Oxygen Isotope Signatures Research Articles

Clumped isotope thermometry of carbonatites as an indicator of diagenetic alteration

We measure the clumped isotopic signature of carbonatites to assess the integrity of the clumped isotope paleothermometer over long timescales (10 7–10 9 years) and the susceptibility of the proxy to closed system re-equilibration of isotopes during burial diagenesis. We find pristine carbonatites that have primary oxygen isotope signatures, along with a Carrara marble standard, do not record clumped isotope signatures lighter than 0.31‰ suggesting atoms of carbon and oxygen freely exchange within the carbonate lattice at temperatures above 250–300 °C. There is no systematic trend in the clumped isotope signature of pristine carbonatites with age, although partial re-equilibration to lower temperatures can occur if a carbonatite has been exposed to burial temperatures for long periods of time. We conclude that the solid-state re-ordering of carbon and oxygen atoms is sufficiently slow to enable the use of clumped isotope paleothermometry on timescales of 10 8 years, but that diagenetic resetting can still occur, even without bulk recrystallization. In addition to the carbonatite data, an inorganic calibration of the clumped isotope paleothermometer for low temperature carbonates (7.5–77 °C) is presented and highlights the need for further inter-lab standardization.

Consequences of larch budmoth outbreaks on the climatic significance of ring width and stable isotopes of larch

Tree-ring widths and stable carbon and oxygen isotopes of five European larch trees from Lotschental, Switzerland were investigated for the period 1900-2004. The objective was to test the suitability of each of these parameters for high-frequency climate reconstructions. This is of special interest with regard to the problem of cyclic larch budmoth (LBM) infestations of alpine larch trees. The results clearly demonstrate that tree-ring width chronologies are not suitable for high-frequency reconstructions because infes- tations lead to variably reduced tree-ring increments, largely suppressing climate signals. On the other hand, the stable isotope chronologies proved less affected by larch budmoth outbreaks, independent of the strength of the infestations. The correlation of the carbon isotopes with summer tem- peratures was especially high (r = 0.73) and with precipi- tation lower but nevertheless significant (r =- 0.43). Oxygen isotopes were also correlated with summer temperature (r = 0.46); however, a certain perturbation of normal oxygen isotope signatures due to LBM outbreaks was evident. Contrary to tree-ring widths, none of the LBM outbreaks caused a significant disturbance of the current year's isotopic climate signal and, most importantly, there were no delayed effects in the following years. Thus, stable carbon isotopes in tree-ring chronologies of the European larch provide an excellent opportunity for high-frequency temperature reconstructions.

Strong seasonal disequilibrium measured between the oxygen isotope signals of leaf and soil CO2 exchange

AbstractThe oxygen isotope composition (δ18O) of atmospheric CO2 is among a very limited number of tools available to constrain estimates of the biospheric gross CO2 fluxes, photosynthesis and respiration at large scales. However, the accuracy of the partitioning strongly depends on the extent of isotopic disequilibrium between the signals carried by these two gross fluxes. Chamber‐based field measurements of total CO2 and CO18O fluxes from foliage and soil can help evaluate and refine our models of isotopic fractionation by plants and soils and validate the extent and pattern of isotopic disequilibrium within terrestrial ecosystems. Owing to sampling limitations in the past, such measurements have been very rare and covered only a few days. In this study, we coupled automated branch and soil chambers with tuneable diode laser absorption spectroscopy techniques to continuously capture the δ18O signals of foliage and soil CO2 exchange in a Pinus pinaster Aït forest in France. Over the growing season, we observed a seasonally persistent isotopic disequilibrium between the δ18O signatures of net CO2 fluxes from leaves and soils, except during rain events when the isotopic imbalance became temporarily weaker. Variations in the δ18O of CO2 exchanged between leaves, soil and the atmosphere were well explained by theory describing changes in the oxygen isotope composition of ecosystem water pools in response to changes in leaf transpiration and soil evaporation.

THE EARLY BARREMIAN WARM PULSE AND THE LATE BARREMIAN COOLING: A HIGH-RESOLUTION GEOCHEMICAL RECORD OF THE BOREAL REALM

A total of 289 belemnite guards was analyzed from the Boreal Barremian (northeastern England and northwestern Germany) for their trace elements and isotopic composition. The oxygen-isotope signature shows similar paleotemperature variations for all seven sections investigated. Three different phases of climatic evolution can be recognized. An earliest Barremian cool phase (Praeoxyteuthis pugio belemnite Zone) is followed by a distinctive warming event in the late early Barremian (Aulacoteuthis spp. belemnite Zone). This Aulacoteuthis warm pulse has a relatively short-term peak (,500 kyr), which accompanies widespread anoxic bottom-water conditions and records the highest temperature throughout the Valanginian–Barremian in the Boreal Realm. The late Barremian (Oxyteuthis brunsvicensis, O. germanica, and O. depressa belemnite Zones) is characterized by relatively positive d 18 O values reflecting a cooling phase. This paleotemperature interpretation of the oxygen data is supported by the Mg/Ca trends. The carbon isotope curve of the Barremian shows an increase from 1%o in the early to 3%o in the late Barremian. This trend may reflect the global signal of the carbon budget of Barremian oceans and be related to the widespread deposition of TOCrich sediments in the southern part of the Boreal Realm and the progradation of the Urgonian carbonate platforms in the Tethys. There is evidence, however, that paleobiological factors may have played a much more important role in the incorporation of stable carbon isotopes than hitherto thought. In a monospecific belemnite population collected from one 30-cm-thick interval, the d13C signal shows a variation of 1.5%o. Ontogenetic changes in the metabolic activity linked to temperature or food changes may explain changes in the fractionation rate, resulting in an ontogenetically controlled biofractionation.

Picea+ Tsuga pollen record as a mirror of oxygen isotope signal? An insight into the Italian long pollen series from Pliocene to Early Pleistocene

Chronostratigraphic control is of substantial importance for palynological studies carried out both on continental and marine sequences. Long pollen records provide an opportunity to examine the response of the vegetation to glacial/interglacial climate changes, and the integration of pollen data with different datasets (biostratigraphic – foraminifers and nanoplankton, δ 18O and magnetostratigraphic data) results in a reliable chronostratigraphic control. Unfortunately, long pollen records with multi-datasets are uncommon, and lack of such integration could generate misinterpretations in chronostratigraphic assignment of the sedimentary successions in which only pollen data are available. To specify the chronostratigraphic placement of sedimentary successions controlled by palynological data solely, an alternative approach is proposed in this paper. The procedure is very simple and practical, being based on the combination of Picea and Tsuga pollen data resulting in a new pollen profile to be visually compared to δ 18O signals from reference sites. Picea and Tsuga are two of the main conifer forest taxa in Italian sedimentary successions of Pliocene to Early Pleistocene age, and their pollen-types can be unambiguously identified at the microscope. Moreover, Picea and Tsuga are elements of peculiar ecological demands, being very responsive to the variations of temperature ( Picea) and precipitation ( Tsuga). As temperature and precipitation are also the main climatic parameters which drive the expansion and retreat of ice-caps, by comparing the profile of the stacked Picea+ Tsuga pollen curve to δ 18O curves from reference sites, a visual resemblance between each other, if present, could be used for chronostratigraphic implications. In order to test and discuss the validity of this approach, the Italian long pollen series of Pliocene–Early Pleistocene age (with Picea and Tsuga concurrently present) are considered in this study.

Stable oxygen isotope records of corals and a sclerosponge in the Western Pacific warm pool

High-resolution measurements of the stable oxygen isotopic signature (δ18O) of two new 10-year Porites lobata coral cores and one previously studied multi-decadal sclerosponge Acanthocheatetes wellsi from the Republic of Palau (7°16′N, 134°31′E) located in the Western Pacific warm pool were analyzed and monthly interpolated time-series records developed. Despite significant differences in collection depth and growth rates, both coral and sclerosponge faithfully recorded the interannual changes of sea surface salinity (SSS) driven by the strong influence of the El Nino Southern Oscillation (ENSO). The strong relationship of coral skeletal δ18O with SSS, but not sea surface temperature or precipitation, confirms previous findings that changes in Palau regional surface water are driven by the advection of water masses into the area over interannual timescales associated with ENSO. Opportunities exist for the expansion of short coral proxy records with longer sclerosponge records to verify the stability of ENSO-induced SSS variability over longer timescales.

Sea-level changes as controlling factor of early diagenesis: the reefal limestones of Adnet (Late Triassic, Northern Calcareous Alps, Austria)

The uppermost Rhaetian Adnet reef is part of the Dachstein carbonate platform and is situated at the transition to the intrashelf Kossen Basin. Its diagenetic evolution is investigated focusing on dissolution cavities in the Tropfbruch quarry of Adnet (near Salzburg) stratigraphically situated immediately below the Triassic–Jurassic boundary. Sea-level changes due to global eustatic trends and regional tectonics are assumed to be the controlling factors in the development of a manifold diagenetic sequence characterized by phases of meteoric dissolution, marine and burial cementation, and internal sedimentation. Despite small-scale variations of the sequence, a superordinate pattern of diagenetic phases could be elaborated. Small-scale eustatic sea-level falls subordinate to a global regression trend caused subaerial exposures of the Adnet reef in the latest Rhaetian to earliest Hettangian. The result was karstification and meteoric dissolution of aragonitic coral skeletons (Retiophyllia) leading to the formation of biomoldic porosity. Coral septa which escaped dissolution were transformed into neomorphic calcite spar under meteoric–phreatic conditions. A first generation of dog-tooth cements precipitated sporadically on the altered coral skeletons. Eustatic sea-level rise in Early to Mid-Hettangian times caused a renewed flooding of the pore space of the Adnet reef by marine water and the influx of a first generation of internal sediments (IS I), derived from the karstified host rock of the Upper Rhaetian reef limestone. These internal sediments are overgrown by radiaxial-fibrous calcites (RFCs) whose oxygen-isotopic signature (δ18O = −1.3 (±0.7)‰) indicates precipitation in deeper (colder) water (18–21°C) due to a first phase of drowning. An intermediate phase of eustatic sea-level lowstand in the Late Hettangian is expressed by dissolution and corrosion of RFCs. Rapid drowning of the Dachstein carbonate platform due to eustatic sea-level rise and tectonic movements took place in the Early Sinemurian and a second generation of internal sediments (IS II) derived from the Lower Sinemurian Adnet Formation is washed into the dissolution cavities. Where IS II is absent, RFCs are overgrown by a second generation of dog-tooth cements with a bright-luminescent outer rim indicating the transition to negative redox conditions in the pore water during shallow burial. Burial diagenesis is represented by blocky calcite cements which occlude the remaining pore space. Depleted oxygen-isotope values and significant Fe contents indicate precipitation under reducing redox conditions and elevated temperatures of 30–50°C at burial depths of 420–870 m. Locally, replacive saddle dolomite is the latest diagenetic phase in the Adnet reef indicating crystallization under hydrothermal influences related to compressional subduction regimes of the Penninic Ocean.

Extinction implications of a chenopod browse diet for a giant Pleistocene kangaroo

Kangaroos are the world's most diverse group of herbivorous marsupials. Following late-Miocene intensification of aridity and seasonality, they radiated across Australia, becoming the continent's ecological equivalents of the artiodactyl ungulates elsewhere. Their diversity peaked during the Pleistocene, but by approximately 45,000 years ago, 90% of larger kangaroos were extinct, along with a range of other giant species. Resolving whether climate change or human arrival was the principal extinction cause remains highly contentious. Here we combine craniodental morphology, stable-isotopic, and dental microwear data to reveal that the largest-ever kangaroo, Procoptodon goliah, was a chenopod browse specialist, which may have had a preference for Atriplex (saltbushes), one of a few dicots using the C(4) photosynthetic pathway. Furthermore, oxygen isotope signatures of P. goliah tooth enamel show that it drank more in low-rainfall areas than its grazing contemporaries, similar to modern saltbush feeders. Saltbushes and chenopod shrublands in general are poorly flammable, so landscape burning by humans is unlikely to have caused a reduction in fodder driving the species to extinction. Aridity is discounted as a primary cause because P. goliah evolved in response to increased aridity and disappeared during an interval wetter than many it survived earlier. Hunting by humans, who were also bound to water, may have been a more decisive factor in the extinction of this giant marsupial.

Interdecadal variation in seawater d13C and d18O recorded in fish otoliths

Stable carbon (Δ13C) and oxygen (Δ18O) in the otolith cores of Atlantic bluefin tuna (Thunnus thynnus) vary temporally, with changes that quantitatively follow interdecadal variation in atmospheric and oceanic reservoirs. Both carbon and oxygen isotopic signatures vary significantly by year of birth over the range investigated (1947‐2006), with Δ13C decreasing and Δ18O increasing (‐2.56 × 10−2‰ and 4.3 × 10−3‰ yr−1, respectively). The rate of change in otolith Δ13C was similar to reported rates of atmospheric Δ13C depletion, attributed to deforestation and the burning of fossil fuels (referred to as the Suess effect), suggesting a close link between atmospheric and oceanic carbon pools. Increases in otolith Δ18O were evident but less pronounced, with observed variation possibly attributable to changing salinity in the Atlantic Ocean. Otolith cores of bluefin tuna effectively track interdecadal trends and record past seawater Δ13C and Δ18O.

Devonian climate and reef evolution: Insights from oxygen isotopes in apatite

Conodonts, microfossils composed of carbonate-fluor apatite, are abundant in Palaeozoic–Triassic sediments and have a high potential to preserve primary oxygen isotope signals. In order to reconstruct the palaeotemperature history of the Devonian, the oxygen isotope composition of apatite phosphate was measured on 639 conodont samples from sequences in Europe, North America and Australia. The Early Devonian (Lochkovian; 416–411 Myr) was characterized by warm tropical temperatures of around 30 °C. A cooling trend started in the Pragian (410 Myr) with intermediate temperatures around 23 to 25 °C reconstructed for the Middle Devonian (397–385 Myr). During the Frasnian (383–375 Myr), temperatures increased again with temperatures to 30 °C calculated for the Frasnian–Famennian transition (375 Myr). During the Famennian (375–359 Myr), surface water temperatures slightly decreased. Reconstructed Devonian palaeotemperatures do not support earlier views suggesting the Middle Devonian was a supergreenhouse interval, an interpretation based partly on the development of extensive tropical coral–stromatoporoid communities during the Middle Devonian. Instead, the Devonian palaeotemperature record suggests that Middle Devonian coral–stromatoporoid reefs flourished during cooler time intervals whereas microbial reefs dominated during the warm to very warm Early and Late Devonian.

Oxygen isotopic signature of the skeletal microstructures in cultured corals: Identification of vital effects

In order to identify vital effect on oxygen isotopic ratio, we analyzed at micrometer size scale skeleton microstructures of a scleractinian coral Acropora, cultured under constant conditions. Measurements focused on the two crystalline units highlighted different isotopic signatures. Massive crystals (centers of calcification: COC) exhibit quasi-constant lowest values whereas fibers, the dominant units exhibit scattered distribution with amplitude of up to 5‰. Fiber oxygen isotopic ratios (δ 18O) range from values similar to instantaneous deposition to equilibrium value. By comparing data obtained on the Acropora specimen and deep-sea corals grown under well constrained conditions, we infer that the scattered δ 18O aragonite fibers indicate precipitation through kinetic precipitation. Thus, we argue for inherent biological feature typical to all coral genera. Modalities of COC formation remain ignored. The different isotopic signature of two mineral microstructures present in close proximity in coral skeleton can only be explained by compartment depositions related to different organic environments. Indeed, multiple secondary electron microscopy (SEM) observations favor interaction between mineral and organic matrix surface. Moreover, atomic forcing microscopy (AFM) investigations demonstrated thermodynamic changes induced by mineralization in presence of organic compounds. The combination of our results with previous published ones from biological studies, allows us proposing a consistent model of fiber skeleton formation. The prerequisite step of mineral growth unit precipitation would be initiated by organic matrix secretion, which defines spatial extension. Specific carriers supply ionic compounds of the crystals to ensure local supersaturation. However, possibly controlled by organic molecules, ionic amount would be limited, implying the supersaturation decline over the time. This could explain the progressive decrease of the coral growth rate. In this case, vital effect should not only bias isotopic fractionation through biological activity but the mechanism of skeleton deposition is imposed by specific chemical and/or physical conditions due to the presence of organic molecules. These conclusions derive from observations performed at high resolution. Therefore, isotopic ratio measured on millimeter scale for paleoclimatic purposes, result of the average of strongly heterogeneous values. It could explain vital effects shown by geochemical time series derived from tropical coral skeleton, including the high species and/or colony variability.

Tracing carbon and oxygen isotope signals from newly assimilated sugars in the leaves to the tree‐ring archive

The analysis of delta(13)C and delta(18)O in tree-ring archives offers retrospective insights into environmental conditions and ecophysiological processes. While photosynthetic carbon isotope discrimination and evaporative oxygen isotope enrichment are well understood, we lack information on how the isotope signal is altered by downstream metabolic processes. In Pinus sylvestris, we traced the isotopic signals from their origin in the leaf water (delta(18)O) or the newly assimilated carbon (delta(13)C), via phloem sugars to the tree-ring, over a time-scale that ranges from hours to a growing season. Seasonally, variable (13)C enrichment of sugars related to phloem loading and transport did lead to uncoupling between delta(13)C in the tree-ring, and the c(i)/c(a) ratio at the leaf level. In contrast, the oxygen isotope signal was transferred from the leaf water to the tree-ring with an expected enrichment of 27 per thousand, with time-lags of approximately 2 weeks and with a 40% exchange between organic oxygen and xylem water oxygen during cellulose synthesis. This integrated overview of the fate of carbon and oxygen isotope signals within the model tree species P. sylvestris provides a novel physiological basis for the interpretation of delta(13)C and delta(18)O in tree-ring ecology.

Influence of Host-Rock Texture on the Morphology of Carbonate Concretions in a Meteoric Diagenetic Environment

Abstract The integrated stratigraphic-petrographic–geochemical study of Quaternary marine terraces that have undergone well-constrained sea-level and water-table oscillation(s) over geologically short time periods can provide important clues on diagenetic processes at the marine–continental interface. The Punta di i Cani marine terrace (Quaternary; Saint-Florent, Corsica) unconformably overlies the well-cemented, low-permeability arenites of the Miocene Monte S. Angelo Formation and is erosively overlain by unlithified, high-permeability Holocene continental sediments. The terrace is composed of a retrogradational, fining-upward succession, from fluvial pebbly sandstone to beach and shoreface deposits. The entire thickness of the terrace hosts concretions made of low-Mg calcite. Concretion geometry is linked to the sedimentological facies of the host sediment: (i) massive fluvial pebbly sandstone at the base of the terrace contains knobby, subhorizontal concretions somewhat resembling turbidite flute casts; (ii) very low-angle cross-bedded, well sorted beach deposits contain pipy, subhorizontal concretions; (iii) trough-crossbedded upper-shoreface deposits in the upper part of the terrace contain a subtle diagenetic lineation on the laminae, locally obliterated by a patchy, isotropic cementation. Overall, the degree of cementation decreases from the bottom to the top of the terrace. Cement chemistry and morphology, as well as stable carbon and oxygen isotopic signatures, indicate that the concretions formed by precipitation of meteoric phreatic water flowing toward the coast. Elongate concretions are invariably parallel to the topographic gradient, consistent with the notion that the growth of concretions was controlled by advection. Smectitic vadose cement is locally present; its crosscutting relationships with calcite cement point to periodic oscillations of the water table. The results of this study (i) show that elongate concretions can be used as interstitial paleoflow indicators and (ii) suggest that concretion geometry in an advection-dominated diagenetic environment can depend on the small-scale permeability distribution, as suggested by the fact that each type of concretion is invariably associated with a specific sedimentary facies.

Oxygen exchange between nitrogen oxides and H 2O can occur during nitrifier pathways

Interpretation of the oxygen isotopic signature of soil-derived N 2O may be flawed when it is based on reaction stoichiometry and fractionation alone. In fact, oxygen (O) exchange between H 2O and intermediates of N 2O production pathways may largely determine this O isotopic signature. Although in our previous work we conclusively proved the occurrence of O exchange during N 2O production by denitrification of NO 3 −, its occurrence in N 2O production pathways by nitrifiers remains unclear. The aim of this study was to examine the likeliness of O exchange during various stages of N 2O production in soil via nitrification, nitrifier denitrification and denitrification. We evaluated a set of scenarios on the presence of such exchange using data from a series of 18O and 15N tracing experiments. The measured actual O incorporation from H 2O into N 2O (AOI) was compared with the theoretical maximum O incorporation (MOI) from various scenarios that differed in their assumptions on the presence of O exchange. We found that scenarios where O exchange was assumed to occur exclusively during denitrification could not explain the observed AOI, as it exceeded the MOI for 9 out of 10 soils. This demonstrates that additional O exchange must have occurred in N 2O production through nitrifier pathways. It remains to be determined in which steps of these pathways O exchange can take place. We conclude that O exchange is likely to be mediated by ammonia oxidizers during NO 2 − reduction (nitrifier denitrification), and that it could possibly occur during NO 2 − oxidation to NO 3 − by nitrite oxidizers as well.

Eogenetic dolomite cementation in lower Permian reservoir sandstones, southern Zagros, Iran

AbstractIn spite of the increasing importance to hydrocarbon exploration of the Palaeozoic succession in southwest Iran (Zagros Belt), few comprehensive diagenetic studies of this interval have been carried out. This is the first study presented on the diagenetic characteristics of the Early Permian Faraghan Formation in its most representative exposures in the Zagros region. This formation is dominated by cross‐stratified sandstones at outcrop, along with stromatolitic dolomite, dolomitized limestones and subordinate siltstone/sandy shales. The formation grades upward into the thick‐bedded carbonates of the Late Permian Dalan Formation. Both formations are major hydrocarbon reservoirs in SW Iran and are time equivalents of the Unayzah and Khuff formations, respectively, in Saudi Arabia. The constituent facies of the Faraghan Formation represent sedimentation in a siliciclastic shallow‐marine environment with foreshore (including tidal flat and numerous tidal channels), estuary, sabkha, shoreface and offshore sub‐environments. In this study, the development of ferroan dolomite cements in the Faraghan Formation is investigated. Geometrically, ferroan dolomite cements occur in laterally extensive, cemented beds in conglomerates and sandstones, stratabound layers and as nodules in sandstones. Their various petrographic characteristics (such as microcrystalline nature, preservation of porosity, formation as the first diagenetic phase on the grains and their high volume) all indicate an eodiagenetic origin. Oxygen isotopic signatures of the cement (−2.10 to −6.42‰) are close to the proposed content for Lower Permian seawater. Development of this cement is attributed to the eodiagenetic environment marked by fresh and seawater mixing. In this setting, sabkha‐type dolomitic grains/lithics acted as nucleation sites facilitating dolomite cement precipitation. Copyright © 2009 John Wiley & Sons, Ltd.This article was published online on 13 May 2009. An error was subsequently identified. This notice is included in the online and print versions to indicate that both have been corrected 17 August 2009.

Pleistocene seasonal temperature variations recorded in the δ18O of Bison priscus teeth

Oxygen isotope analysis of phosphate in tooth enamel of mammals ( δ 18O p) constitutes a valuable method to reconstruct past air temperatures in continental environments. The method is based on interdependent relationships between the δ 18O of apatite phosphate, body fluids, environmental waters and air temperatures. Continuous tooth growth and absence of enamel remodelling in bovid teeth ensures a reliable record of the intra-annual variability of air temperature through an incremental δ 18O analysis from apex to cervix. This method has been applied to Bison priscus dental remains of the late Middle Pleistocene from the fossiliferous layer of a cave at Coudoulous I in South-Western France (Layer 4). The stacked oxygen isotope signal obtained by combining 9 bison teeth shows sinusoidal variations (15.0‰ to 19.1‰ V-SMOW) of seasonal origin over 2.5 yr. The corresponding computed MAT of 9 ± 3 °C is about 4 °C lower than at present. Seasons appear more contrasted in Coudoulous I during Layer 4 deposition with summers as warm as present ones (19 ± 3 °C) and significantly colder winters about 0 ± 3 °C compared to 6 ± 1 °C at present.

In situ U–Pb, O and Hf isotopic compositions of zircon and olivine from Eoarchaean rocks, West Greenland: New insights to making old crust

The sources and petrogenetic processes that generated some of the Earth’s oldest continental crust have been more tightly constrained via an integrated, in situ (U–Pb, O and Hf) isotopic approach. The minerals analysed were representative zircon from four Eoarchaean TTG tonalites and two felsic volcanic rocks, and olivine from one harzburgite/dunite of the Itsaq Gneiss Complex (IGC), southern West Greenland. The samples were carefully chosen from localities with least migmatisation, metasomatism and strain. Zircon was thoroughly characterized prior to analysis using cathodoluminescence, scanning electron, reflected and transmitted light imaging. The zircon from all but one sample showed only minor post-magmatic recrystallisation. 207Pb/ 206Pb dating of oscillatory-zoned zircon using SHRIMP RG ( n = 142) indicates derivation of the felsic igneous rocks from different batches of magma at 3.88, 3.85, 3.81, 3.80 and 3.69 Ga. Analyses of 18O/ 16O compositions of olivine from a harzburgite/dunite ( n = 8) using SHRIMP II in multi-collector mode, indicate that the oxygen isotopic composition of this sample of Eoarchaean mantle ( δ 18O Ol = 6.0 ± 0.4‰) was slightly enriched in 18O, but not significantly different from that of the modern mantle. Zircon δ 18O measurements from the six felsic rocks ( n = 93) record mean or weighted mean compositions ranging from 4.9 ± 0.7‰ to 5.1 ± 0.4‰, with recrystallised domains showing no indication of oxygen isotopic exchange during younger tectonothermal events. δ 18O Zr compositions indicate that the primary magmas were largely in equilibrium with the mantle or mantle-derived melts generated at similar high temperatures, while calculated tonalite δ 18O WR compositions (6.7–6.9‰) resemble those of modern adakites. LA-MC-ICPMS zircon 176Hf/ 177Hf analyses were obtained from six samples ( n = 122). Five samples record weighted mean initial ε Hf compositions ranging from to 0.5 ± 0.6 to −0.1 ± 0.7 (calculated using λ 176Lu = 1.867 × 10 −11 yr −1), while one sample records a composition of 1.3 ± 0.7, indicating the magmas were generated from a reservoir with a time averaged, near chondritic Lu/Hf. The derivation of TTG magmas from a chondritic Lu/Hf source implies either that there was not voluminous continental crustal growth nor major mantle differentiation leading to Lu/Hf fractionation during the Hadean or Eoarchaean, or alternatively that rapid recycling of an early formed crust allowed the early mantle to maintain a chondritic Lu/Hf. Previous studies have demonstrated that ancient TTG rocks were mostly produced by dehydration melting of mafic rocks within the stability field of garnet, probably in flatly-subducted or buried oceanic crust. The oxygen isotopic signatures measured here at high spatial resolution allow the source materials to be better defined. Melting of a mixed mafic source consisting of ∼80% unaltered gabbro ( δ 18O WR = 5.5‰) with ∼20% hydrothermally altered gabbro/basalt ( δ 18O WR = 4.0‰) would produce tonalite magmas within the average compositional range observed. 18O-enriched components such as altered shallow basaltic oceanic crust and pelagic or continental sediments were not present in the sources of these TTG melts. The absence of high 18O signatures may indicate either the rarity of low temperature altered sediments, or their effective removal from the down-going slab.

26Al AND THE FORMATION OF THE SOLAR SYSTEM FROM A MOLECULAR CLOUD CONTAMINATED BY WOLF-RAYET WINDS

In agreement with previous work, we show that the presence of the short-lived radionuclide Al-26 in the early Solar System was unlikely (<2% a priori probability) to be the result of direct introduction of supernova ejecta into the gaseous disk during the Class II stage of protosolar evolution. We also show that any Bondi-Hoyle accretion of contaminated residual gas from the natal star cluster made a negligible contribution to the primordial Al-26 inventory of the Solar System. These results are consistent with the absence of the oxygen isotopic signature expected with any late introduction of supernova ejecta into the protoplanetary disk. Instead, the presence of Al-26 in the oldest Solar System solids (calcium-aluminum-rich inclusions or CAIs) and its apparent uniform distribution with the inferred canonical Al-26/Al-27 ratio of (4.5-5)E-5 support the inheritance of Al-26 from the parent giant molecular cloud. We propose that this radionuclide originated in a prior generation of massive stars that formed in the same host molecular cloud as the Sun and contaminated that cloud by Wolf-Rayet winds. We calculated the Galactic distribution of Al-26/Al-27 ratios that arise from such contamination using the established embedded cluster mass and stellar initial mass functions, published nucleosynthetic yields from the winds of massive stars, and by assuming rapid and uniform mixing into the cloud. Although our model predicts that the majority of stellar systems contain no Al-26 from massive stars, and that the a priori probability that the Al-26/Al-27 ratio will reach or exceed the canonical Solar System value is only ~6%, the maximum in the distribution of non-zero values is close to the canonical ratio.

Compound- and enzyme-specific phosphodiester hydrolysis mechanisms revealed by δ18O of dissolved inorganic phosphate: Implications for marine P cycling

We have studied the oxygen isotope signature of inorganic phosphate (P i) generated by hydrolysis of nucleic acid phosphodiester (P-diester) compounds by cell-free enzymes (Deoxyribonuclease 1, Phosphodiesterase 1, Alkaline phosphatase) and microbial cultures at natural isotopic abundances. We demonstrate that the diesterase-catalyzed hydrolytic step leads to incorporation of at least one water O into released P i for a total of two O atoms from water incorporated into P i released from P-diesters. In the presence of Phosphodiesterase 1, 16O is preferentially incorporated into nucleotides released from DNA; whereas 18O is preferentially incorporated into nucleotides released from RNA. A strong consistency between predicted O-isotope regeneration signatures based on results of cell-free enzyme experiments and measured isotopic signatures from independent experiments with E. coli cultures was observed and confirms proposed models for phosphoester hydrolysis. Results from these studies made at natural 18O abundance levels provide a new tool, enzyme-specific O-isotope fractionation, for investigations of organophosphate metabolism and phosphorus cycling pathways in natural aquatic systems.

The δ18O of dissolved O2 as a tracer of mixing and respiration in the mesopelagic ocean

The isotopic composition of dissolved oxygen in the mesopelagic ocean is a unique tracer of respiration and transport. New δ18O of O2 data from the tropical South Atlantic oxygen minimum zone are presented and compared to global δ18O data. The δ18O variability in oxygen poor waters is attributed to differences in physical and biogeochemical processes. Simple respiration‐transport models show that both isopycnal diffusion and advection must be properly considered when interpreting oxygen isotope signatures along an isopycnal surface. We estimate rates of respiration and oxygen isotope fractionation for the study region using a two‐dimensional (2‐D) isopycnal and 1‐D diapycnal model. Estimated respiration rates are consistent with previous studies. However, to account for observed δ18O values at low [O2], model solutions need to invoke either very low [O2] that have not been observed in the South Atlantic or an isotope effect that is lower than values measured in the laboratory or euphotic zone.