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

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 (<−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. We consider alternative interpretations for generating changes in the δ⁴⁴Ca of ancient carbonate rocks including enhanced continental weathering, increases in evaporite deposition, and changes in the style of dolomitisation.