Reconstructing early Earth environments

The most extreme climate transitions in Earth history are recorded by the juxtaposition of Neoproterozoic glacial deposits with overlying “cap” carbonate beds. Some of the most remarkable sedimentary structures within these beds are sharp-crested (trochoidal) bedforms with regular spacing of up to several meters, which are often interpreted as giant wave ripples formed under extreme wave conditions in a nonuniform postglacial climate. In a Geology article we evaluate this hypothesis using a new bedform stability diagram for symmetric oscillatory flows that indicates that the first-order control on the formation of trochoidal rather than hummocky bedforms is sediment size, not wave climate. New measurements of bedform wavelengths and particle sizes from the ca. 635 Ma Nuccaleena Fm, Australia, indicate that the giant ripples are generally composed of coarse to very coarse sand, with most observations falling within the trochoidal bedform stability phase space for normal wave climates. Moreover, numerical simulations of flow over fixed bedforms show that symmetric trochoidal ripples with a nearly vertical angle of climb may be produced over long time periods with variable wave climates in conjunction with rapid seabed cementation. These data reveal that, rather than extreme wave conditions, the giant wave ripples are a consequence of the unusual mode of carbonate precipitation during a global carbon-cycle perturbation unprecedented in Earth history.