δ18O + 12.075, with a slope and intercept suggesting precipitation from air that has undergone condensation, air masses with variable moisture or recycled moisture. The average δ18O is −2.57‰ (−12.23 to 1.2‰) and δ2H is −7.94‰ (−83.39 to 18.32‰). This data set was collected by the implementation of an isotopic monitoring network, gathering information useful as a tool for better understanding of the hydrology and hydrogeochemistry of karstic aquifers, and to develop proxies for paleohydrology, ecohydrology, climate change and paleoclimate studies in the Gulf of Mexico and the Great Caribbean Area.

Lake water based isoscape in central-south Chile reflects meteoric water

Warming across the globe is expected to alter the strength and amount of regional precipitation, but there is uncertainty associated with the magnitude of these expected changes, and also how these changes in temperature and the hydrologic cycle will affect humans. For example, the climate in central-south Chile is projected to become significantly warmer and drier over the next several decades in response to anthropogenically driven warming, but these anthropogenic changes are superimposed on natural climate variability. The stable isotope composition of meteoric water provides significant information regarding the moisture source, pathways, and rain-out history of an air mass, but precipitation samples suitable for stable isotope measurements require long-term placement of field equipment making them difficult to obtain. The International Atomic Energy Agency (IAEA) Global Network of Isotopes in Precipitation (GNIP) stations generate isotopic and ancillary data of precipitation from many locations around the world, but remote areas of developing countries like Chile typically have sparse networks of meteorological stations, which inhibit our ability to accurately model regional precipitation. Central-south Chile, in particular, has a sparse network of GNIP stations and, as a result, the isotopic composition of meteoric water is underrepresented in the global database complicating efforts to constrain modern day hydroclimate variability as well as paleohydrologic reconstruction for southern South America. In this study, we measured the stable isotope compositions of hydrogen (δ2H) and oxygen (δ18O) in surface lacustrine waters of central-south Chile to determine what physical and/or climatic features are the dominant controls on lacustrine δ18O and δ2H composition, assess whether or not the isotopic composition of the lakes record time-averaged isotope composition of meteoric water, and determine whether an isoscape map based on lake surface waters could predict the H and O isotope compositions of precipitation at the few GNIP stations in the region.

Triple Oxygen Isotope Paleoaltimetry of Crystalline Rocks

Triple oxygen isotopes of hydrothermally altered minerals from crystalline rocks can be used to determine past elevations of mountain ranges. This method uses all three isotopes of oxygen (16O, 17O, and 18O) to create arrays that can be extrapolated back to the meteoric water line. One advantage of this technique is that it relies only on oxygen isotopes in contrast to previous studies that use oxygen and hydrogen isotopes to determine the isotopic composition of meteoric waters. Our analysis suggests that hydrogen isotopes may exchange with ambient fluids. Triple oxygen isotopes provide an independent check on the reliability of hydrogen isotope studies.

INTERCONECTIVIDADE DE AQUÍFEROS E OS EFEITOS DA DRENAGEM ÁCIDA DE MINA NA QUALIDADE DAS ÁGUAS DA BACIA CARBONÍFERA DE SANTA CATARINA (SC)

A atividade carbonífera no sul de Santa Catarina se desenvolve há mais de um século e a falta de gerenciamento ambiental nos empreendimentos mineiros causou significativas alterações de qualidade nos recursos hídricos da Bacia Hidrográfica do Araranguá. Além da alteração na qualidade das águas, ocorreram fenômenos de colapso e abatimento de minas subterrâneas que ocasionaram fraturamento das camadas sobrejacentes, e permitiram a ampliação dos processos de circulação dos sistemas aquíferos com corpos hídricos superficiais. Com objetivo de melhorar o entendimento destes processos, foram analisadas amostras das águas de 9 poços do Sistema Aquífero Rio Bonito, 4 poços do Sistema Aquífero de Leques Aluviais e 6 amostras de águas superficiais das mesmas microbacias onde estão instalados os poços. As análises hidroquímicas indicam uma predominância de águas bicarbonatadas cálcicas e sódicas, que quando em contato com a drenagem ácida de mina são transformadas em águas sulfatadas. A composição isotópica em termos de isótopos de oxigênio e de hidrogênio das amostras evidencia semelhança com a composição isotópica das águas meteóricas. Este fato contribui com a hipótese da forte interação da água subterrânea com as águas freáticas e superficiais, que ocorre devido à tectônica rúptil que afeta as litologias gonduânicas, e é intensificada pela presença das minas de subsolo.

The isotopic composition of meteoric water along altitudinal transects in the Tian Shan of Central Asia

The Tian Shan in Central Asia are a unique mountain range in that they are in the world's most continental location. Seasonal precipitation in the northern Tian Shan is segregated into distinct elevation bands where high elevations receive precipitation primarily during summer and low elevations to the north receive precipitation primarily during the late winter and spring. In this study, we sampled stream water along multiple altitudinal transects to determine the effect unique seasonal patterns of precipitation have on the isotopic composition of surface water. Our results suggest that the northern Tian Shan exhibits an isotopic lapse rate for waters sampled in late spring, but not for those sampled in late summer, when stream water budgets are dominated by high elevation precipitation and snow melt. Deuterium excess results suggest that subcloud evaporation significantly affects the isotopic composition of precipitation at low elevations in spring and that sublimation of snow has a minor impact on δ18O values of summer melt water. Because high and low elevation δ18O values are similar, conventional paleoaltimetry based on Rayleigh distillation of an air mass is not applicable to the Kyrgyz Tian Shan. Stream water proxies from the rock record are likely to reflect changes in the seasonal distribution of precipitation which occur on the same spatial scale as altitudinal changes. These results highlight the need to understand modern controls on local stable isotopes of meteoric water in reconstructions of past climate or topography using geologic proxy materials.

The Sensitivity of Terrestrial δ18O Gradients to Hydroclimate Evolution

AbstractTerrestrial gradients in the oxygen isotopic composition of meteoric water (δ18O), as reconstructed through proxies, reflect characteristics of ancient hydrologic conditions. These gradients are primarily influenced by the atmospheric transport of water vapor and the balance of precipitation and evapotranspiration, which are linked to climate and topography. We incorporate these effects into a one‐dimensional model that predicts the spatial evolution of δ18O based on local topography and the regional water‐energy budget. Specifically, we build on existing reactive transport models by incorporating parameterizations of orographic precipitation and energetic constraints on evapotranspiration following the Budyko water balance framework. We test our model on three modern transects that represent topographically distinct environments. These are the Amazon Basin (lowlands), the Cascade Range (mountains), and the eastern Himalayan Range (lowlands and mountains). Comparisons among these gradients demonstrate that the topographic regime determines how sensitive isotope records are to hydroclimate evolution. As a result, isotope records differentially express signatures of topography and the regional water balance, and we present a quantitative framework to predict this trade‐off. Finally, we link these effects to climate evolution and discuss how our model may help disentangle topographic and climatic signals through Earth history.

A Cenozoic terrestrial paleoclimate record from He dating and stable isotope geochemistry of goethites from Western Australia

Supergene goethites from the Hamersley region of Western Australia were analyzed for δ^(18)O and the results combined with both new and previously published (U-Th)/He ages to assess the paleoenvironmental signal recorded by surficial goethite in a long-term continental archive. Δ^(18)O_(goe) values increase monotonically by ∼3‰ between 71 Ma and 5 Ma and appear little affected by changes in temperature or other characteristics of depositional environment. The temporal trend likely reflects the isotopic composition of meteoric water from which the goethite formed, which varied as Australia migrated ∼3000 km northward over the Cenozoic. Translating age to paleolatitude reveals that the δ^(18)O_(goe) data are consistent with a latitudinal gradient in the δ^(18)O_(precip) that, throughout the Cenozoic, remained largely unchanged from the modern gradient. There is a 5‰ offset between the latitudinal trends of δ^(18)O_(goe) and modern precipitation, approximately as expected from the water-goethite fractionation factor. Temporal variation in the δ^(18)O of local meteoric water arising from changes in moisture source or pathway are not evident in the goethites, at least in part because topography is subdued and has evolved little in this tectonically quiescent landscape. Our results provide evidence that the efficiency of water transport from equator to pole that governs the latitudinal δ^(18)O gradient of precipitation has been roughly constant over southern temperate latitudes for the entirety of the Cenozoic.

Stable isotope variations (δ18O and δD) in modern waters across the Andean Plateau

Environmental parameters that influence the isotopic composition of meteoric water (δ18O and δD) are well characterized up the windward side of mountains, where orographic precipitation results in a predictable relationship between the isotopic composition of precipitation and elevation. The topographic and climatic evolution of the Andean Plateau and surrounding regions has been studied extensively by exploiting this relationship through the use of paleowater proxies. However, interpretation on the plateau itself is challenged by a poor understanding of processes that fractionate isotopes during vapor transport and rainout, and by the relative contribution of unique moisture sources. Here, we present an extensive dataset of modern surface water samples for the northern Andean Plateau and surrounding regions to elucidate patterns and causes of isotope fractionation in this continental environment. These data show a progressive increase in δ18O of stream water west of the Eastern Cordillera (∼1‰/70km), almost identical to the rate observed across the Tibetan Plateau, attributed to a larger fraction of recycled water in precipitation and/or increased evaporative enrichment downwind. This may lead to underestimates of paleoelevation, particularly for sites deep into the rainshadow of the Eastern Cordilleran crest. That said, elevation is a primary control on the isotopic composition of surface waters across the entire Andean Plateau and its flanks when considering the most negative δ18O values, highlighting the need for sufficiently large datasets to distinguish minimally evaporated samples. There is a general increase in δ18O on the plateau from north to south, concomitant with an increase in aridity and decrease in convective moistening (amount effect). Lastly, stable isotope and seasonal precipitation patterns suggest easterlies provide the vast majority of moisture that falls as precipitation across the Andean Plateau and Western Cordillera, from Peru to northern Bolivia (−13° to −20° latitude), with Pacific-derived moisture contributing a minor amount at low elevations near the coast.

Microbial controls on the origin and evolution of coal seam gases and production waters of the Walloon Subgroup; Surat Basin, Australia

The Walloon Subgroup coal seam gas (CSG) play in the Surat Basin, Queensland, is Australia's pre-eminent onshore gas field. Concerted multi-disciplinary research is underway investigating the distribution, origin and composition of waters and gases in this dominantly microbial CSG reservoir, to guide both continued production and potential microbially enhanced coal bed methane (MECoM) applications. However, prior to the present research, a detailed study of co-produced waters and gases from across the Surat Basin was not available in the public domain. This study tested whether co-produced water compositional and stable isotopic data show relationships with production gas stable isotope compositions, to elucidate further evidence for microbial CO2 reduction and explore the down-dip geochemical evolution of Walloon coal bed waters and gases. A total of 41 wells were sampled with 50 water and 25 gas samples spanning the 3 major production areas of the Surat Basin. Detailed isotopic and hydrochemical analysis of these samples revealed distinct spatial trends between the different production locales. Water compositions were distinct for each of the production regions reflecting the different lithologies of adjacent recharge zones, differing fluid–rock interactions, likely different microbial consortia, and the extent of methanogenesis. On the western side of the basin near Roma, waters were the ‘freshest’ with the lowest median values for alkalinity (861mg/L), and Cl− (588mg/L) and a δ13CDIC of 14.2‰. On the eastern side of the basin, the Kogan Nose waters were the most saline with the highest median values for Na+ (1955mg/L), Cl− (2280mg/L) and δ13CDIC (20.0‰). Also in the east, in the present gas fairway, the Undulla Nose waters had the highest median alkalinity (1841mg/L) and were found to have a Na+ excess (median=1050mg/L) and a lower than expected median δ13CDIC (14.0‰). Co-mingled, produced methane carbon isotope values (δ13C −57.0‰ to −44.5‰) from both the upper (Juandah) and lower (Taroom) coal measures plot within the mixed ‘thermogenic/microbial’ genetic field. By contrast, deuterium isotopic difference [Δ2H(H2O–CH4)] values and cross-plots of δ2H–H2O and δ18O–H2O suggest that microbially mediated CO2 reduction is the dominant methane generation process in situ. At a given depth, the Undulla Nose waters in the east are more depleted in 2H and 18O than elsewhere in the Surat Basin, which may suggest these samples have been more heavily impacted by water–rock–microbial reactions. 14C values from the 3 production regions (0.115 to 1.769pmC; age: 32,400 to >50,000years before present (B.P.)) suggest that Walloon coals likely recharged in the last ~50,000years (limit of radiocarbon dating). Consistent with these dates, δ2H–H2O and δ18O–H2O values for the Surat Basin (δ2H −32‰ to −56‰, δ18O −5.9‰ to −9.0‰) echo the stable isotopic composition of meteoric waters during the initial part of the last glacial period in southeast Queensland. Based on a strong correlation between δ2H–CH4 and δ2H–H2O, we suggest that methane was generated since the Late Pleistocene. PCA analysis showed a degree of positive correlation between total alkalinity and both the δ13CDIC (median 14.2‰) and δ13C–CH4 (median −52.1‰) vectors that is consistent with finite reservoir effects. The results inform ongoing studies of gas distribution and origins and MECoM potential in the Surat Basin, and underpin a broader study examining aquifer interactions.

Leaf wax stable isotopes from Northern Tibetan Plateau: Implications for uplift and climate since 15 Ma

The growth of Tibetan Plateau is considered to have played a key role during the evolution of Asian climate. Our understanding of the relationship between the plateau growth and Asian climate changes is limited, however, due to the scarcity of well-dated sedimentary sequences that could provide parallel information of the evolution of elevation and climate. Here, we report a high-resolution time series record of the stable hydrogen isotopic composition of leaf-wax n-alkanes (δDn-alk) from a continuous Neogene stratigraphic sequence (15–1.8 Ma) from the Qaidam basin on the northern Tibetan Plateau. These data are used to reconstruct the isotopic composition of meteoric waters (δDm) and subsequently applied to interpret the history of paleotopography and climate in Qaidam.Our results indicate four stages in the evolution of hydrology in the Qaidam basin. In Stage I (15 Ma to 10.4 Ma), δDm gradually decreases from −24.9‰ to −75.5‰, synchronous with a period of active tectonism. The estimated topographic growth of 2.1±0.3 km is comparable to the height of Qaidam basin relative to the foreland Hexi Corridor. We note that C3 plants were dominant in this region since the Miocene; we take this as independent evidence that this area was mountainous before the C4 expansion in late Miocene and Pliocene. δDm variability in subsequent stages appears to be related to shifts in dry and moist conditions and independent of topographical changes — a conclusion supported by other independent climatic records on the Tibetan Plateau. High δDm values in Stage II (10.4 Ma to 6.9 Ma) are related to severe aridity, and Stage III (6.9 Ma to 4.1 Ma) is marked by low δDm values, suggestive of moist conditions related to the strengthening East Asia Summer Monsoon. High δDm values in Stage IV (4.1 Ma to 1.8 Ma) reflect a climate, drier than the present.

Lignin methoxyl hydrogen isotope ratios in a coastal ecosystem

Stable hydrogen isotope ratios of plant lignin methoxyl groups have recently been shown to record the hydrogen isotopic composition of meteoric water. Here we extend this technique towards tracing water source variations across a saltwater to freshwater gradient in a coastal, subtropical forest ecosystem. We measure the hydrogen isotopic composition of xylem water (δDxw) and methoxyl hydrogen (δDmethoxyl) to calculate fractionations for coastal mangrove, buttonwood and hammock tree species in Sugarloaf Key, as well as buttonwoods from Miami, both in Florida, USA. Prior studies of the isotopic composition of cellulose and plant leaf waxes in coastal ecosystems have yielded only a weak correlation to source waters, attributed to leaf water effects. Here we find δDmethoxyl values range from −230‰ to −130‰, across a 40‰ range in δDxw with a regression equation of δDmethoxyl ‰=1.8*δDxw−178‰ (R2=0.48, p<0.0001, n=74). This is comparable within error to the earlier published relationship for terrestrial trees which was defined across a much larger 125‰ isotopic range in precipitation. Analytical precision for measurements of δD values of pure CH3I by gas chromatography–pyrolysis–isotope ratio mass spectrometry (GC–P–IRMS) is σ=6‰ (n=31), which is considerably better than for CH3I liberated through cleavage with HI from lignin with σ=18‰ (n=26). Our results establish that δDmethoxyl can record water sources and salinity incursion in coastal ecosystems, where variations sufficiently exceed method uncertainties (i.e., applications with δD excursions >50‰). For the first time, we also report yields of propyl iodide, which may indicate lignin synthesis of propoxyl groups under salt-stress.

Environmental conditions vs. landscape. Assessment of the factors that influence small mammal fauna distribution in Southern Iberia during the latest Messinian by mean of stable isotopes

Tropical conditions during the late Miocene in southwestern Europe influenced the continental environments. Although there are previous qualitative environmental interpretations of these continental landscapes during the late Miocene, quantitative environmental and paleohydrological data are scarce. A very long sequence of small mammals allowed to date the different continental drainage stages of the Granada Basin from latest Tortonian to the Holocene and to reconstruct qualitatively its environmental evolution. The study of the isotopic record of these fossil small mammals and sediments in the stratigraphic sequence of a latest Messinian paleo-lake in the Granada Basin has provided quantitative environmental and paleohydrological data, in order to know if the fluctuations in the faunal distribution were mainly influenced by the paleogeographic configuration, such as changes in the landscape of the basin, or by the climatic conditions. Open habitats with C3 plants predominated. Estimated quantitative data suggest an oxygen isotopic composition of meteoric waters ranging from −4.9 to −3.9‰, past temperatures from ~2.5°C to 4.7°C higher than the current ones, and summer relative paleohumidity from ~60% to ~70%. In this context, changes in the landscape, such as the reduction of the lacustrine system caused by tectonic activity, would affect taxa with high dependence on humid conditions.

High regional climate sensitivity over continental China constrained by glacial-recent changes in temperature and the hydrological cycle

The East Asian monsoon is one of Earth's most significant climatic phenomena, and numerous paleoclimate archives have revealed that it exhibits variations on orbital and suborbital time scales. Quantitative constraints on the climate changes associated with these past variations are limited, yet are needed to constrain sensitivity of the region to changes in greenhouse gas levels. Here, we show central China is a region that experienced a much larger temperature change since the Last Glacial Maximum than typically simulated by climate models. We applied clumped isotope thermometry to carbonates from the central Chinese Loess Plateau to reconstruct temperature and water isotope shifts from the Last Glacial Maximum to present. We find a summertime temperature change of 6-7 °C that is reproduced by climate model simulations presented here. Proxy data reveal evidence for a shift to lighter isotopic composition of meteoric waters in glacial times, which is also captured by our model. Analysis of model outputs suggests that glacial cooling over continental China is significantly amplified by the influence of stationary waves, which, in turn, are enhanced by continental ice sheets. These results not only support high regional climate sensitivity in Central China but highlight the fundamental role of planetary-scale atmospheric dynamics in the sensitivity of regional climates to continental glaciation, changing greenhouse gas levels, and insolation.

Isotopic composition of precipitation and groundwater in Sicily, Italy

The isotopic composition of meteoric water in Sicily, Italy was investigated from May 2004 until June 2006. Samples were sampled monthly from a network of 50 rain gauges. During the same period 580 groundwater samples were collected from springs and wells to obtain insight into the isotopic composition of the water circulating in the main aquifers of the area. The mean weighted precipitation values were used to define the weighted local meteoric water line for five different sectors of Sicily. The use of Geographical Information System tools, coupled with isotopic vertical gradients, allowed designing an isotopic contour map of precipitation in Sicily. The defined meteoric compositions were highly consistent with most of the groundwater samples in each sector. However, in some areas fractionation processes occurring during and after rainfall slightly modify the isotopic composition of the groundwater. The obtained data set defines the present day isotopic composition of meteoric water in the central Mediterranean area and provides baseline values for future climatic and/or isotope-based hydrology studies.

Effect of origin, breeding and processing conditions on the isotope ratios of bioelements in dry-cured ham

The stable isotope ratios (SIR) of the bioelements (2H/1H, 13C/12C, 15N/14N, 18O/16O, 34S/32S) of the defatted dry matter and marbling and subcutaneous fat fractions, were assessed on 86 ham samples belonging to six different types, with the aim of ascertaining the effect of origin and production system on 11 isotopic ratios. The ham types were obtained from pigs reared in three regions, examining in every location one different production factor at two levels of expression: pig genotype (local breed vs. industrial hybrid) in Friuli (Italy), pig feeding regime (Bellota vs. Campo) in Extremadura (Spain) and ham seasoning time (mid vs. end) in Emilia (Italy). The isotopic composition of meteoric water and the dietary abundance of C4 plants allowed to distinguish Italian PDO from Spanish hams. The contrasting treatments within the regional batches generated promising differences in SIR, potentially useful for tracing the whole ham production system, including the processing procedure.

Are two elements better than one? Dual isotope‐ratio detrending of evaporative effects on lake carbonate paleoelevation proxies

Stable isotope‐based proxy methods enhance our ability to interpret geodynamical histories for tectonic provinces via paleoelevational reconstructions. These methods require that the unmodified isotopic composition of meteoric water is recorded by authigenic minerals, a critical assumption that has not been tested across wide‐ranging environmental and topographical contexts. Here, we show that Quaternary lake carbonateδ18O values are not strongly, nor significantly, correlated with regional elevation due to the isotopic modification of in‐flow waters following entry into the lake environment. These modifications are largely caused by surface water evaporation, and can result in &gt;3 km errors in paleoelevation estimates if not accounted for. However, our analysis suggests that positive shifts in surface waterδ18O are accompanied by similar magnitude shifts in δ13C‐DIC. This positive co‐variation inδ18O and δ13C may be used to detrend lake carbonate compositions for the effects of surface water evaporation using a parameter we define here as the ‘13C‐excess’. When Tibetan lakes are excluded from the data set,13C‐excess values are significantly correlated with mean up‐slope hypsometric altitude with an error of ±500 m. Application of the13C‐excess approach to Cenozoic lake carbonate records from the western U.S. Cordillera both challenges and reinforces previous paleoelevational interpretations based onδ18O alone.

D/H variation in terrestrial lipids from Santa Barbara Basin over the past 1400 years: A preliminary assessment of paleoclimatic relevance

We analyzed D/H ratios of common terrestrial leaf wax lipids in a 1400 year sediment core from the Santa Barbara Basin (SBB) to test whether they accurately record terrestrial climate in Southern California. The D/H ratios of long chain n-alkanes vary substantially with depth, but are poorly correlated with other terrestrial climate proxies. Interference from fossil hydrocarbons may be at least partly responsible. Long chain n-alkanoic acids exhibit nearly constant downcore D/H ratio values. This constancy in the face of known climatic shifts presumably reflects a substantial residence time for leaf wax compounds in terrestrial soil and/or on the basin flanks. Alternatively, the isotopic composition of meteoric waters in Southern California may not covary with climate, particularly aridity. However, the δD values of n-C 22 and n-C 24 fatty acids, commonly attributed to terrestrial aquatic sources, are partially correlated with Southern California winter Palmer Drought Severity Index, a tree ring-based climatic proxy ( R 2 0.25; p < 0.01) on multi-centennial scales with an inferred ca. 215 year time lag. The improved correlation of these biomarkers can be explained by the fact that they are not stored in terrestrial soil nor are subject to interference from fossil hydrocarbons. Our study indicates that the SBB is unlikely to preserve high resolution leaf wax D/H records that can serve as quantitative paleoclimate proxies, though some qualitative information may be retained. More generally, the sources of lipids in marginal marine basins need to be carefully evaluated prior to attempting paleoclimate reconstruction based on the leaf wax D/H proxy.

Estimating the Oxygen Isotopic Composition of Equatorial Precipitation During the Mid-Cretaceous

Abstract Recent studies utilize the oxygen isotopic composition of pedogenic carbonates (siderite and calcite) to evaluate empirically the global hydrologic cycle during the Cretaceous. Pedogenic carbonates are used as a proxy for the isotopic composition of meteoric waters in order to estimate precipitation rates in the Aptian–Albian via a mass-balance model. These modeling studies have previously been limited to mid-latitude to high-latitude data, and thus a more extensive latitudinal data set is needed to produce a more globally constrained model. This study provides an equatorial (~ 2° S paleolatitude) data set from pedogenic sphaerosiderites of the Caballos Formation from the Upper Magdalena Valley, Colombia. In addition, macroscopic and microscopic morphological features of paleosols were used to identify ancient soil conditions. The gleyed appearance and presence of sphaerosiderites suggest that soils were dominated by water-saturated conditions, with occasional incursions of marine-influenced water resulting in pyrite precipitation. This observation constrains the sampling sites to paleoelevations near sea level. Paleosols are interpreted to have been Entisols, Inceptisols, and Ultisols similar to soils in coastal equatorial regions of modern Colombia. Relatively invariant δ18O values and more variable δ13C values produce vertical trends in δ13C vs. δ18O space called meteoric sphaerosiderite lines. The average oxygen isotopic composition for these trends is −4.41 ± 0.37‰ (VPDB). The average oxygen isotopic composition of meteoric waters precipitating the sphaerosiderites is estimated at −4.56 ± 0.38‰ (VSMOW). These are slightly more enriched than modern and modeled Cretaceous estimates of equatorial precipitation δ18O but are generally in agreement with expected oxygen isotopic compositions of precipitation for this paleolatitude.