STABLE NI ISOTOPE FRACTIONATION IN SYSTEMS RELEVANT TO BANDED IRON FORMATION HALEIGH HOWE*, LEV SPIVAK-BIRNDORF, DERRICK NEWKIRK, LAURA WASYLENKI INDIANA UNIVERSITY, BLOOMINGTON, USA, [email protected]
Summary • In both adsorption and coprecipitation experiments light isotopes of Ni are
preferentially associated with ferrihydrite.
• Adsorption and coprecipitation experiments have the same magnitude of
fractionation suggesting the mechanism of Ni incorporation is similar in both
processes.
• The observed fractionations indicate an equilibrium fractionation, thus it may be
possible for the Ni isotopes of the water mass to be calculated from Ni isotope
composition of BIF.
• The impact on ferrihydrite crystal age and structure on Ni isotope fractionation during
adsorption is currently being investigated.
Introduction
An important event in the evolution of life was the rise of atmospheric oxygen during the Proterozoic. Associated with the rise in O2 was a decline in atmospheric methane concentrations, possibly due to decline of methanogenic Archaea.
Methanogens use Ni for cofactor F430, a catalyst during methanogenesis. To confirm Konhauser’s hypothesis, a proxy for methanogen productivity is necessary, in order to determine whether a decline in methanogen populations correlated with the observed decrease in Ni in rocks from the Archaean. Ni isotope ratios recorded in BIF (oceanic sediments consisting of layered iron oxides and cherts) may provide evidence of a decline in methane production.
Based on Ni concentrations in banded iron formations (BIF), Konhauser et al. (2009) hypothesized that mantle cooling during the Archaean reduced the amount of Ni present in igneous rocks and oceans, causing a Ni shortage for methanogens.
Cameron et al. (2009) have shown that methanogens preferentially assimilate light Ni isotopes. Thus Ni isotopes in BIF have potential use as biomarkers for methanogenesis.
Experiments
Cofactor F430 Jaun and Thauer (2007)
Konhauser et al.(2009)
Cameron et al. (2009)
• K.J. Zahnle, M.W. Claire, D.C. Catling, The loss of mass-independent fractionation of sulfur due to a Paleoproterozoic collapse of atmospheric methane, Geobiology, 4, 271-283 • Kurt Konhauser, Ernesto Pecoits, Stefan V. Lalonde, Dominic Papineau, Euan G. Nisbet, Mark E. Barley, Nicholas T. Arndt, Kevin Zahnle, Balz S. Kamber, Oceanic nickel depletion and
a methanogen famine before the Great Oxidation Event, 2009, Nature, 458, 750-753 • Bernhard Jaun, Rudolf K. Thauer, Methyl-coenzyme M reductase and its nickel corphin coenzyme F430 in Methanogenic Archaea, Nickel and its Surprising Impact in Nature, 2, ch8 • V. Cameron, D. Vance, C. Archer, C.H. House, 2009, A Biomarker Based on the Stable Isotopes of Nickel, Proceedings of the National Academy of Sciences, 106, 10944-10948 • U. Schwertmann, R.M. Cornell, Iron Oxides in the Laboratory, VCH. NY, NY. 1991
References
http://geology.indiana.edu/wasylenki/sesamelab/
In order to understand how Ni isotopes are recorded by BIF, abiotic mechanisms of Ni isotope fractionation must be investigated. During BIF deposition ferrihydrite was the dominant Fe oxide precipitating. Thus we investigated experimentally the relationship between Ni isotopes in solution and Ni associated with ferrihydrite.
Light isotopes of Ni preferentially adsorb to the surface of ferrihydrite nanoparticles, whereas heavy isotopes remain in solution. The magnitude of fractionation between adsorbed Ni and aqueous Ni is independent of %Ni adsorbed, suggesting Ni isotope fractionation is an equilibrium processes. The average Δ60NiLiquid-Solid is 0.33‰.
Light isotopes of Ni are preferentially associated with ferrihydrite, and heavy isotopes remain in solution. Similar to the adsorption results, the magnitude of fractionation in each experiment is independent of % Ni coprecipitated with ferrihydrite. Coprecipitation of Ni with ferrihydrite also causes equilibrium fractionation with an average Δ60NiLiquid-Solid of 0.33‰.
We conducted two series of experiments: adsorption of aqueous Ni onto surfaces of synthetic ferrihydrite and coprecipitation of aqueous Ni with ferrihydrite. In adsorption experiments varying amounts of Ni solution and ferrihydrite suspension were mixed, and shaken for 24 hours, then filtered. Ferrihydrite stock was precipitated 1-2 hours prior to use in adsorption experiments. In coprecipitation experiments aqueous Ni and Fe were mixed prior to precipitation of ferrihydrite. All experiments had pH 8-8.5. Recovered liquids and solids were passed through ion exchange columns to isolate Ni from other elements. Purified Ni samples were then analyzed using mass spectrometry.
Acknowledgements • NASA Exobiology Grant • Dr. David Bish for assisting with X-ray diffraction data • Steve Romaniello for assistance with data processing code
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δ60N
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% Ni adsorbed to ferrihydrite
Adsorption
LiquidSolid
Adsorption and Coprecipitation Experiment Results
Aged Ferrihydrite Adsorption Experiments
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% Ni coprecipitated with ferrihydrite
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% N
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Days ferrihydrite aged prior to adding Ni
Adsorption The impact of ferrihydrite age upon Ni isotope fractionation during adsorption is being investigated. A series of adsorption experiments were prepared using the same quantities of water, Ni stock, and ferrihydrite stock. The ferrihydrite stock was allowed to age for 30 days. Each experiment sampled the ferrihydrite stock at a different time. X-ray diffraction patterns were collected on the day of each experiment to track changes in the crystallinity of the ferrihydrite stock with time. A significant decrease in Ni sorption to ferrihydrite from 76% to 25% occurred within 24 hours. Ferrihydrite stock aged between 1-30 days sorbed on average 24% of Ni.
Starting composition
Starting composition
1 hour
2 days
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30 days
X-ray diffraction patterns of the ferrihydrite stock show that significant changes to the crystal structure occurred during the 30 day experiment. No significant changes to the ferrihydrite pattern were detected during the first seven days of aging. The change in amount of Ni sorbed to ferrihydrite occurred prior to changes in the crystal structure of the bulk stock. Ni isotopes of the experiments will be measured soon to determine if any changes in isotope composition occurred as result of changes in bulk crystal structure.
h g g
g h h
h
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g = goethite peak h = hematite peak
ferrihydrite peak Artifact of mount
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