Global perturbation of the marine Ca isotopic composition in the aftermath of the Marinoan global glaciation

Abstract

Ca isotopic compositions of Marinoan post-glacial carbonate successions in Brazil and NW Canada were measured. Both basal dolostones display δ 44/40Ca values between 1 and 0.7‰, overlying limestones show a negative Ca isotope excursion to values around 0.1‰, and δ 44/40Ca values rapidly increase up-section to near 2.0‰. In the Brazilian successions, those high δ 44/40Ca values rapidly decrease and stabilize to values between 0.6 and 0.9‰. These Ca isotope secular variation trends are unlike those of Sturtian post-glacial carbonate successions, but similar to those of Marinoan post-glacial carbonate successions in Namibia, suggesting that the perturbation of the marine Ca cycle was global. This recommends Ca isotope stratigraphy as a tool to correlate Neoproterozoic post-glacial carbonate successions worldwide. While the lowermost and uppermost strata have δ 44/40Ca values typical of Phanerozoic carbonates, the extremes, 0.1 and 2.0‰, have not been thus far reported for other marine carbonates. These extreme values suggest a short-lived, non-actualistic perturbation in the marine Ca cycle. Simple box modelling of the Marinoan post-glacial marine Ca cycle can reproduce the extreme values only by postulating a two-step process, with Ca input initially exceeding Ca removal trough carbonate precipitation, followed by precipitation overtaking a decreased Ca input.