Eugeni Barkan

Triple-isotope composition of atmospheric oxygen as a tracer of biosphere productivity

Oxygen has three naturally occurring isotopes, of mass numbers 16, 17 and 18. Their ratio in atmospheric O2 depends primarily on the isotopic composition of photosynthetically produced O2 from terrestrial and aquatic plants, and on isotopic fractionation due to respiration. These processes fractionate isotopes in a mass-dependent way, such that 17O enrichment would be approximately half of the 18O enrichment relative to 16O. But some photochemical reactions in the stratosphere give rise to a mass-independent isotope fractionation, producing approximately equal 17O and 18O enrichments in stratospheric ozone and carbon dioxide,, and consequently driving an atmospheric O2 isotope anomaly. Here we present an experimentally based estimate of the size of the 17O/16O anomaly in tropospheric O2, and argue that it largely reflects the influences of biospheric cycling and stratospheric photochemical processes. We propose that because the biosphere removes the isotopically anomalous stratosphere-derived O2 by respiration, and replaces it with isotopically ‘normal’ oxygen by photosynthesis, the magnitude of the tropospheric 17O anomaly can be used as a tracer of global biosphere production. We use measurements of the triple-isotope composition of O2 trapped in bubbles in polar ice to estimate global biosphere productivity at various times over the past 82,000 years. In a second application, we use the isotopic signature of oxygen dissolved in aquatic systems to estimate gross primary production on broad time and space scales.

High precision measurements of 17O/16O and 18O/16O ratios in H2O.

RATIONALE Measurements of δ(17)O and δ(18)O values of tropospheric CO(2) are of great importance. However, to be useful, such measurements must be an order of magnitude more precise than in current published literature. With this purpose we developed a new method for high-precision mass spectrometric measurements of (17)O/(16)O and (18)O/(16)O ratios in CO(2), which is presented in this study. METHODS The method is based on isotopic exchange equilibration between H(2)O and CO(2) in sealed glass ampoules followed by water fluorination to produce O(2). Dual inlet isotope ratio mass spectrometric (IRMS) measurements of the δ(17)O and δ(18)O values of this O(2) allow δ(17)O values of CO(2) to be obtained with very high precision (0.01 to 0.03 ‰). This method requires about 70 µmol of CO(2). RESULTS Measurements of standard CO(2) gas and atmospheric CO(2) yield reproducibility of 0.01 to 0.03 ‰ for both δ(17)O and δ(18)O values, and 5 per meg for (17)O(excess). Fractionation factors (17)α and (18)α were determined in the H(2)O-CO(2) equilibrium at 25 °C as 1.021254 ± 0.00004 and 1.041036 ± 0.00008, respectively, and the ratio ln(17)α/ln(18)α as 0.5229 ± 0.0001. CONCLUSIONS The results demonstrate that the new method is the most accurate analytical procedure ever presented for measurements of (17)O(excess) of CO(2) gas. It is suitable for measurements of CO(2) extracted from relatively small samples of air (~5 L), and is thus useful for monitoring the (17)O(excess) of CO(2) on a broad global scale.

Fractionation of the Three Stable Oxygen Isotopes by Oxygen-Producing and Oxygen-Consuming Reactions in Photosynthetic Organisms

The triple isotope composition (δ17O and δ18O) of dissolved O2 in the ocean and in ice cores was recently used to assess the primary productivity over broad spatial and temporal scales. However, assessment of the productivity with the aid of this method must rely on accurate measurements of the 17O/16O versus 18O/16O relationship in each of the main oxygen-producing and -consuming reactions. Data obtained here showed that cleavage of water in photosystem II did not fractionate oxygen isotopes; the δ18O and δ17O of the O2 evolved were essentially identical to those of the substrate water. The fractionation slopes for the oxygenase reaction of Rubisco and respiration were identical (0.518 ± 0.001) and that of glycolate oxidation was 0.503 ± 0.002. There was a considerable difference in the slopes of O2 photoreduction (the Mehler reaction) in the cyanobacterium Synechocystis sp. strain PCC 6803 (0.497 ± 0.004) and that of pea (Pisum sativum) thylakoids (0.526 ± 0.001). These values provided clear and independent evidence that the mechanism of O2 photoreduction differs between higher plants and cyanobacteria. We used our method to assess the magnitude of O2 photoreduction in cyanobacterial cells maintained under conditions where photorespiration was negligible. It was found that electron flow to O2 can be as high as 40% that leaving photosystem II, whereas respiratory activity in the light is only 6%. The implications of our findings to the evaluation of specific O2-producing or -consuming reactions, in vivo, are discussed.

Triple isotope composition of oxygen in atmospheric water vapor

[1] Recently, an excess of 17 O ( 17 O-excess) has been demonstrated in meteoric water and ice cores. Based on theory and experiments, it has been suggested that this excess originates from evaporation of ocean water into under-saturated air. However, there has never been direct demonstration of this excess in marine vapor. Here, we present results of the first measurements of δ 17 O and δ 18 O in vapor samples collected over the South Indian and the Southern Oceans. Our data show the existence of 17 O-excess in marine vapor and also clear negative correlation between 17 O-excess and relative humidity. Thus, 17 O-excess is useful for constraining oceanic humidity in hydrological and climatic models. Using the obtained values of 17 O-excess, we estimated the fractionation factor between H 18 2 O and H 16 2 O for diffusion in air above the ocean ( 18 α diff ). The new estimation of 18 α diff (1.008) is larger than the widely accepted value in hydrological studies.

Contribution of soil respiration in tropical, temperate, and boreal forests to the 18O enrichment of atmospheric O2

[1] The 18 O content of atmospheric O2 is an important tracer for past changes in the biosphere. Its quantitative use depends on knowledge of the discrimination against 18 O associated with the various O2 consumption processes. Here we evaluated, for the first time, the in situ 18 O discrimination associated with soil respiration in natural ecosystems. The discrimination was estimated from the measured [O2] and d 18 Oo f O2 in the soilair. The discriminations that were found are 10.1 ± 1.5%, 17.8 ± 1.0%, and 22.5 ± 3.6%, for tropical, temperate, and boreal forests, respectively, 17.9 ± 2.5% for Mediterranean woodland, and 15.4 ± 1.6% for tropical shrub land. Current understanding of the isotopic composition of atmospheric O2 is based on the assumption that the magnitude of the fractionation in soil respiration is identical to that of dark respiration through the cytochrome pathway alone (� 18%). The discrimination we found in the tropical sites is significantly lower, and is explained by slow diffusion in soil aggregates and root tissues that limits the O2 concentration in the consumption sites. The high discrimination in the boreal sites may be the result of high engagement of the alternative oxidase pathway (AOX), which has high discrimination associated with it (� 27%). The intermediate discrimination (� 18%) in the temperate and Mediterranean sites can be explained by the opposing effects of AOX and diffusion limitation that cancel out. Since soil respiration is a major component of the global oxygen uptake, the contribution of large variations in the discrimination, observed here, to the global Dole Effect should be considered in global scale studies. INDEX TERMS: 0315 Atmospheric Composition and Structure: Biosphere/ atmosphere interactions; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 1040 Geochemistry: Isotopic composition/chemistry; 1615 Global Change: Biogeochemical processes (4805); KEYWORDS: Dole Effect, oxygen isotopes, soil respiration Citation: Angert, A., et al., Contribution of soil respiration in tropical, temperate, and boreal forests to the 18 O enrichment of

Conversion of O2 into CO2 for High-Precision Oxygen Isotope Measurements

An improved procedure of (18)O/(16)O ratio measurements by means of oxygen conversion to CO(2) is developed, which allows one to obtain the true δ(18)O values with a precision of ±0.05‰ in oxygen samples down to 7 μmol. The isotopic exchange between quartz glass and oxygen gas was measured in the temperature range of 600-900 °C, and it was found to be less than 0.2%.

Reply to comment by Martin F. Miller on “Record of δ18O and 17O-excess in ice from Vostok Antarctica during the last 150,000 years”

that this excess was lower in glacial than in interglacial times. We further suggested that the change in 17 O-excess indicates smaller effect of kinetics signifying higher normalized humidity in the oceanic source region during glacials. In his comment, Miller [2008] questions the validity of these conclusions on three grounds. First, he emphasizes that the measurements are reported with respect to the VSMOW standard and not with respect to ocean water and therefore 17 O-excess with respect to the ocean may not exist. We note, however, that this point was raised by us [Landais et al., 2008, p. 3] and we emphasized the need for precise calibration with respect to seawater. Recently, we [Luz and Barkan, 2008] have made precise calibration and reported that the triple oxygen-isotope composition of VSMOW is similar to seawater. Therefore, we conclude that Antarctic precipitation unambiguously contains 17 O-excess with respect to the ocean.

The Triple Isotopic Composition of Oxygen in Leaf Water and Its Implications for Quantifying Biosphere Productivity

Publisher Summary Among the main processes that affect global climate changes, are the interactions between the atmosphere and biosphere. Climatic conditions control the biosphere productivity and, in turn, vegetation strongly influences climate through the emission and consumption of greenhouse gases and through the terrestrial albedo. The only effective way to understand these interactions is based on examination of the past climate changes and the associated biosphere evolution. This chapter describes some of the basic principles underlying the mass-dependent and mass-independent fractionation in the oxygen cycle. It then details how the change in the global oxygen-based biosphere productivity can be inferred from the past changes in the triple isotopic composition of O2. The necessity to know precisely the relationship between δ17O and δ18O during leaf transpiration, is emphasized. The results of the experimental studies on transpiration isotope effects that include— variations along a leaf, daily variations, the influence of plant species, and the effects of environmental and climatic conditions— are reported. Finally, these results are used to perform a global budget of the three isotopes of oxygen in the atmosphere and show the implications for the estimate of the ratio between the last glacial maximum (LGM) and the present-day oxygen biosphere productivities.

Species-specific imprint of the phytoplankton assemblage on carbon isotopes and the carbon cycle in Lake Kinneret, Israel

Abstract Lakes undergoing major changes in phytoplankton species composition are likely to undergo changes in carbon (C) cycling. In this study we used stable C isotopes to understand how the C cycle of Lake Kinneret, Israel, responded to documented changes in phytoplankton species composition. We compared the annual δ13C cycle of particulate organic matter from surface water (POMsurf) between (1) years in which a massive spring bloom of the dinoflagellate Peridinium gatunense occurred (Peridinium years) and (2) years in which it did not (non-Peridinium years). In non- Peridinium years, the spring δ13C–POMsurf maxima were lower by 3.3‰. These spring δ13C maxima were even lower in POM sinking into sediment traps and in zooplankton (lower by 6.8 and 6.9‰, respectively). These differences in the isotopic composition of the major organic C components in the lake represent ecosystem-level responses to the presence or absence of the key blooming species P. gatunense. When present, the intensive, almost monospecific bloom lowers the concentrations of CO2(aq), causing a reduction in the isotopic fractionation of the algae (higher δ13C of POMsurf) and massive precipitation of calcium carbonate (CaCO3). In non-Peridinium years, the phytoplankton cannot deplete CO2(aq) to similar levels; the algae maintain higher isotopic fractionation, leading to lower δ13C maxima. These changes are reflected higher up in the food web (zooplankton) and in sedimenting organic matter. The consequences for the ecosystem in non-Peridinium years are lower export of both organic and inorganic C.

High-precision measurements of δ17O and 17Oexcess of NBS19 and NBS18

RATIONALE Measurements of oxygen-17 excess ((17)Oexcess) in carbonates have become of great importance. However, to compare results obtained by different laboratories, it is necessary to normalize them to international standards. With this purpose in mind, we measured accurate and high precision δ(17)O and (17)Oexcess values for NBS19 and NBS18, two international standards, for which δ(18)O values are already widely used as the references for carbonates. METHODS The measurements are based on isotopic exchange at steady state between O2 and CO2 over hot platinum sponge at 750°C. Dual-inlet isotope ratio mass spectrometry (IRMS) measurements of the δ(17)O and δ(18)O values of this O2 allow δ(17)O values of CO2 to be obtained with very high precision (0.01 to 0.03‰) and, correspondingly, accurate (17)Oexcess values with a precision of 5 per meg. RESULTS We measured, for the first time, the δ(17)O values and (17)Oexcess of CO2 liberated from NBS19 at 25°C (39.196 ± 0.026; 20.276 ± 0.015‰ and -227 ± 4 per meg, respectively) and NBS18 (17.591 ± 0.041‰; 9.253 ± 0.018‰ and 3 ± 5 per meg, respectively). The values are given versus VSMOW. CONCLUSIONS Accurate values of δ(17)O and (17)Oexcess of the international standards NBS19-CO2 and NBS18-CO2 are now available. The new values should be used for normalization of measured oxygen isotope ratios of carbonates to allow meaningful comparison of results among different laboratories.