TY - JOUR
T1 - Calcium isotopic fractionation in microbially mediated gypsum precipitates
AU - Harouaka, Khadouja
AU - Mansor, Muammar
AU - Macalady, Jennifer L.
AU - Fantle, Matthew S.
N1 - Funding Information:
M.S.F. acknowledges support from NSF grant EAR-0959092 and NASA grant NASA-NAI-NNA09DA76A (Penn State Astrobiology Center, PSARC). J.L.M acknowledges support from the National Science Foundation (NSF EAR-0525503) and NASA grant NASA-NAI-NNA09DA76A (Penn State Astrobiology Center, PSARC). K.H. acknowledges a research assistantship support from PSARC, the NASA Earth and Space Science Graduate Fellowship (NESSF), and invaluable technical support from Ana Kolevica (GEOMAR) and Henry Gong, Julie Anderson, Denny Walizer, Karol Confer, Matthew Gonzales, Dr. Huimin Yu, and Dr. Scott Hynek (Penn State). K.H. appreciates valuable advice on Ca isotope measurements on the Neptune from Stephen Romaniello and Gwyneth Gordon at Arizona State University. The authors deeply appreciate the opportunity to conduct analyses in the laboratory of Dr. Anton Eisenhauer (GEOMAR), without whose generosity the Frasassi Ca isotope data would not have been collected. M.S.F. thanks Shaun Brown (Berkeley) profusely for his assistance with the gypsum analyses at the CIG, and Dr. Donald J. DePaolo for generously allowing access to the CIG facilities. The authors also thank Sandro Galdenzi for interesting and useful discussions of the caves and for providing samples. We also thank Alessandro Montanari (Osservatorio Geologico di Coldigioco) for laboratory space and logistical support during field work in Italy, and speleologists Sandro Mariani and Simone Cerioni for assistance in the field. Finally, the authors thank three anonymous reviewers and AE Jacobson for their constructive and detailed reviews, which improved the manuscript.
PY - 2016/7/1
Y1 - 2016/7/1
N2 - Gypsum (CaSO4·2H2O) precipitation experiments were carried out at low pH in the presence of the sulfur oxidizing bacterium Acidithiobacillus thiooxidans. The observed Ca isotopic fractionation (expressed as δ44/40Cas-f = δ44/40Casolid-δ44/40Cafluid) at the end of each experimental time period (~50 to 60 days) was -1.41‰ to -1.09‰ in the biotic experiments, -1.09‰ in the killed control, and -1.01‰ to -0.88‰ in the abiotic controls. As there were no strong differences in the solution chemistry and the rate at which gypsum precipitated in the biotic and abiotic controls, we deduce a biological Ca isotope effect on the order of -0.3‰. The isotope effect correlates with a difference in crystal aspect ratios between the biotic experiments (8.05 ± 3.99) and abiotic controls (31.9 ± 8.40). We hypothesize that soluble and/or insoluble organic compounds selectively inhibit crystal growth at specific crystal faces, and that the growth inhibition affects the fractionation factor associated with gypsum precipitation. The experimental results help explain Ca isotopic variability in gypsum sampled from a sulfidic cave system, in which gypsum crystals exhibiting a diversity of morphologies (microcrystalline to cm-scale needles) have a broad range of δ44/40Ca values (~1.2-0.4‰) relative to the limestone wall (δ44/40Ca = 1.3‰). In light of the laboratory experiments, the variation in Ca isotope values in the caves can be interpreted as a consequence of gypsum precipitation in the presence of microbial organic matter and subsequent isotopic re-equilibration with the Ca source.
AB - Gypsum (CaSO4·2H2O) precipitation experiments were carried out at low pH in the presence of the sulfur oxidizing bacterium Acidithiobacillus thiooxidans. The observed Ca isotopic fractionation (expressed as δ44/40Cas-f = δ44/40Casolid-δ44/40Cafluid) at the end of each experimental time period (~50 to 60 days) was -1.41‰ to -1.09‰ in the biotic experiments, -1.09‰ in the killed control, and -1.01‰ to -0.88‰ in the abiotic controls. As there were no strong differences in the solution chemistry and the rate at which gypsum precipitated in the biotic and abiotic controls, we deduce a biological Ca isotope effect on the order of -0.3‰. The isotope effect correlates with a difference in crystal aspect ratios between the biotic experiments (8.05 ± 3.99) and abiotic controls (31.9 ± 8.40). We hypothesize that soluble and/or insoluble organic compounds selectively inhibit crystal growth at specific crystal faces, and that the growth inhibition affects the fractionation factor associated with gypsum precipitation. The experimental results help explain Ca isotopic variability in gypsum sampled from a sulfidic cave system, in which gypsum crystals exhibiting a diversity of morphologies (microcrystalline to cm-scale needles) have a broad range of δ44/40Ca values (~1.2-0.4‰) relative to the limestone wall (δ44/40Ca = 1.3‰). In light of the laboratory experiments, the variation in Ca isotope values in the caves can be interpreted as a consequence of gypsum precipitation in the presence of microbial organic matter and subsequent isotopic re-equilibration with the Ca source.
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U2 - 10.1016/j.gca.2016.03.003
DO - 10.1016/j.gca.2016.03.003
M3 - Article
AN - SCOPUS:84964829638
SN - 0016-7037
VL - 184
SP - 114
EP - 131
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
ER -