Isotopic Discrimination Factors (Delta C-13 and Delta N-15) between Tissues and Diet of the Broad-Snouted Caiman (Caiman latirostris)

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Whole-rock geochemistry and Sr-Nd isotopic composition of the pre-rift sequence of the Camamu Basin, northeastern Brazil

Whole-rock geochemistry, combined with Sr-Nd isotopic composition of pelitic sedimentary rocks, have been considered to be useful parameters to estimate not only their provenance but also to make inferences about their depositional environment as well as the weathering processes they have been through. The basal sedimentary units of the basins of the northeastern Brazilian continental margin, particularly those of the pre-rift sequence, have been subject of interest of studies based on chemical and isotopic data, since they lack fossil content to establish their age and, therefore, stratigraphic correlations are difficult. The major and trace element contents as well as Sr-Nd isotopic compositions of whole-rock shale samples from five outcrops attributed to the pre-rift supersequence of the Camamu Basin were analyzed with the purpose of characterizing and obtaining further information that would allow a better correlation between the sites studied. The geochemical data suggest that the rocks exposed in the studied outcrops are part of the same sedimentary unit and that they might be correlated to the Capianga Member of the Alianca Formation of the Reconcavo Basin, exposed to the north of the Camamu Basin. The chemical index of alteration (CIA) suggests conditions associated with a humid tropical/subtropical climate at the time of deposition. Nd isotopic compositions indicate provenance from the Paleoproterozoic rocks of the Sao Francisco craton. The results presented here, therefore, show that the combined use of chemical and isotopic analyses may be of great interest to characterize and correlate lithologically homogeneous clastic sedimentary sequences. (C) 2012 Elsevier Ltd. All rights reserved.

Oceanic crust and island arc formation in Central Asia during late Neoproterozoic times - evidence from petrological and geochemical studies

Die vorliegende Arbeit behandelt die Entwicklung des 570 Ma alten, neoproterozoischen Agardagh - Tes-Chem Ophioliths (ATCO) in Zentralasien. Dieser Ophiolith liegt südwestlich des Baikalsees (50.5° N, 95° E) und wurde im frühen Stadium der Akkretion des Zentralasiatischen Mobilgürtels auf den nordwestlichen Rand des Tuvinisch-Mongolischen Mikrokontinentes aufgeschoben. Bei dem Zentralasiatische Mobilgürtel handelt es sich um einen riesigen Akkretions-Subduktionskomplex, der heute das größte zusammenhängende Orogen der Erde darstellt. Im Rahmen dieser Arbeit wurden eine Reihe plutonischer und vulkanischer Gesteine, sowie verschiedene Mantelgesteine des ATCO mittels mikroanalytischer und geochemischer Verfahren untersucht (Elektronenstrahlmikrosonde, Ionenstrahlmikrosonde, Spurenelement- und Isotopengeochemie). Die Auswertung dieser Daten ermöglichte die Entwicklung eines geodynamisch-petrologischen Modells zur Entstehung des ATCO. Die vulkanischen Gesteine lassen sich aufgrund ihrer Spurenelement- und Isotopenzusammensetzung in inselbogenbezogene und back-arc Becken bezogene Gesteine (IA-Gesteine und BAB-Gesteine) unterscheiden. Darüber hinaus gibt es eine weitere, nicht eindeutig zuzuordnende Gruppe, die hauptsächlich mafische Gänge umfasst. Der grösste Teil der untersuchen Vulkanite gehört zur Gruppe der IA-Gesteine. Es handelt sich um Al-reiche Basalte und basaltische Andesite, welche aus einem evolvierten Stammmagma mit Mg# 0.60, Cr ~ 180 µg/g und Ni ~ 95 µg/g hauptsächlich durch Klinopyroxenfraktionierung entstanden sind. Das Stammmagma selbst entstand durch Fraktionierung von ca. 12 % Olivin und geringen Anteilen von Cr-Spinell aus einer primären, aus dem Mantel abgeleiteten Schmelze. Die IA-Gesteine haben hohe Konzentrationen an inkompatiblen Spurenelementen (leichte-(L)- Seltenerdelement-(SEE)-Konzentrationen etwa 100-fach chondritisch, chondrit-normierte (La/Yb)c von 14.6 - 5.1), negative Nb-Anomalien (Nb/La = 0.37 - 0.62) und niedrige Zr/Nb Verhältnisse (7 - 14) relativ zu den BAB-Gesteinen. Initiale eNd Werte liegen bei etwa +5.5, initiale Bleiisotopenverhältnisse sind: 206Pb/204Pb = 17.39 - 18.45, 207Pb/204Pb = 15.49 - 15.61, 208Pb/204Pb = 37.06 - 38.05. Die Anreicherung lithophiler inkompatibler Spurenelemente (LILE) in dieser Gruppe ist signifikant (Ba/La = 11 - 130) und zeigt den Einfluss subduzierter Komponenten an. Die BAB-Gesteine repräsentieren Schmelzen, die sehr wahrscheinlich aus der gleichen Mantelquelle wie die IA-Gesteine stammen, aber durch höhere Aufschmelzgrade (8 - 15 %) und ohne den Einfluss subduzierter Komponenten entstanden sind. Sie haben niedrigere Konzentrationen an inkompatiblen Spurenelementen, flache SEE-Muster ((La/Yb)c = 0.6 - 2.4) und höhere initiale eNd Werte zwischen +7.8 und +8.5. Nb Anomalien existieren nicht und Zr/Nb Verhältnisse sind hoch (21 - 48). Um die geochemische Entwicklung der vulkanischen Gesteine des ATCO zu erklären, sind mindestens drei Komponenten erforderlich: (1) eine angereicherte, ozeaninselbasalt-ähnliche Komponente mit hoher Nb Konzentration über ~ 30 µg/g, einem niedrigen Zr/Nb Verhältnis (ca. 6.5), einem niedrigen initialen eNd Wert (um 0), aber mit radiogenen 206Pb/204Pb-, 207Pb/204Pb- und 208Pb/204Pb-Verhältnissen; (2) eine N-MORB ähnliche back-arc Becken Komponente mit flachem SEE-Muster und einem hohen initialen eNd Wert von mindestens +8.5, und (3) eine Inselbogen-Komponente aus einer verarmten Mantelquelle, welche durch die abtauchende Platte geochemisch modifiziert wurde. Die geochemische Entstehung der ATCO Vulkanite lässt sich dann am besten durch eine Kombination aus Quellenkontamination, fraktionierte Kristallisation und Magmenmischung erklären. Geodynamisch gesehen entstand der ATCO sehr wahrscheinlich in einem intraozeanischen Inselbogen - back-arc System. Bei den untersuchten Plutoniten handelt es sich um ultramafische Kumulate (Wehrlite und Pyroxenite) sowie um gabbroische Plutonite (Olivin-Gabbros bis Diorite). Die geochemischen Charakteristika der mafischen Plutonite sind deutlich unterschiedlich zu denen der vulkanischen Gesteine, weshalb sie sehr wahrscheinlich ein späteres Entwicklungsstadium des ATCO repräsentieren. Die Spurenelement-Konzentrationen in den Klinopyroxenen der ultramafischen Kumulate sind extrem niedrig, mit etwa 0.1- bis 1-fach chondritischen SEE-Konzentrationen und mit deutlich LSEE-verarmten Mustern ((La/Yb)c = 0.27 - 0.52). Berechnete Gleichgewichtsschmelzen der ultramafischen Kumulate zeigen grosse Ähnlichkeit zu primären boninitischen Schmelzen. Die primären Magmen waren daher boninitischer Zusammensetzung und entstanden in dem durch vorausgegangene Schmelzprozesse stark verarmten Mantelkeil über einer Subduktionszone. Niedrige Spurenelement-Konzentrationen zeigen einen geringen Einfluss der abtauchenden Platte an. Die Spurenelement-Konzentrationen der Gabbros sind ebenfalls niedrig, mit etwa 0.5 - 10-fach chondritischen SEE-Konzentrationen und mit variablen SEE-Mustern ((La/Yb)c = 0.25 - 2.6). Analog zu den Vulkaniten der IA-Gruppe haben alle Gabbros eine negative Nb-Anomalie mit Nb/La = 0.01 - 0.31. Die initialen eNd Werte der Gabbros variieren zwischen +4.8 und +7.1, mit einem Mittelwert von +5.9, und sind damit identisch mit denen der IA-Vulkanite. Bei den untersuchten Mantelgesteinen handelt es sich um teilweise serpentinisierte Dunite und Harzburgite, die alle durch hohe Mg/Si- und niedrige Al/Si-Verhältnisse gekennzeichnet sind. Dies zeigt einen refraktären Charakter an und steht in guter Übereinstimmung mit den hohen Cr-Zahlen (Cr#) der Spinelle (bis zu Cr# = 0.83), auf deren Basis der Aufschmelzgrad der residuellen Mantelgesteine berechnet wurde. Dieser beträgt etwa 25 %. Die geochemische Zusammensetzung und die petrologischen Daten der Ultramafite und Gabbros lassen sich am besten erklären, wenn man für die Entstehung dieser Gesteine einen zweistufigen Prozess annimmt. In einer ersten Stufe entstanden die ultramafischen Kumulate unter hohem Druck in einer Magmenkammer an der Krustenbasis, hauptsächlich durch Klinopyroxen-Fraktionierung. Bei dieser Magmenkammer handelte es sich um ein offenes System, dem von unten laufend neue Schmelze zugeführt wurde, und aus dem im oberen Bereich evolviertere Schmelzen geringerer Dichte entwichen. Diese evolvierten Schmelzen stiegen in flachere krustale Bereiche auf und bildeten dort meist isolierte Intrusionskörper. Diese Intrusionskörper erstarrten ohne Magmen-Nachschub, weshalb petrographisch sehr unterschiedliche Gesteine entstehen konnten. Eine geochemische Modifikation der abkühlenden Schmelzen erfolgte allerdings durch die Assimilation von Nebengestein. Da innerhalb der Gabbros keine signifikante Variation der initalen eNd Werte existiert, handelte es sich bei dem assimilierten Material hauptsächlich um vulkanische Gesteine des ATCO und nicht um ältere, möglicherweise kontinentale Kruste.

Isotope geochemical and geochronologic studies of basement units in the Central Western Carpathians (Slovakia)

In dieser Arbeit werden geochronologische und isotopen-geochemische Daten zur Entwicklung der Zentralen Westlichen Karpathen präsentiert. Die Karpathen bilden die östliche Fortsetzung der Alpen und können in drei Alpine Grundgebirgsdecken unterteilt werden, von denen zwei, die Veporische und die Gemerische, bearbeitet wurden. In der Veporischen Einheit wurden polymetamorphe Grundgebirgseinheiten untersucht, um deren genaue Altersstellung zu definieren und sie isotopengeochemisch zu klassifizieren. Dagegen wurde in der der Gemerischen Einheit, welche die Veporische Einheit überlagert, ein spezialisierter S-Typ Granit im Detail untersucht, um die petrogenetischen Prozesse, die zur magmatischen Entwicklung dieses Granits geführt haben, zu identifizieren. U-Pb Datierungen an Zirkonen der Veporischen Grundgebirgseinheiten zeigen für die gesamte Veporische Einheit ordovizische Entsehungsalter an (440-470 Ma). Diese Datierungen revidieren publizierte kambrische Entstehungsalter dieses Grundgebirges. Die Isotopensignatur (epsilon Nd und 87Sr/86Sr) der ordovizischen Grundgebirgseinheiten, bestehend aus stark überprägten Amphiboliten und Gneissen, ist von der Signatur der sich im Norden anschliessenden Tatrischen Einheit gut unterscheidbar. Die Bleiisotopenzusammensetzung dieser Gesteine ist stark krustal geprägt und überschneidet sich mit der der Tatrischen Einheit. Zusammen mit den T-DM Altern sind diese Einheiten vergleichbar mit prävariskischen Einheiten der Alpen. Somit kann das ordovizische Grundgebirge zu den peri-Gondwana Terranen gezählt werden, die an einem aktiven Kontinentalrand im Norden von Gondwana gebildet wurden. In den Gesteinen der Veporischen Einheit wurde im Weiteren eine starke metamorphe überprägung und intensiver felsischer Magmatismus karbonischen Alters erkannt (320-350 Ma). Dieses Ereignis ist zeitgleich mit dem Magmatismus, welcher hauptsächlich in der sich im Norden anschliessenden Tatrischen Einheit beobachtet wird. Dieser gehört der variskischen Orogenese an. Intensive alpine Deformation und Metamorphose konnte in der südlichen Veporischen Einheit anhand der Einzelzirkondatierungen und der Isotopendaten der ordovizischen Einheiten nachgewiesen werden. Am Dlha Dolina Granit in der Gemerischen Einheit können starke Fraktionierungs- und Auto-Metasomatose-Effekte beobachtet werden. Durch die magmatische Fraktionierung wird eine Anreicherung der SEE erzeugt, wogegen die Metasomatose die SEE stark verarmt. Es kommt sogar zur Ausbildung eines Tetraden Effektes im SEE Muster, welche den starken Einfluss von Fluiden während der spät-magmatischen Phase belegt. Gesamtgesteins Pb-Pb Daten beschränken das minimale Intrusionsalter dieses Granites auf 240 Ma. Dieses Alter ist in guter übereinstimmung mit den Sr-Isotopendaten der magmatisch dominierten Gesteine, wohingegen die stark metasomatisch geprägten Gesteine ein zu radiogenes 87Sr/86Sri aufweisen. Während dieser Arbeit wurde intensiv mit der Blei-Isotopenzusammensetzung von Gesamtgesteinsproben gearbeitet. Um die Auswertung dieser Daten optimieren zu können wurde ein Computerscript für das GPL Programm Octave erstellt. Die Hauptaufgabe dieses Scripts besteht darin, Regressionen für geochronologische Anwendungen gemäss York (1969) zu berechnen. Ausserdem können mu und kappa-Werte für diese Regressionen berechnet und eine Hauptkomponentenanalyse, welche hilfreich für den Vergleich von zwei Datensätzen ist, durchgeführt werden. Am Ende der vorliegenden Arbeit wird die analytische Methode für einen Mikrowellen beschleunigten Säureaufschluss von granitoidem Material zur Bestimmung der Sr- und Nd-Isotopenzusammensetzung und der Elementkonzentrationen vorgestellt. Diese kombinierte Methode nutzt ein TIMS für die Sr und Nd Isotopenmessungen und eine Einzelkollektor-ICPMS zur Bestimmung der SEE, Rb und Sr Konzentrationen, welche mithilfe von relativen Sensitivitätsfaktoren gegenüber einem internen Standard quantifiziert werden. Diese Methode wird durch Messungen von internationalen Referenzmaterialien bewertet. Die Ergebnisse zeigen eine Reproduzierbarkeit von <10% für die Elementkonzentrationen und von <5% für Elementverhältnisse.

NMR studies on supramolecular aggregates in solution and the solid state

The aim of this work presented here is the characterization of structure and dynamics of different types of supramolecular systems by advanced NMR spectroscopy. One of the characteristic features of NMR spectroscopy is based on its high selectivity. Thus, it is desirable to exploit this technique for studying structure and dynamics of large supramolecular systems without isotopic enrichment. The observed resonance frequencies are not only isotope specific but also influenced by local fields, in particular by the distribution of electron density around the investigated nucleus. Barbituric acid are well known for forming strongly hydrogen-bonded complexes with variety of adenine derivatives. The prototropic tautomerism of this material facilitates an adjustment to complementary bases containing a DDA(A = hydrogen bond acceptor site, D = hydrogen bond donor site) or ADA sequences, thereby yielding strongly hydrogen-bonded complexes. In this contribution solid-state structures of the enolizable chromophor "1-n-butyl-5-(4-nitrophenyl)-barbituric acid" that features adjustable hydrogen-bonding properties and the molecular assemblies with three different strength of bases (Proton sponge, adenine mimetic 2,6-diaminopyridine (DAP) and 2,6-diacetamidopyridine (DAC)) are studied. Diffusion NMR spectroscopy gives information over such interactions and has become the method of choice for measuring the diffusion coefficient, thereby reflecting the effective size and shape of a molecular species. In this work the investigation of supramolecular aggregates in solution state by means of DOSY NMR techniques are performed. The underlying principles of DOSY NMR experiment are discussed briefly and more importantly two applications demonstrating the potential of this method are focused on. Calix[n]arenes have gained a rather prominent position, both as host materials and as platforms to design specific receptors. In this respect, several different capsular contents of tetra urea calix[4]arenes (benzene, benzene-d6, 1-fluorobenzene, 1-fluorobenzene-d5, 1,4-difluorobenzene, and cobaltocenium) are studied by solid state NMR spectroscopy. In the solid state, the study of the interaction between tetra urea calix[4]arenes and guest is simplified by the fact that the guests molecule remains complexed and positioned within the cavity, thus allowing a more direct investigation of the host-guest interactions.

Water isotopic ratios from a continuously melted ice core sample

A new technique for on-line high resolution isotopic analysis of liquid water, tailored for ice core studies is presented. We built an interface between a Wavelength Scanned Cavity Ring Down Spectrometer (WS-CRDS) purchased from Picarro Inc. and a Continuous Flow Analysis (CFA) system. The system offers the possibility to perform simultaneuous water isotopic analysis of δ18O and δD on a continuous stream of liquid water as generated from a continuously melted ice rod. Injection of sub μl amounts of liquid water is achieved by pumping sample through a fused silica capillary and instantaneously vaporizing it with 100% efficiency in a~home made oven at a temperature of 170 °C. A calibration procedure allows for proper reporting of the data on the VSMOW–SLAP scale. We apply the necessary corrections based on the assessed performance of the system regarding instrumental drifts and dependance on the water concentration in the optical cavity. The melt rates are monitored in order to assign a depth scale to the measured isotopic profiles. Application of spectral methods yields the combined uncertainty of the system at below 0.1‰ and 0.5‰ for δ18O and δD, respectively. This performance is comparable to that achieved with mass spectrometry. Dispersion of the sample in the transfer lines limits the temporal resolution of the technique. In this work we investigate and assess these dispersion effects. By using an optimal filtering method we show how the measured profiles can be corrected for the smoothing effects resulting from the sample dispersion. Considering the significant advantages the technique offers, i.e. simultaneuous measurement of δ18O and δD, potentially in combination with chemical components that are traditionally measured on CFA systems, notable reduction on analysis time and power consumption, we consider it as an alternative to traditional isotope ratio mass spectrometry with the possibility to be deployed for field ice core studies. We present data acquired in the field during the 2010 season as part of the NEEM deep ice core drilling project in North Greenland.

Calcium isotopic composition of high-latitude proxy carrier Neogloboquadrina pachyderma (sin.)

The accurate reconstruction of sea surface temperature (SST) history in climate-sensitive regions (e.g. tropical and polar oceans) became a challenging task in palaeoceanographic research. Biogenic shell carbonate SST proxies successfully developed for tropical regions often fail in cool water environments. Their major regional shortcomings and the cryptic diversity now found within the major high latitude proxy carrier Neogloboquadrina pachyderma (sin.) highlight an urgent need to explore complementary SST proxies for these cool-water regions. Here we incorporate the genetic component into a calibration study of a new SST proxy for the high latitudes. We found that the calcium isotopic composition (δ44/40Ca) of calcite from genotyped net catches and core-top samples of the planktonic foraminifera Neogloboquadrina pachyderma (sin.) is related to temperature and unaffected by genetic variations. The temperature sensitivity has been found to be 0.17 (±0.02)‰ per 1°C, highlighting its potential for downcore applications in open marine cool-water environments. Our results further indicate that in extreme polar environments, below a critical threshold temperature of 2.0 (±0.5)°C associated with salinities below 33.0 (±0.5)‰, a prominent shift in biomineralization affects the δ44/40Ca of genotyped and core-top N. pachyderma (sin.), becoming insensitive to temperature. These findings highlight the need of more systematic calibration studies on single planktonic foraminiferal species in order to unravel species-specific factors influencing the temperature sensitivity of Ca isotope fractionation and to validate the proxies' applicability.

Organic vs. Conventional Grassland Management: Do N-15 and C-13 Isotopic Signatures of Hay and Soil Samples Differ?

Distinguishing organic and conventional products is a major issue of food security and authenticity. Previous studies successfully used stable isotopes to separate organic and conventional products, but up to now, this approach was not tested for organic grassland hay and soil. Moreover, isotopic abundances could be a powerful tool to elucidate differences in ecosystem functioning and driving mechanisms of element cycling in organic and conventional management systems. Here, we studied the delta N-15 and delta C-13 isotopic composition of soil and hay samples of 21 organic and 34 conventional grasslands in two German regions. We also used Delta delta N-15 (delta N-15 plant - delta N-15 soil) to characterize nitrogen dynamics. In order to detect temporal trends, isotopic abundances in organic grasslands were related to the time since certification. Furthermore, discriminant analysis was used to test whether the respective management type can be deduced from observed isotopic abundances. Isotopic analyses revealed no significant differences in delta C-13 in hay and delta C-13 in both soil and hay between management types, but showed that delta C-13 abundances were significantly lower in soil of organic compared to conventional grasslands. delta C-15 values implied that management types did not substantially differ in nitrogen cycling. Only delta C-13 in soil and hay showed significant negative relationships with the time since certification. Thus, our result suggest that organic grasslands suffered less from drought stress compared to conventional grasslands most likely due to a benefit of higher plant species richness, as previously shown by manipulative biodiversity experiments. Finally, it was possible to correctly classify about two third of the samples according to their management using isotopic abundances in soil and hay. However, as more than half of the organic samples were incorrectly classified, we infer that more research is needed to improve this approach before it can be efficiently used in practice.

The abundance and isotopic composition of Cd in iron meteorites

Cadmium is a highly volatile element and its abundance in meteorites may help better understand volatility-controlled processes in the solar nebula and on meteorite parent bodies. The large thermal neutron capture cross section of 113Cd suggests that Cd isotopes might be well suited to quantify neutron fluences in extraterrestrial materials. The aims of this study were (1) to evaluate the range and magnitude of Cd concentrations in magmatic iron meteorites, and (2) to assess the potential of Cd isotopes as a neutron dosimeter for iron meteorites. Our new Cd concentration data determined by isotope dilution demonstrate that Cd concentrations in iron meteorites are significantly lower than in some previous studies. In contrast to large systematic variations in the concentration of moderately volatile elements like Ga and Ge, there is neither systematic variation in Cd concentration amongst troilites, nor amongst metal phases of different iron meteorite groups. Instead, Cd is strongly depleted in all iron meteorite groups, implying that the parent bodies accreted well above the condensation temperature of Cd (i.e., ≈650 K) and thus incorporated only minimal amounts of highly volatile elements. No Cd isotope anomalies were found, whereas Pt and W isotope anomalies for the same iron meteorite samples indicate a significant fluence of epithermal and higher energetic neutrons. This observation demonstrates that owing to the high Fe concentrations in iron meteorites, neutron capture mainly occurs at epithermal and higher energies. The combined Cd-Pt-W isotope results from this study thus demonstrate that the relative magnitude of neutron capture-induced isotope anomalies is strongly affected by the chemical composition of the irradiated material. The resulting low fluence of thermal neutrons in iron meteorites and their very low Cd concentrations make Cd isotopes unsuitable as a neutron dosimeter for iron meteorites.

Microstructural, chemical and isotopic evidence for the origin of late neolithic leather recovered from an ice field in the Swiss Alps

Archaeological leather samples recovered from the ice field at the Schnidejoch Pass (altitude 2756 m amsl) in the western Swiss Alps were studied using optical, chemical molecular and isotopic (δ13C and δ15N of the bulk leather, and compound-specific δ13C analyses of the organic-solvent extracted fatty acids) methods to obtain insight into the origin of the leather and ancient tanning procedures. For comparison, leathers from modern native animals in alpine environment (red deer, goat, sheep, chamois, and calf/cow) were analyzed using the same approach. Optical and electron microscopically comparisons of Schnidejoch and modern leathers showed that the gross structure (pattern of collagen fibrils and intra-fibrils material) of archaeological leather had survived essentially intact for five millennia. The SEM studies of the hairs from the most important archaeological find, a Neolithic leather legging, show a wave structure of the hair cuticle, which is a diagnostic feature for goatskins. The variations of the bulk δ13C and δ15N values, and δ13C values of the main fatty acids are within the range expected for pre-industrial temperate C3 environment. The archaeological leather samples contain a mixture of indigenous (from the animal) and exogenous plant/animal lipids. An important amount of waxy n-alkanes, n-alkan-1-ols and phytosterols (β-sitosterol, sitostanol) in all samples, and abundant biomarker of conifers (nonacosan-10-ol) in the legging leathers clearly indicate that the Neolithic people were active in a subalpine coniferous forest, and that they used an aqueous extract of diverse plant material for tanning leather.

The stable carbon isotopic composition of Daphnia ephippia in small, temperate lakes reflects in-lake methane availability

Daphnia can ingest methane-oxidizing bacteria and incorporate methanogenic carbon into their biomass, leading to low stable carbon isotope ratios (expressed as δ13C values) of their tissue. Therefore, δ13C analysis of Daphnia resting eggs (ephippia) in lake sediment records can potentially be used to reconstruct past in-lake availability of methane (CH4). However, detailed multilake studies demonstrating that δ13C values of recently deposited Daphnia ephippia (δ13Cephippia) are systematically related to in-lake CH4 concentrations (CH4aq) are still missing. We measured δ13Cephippia from surface sediments of 15 small lakes in Europe, and compared these values with late-summer CH4aq. δ13Cephippia ranged from −51.6‰ to −25.9‰, and was strongly correlated with CH4aq in the surface water and above the sediment (r −0.73 and −0.77, respectively), whereas a negative rather than the expected positive correlation was found with δ13C values of carbon dioxide (CO2) (r −0.54), and no correlation was observed with CO2aq. At eight sites, offsets between δ13 CCO2 and δ13Cephippia exceeded offsets between δ13 CCO2 and δ13Calgae reported in literature. δ13Cephippia was positively correlated with δ13C values of sedimentary organic matter (r 0.54), but up to 20.7‰ lower in all except one of the lakes (average −6.1‰). We conclude that incorporation of methanogenic carbon prior to ephippia formation must have been widespread by Daphnia in our study lakes, especially those with high CH4aq. Our results suggest a systematic relationship between δ13Cephippia values and CH4aq in small temperate lakes, and that δ13Cephippia analysis on sediment records may provide insights into past changes in in-lake CH4aq.

The stable isotopic composition of Daphnia ephippia reflects changes in δ13C and δ18O values of food and water

The stable isotopic composition of fossil resting eggs (ephippia) of Daphnia spp. is being used to reconstruct past environmental conditions in lake ecosystems. However, the underlying assumption that the stable isotopic composition of the ephippia reflects the stable isotopic composition of the parent Daphnia, of their diet and of the environmental water have yet to be confirmed in a controlled experimental setting. We performed experiments with Daphnia pulicaria cultures, which included a control treatment conducted at 12 °C in filtered lake water and with a diet of fresh algae and three treatments in which we manipulated the stable carbon isotopic composition (δ13C value) of the algae, stable oxygen isotopic composition (δ18O value) of the water and the water temperature, respectively. The stable nitrogen isotopic composition (δ15N value) of the algae was similar for all treatments. At 12 °C, differences in algal δ13C values and in δ18O values of water were reflected in those of Daphnia. The differences between ephippia and Daphnia stable isotope ratios were similar in the different treatments (δ13C: +0.2 ± 0.4 ‰ (standard deviation); δ15N: −1.6 ± 0.4 ‰; δ18O: −0.9 ± 0.4 ‰), indicating that changes in dietary δ13C values and in δ18O values of water are passed on to these fossilizing structures. A higher water temperature (20 °C) resulted in lower δ13C values in Daphnia and ephippia than in the other treatments with the same food source and in a minor change in the difference between δ13C values of ephippia and Daphnia (to −1.3 ± 0.3 ‰). This may have been due to microbial processes or increased algal respiration rates in the experimental containers, which may not affect Daphnia in natural environments. There was no significant difference in the offset between δ18O and δ15N values of ephippia and Daphnia between the 12 and 20 °C treatments, but the δ18O values of Daphnia and ephippia were on average 1.2 ‰ lower at 20 °C than at 12 °C. We conclude that the stable isotopic composition of Daphnia ephippia provides information on that of the parent Daphnia and of the food and water they were exposed to, with small offsets between Daphnia and ephippia relative to variations in Daphnia stable isotopic composition reported from downcore studies. However, our experiments also indicate that temperature may have a minor influence on the δ13C, δ15N and δ18O values of Daphnia body tissue and ephippia. This aspect deserves attention in further controlled experiments.

Correlated cosmogenic W and Os isotopic variations in Carbo and implications for Hf-W chronology

An obstacle for establishing the chronology of iron meteorite formation using 182Hf-182W systematics (t1/2 = 8.9 Myr) is to find proper neutron fluence monitors to correct for cosmic ray modification of W isotopic composition. Recent studies showed that siderophile elements such as Pt and Os could serve such a purpose. To test and calibrate these neutron dosimeters, the isotopic compositions of W and Os were measured in a slab of the IID iron meteorite Carbo. This slab has a well-characterized noble gas depth profile reflecting different degrees of shielding to cosmic rays. The results show that W and Os isotopic ratios correlate with distance from the pre-atmospheric center. Negative correlations, barely resolved within error, were found between epsilo190Os-epsilo189Os and epsilo186Os-epsilo189Os with slopes of -0.64 ± 0.45 and -1.8(+1.9/-2.1), respectively. These Os isotope correlations broadly agree with model predictions for capture of secondary neutrons produced by cosmic ray irradiation and results reported previously for other groups of iron meteorites. Correlations were also found between epsilo182W-epsilo189Os (slope = 1.02 ± 0.37) and epsilo182W-epsilo190Os (slope = -1.38 ± 0.58). Intercepts of these two correlations yield pre-exposure epsilo182W values of -3.32 ± 0.51 and -3.62 ± 0.23, respectively (weighted average epsilo182W = -3.57 ± 0.21). This value relies on a large extrapolation leading to a large uncertainty but gives a metal-silicate segregation age of -0.5 ± 2.4 Myr after formation of the solar system. Combining the iron meteorite measurements with simulations of cosmogenic effects in iron meteorites, equations are presented to calculate and correct for cosmogenic effects on 182W using Os isotopes.

Oxygen and carbon isotopic composition of shells from Radix sp. in lakes across the Tibetan Plateau

The Tibetan Plateau (TP), including its surrounding mountain ranges, represents the largest store of ice outside the polar regions. It hosts numerous lakes as well as the head waters of major Asian rivers, on which billions of people depend, and it is particularly sensitive to climate change. The moisture transport to the TP is controlled by the Indian and Pacific monsoon and the Westerlies. Understanding the evolution of the interaction of these circulation systems requires studies on climate archives in different spatial and temporal contexts. The objective of this study is to learn more about the interannual variability of precipitation patterns across the TP and how different hydrologic systems react to different climatic factors. Aragonite shells of the aquatic gastropod Radix, which is widely distributed in the region, may represent suitable archives for inferring hydrologic and climatic signals in particularly high resolution. Therefore, sclerochronological studies of d18O and d13C ratios in Radix shells from seven lakes were conducted, each representing a different hydrologic and climatic setting, on a transect from the Pamirs across the TP. The shell patterns exhibit an increasing influence of precipitation and a decreasing influence of evaporation on the isotope compositions from west to east. d18O values of shells from lakes on the eastern and central TP (Donggi Cona, Yamdrok Yumco, Tarab Co) mirror monsoon signals, indicated by more negative values and higher variabilities compared to the more western lakes (Karakul, Bangong/Nyak, Manasarovar). In Yadang Co, located on the central southern TP, the monsoon rains did not reach the lake in the sampling year, although it is located in a region which is usually affected by monsoon circulation. The d18O values are used to differentiate the annual hydrological cycle into ice cover period, melt water period, precipitation period and evaporation period. d13C compositions in the shells particularly depend on specific habitats, which vary in biological productivity and in carbon sources. d18O and d13C patterns show a positive covariance in shells originating from large closed basins. The results show that Radix shells mirror general climatic differences between the seven lake regions. These differences reflect both regional and local climate signals in sub-seasonal resolution, without noticeable dependence on the particular lake system.