SLOW NET SEDIMENT ACCUMULATION SETS SNOWBALL EARTH APART FROM ALL YOUNGER GLACIAL EPISODES (Invited Presentation)

The Snowball Earth events of the Cryogenian Era (~720 to 635 Ma) record extreme and long-lived glacial episodes. Although local glacial depositional process is reliably inferred from small-scale sedimentary features that resemble Cenozoic glacial deposits, this clearly does not imply comparable larger-scale or longer-term conditions – Cryogenian ice cover extended from polar to tropical latitudes and may have persisted for tens of millions of years. One of the predictions for Snowball Earth is a weakened or failed hydrological cycle; this would likely reduce sediment flux to the oceans. We test this idea by comparing accumulation rates (>500) for Cryogenian Snowball Earth episodes with a comprehensive rate database (>10,000 records) for glacial episodes of all ages and durations. We find that median accumulation rates of Cryogenian glacial sequences are 4 to 15 times slower than expected for Phanerozoic glacimarine deposits. Crucially, our comparison recognizes that expected Phanerozoic accumulation rates decrease systematically when averaged over longer intervals. Rates with which to compare Marinoan and Sturtian episodes (5 and 57 m.y. duration, respectively) were chosen to account for this dependence on time span. Two other factors affect modern (Cenozoic) glacimarine accumulation rates: they slow with increasing latitude from temperate to polar climates and with increasing distance from the ice margin. Snowball Earth accumulation rates are even slower than expected for high-latitude Cenozoic glacial deposits (despite evidence of deposition at low latitudes) and also slower than expected for distal glacimarine abyssal muds with ice-rafted debris. An anomalously low net sediment flux would be expected under a weakened hydrologic cycle in a “hard” Snowball Earth scenario, which provides one possible explanation. Whatever the cause, Cryogenian glacial episodes intriguingly record the lowest net glacimarine accumulation rates recorded in Earth’s history of glacial climates.