Neoproterozoic glaciation: Reconciling low palaeolatitudes and the geologic record

Abstract

Paleomagnetic data for Neoproterozoic glacial deposits in South Australia and else-where verify glaciomarine deposition near the paleoequator. Tidal rhythmites from such deposits in South Australia display symmetrical ripples indicating decades of continuous wave activity, and also record the annual oscillation of sea level. In low and moderate latitudes the annual oscillation of sea level results mostly from seasonal changes in heat content of the sea, indicating extensive and long-lived open seas in low latitudes during Neoproterozoic glaciations. Neoproterozoic periglacial sand wedges 3+ m deep, marking polygons 10–30 m across, are closely comparable to periglacial wedges in present high latitudes and imply large (~40°C) seasonal changes of mean monthly temperature near the paleoequator. Periglacial wedges did not form at high elevations on the Pleistocene equator where temperatures were well below 0°C throughout the year and temperature fluctuations were mainly diurnal, which militates against diurnal fluctuations as the cause of the Neoproterozoic wedges. An extremely large (50%) octupole component of the geomagnetic field is required to make true moderate latitudes appear paleoequatorial, whereas Proterozoic paleomagnetic data suggest a maximum octupole component of ≤30%. A snowball or slushball Earth is difficult to reconcile with open seas and large seasonal temperature-changes in low paleolatitudes. A Proterozoic high obliquity (>54°) resulting from the Moon-producing single giant impact may explain glaciation and strong seasonality on the equator, but a mechanism is required to subsequently reduce the obliquity. At present the Neoproterozoic paleomagnetic and glacial records cannot be reconciled satisfactorily, demanding further wide-ranging research.