TY - JOUR
T1 - Preliminary exploration of molybdenum isotope fractionation during coprecipitation of molybdate with abiotic and microbial calcite
AU - Chen, Xinming
AU - Romaniello, Stephen J.
AU - Anbar, Ariel D.
N1 - Funding Information:
This work was supported by the U.S. National Science Foundation (Grant OCE-0952394 ), the NASA Exobiology Program , and FESD “Dynamics of Earth System Oxygenation” ( NSF EAR 1338810 to Anbar). The authors would like to thank students Jonah Boucher, Jennifer Crawford, Eric Nieminen, Kai Scarangella, Anneka Sheppard, Stephen Wisser, and Alyssa Sherry for their help with field work and sample processing in Prof. McCormick's Lab, and the New York State Parks, the Green Lake Park Staff and Rangers for their help. The authors also appreciate Matthew O. Clarkson and another anonymous reviewer for their detailed comments that help improve this manuscript.
Funding Information:
This work was supported by the U.S. National Science Foundation (Grant OCE-0952394), the NASA Exobiology Program, and FESD ?Dynamics of Earth System Oxygenation? (NSF EAR 1338810 to Anbar). The authors would like to thank students Jonah Boucher, Jennifer Crawford, Eric Nieminen, Kai Scarangella, Anneka Sheppard, Stephen Wisser, and Alyssa Sherry for their help with field work and sample processing in Prof. McCormick's Lab, and the New York State Parks, the Green Lake Park Staff and Rangers for their help. The authors also appreciate Matthew O. Clarkson and another anonymous reviewer for their detailed comments that help improve this manuscript.
Publisher Copyright:
© 2021 Elsevier B.V.
PY - 2021/4/5
Y1 - 2021/4/5
N2 - Molybdenum isotopes in marine carbonates have been proposed to track the redox conditions of oceans through time. Large variations of Mo concentration and isotopic composition were observed in primary biogenic carbonates and shallow-water carbonate sediments from modern oceans. However, it remains unknown whether the abiotic calcium carbonate precipitation process fractionates Mo isotopes and affects Mo isotopic composition in marine carbonates. To better understand carbonate Mo isotope proxy, we conducted abiotic calcite coprecipitation experiment to determine Mo partition coefficient in calcite and Mo isotope fractionation during molybdate incorporation into calcite. We also investigated Mo isotopic compositions in microbial calcite induced by photosynthetic activity of cyanobacteria and diatoms in the surface water of Fayetteville Green Lake (FGL), New York, USA. Our calcite coprecipitation experiment, for the first time, determined the Mo partition coefficient [Figure presented] = 1.49 × 10−5 in calcite at pH ~8.3. We observed a relatively small Mo isotope fractionation of 0.15 ± 0.06‰ with light Mo isotopes preferentially incorporated into calcite. The Mo isotope fractionation observed in our experiment most likely results from equilibrium isotope fractionation among different Mo species and preferential incorporation of the isotopically light Mo(VI) species (MoO42−) into calcite. Microbial calcite formed in FGL fractionated Mo isotopes in the same direction as abiotic calcite coprecipitation experiment but with significantly larger isotope fractionations (0.82–2.45‰) and significantly higher Mo concentrations (0.04–0.28 ppm vs. the inferred 0.002 ppm Mo from our abiotic calcite coprecipitation experiment). Molybdenum isotopic composition decreased with Mn/Fe ratios from +1.56 to −0.07‰, suggesting that Fe and Mn oxides dominantly control Mo isotope fractionation in these microbial calcite samples. Additionally, isotopically light Mo associated with organic matter (e.g., live or dead microorganisms like cyanobacteria) might also contribute to the light Mo isotopic composition in microbial calcite. The relatively small Mo isotope fractionation (0.15 ± 0.06‰) and Mo partition coefficient (1.49 × 10−5) in our abiotic calcite coprecipitation experiment indicate that calcite precipitation process has negligible effects on Mo isotopic composition in primary biogenic calcite precipitates. In contrast, Mo associated with organic matter, detrital particles, and Fe and Mn oxides, and carbonate diagenesis strongly affect Mo isotopic compositions in calcium carbonate precipitates in nature. Caution should be taken when applying Mo isotopes in ancient carbonates rocks to track seawater redox conditions.
AB - Molybdenum isotopes in marine carbonates have been proposed to track the redox conditions of oceans through time. Large variations of Mo concentration and isotopic composition were observed in primary biogenic carbonates and shallow-water carbonate sediments from modern oceans. However, it remains unknown whether the abiotic calcium carbonate precipitation process fractionates Mo isotopes and affects Mo isotopic composition in marine carbonates. To better understand carbonate Mo isotope proxy, we conducted abiotic calcite coprecipitation experiment to determine Mo partition coefficient in calcite and Mo isotope fractionation during molybdate incorporation into calcite. We also investigated Mo isotopic compositions in microbial calcite induced by photosynthetic activity of cyanobacteria and diatoms in the surface water of Fayetteville Green Lake (FGL), New York, USA. Our calcite coprecipitation experiment, for the first time, determined the Mo partition coefficient [Figure presented] = 1.49 × 10−5 in calcite at pH ~8.3. We observed a relatively small Mo isotope fractionation of 0.15 ± 0.06‰ with light Mo isotopes preferentially incorporated into calcite. The Mo isotope fractionation observed in our experiment most likely results from equilibrium isotope fractionation among different Mo species and preferential incorporation of the isotopically light Mo(VI) species (MoO42−) into calcite. Microbial calcite formed in FGL fractionated Mo isotopes in the same direction as abiotic calcite coprecipitation experiment but with significantly larger isotope fractionations (0.82–2.45‰) and significantly higher Mo concentrations (0.04–0.28 ppm vs. the inferred 0.002 ppm Mo from our abiotic calcite coprecipitation experiment). Molybdenum isotopic composition decreased with Mn/Fe ratios from +1.56 to −0.07‰, suggesting that Fe and Mn oxides dominantly control Mo isotope fractionation in these microbial calcite samples. Additionally, isotopically light Mo associated with organic matter (e.g., live or dead microorganisms like cyanobacteria) might also contribute to the light Mo isotopic composition in microbial calcite. The relatively small Mo isotope fractionation (0.15 ± 0.06‰) and Mo partition coefficient (1.49 × 10−5) in our abiotic calcite coprecipitation experiment indicate that calcite precipitation process has negligible effects on Mo isotopic composition in primary biogenic calcite precipitates. In contrast, Mo associated with organic matter, detrital particles, and Fe and Mn oxides, and carbonate diagenesis strongly affect Mo isotopic compositions in calcium carbonate precipitates in nature. Caution should be taken when applying Mo isotopes in ancient carbonates rocks to track seawater redox conditions.
KW - Abiotic calcite
KW - Biological effects
KW - Fayetteville Green Lake
KW - Fe and Mn oxides
KW - Isotope fractionation
KW - Microbial calcite
KW - Molybdenum
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U2 - 10.1016/j.chemgeo.2021.120102
DO - 10.1016/j.chemgeo.2021.120102
M3 - Article
AN - SCOPUS:85100402708
SN - 0009-2541
VL - 566
JO - Chemical Geology
JF - Chemical Geology
M1 - 120102
ER -