scholarly journals Investigating chromium cycling in global oxygen deficient zones with chromium isotopes

2021 ◽  
Author(s):  
◽  
Tianyi Huang
Keyword(s):  
North Pacific ◽  
Isotope Ratio ◽  
Arabian Sea ◽  
Isotope Effects ◽  
Redox Conditions ◽  
Processing Conditions ◽  
Fractionation Factor ◽  
Redox Species ◽  
Seawater Samples ◽  
Isotope Partitioning

Chromium (Cr) isotopes have shown great potential as a paleo-redox proxy to trace the redox conditions of ancient oceans and atmosphere. However, its cycling in modern environments is poorly constrained. In my thesis, I attempt to fill in the gap of our understanding of chromium cycling in the modern ocean, with a focus on the redox processes in global oxygen deficient zones (ODZs). Firstly, we developed a method to analyze Cr isotopes of different Cr redox species. Tests on processing conditions demonstrated its robustness in obtaining accurate Cr isotope data. It is applicable to both frozen and fresh samples. This method allows us to investigate the redox cycling of Cr that is hard to unravel by existing total Cr methods. Secondly, in the Eastern Tropical North Pacific (ETNP), Eastern Tropical South Pacific (ETSP) and Arabian Sea ODZs, their total dissolved Cr profiles show preferential reduction of isotopically light Cr(VI) to Cr(III), which is scavenged and exported to deeper oceans. Applying our new method to ETNP and ETSP ODZ seawater samples, we observed Cr(VI) reduction in both ODZs with a similar fractionation factor. This indicates similar mechanisms may be controlling Cr(VI) reduction in the two ODZs. Cr(III) maximum coincides with Fe(II) and secondary nitrite maximums in the upper core of both ODZs. Shipboard incubations with spiked Fe(II) showed fast Cr(VI) reduction occurring in the ETNP ODZ. But neither Fe(II) nor microbes were reducing Cr(VI) directly. Thirdly, we calculated the isotope effects of Cr scavenging in the ETNP and ETSP ODZs. Thetwo ODZs show a similar isotope partitioning during Cr scavenging. And spatial variability is observed in the ETNP ODZ. Our calculated scavenged Cr isotope ratio is lighter than that of the total dissolved Cr from the same depth. It is also comparable to that of reducing or anoxic sediments, which implies that Cr isotopes can be used as an archive for local redox conditions.

scholarly journals Nitrogen cycling in shallow low-oxygen coastal waters off Peru from nitrite and nitrate nitrogen and oxygen isotopes

2016 ◽  
Vol 13 (5) ◽  
pp. 1453-1468 ◽  
Author(s):  
Happy Hu ◽  
Annie Bourbonnais ◽  
Jennifer Larkum ◽  
Hermann W. Bange ◽  
Mark A. Altabet
Keyword(s):  
Isotope Fractionation ◽  
Coastal Upwelling ◽  
Isotope Effects ◽  
Low Oxygen ◽  
N Loss ◽  
Fractionation Factor ◽  
High Productivity ◽  
O Isotopes ◽  
Subsequent Loss ◽  
Matter Flux

Abstract. O2 deficient zones (ODZs) of the world's oceans are important locations for microbial dissimilatory nitrate (NO3−) reduction and subsequent loss of combined nitrogen (N) to biogenic N2 gas. ODZs are generally coupled to regions of high productivity leading to high rates of N-loss as found in the coastal upwelling region off Peru. Stable N and O isotope ratios can be used as natural tracers of ODZ N-cycling because of distinct kinetic isotope effects associated with microbially mediated N-cycle transformations. Here we present NO3− and nitrite (NO2−) stable isotope data from the nearshore upwelling region off Callao, Peru. Subsurface oxygen was generally depleted below about 30 m depth with concentrations less than 10 µM, while NO2− concentrations were high, ranging from 6 to 10 µM, and NO3− was in places strongly depleted to near 0 µM. We observed for the first time a positive linear relationship between NO2−δ15N and δ18O at our coastal stations, analogous to that of NO3− N and O isotopes during NO3− uptake and dissimilatory reduction. This relationship is likely the result of rapid NO2− turnover due to higher organic matter flux in these coastal upwelling waters. No such relationship was observed at offshore stations where slower turnover of NO2− facilitates dominance of isotope exchange with water. We also evaluate the overall isotope fractionation effect for N-loss in this system using several approaches that vary in their underlying assumptions. While there are differences in apparent fractionation factor (ε) for N-loss as calculated from the δ15N of NO3−, dissolved inorganic N, or biogenic N2, values for ε are generally much lower than previously reported, reaching as low as 6.5 ‰. A possible explanation is the influence of sedimentary N-loss at our inshore stations which incurs highly suppressed isotope fractionation.

Triple isotope effects accompanying evaporation of water: new insights from laboratory experiments

2020 ◽  
Author(s):  
Anna Pierchala ◽  
Kazimierz Rozanski ◽  
Marek Dulinski ◽  
Zbigniew Gorczyca ◽  
Robert Czub
Keyword(s):  
Surface Water ◽  
Liquid Phase ◽  
Laboratory Experiments ◽  
Isotope Effects ◽  
Water Bodies ◽  
Fractionation Factor ◽  
Preparation Methods ◽  
Mass Spectrom ◽  
Health Studies ◽  
Surface Water Bodies

<p>Stable isotopes of hydrogen and oxygen (<sup>2</sup>H and <sup>18</sup>O) are often used for quantification of water budgets of lakes and other surface water bodies, in particular for the assessment of underground components of those budgets [1]. Recent advances in laser spectroscopy enabled simultaneous analyses of <sup>2</sup>H, <sup>18</sup>O and <sup>17</sup>O content in water, with measurement uncertainties comparable (δ<sup>18</sup>O) or surpassing (δ<sup>2</sup>H) those routinely achieved by off-line sample preparation methods combined with conventional IRMS technique [2]. This open up the doors for improving reliability of isotope-aided budgets of surface water bodies by adding third isotope tracer (<sup>17</sup>O). This, however, requires adequate information on triple isotope effects accompanying evaporation of water, in particular the kinetic isotope effect related to evaporation of <sup>1</sup>H<sub>2</sub><sup>17</sup>O isotopologue.</p><p>Here we present the results of dedicated laboratory experiments aimed at quantification of triple isotope effects accompanying evaporation of water under fully developed diffusive sublayer [3]. Identical containers with predefined mass of water of known isotopic composition were placed in an isolated chamber with controlled atmosphere during the experiment (temperature, relative humidity). The chamber was flushed with synthetic air. At regular time intervals (approximately one week) containers were removed one by one from the chamber, the remaining mass of water in the removed container was determined gravimetrically, and stored for subsequent isotope analyses. The flow rate was adjusted at each step of the process to keep humidity inside the chamber constant. Evaporation continued until approximately half of the initial mass of water was removed from the containers. The experiment was repeated under diiferent conditions inside the chamber (two different temperatures and three different values of relative humidty).</p><p>The results of the experiments were interpreted in the framework of Craig-Gordon model of evaporation [3]. It turned out that the assumption often used in the description of isotopic effects accompanying evaporation that liquid phase is isotopically homogeneous during the process, leads to conflicting results for three isotope systems in use. However, if surface enrichment of the liquid phase, different for each heavy isotopologue (<sup>1</sup>H<sup>2</sup>H<sup>16</sup>O, <sup>1</sup>H<sub>2</sub><sup>18</sup>O, <sup>1</sup>H<sub>2</sub><sup>17</sup>O) is included in the model, consistent results for all three isotopes can be achieved, with calculated kinetic fractionation factor for <sup>1</sup>H<sub>2</sub><sup>17</sup>O isotopologue equal 14.76 ± 0.48 ‰,. This value agrees, within the quoted uncertainty, with the value of 14.60 ± 0.30 ‰ obtained by Barkan and Luz [4].  </p><p>Acknowledgements: The presented work was supported by National Science Centre (research grant No. 2016/23/B/ST10/00909) and by the Ministry of Science and Higher Education (project no. 16.16.220.842 B02)</p><p>References:<br>[1]   Rozanski K. Froehlich K. Mook WG. Technical Documents in Hydrology, No. 39, Vol. III, UNESCO, Paris, 2001 117 pp.<br>[2]   Pierchala A, Rozanski K, Dulinski M, Gorczyca Z, Marzec M, Czub R, Isotopes in Environmental and Health Studies, 2019 (55) 290-307.<br>[3]   Horita, J. Rozanski K. Cohen S. 2007. Isotopes in Environmental and Health Studies, 2007 (44) 23-49.<br>[4]   Barkan E. Luz B. Rapid Commun. Mass Spectrom., 2007(21) 2999-3005.</p>

Isotopic composition of nitrate in the central Arabian Sea and eastern tropical North Pacific: A tracer for mixing and nitrogen cycles

1998 ◽  
Vol 43 (7) ◽  
pp. 1680-1689 ◽  
Author(s):  
Jay A. Brandes ◽  
Allan H. Devol ◽  
T. Yoshinari ◽  
D. A. Jayakumar ◽  
S. W. A. Naqvi
Keyword(s):  
Isotopic Composition ◽  
North Pacific ◽  
Arabian Sea ◽  
Nitrogen Cycles

scholarly journals 210Po/210Pb Disequilibria in the Eastern Tropical North Pacific

2021 ◽  
Vol 8 ◽  
Author(s):  
Qiang Ma ◽  
Yusheng Qiu ◽  
Run Zhang ◽  
E Lv ◽  
Yipu Huang ◽  
...  
Keyword(s):  
Water Column ◽  
North Pacific ◽  
Biological Activities ◽  
Small Scale ◽  
Horizontal Transport ◽  
Activity Ratios ◽  
Scale Particle ◽  
Seawater Samples ◽  
Scale Variation

The 210Po/210Pb disequilibrium was attempted to reveal the small-scale particle dynamics in the eastern tropical North Pacific. Seawater samples in the full water column were collected from three sites in the Tehuantepec bowl near the East Pacific Ridge for determination of dissolved and particulate 210Po and 210Pb. Our results show that TPo/TPb activity ratios in the full water column at the three sites are less than 1, with an average of 0.56, indicating that the total 210Po in the oligotrophic sea is significantly deficient. The activity ratios of DPo/DPb in the dissolved phase are less than 1, while those in the particulate phase are greater than 1 (except for the bottom 300 m), indicating fractionation between 210Po and 210Pb in the scavenging process. A negative linear relationship between 210Po deficit and silicate proves that biological activities are responsible for 210Po deficiency in the upper 200 m. However, the deficit of 210Po in the bottom 300 m may be caused by the horizontal transport of the hydrothermal plume. After correcting the horizontal contribution, the removal rates of 210Po for the 200–1,500 m and the bottom 300 m layers increased by 7.5–21 and 26.1–29.5%, respectively. Correspondingly, the variation range of the residence time of a total 210Po became smaller. Our calculations suggest that horizontal transport is acting as a stabilizer for small-scale variation in the 210Po deficit in the eastern tropical North Pacific. Our study highlights the need to pay more attention to the small-scale variation of 210Po deficit when applying 210Po/210Pb disequilibria to trace biogeochemical processes, and the mechanism responsible for this variation deserves further study.

scholarly journals Iron-Manganese System for Preparation of Radiocarbon Ams Targets: Characterization of Procedural Chemical-Isotopic Blanks and Fractionation

Radiocarbon ◽  
1997 ◽  
Vol 39 (3) ◽  
pp. 269-283 ◽  
Author(s):  
R. Michael Verkouteren ◽  
Donna B. Klinedinst ◽  
Lloyd A. Currie
Keyword(s):  
Sample Size ◽  
Isotope Effects ◽  
Isotopic Fractionation ◽  
Chemical Yield ◽  
Standard Reference Materials ◽  
Methane Formation ◽  
Fractionation Factor ◽  
Beam Geometry ◽  
C Fibers ◽  
Counting Statistics

We report a practical system to mass-produce accelerator mass spectrometry (AMS) targets with 10–100 μg carbon samples. Carbon dioxide is reduced quantitatively to graphite on iron fibers via manganese metal, and the Fe-C fibers are melted into a bead suitable for AMS. Pretreatment, reduction and melting processes occur in sealed quartz tubes, allowing parallel processing for otherwise time-intensive procedures.Chemical and isotopic (13C, 14C) blanks, target yields and isotopic fractionation were investigated with respect to levels of sample size, amounts of Fe and Mn, pretreatment and reduction time, and hydrogen pressure. With 7-day pretreatments, carbon blanks exhibited a lognormal mass distribution of 1.44 μg (central mean) with a dispersion of 0.50 μg (standard deviation). Reductions of 10 μg carbon onto targets were complete in 3–6 h with all targets, after correction for the blank, reflecting the 13C signature of the starting material. The 100 μg carbon samples required at least 15 h for reduction; shorter durations resulted in isotopic fractionation as a function of chemical yield. The trend in the 13C data suggested the presence of kinetic isotope effects during the reduction. The observed CO2-graphite 13C fractionation factor was 3–4% smaller than the equilibrium value in the simple Rayleigh model. The presence of hydrogen promoted methane formation in yields up to 25%.Fe-C beaded targets were made from NIST Standard Reference Materials and compared with graphitic standards. Although the 12C ion currents from the beads were one to two orders of magnitude lower than currents from the graphite, measurements of the beaded standards were reproducible and internally consistent. Measurement reproducibility was limited mainly by Poisson counting statistics and blank variability, translating to 14C uncertainties of 5–1% for 10–100 μg carbon samples, respectively. A bias of 5–7% (relative) was observed between the beaded and graphitic targets, possibly due to variations in sputtering fractionation dependent on sample size, chemical form and beam geometry.

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