CO2 Isotope Research Articles

Maintaining consistent traceability in high-precision isotope measurements of CO<sub>2</sub>: a way to verify atmospheric trends of δ<sup>13</sup>C and δ<sup>18</sup>O

Abstract. Maintaining consistent traceability of high-precision measurements of CO2 isotopes is critical in order to obtain accurate atmospheric trends of δ13C and δ18O (in CO2). Although a number of laboratories/organizations around the world have been conducting baseline measurements of atmospheric CO2 isotopes for several decades, reports on the traceability and maintenance are rare. In this paper, a principle and an approach for maintaining consistent traceability in high-precision isotope measurements (δ13C and δ18O) of atmospheric CO2 are described. The concept of Big Delta is introduced and its role in maintaining traceability of the isotope measurements is described and discussed extensively. The uncertainties of the traceability have been estimated based on annual calibration records over the last 10 yr. The overall uncertainties of CO2 isotope measurements for individual ambient samples analyzed by the program at Environment Canada have been estimated (excluding these associated with the sampling). The values are 0.02 and 0.05‰ in δ13C and δ18O, respectively, which are close to the World Meteorological Organization (WMO) targets for data compatibility. The annual rates of change in δ13C and δ18O of the primary anchor (which links the flask measurements back to the VPDB-CO2 scale) are close to zero (−0.0016 ± 0.0012‰, and −0.006 ± 0.003‰ per year, respectively) over a period of 10 yr (2001–2011). The average annual changes of δ13C and δ18O in air CO2 at Alert GAW station over the period from 1999 to 2010 have been evaluated and confirmed; they are −0.025 ± 0.003‰ and 0.000 ± 0.010‰, respectively. The results are consistent with a continuous contribution of fossil fuel CO2 to the atmosphere, having a trend toward more negative in δ13C, whereas the lack of change in δ18O likely reflects the influence from the global hydrologic cycle. The total change of δ13C and δ18O during this period is ~0.27‰ and ~0.00‰, respectively. Finally, the challenges and recommendations as strategies to maintain a consistent traceability are described.

Tracking isotopic signatures of CO<sub>2</sub> at the high altitude site Jungfraujoch with laser spectroscopy: analytical improvements and representative results

Abstract. We present the continuous data record of atmospheric CO2 isotopes measured by laser absorption spectroscopy for an almost four year period at the High Altitude Research Station Jungfraujoch (3580 m a.s.l.), Switzerland. The mean annual cycles derived from data of December 2008 to September 2012 exhibit peak-to-peak amplitudes of 11.0 μmol mol−1 for CO2, 0.60‰ for δ13C and 0.81‰ for δ18O. The high temporal resolution of the measurements also allow us to capture variations on hourly and diurnal timescales. For CO2 the mean diurnal peak-to-peak amplitude is about 1 μmol mol−1 in spring, autumn and winter and about 2 μmol mol−1 in summer. The mean diurnal variability in the isotope ratios is largest during the summer months too, with an amplitude of about 0.1‰ both in the δ13C and δ18O, and a smaller or no discernible diurnal cycle during the other seasons. The day-to-day variability, however, is much larger and depends on the origin of the air masses arriving at Jungfraujoch. Backward Lagrangian particle dispersion model simulations revealed a close link between air composition and prevailing transport regimes and could be used to explain part of the observed variability in terms of transport history and influence region. A footprint clustering showed significantly different wintertime CO2, δ13C and δ18O values depending on the origin and surface residence times of the air masses. Several major updates on the instrument and the calibration procedures were performed in order to further improve the data quality. We describe the new measurement and calibration setup in detail and demonstrate the enhanced performance of the analyzer. A measurement precision of about 0.02‰ for both isotope ratios has been obtained for an averaging time of 10 min, while the accuracy was estimated to be 0.1‰, including the uncertainty of the calibration gases.

Jena Reference Air Set (JRAS): a multi-point scale anchor for isotope measurements of CO<sub>2</sub> in air

Abstract. The need for a unifying scale anchor for isotopes of CO2 in air was brought to light at the 11th WMO/IAEA Meeting of Experts on Carbon Dioxide in Tokyo 2001. During discussions about persistent discrepancies in isotope measurements between the worlds leading laboratories, it was concluded that a unifying scale anchor for Vienna Pee Dee Belemnite (VPDB) of CO2 in air was desperately needed. Ten years later, at the 2011 Meeting of Experts on Carbon Dioxide in Wellington, it was recommended that the Jena Reference Air Set (JRAS) become the official scale anchor for isotope measurements of CO2 in air (Brailsford, 2012). The source of CO2 used for JRAS is two calcites. After releasing CO2 by reaction with phosphoric acid, the gases are mixed into CO2-free air. This procedure ensures both isotopic stability and longevity of the CO2. That the reference CO2 is generated from calcites and supplied as an air mixture is unique to JRAS. This is made to ensure that any measurement bias arising from the extraction procedure is eliminated. As every laboratory has its own procedure for extracting the CO2, this is of paramount importance if the local scales are to be unified with a common anchor. For a period of four years, JRAS has been evaluated through the IMECC1 program, which made it possible to distribute sets of JRAS gases to 13 laboratories worldwide. A summary of data from the six laboratories that have reported the full set of results is given here along with a description of the production and maintenance of the JRAS scale anchors. 1 IMECC refers to the EU project "Infrastructure for Measurements of the European Carbon Cycle" (http://imecc.ipsl.jussieu.fr/).

Multivariate determination of 13CO2/12CO2 ratios in exhaled mouse breath with mid-infrared hollow waveguide gas sensors

The (12)CO2/(13)CO2 isotope ratio is a well-known marker in breath for a variety of biochemical processes and enables monitoring, e.g., of the glucose metabolism during sepsis. Using animal models-here, at a mouse intensive care unit-the simultaneous determination of (12)CO2 and (13)CO2 within small volumes of mouse breath was enabled by coupling a novel low-volume hollow waveguide gas cell to a compact Fourier transform infrared spectrometer combined with multivariate data evaluation based on partial least squares regression along with optimized data preprocessing routines.

Sr[sbnd]Nd[sbnd]Pb isotopic compositions of Early Cretaceous granitoids from the Dabie orogen: Constraints on the recycled lower continental crust

In order to characterize the recycled lower continental crust (LCC) in the Dabie orogen, 17 Early Cretaceous low-Mg adakitic (LMA) and 9 normal (non-adakitic) granitoids have been investigated for SrNdPb isotopes. Combined with literature data, LMA have low ƐNd(t) (−27.8 to −14.7) and 206Pb/204Pb(i) (15.69–17.16) and low to moderately high 87Sr/86Sr(i) (0.7066 to 0.7087) ratios. Normal granitoids yield isotope ratios similar to adakitic rocks, except a few with 87Sr/86Sr (i) up to 0.7105. Dabie LMA define a linear trend parallel to the North Hemisphere Reference Line (NHRL) in a 208Pb/204Pb(i)– 206Pb/204Pb(i) diagram. For a given 206Pb/204Pb(i), the 208Pb/204Pb(i) or ∆8/4 (152–217) of Dabie LMA are close to the majority of UHP gneisses and the Neoproterozoic mafic rocks from the northern margin of the South China Block (SCB), but significantly higher than adakitic rocks from the North China Block (∆8/4<150). Considering the commonly present Neoproterozoic inherited zircons, we suggest that the LMA in the Dabie orogen are derived from a thickened LCC that could be dominantly composed of ancient SCB lower crust.The SrNdPb isotopic composition of LMA are similar to those of Post-Collisional Mafic Igneous rocks (PCMI) from the Dabie orogen, but different to exhumed UHP rocks. The SrNdPb isotopic system of the mantle source of the PCMI could be enriched in components dominantly from delaminated LCC of the Dabie orogen instead of deeply subducted continental crust, which is in contrast to O- and C-isotope data by [Zhao, Z.F., Zheng, Y.F., Wei, C.S., Wu, Y.B., Chen, F.K., and Jahn, B.M., 2005. Zircon UPb age, element and CO isotope geochemistry of post-collisional mafic–ultramafic rocks from the Dabie orogen in east-central China. Lithos 83(1–2), 1–28; Dai, L.Q., Zhao, Z.F., Zheng, Y.F., Li, Q.L., Yang, Y.H., and Dai, M.N., 2011. Zircon HfO isotope evidence for crust–mantle interaction during continental deep subduction. Earth and Planetary Science Letters 308, 229–244]. Combining the SrNdPbHf and OC isotopic data, two types of commonly present magma components have been identified in the PCMI, reflecting crust–mantle interaction during the Triassic subduction and the Early Cretaceous orogen collapse, respectively.

Geochemical constraints on the origin of post-depositional fluids in sedimentary carbonates of the Ediacaran system in South China

In order to evaluate the preservation of primary geochemical signatures in sedimentary carbonates and the effect of post-depositional fluids, a combined study of CO isotopes and trace elements was carried out for different microfacies of the Lantian carbonates in the southern Anhui, which is equivalent to the Doushantuo Formation of the Ediacaran system elsewhere in South China. Stratigraphically the Lantian carbonate can be subdivided into the Upper and Lower Units. For the Upper Unit (UU), petrographic observations indicate that the wallrock carbonates experienced only limited recrystallization. The REE+Y patterns of wallrock carbonates with CaO contents higher than 53% are consistent with those treated by the acetic acid solution, representing the primary records of carbonates deposition. The veins and wallrock carbonates in this unit exhibit consistent REE+Y patterns and similar δ13C and δ18O values, suggesting the same origin of their depositional fluids. In contrast, there are significant differences in the geochemical signatures between veins/cements and wallrock carbonates in the Lower Unit (LU), suggesting that different origins of their depositional fluid.The REE+Y patterns for calcite veins and cements in the both units indicate incorporation of terrigenous materials into post-depositional fluids. With respect to correlations between δ13C and δ18O values, negative one occurs in the UU, whereas positive one occurs in the LU. Unusually low δ18O values of −28.6‰ to −22.0‰ for the veins and cements in the both units indicate that the post-depositional fluids were derived from negative δ18O continental freshwater subsequent to the Gaskiers and Marinoan iceages. The REE+Y patterns and δ18O variations for the veins in UU suggest the parent fluid contained a component of continental deglacial meltwater. The unusually low δ18O values also occur in the veins and cements of LU, indicating a significant contribution from meteoric water. On the other hand, the δ13C values for the veins and cements of the both units suggest that the same carbon sources as the wallrock. The present results provide insights into the preservation of primary geochemical records in sedimentary carbonates and into the effect of post-depositional processes on the Precambrian carbonates. In particular, the origin of post-depositional fluids can be distinguished from the depositional fluids by means of the combined trace element and stable isotope tracing.

Planetary fertility during the past 400 ka based on the triple isotope composition of O&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; in trapped gases from the Vostok ice core

Abstract. The productivity of the biosphere leaves its imprint on the isotopic composition of atmospheric oxygen. Ultimately, atmospheric oxygen, through photosynthesis, originates from seawater. Fractionations during the passage from seawater to atmospheric O2 and during respiration affect δ17O approximately half as much as δ18O. An "anomalous" (also termed mass independent) fractionation process changes δ17O about 1.7 times as much as δ18O during isotope exchange between O2 and CO2 in the stratosphere. The relative rates of biological O2 production and stratospheric processing determine the relationship between δ17O and δ18O of O2 in the atmosphere. Variations of this relationship thus allow us to estimate changes in the rate of O2 production by photosynthesis versus the rate of O2–CO2 isotope exchange in the stratosphere. However, the analysis of the 17O anomaly is complicated because each hydrological and biological process fractionates δ17O and δ18O in slightly different proportions. In this study we present O2 isotope data covering the last 400 ka (thousand years) from the Vostok ice core. We reconstruct oxygen productivities from the triple isotope composition of atmospheric oxygen with a box model. Our steady state model for the oxygen cycle takes into account fractionation during photosynthesis and respiration by the land and ocean biosphere, fractionation during the hydrologic cycle, and fractionation when oxygen passes through the stratosphere. We consider changes of fractionation factors linked to climate variations, taking into account the span of estimates of the main factors affecting our calculations. We find that ocean oxygen productivity was within 20% of the modern value throughout the last 400 ka. Given the presumed reduction in terrestrial oxygen productivity, the total oxygen production during glacials was likely reduced.

Using cavity ringdown spectroscopy for continuous monitoring of δ13C(CO2) and ƒCO2 in the surface ocean

The role of the global surface ocean as a source and sink for atmospheric carbon dioxide and the flux strengths between the ocean and the atmosphere can be quantified by measuring the fugacity of CO2 (ƒCO2) as well as the dissolved inorganic carbon (DIC) concentration and its isotopic composition in surface seawater. In this work, the potential of continuous wave cavity ringdown spectroscopy (cw‐CRDS) for autonomous underway measurements of ƒCO2 and the stable carbon isotope ratio of DIC [δ13C(DIC)] is explored. For the first time, by using a conventional air‐sea equilibrator setup, both quantities were continuously and simultaneously recorded during a field deployment on two research cruises following meridional transects across the Atlantic Ocean (Bremerhaven, Germany—Punta Arenas, Chile). Data are compared against reference measurements by an established underway CO2 monitoring system and isotope ratio mass spectrometric analysis of individual water samples. Agreement within ΔƒCO2 = 0.35 µatm for atmospheric and ΔƒCO2 = 2.5 µatm and Δδ13C(DIC) = 0.33‰ for seawater measurements have been achieved. Whereas “calibration‐free” ƒCO2 monitoring is feasible, the measurement of accurate isotope ratios relies on running reference standards on a daily basis. Overall, the installed CRDS/equilibrator system was shown to be capable of reliable online monitoring of ƒCO2, equilibrium δ13C(CO2), δ13C(DIC), and pO2 aboard moving research vessels, thus making possible corresponding measurements with high spatial and temporal resolution.

Evolution of the geothermal fluids at Los Azufres, Mexico, as traced by noble gas isotopes, δ18O, δD, δ13C and 87Sr/86Sr

Isotopes of noble gases, CO2, H2O and Sr were measured in 10 geothermal wells and 8 hot springs, fumaroles and mud volcanoes at Los Azufres, the second most important geothermal field in Mexico. The aim of this study is to provide additional information on fluid circulation in the field and surrounding areas (Araró hot springs), as well as on physical processes such as boiling, steam separation and invasion of re-injected brines following over 25years of geothermal fluid exploitation. Mantle helium dominates in fluids from the northern production zone of Marítaro, with measured 3He/4He ratios up to 7 Ra (where Ra is the atmospheric ratio of 1.386×10−6). 3He/4He ratios are positively correlated with 87Sr/86Sr ratios and with δD and δ18O. These relationships suggest that Los Azufres fluids represent a mixing between primary magmatic 3He-rich fluids and groundwater currently discharging at Araró hot springs and enriched in radiogenic 4He acquired from Miocene andesites. Unusually high He ratios together with radiogenic Sr isotopic ratios suggest that thermal waters acquired mantle He from deep-seated parent magmas and radiogenic Sr possibly during their uprising through the metamorphic basement. 40Ar/36Ar ratios of 366 to 429 measured in two wells indicate either mantle-derived argon or a radiogenic 40Ar in situ component, suggesting the local presence of an older crustal fluid component in the northern part of the field. Ne, Kr and Xe are entirely of atmospheric origin, but processes of boiling, steam separation and re-injection of used brines have led to fractionation of their elemental abundances. Comparison with previous studies suggests that the boiling zone in the northern production zone is currently extending further north (Marítaro hot springs). In the southwestern productive zone, re-injected brines might account for more than 90% of the exploited fluids.

A 1 mm SPECTRAL LINE SURVEY TOWARD GLIMPSE EXTENDED GREEN OBJECTS (EGOs)

A northern subsample of 89 Spitzer GLIMPSE extended green objects (EGOs), the candidate massive young stellar objects, are surveyed for molecular lines in two 1-GHz ranges: 251.5- 252.5 and 260.188-261.188 GHz. A comprehensive catalog of observed molecular line data and spectral plots are presented. Eight molecular species are undoubtedly detected: H13CO+, SiO, SO, CH3OH, CH3OCH3, CH3CH2CN, HCOOCH3, and HN13C. H13CO+ 3-2 line is detected in 70 EGOs among which 37 ones also show SiO 6-5 line, demonstrating their association to dense gas and supporting the outflow interpretation of the extended 4.5 um excess emission. Our major dense gas and outflow tracers (H13CO+, SiO, SO and CH3OH) are combined with our previous survey of 13CO, 12CO and C18O 1-0 toward the same sample of EGOs for a multi-line multi- cloud analysis of line width and luminosity correlations. Good log-linear correlations are found among all considered line luminosities, which requires a universal similarity of density and thermal structures and probably of shock properties among all EGO clouds to explain. It also requires that the shocks should be produced within the natal clouds of the EGOs. Diverse degrees of correlation are found among the line widths. However, both the line width and luminosity correlations tend to progressively worsen across larger cloud subcomponent size-scales, depicting the increase of randomness across cloud subcomponent sizes. Moreover, the line width correlations among the three isotopic CO 1-0 lines show data scatter as linear functions of the line width itself, indicating that the velocity randomness also increases with whole-cloud sizes and has some regularity behind.

Determination of the Rate of CO2–CO Reaction with Magnetite by Isotope Exchange Technique

AbstractTo clarify the mechanism and get the accurate kinetic parameters of CO2–CO reaction with magnetite, 13CO2–CO isotope exchange technique was used to determine the rate constants of CO2 dissociation on the surface of magnetite from 1073 to 1373 K. The real interfacial rate constant was estimated by considering the gas phase mass transfer along a plate. The relationship of the rate constant and CO2/CO ratio was expressed by the formula of kc = k0 (CO2/CO)−1−m, m represents the effect of CO2/CO ratio on the activity of oxygen in iron oxide. The apparent activation energy of the reaction between CO2–CO gas and magnetite was calculated to 161, 175 ± 15, 198, and 197 ± 16 kJ mol−1 for CO2/CO ratios of 4.0, 5.0, 6.0, and 10.0, respectively. This dependence relationship may be caused by the decrease of free electron in magnetite phase with the increase of CO2/CO ratio.

12Cii] and [13C ii] 158μm emission from NGC 2024: Large column densities of ionized carbon

Context: We analyze the NGC 2024 HII region and molecular cloud interface using [12CII] and [13CII] observations. Aims: We attempt to gain insight into the physical structure of the interface layer between the molecular cloud and the HII region. Methods. Observations of [12CII] and [13CII] emission at 158 {\mu}m with high spatial and spectral resolution allow us to study the detailed structure of the ionization front and estimate the column densities and temperatures of the ionized carbon layer in the PDR. Results: The [12CII] emission closely follows the distribution of the 8 mum continuum. Across most of the source, the spectral lines have two velocity peaks similar to lines of rare CO isotopes. The [13CII] emission is detected near the edge-on ionization front. It has only a single velocity component, which implies that the [12CII] line shape is caused by self-absorption. An anomalous hyperfine line-intensity ratio observed in [13CII] cannot yet be explained. Conclusions: Our analysis of the two isotopes results in a total column density of N(H)~1.6\times10^23 cm^-2 in the gas emitting the [CII] line. A large fraction of this gas has to be at a temperature of several hundred K. The self-absorption is caused by a cooler (T<=100 K) foreground component containing a column density of N(H)~10^22 cm^-2.

The structure of hot gas in Cepheus B

By observing radiation-affected gas in the Cepheus B molecular cloud we probe whether the sequential star formation in this source is triggered by the radiation from newly formed stars. We used the dual band receiver GREAT onboard SOFIA to map [C II] and CO 13--12 and 11--10 in Cep B and compared the spatial distribution and the spectral profiles with complementary ground-based data of low-$J$ transitions of CO isotopes, atomic carbon, and the radio continuum. The interaction of the radiation from the neighboring OB association creates a large photon-dominated region (PDR) at the surface of the molecular cloud traced through the photoevaporation of C^+. Bright internal PDRs of hot gas are created around the embedded young stars, where we detect evidence of the compression of material and local velocity changes; however, on the global scale we find no indications that the dense molecular material is dynamically affected.

The Toarcian in the Subbetic basin (southern Spain): Bio-events (ammonite and calcareous nannofossils) and carbon-isotope stratigraphy

A detailed carbon-isotope stratigraphic study for the uppermost Pliensbachian‒lowermost Aalenian interval in the Median Subbetic palaeogeographic domain (External zones of the Betic Cordillera, southern Spain) has been carried out. During the Early Jurassic, the Median Subbetic, which represents a typical basin of the Hispanic Corridor connecting the Tethys and the Eastern Pacific, was located in the westernmost Tethys. The analyzed sections encompass the entire Toarcian stage as represented in the southern Iberian palaeomargin. Rocks are mainly rhythmic sequences of grey marls and marly limestones containing a rich ammonite fauna, nannofossils, and benthic foraminifers—all these provide an accurate biostratigraphic control. The lower and upper Toarcian boundaries are well represented in some of these sections and therefore represent optimal sites to link the carbon-isotope curves to ammonite zones, and to nannofossil events. δ13C values of bulk carbonates from the different localities of the Subbetic basin have similar variations from the uppermost Pliensbachian to the lowermost Aalenian, suggesting changes in the original DIC carbon isotope composition along the Hispanic corridor. The transition from Pliensbachian to Toarcian is marked by increasing δ13C values from ~1.2 to 2.0‰, interrupted in the Serpentinum Zone by a negative shift concomitant with the Toarcian oceanic anoxic event (T-OAE), with the major ammonite extinction event of the Toarcian, and an important turnover of calcareous nannoplankton. The negative shift observed in the Serpentinum Zone confirms the global perturbation of the carbon cycling documented along the Tethys and the palaeo-Pacific in organic material and in marine carbonates. However, the amplitude of the negative excursion (~ −1. 5‰) is not compatible with an isotopic homogeneous seawater DIC and/or CO2 atmospheric reservoirs. The interval from the middle to the top of the Toarcian δ13C shows relatively constant values, minor ammonite turnovers, and is associated with increasing diversity of calcareous nannoplankton.

Molecular outflows toward methanol masers: detection techniques and their properties.

AbstractClass II methanol masers are thought to trace the brief phase in the evolution of a massive YSO, where outflows are expected to occur. Molecular line maps of the CO isotopes of a subset of 6.7 GHz sources from the MMB catalogue were observed with the JCMT telescope. Utilising optically thick 12CO, a search was done to detect broadened line wings (initially only on the source G20.08-0.13). The physical parameters of these detected lobes were then calculated.

CO 2–rock–brine interactions in Lower Tuscaloosa Formation at Cranfield CO 2 sequestration site, Mississippi, U.S.A.

A highly integrated geochemical program was conducted at the Cranfield CO 2-enhanced oil recovery (EOR) and sequestration site, Mississippi, U.S.A.. The program included extensive field geochemical monitoring, a detailed petrographic study, and an autoclave experiment under in situ reservoir conditions. Results show that mineral reactions in the Lower Tuscaloosa reservoir were minor during CO 2 injection. Brine chemistry remained largely unchanged, which contrasts with significant changes observed in other field tests. Field fluid sampling and laboratory experiments show consistently slow reactions. Carbon isotopic composition and CO 2 content in the gas phase reveal simple two-end-member mixing between injected and original formation gas. We conclude that the reservoir rock, which is composed mainly of minerals with low reactivity (average quartz 79.4%, chlorite 11.8%, kaolinite 3.1%, illite 1.3%, concretionary calcite and dolomite 1.5%, and feldspar 0.2%), is relatively unreactive to CO 2. The significance of low reactivity is both positive, in that the reservoir is not impacted, and negative, in that mineral trapping is insignificant.

The efficiency of the CO2‐concentrating mechanism during single‐cell C4 photosynthesis

The photosynthetic efficiency of the CO(2)-concentrating mechanism in two forms of single-cell C(4) photosynthesis in the family Chenopodiaceae was characterized. The Bienertioid-type single-cell C(4) uses peripheral and central cytoplasmic compartments (Bienertia sinuspersici), while the Borszczowioid single-cell C(4) uses distal and proximal compartments of the cell (Suaeda aralocaspica). C(4) photosynthesis within a single-cell raises questions about the efficiency of this type of CO(2) -concentrating mechanism compared with the Kranz-type. We used measurements of leaf CO(2) isotope exchange (Δ(13) C) to compare the efficiency of the single-cell and Kranz-type forms of C(4) photosynthesis under various temperature and light conditions. Comparisons were made between the single-cell C(4) and a sister Kranz form, S. eltonica[NAD malic enzyme (NAD ME) type], and with Flaveria bidentis[NADP malic enzyme (NADP-ME) type with Kranz Atriplicoid anatomy]. There were similar levels of Δ(13) C discrimination and CO(2) leakiness (Φ) in the single-cell species compared with the Kranz-type. Increasing leaf temperature (25 to 30 °C) and light intensity caused a decrease in Δ(13) C and Φ across all C(4) types. Notably, B. sinuspersici had higher Δ(13) C and Φ than S. aralocaspica under lower light. These results demonstrate that rates of photosynthesis and efficiency of the CO(2) -concentrating mechanisms in single-cell C(4) plants are similar to those in Kranz-type.

Theoretical calculations of the reaction cross-sections for proton-induced reactions on natural copper using ALICE-IPPE code

A theoretical study of the nuclear-reaction cross sections for proton-induced reactions on (63)Cu and (65)Cu was performed in the proton energy range from threshold values up to 50MeV. The produced nuclei were different isotopes of Zn, Cu, Ni, Co and Mn, some of which have important applications. The reaction cross-section calculations were performed using the ALICE-IPPE code, which depends on the pre-equilibrium compound nucleus model. This code is suitable for the studied energy and isotopic mass ranges. Approximately 14 excitation functions for the different reactions have been constructed from the calculated cross-section values. The excitation function curves for the proton reactions with natural copper targets have been constructed from those for enriched targets using the natural abundance of the copper isotopes. Comparisons between the calculated excitation functions with those previously experimentally measured are given whenever the experimental values were available. Some statistical parameters were introduced to control the quality of the fitting between both the experimental and the theoretical calculated cross-section values.

Interannual variability in the oxygen isotopes of atmospheric CO2 driven by El Niño

The stable isotope ratios of atmospheric CO(2) ((18)O/(16)O and (13)C/(12)C) have been monitored since 1977 to improve our understanding of the global carbon cycle, because biosphere-atmosphere exchange fluxes affect the different atomic masses in a measurable way. Interpreting the (18)O/(16)O variability has proved difficult, however, because oxygen isotopes in CO(2) are influenced by both the carbon cycle and the water cycle. Previous attention focused on the decreasing (18)O/(16)O ratio in the 1990s, observed by the global Cooperative Air Sampling Network of the US National Oceanic and Atmospheric Administration Earth System Research Laboratory. This decrease was attributed variously to a number of processes including an increase in Northern Hemisphere soil respiration; a global increase in C(4) crops at the expense of C(3) forests; and environmental conditions, such as atmospheric turbulence and solar radiation, that affect CO(2) exchange between leaves and the atmosphere. Here we present 30 years' worth of data on (18)O/(16)O in CO(2) from the Scripps Institution of Oceanography global flask network and show that the interannual variability is strongly related to the El Niño/Southern Oscillation. We suggest that the redistribution of moisture and rainfall in the tropics during an El Niño increases the (18)O/(16)O ratio of precipitation and plant water, and that this signal is then passed on to atmospheric CO(2) by biosphere-atmosphere gas exchange. We show how the decay time of the El Niño anomaly in this data set can be useful in constraining global gross primary production. Our analysis shows a rapid recovery from El Niño events, implying a shorter cycling time of CO(2) with respect to the terrestrial biosphere and oceans than previously estimated. Our analysis suggests that current estimates of global gross primary production, of 120 petagrams of carbon per year, may be too low, and that a best guess of 150-175 petagrams of carbon per year better reflects the observed rapid cycling of CO(2). Although still tentative, such a revision would present a new benchmark by which to evaluate global biospheric carbon cycling models.

The diel imprint of leaf metabolism on the δ13C signal of soil respiration under control and drought conditions

Recent (13) CO(2) canopy pulse chase labeling studies revealed that photosynthesis influences the carbon isotopic composition of soil respired CO(2) (δ(13) C(SR)) even on a diel timescale. However, the driving mechanisms underlying these short-term responses remain unclear, in particular under drought conditions. The gas exchange of CO(2) isotopes of canopy and soil was monitored in drought/nondrought-stressed beech (Fagus sylvatica) saplings after (13) CO(2) canopy pulse labeling. A combined canopy/soil chamber system with gas-tight separated soil and canopy compartments was coupled to a laser spectrometer measuring mixing ratios and isotopic composition of CO(2) in air at high temporal resolution. The measured δ(13) C(SR) signal was then explained and substantiated by a mechanistic carbon allocation model. Leaf metabolism had a strong imprint on diel cycles in control plants, as a result of an alternating substrate supply switching between sugar and transient starch. By contrast, diel cycles in drought-stressed plants were determined by the relative contributions of autotrophic and heterotrophic respiration throughout the day. Drought reduced the speed of the link between photosynthesis and soil respiration by a factor of c. 2.5, depending on the photosynthetic rate. Drought slows the coupling between photosynthesis and soil respiration and alters the underlying mechanism causing diel variations of δ(13) C(SR).