IRIDIUM ANOMALIES AT THE STURTIAN AND MARINOAN DIAMICTITE-CAP CARBONATE BOUNDARIES, CONGO CRATON: ESTIMATION OF THE DURATION AND INTENSITY OF NEOPROTEROZOIC SNOWBALL EARTH GLACIATIONS

The Neoproterozoic Era (1000-540 Ma) is characterized by at least two ice ages, one around 700 Ma (Sturtian glaciation) and the second around 600 Ma (Marinoan glaciation) ago, during which a large part of the Earth’s surface – probably the whole Earth - was covered by ice. The commonly favored models suggest that the glaciations ended in a super-greenhouse environment, which led to rapid melting of the ice cover and precipitation of the so-called cap carbonates. If the Earth were covered with ice, then extraterrestrial material (mostly in the form of interplanetary dust particles, but also from impact events) would have accumulated on and within the ice. During rapid melting, the extraterrestrial material would have been precipitated at the base of the cap carbonate deposits. We determined the content of the siderophile element iridium as a proxy of the content of extraterrestrial deposits. Three drillcores, two from Kipushi (Congo) and one from Chambishi (Zambia) from the Katangan succession on the Eastern Congo craton, show typical sequences of Neoproterozoic glacial deposits overlain by thick cap carbonate sequences. We measured a complete set of major and trace element contents, and isotopic ratios of carbon and sulfur, for a large sample set from these three cores. Ir anomalies were found in all cap carbonate successions, but only above the Marinoan ice age (time) could an estimation of the duration of the glacial episode be made. Such measurements at the transition between Neoproterozoic diamictites and cap carbonates allows us to estimate the duration of the Marinoan glacial episode at ~ 3 Ma. Ir anomalies in the cap carbonate following the Sturtian glaciation (time) are not located immediately above the glacial deposits and span a wider interval in the cap dolostone succession, compared to the sharp Marinoan Ir anomaly. This suggests that the sea was covered with ice during the Marinoan glaciation, capturing extraterrestrial Ir, which was then immediately deposited upon ice melting. In the Sturtian, the Ir anomaly is broader, suggesting that the sea was not covered with ice throughout the glacial epoch. Thus, the Sturtian Ir anomaly in the cap dolostone succession is the result of extraterrestrial Ir deposited on continental ice during a more gradual glacial regression, which was later transported by rivers into the sea.