Calcium isotopes as a record of the marine calcium cycle versus carbonate diagenesis during the late Ediacaran

•δ44Ca from limestones aged ∼550–539 Ma that host the earliest skeletal animal fossils.•Negative shift in δ44Ca that lasted at least 11–14Myrs.•Unlikely to record a transition towards more sediment-buffered carbonate diagenesis.•May record enhanced continental weathering or evaporite deposition.•May...

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Published in:Chemical geology 2019-12, Vol.529, p.119319, Article 119319
Main Authors: Tostevin, Rosalie, Bradbury, Harold J., Shields, Graham A., Wood, Rachel A., Bowyer, Fred, Penny, Amelia M., Turchyn, Alexandra V.
Format: Article
Language:English
Subjects:
Analytical methods
Biomineralisation
Calcium isotopes
Carbonate rocks
Ediacaran
Marine calcium cycle
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container_start_page 119319
container_title Chemical geology
container_volume 529
creator Tostevin, Rosalie
Bradbury, Harold J.
Shields, Graham A.
Wood, Rachel A.
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Penny, Amelia M.
Turchyn, Alexandra V.
description •δ44Ca from limestones aged ∼550–539 Ma that host the earliest skeletal animal fossils.•Negative shift in δ44Ca that lasted at least 11–14Myrs.•Unlikely to record a transition towards more sediment-buffered carbonate diagenesis.•May record enhanced continental weathering or evaporite deposition.•May record a change in timing of dolomitisation. Calcium isotope ratios in ancient carbonate rocks can provide insight into the global marine calcium cycle as well as local conditions during carbonate mineral precipitation and diagenesis. We compare two extraction techniques for the separation of calcium from other ions before δ44Ca analysis, using an automated ion chromatograph and using manual gravity columns. The two techniques produce the same δ44Ca within error (2σ). We present 31 δ44Ca analyses of carbonate rocks from the Nama Group, Namibia, which record a negative shift in δ44Ca of 0.35‰ between ∼550 and ∼547 Ma, from −1.25‰ to −1.60‰, followed by persistently low δ44Ca (−1.48±0.06‰) between ∼547 and 539 Ma. Very low δ44Ca (
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Calcium isotope ratios in ancient carbonate rocks can provide insight into the global marine calcium cycle as well as local conditions during carbonate mineral precipitation and diagenesis. We compare two extraction techniques for the separation of calcium from other ions before δ44Ca analysis, using an automated ion chromatograph and using manual gravity columns. The two techniques produce the same δ44Ca within error (2σ). We present 31 δ44Ca analyses of carbonate rocks from the Nama Group, Namibia, which record a negative shift in δ44Ca of 0.35‰ between ∼550 and ∼547 Ma, from −1.25‰ to −1.60‰, followed by persistently low δ44Ca (−1.48±0.06‰) between ∼547 and 539 Ma. Very low δ44Ca (&lt;−1.5‰) are commonly interpreted to represent the preservation of local aragonite that has recrystallized to calcite under sediment-buffered conditions (where the composition of the diagenetic carbonate product is determined mainly by the original sediments). The shift in δ44Ca across the Nama Group could therefore represent a change from fluid-buffered diagenesis (where the composition of the diagenetic carbonate mineral is determined mainly by the fluid) to sediment-buffered diagenesis. However, this interpretation is not consistent with either potential geochemical indicators of diagenesis (e.g., δ18O), or changes in large-scale fluid-flow as predicted from sequence stratigraphy. 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Calcium isotope ratios in ancient carbonate rocks can provide insight into the global marine calcium cycle as well as local conditions during carbonate mineral precipitation and diagenesis. We compare two extraction techniques for the separation of calcium from other ions before δ44Ca analysis, using an automated ion chromatograph and using manual gravity columns. The two techniques produce the same δ44Ca within error (2σ). We present 31 δ44Ca analyses of carbonate rocks from the Nama Group, Namibia, which record a negative shift in δ44Ca of 0.35‰ between ∼550 and ∼547 Ma, from −1.25‰ to −1.60‰, followed by persistently low δ44Ca (−1.48±0.06‰) between ∼547 and 539 Ma. Very low δ44Ca (&lt;−1.5‰) are commonly interpreted to represent the preservation of local aragonite that has recrystallized to calcite under sediment-buffered conditions (where the composition of the diagenetic carbonate product is determined mainly by the original sediments). The shift in δ44Ca across the Nama Group could therefore represent a change from fluid-buffered diagenesis (where the composition of the diagenetic carbonate mineral is determined mainly by the fluid) to sediment-buffered diagenesis. However, this interpretation is not consistent with either potential geochemical indicators of diagenesis (e.g., δ18O), or changes in large-scale fluid-flow as predicted from sequence stratigraphy. 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subjects Analytical methods
Biomineralisation
Calcium isotopes
Carbonate rocks
Ediacaran
Marine calcium cycle
title Calcium isotopes as a record of the marine calcium cycle versus carbonate diagenesis during the late Ediacaran
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