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

• 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
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2019.119319

•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 (