Neoproterozoic marine chemostratigraphy, or eustatic sea level change?

Abstract

Paleosols are characterized with new chemical and petrographic data from the Tonian (776–729 Ma) Chuar Group in the eastern Grand Canyon, Arizona. Paleosols have been discovered at 66 stratigraphic levels in a 1622 m sequence largely of marine shales and thin intertidal stromatolitic dolostones. A purple paleosol association includes gypsum “desert roses” in profiles like Gypsids of hyperarid climates. A red paleosol association includes calcareous nodules profiles like Calcids of semiarid climates. Degree of weathering and phosphorus depletion of the paleosols is evidence of microbial-earth communities gaining in productivity during the Tonian. The gypsic to calcic soil transition in the modern Atacama Desert of Chile corresponds with a transition from simple actinobacterial-cyanobacterial microbial earths to complex algal-fungal communities of markedly greater weathering effectiveness and carbon sequestration. A similar change in biological weathering may have induced Snowball Earth 12 Myrs after deposition of the Chuar Group. Paleosols are found only at levels of the Chuar Group where δ13C isotopic composition of both organic matter and of carbonate are below marine and mantle values. Although such inflections in isotopic time series have been considered chemostratigraphic markers of global oceanic composition, in this case we interpret them to reflect a change in sea level from shallow marine to terrestrial. A notable inflection at about 752 Ma in the Chuar Group corresponds with the recently recognized Konnarock Glaciation of Virginia, and may have been a time of global sea-level fall. Correlation of Precambrian marine sequences by wiggle matching isotopic time series may still be effective, but because of sea-level change rather than for the chemical oceanographic reasons originally envisaged.