“icehouse” (cold) Climates

Earth's climate has changed, within life-sustaining bounds, from warm to cool intervals, on scales from thousands to hundreds of millions of years. In the Phanerozoic Eon there have been three intervals of glaciation (Ordovician, Carboniferous and Cenozoic) lasting tens of millions of years, with ice down to sea level at mid-latitudes (Frakes et al., 1992; Crowell, 1999). These cool " ice-house " intervals were generally times of lower sea level, lower CO 2 percentage in the atmosphere, less net photosynthesis and carbon burial, and less oceanic volcanism than during alternating " greenhouse " intervals (Fischer, 1986). The transitions from Pha-nerozoic icehouse to greenhouse intervals were synchronous with some biotic crises or mass extinction events, reflecting complex feedbacks between the biosphere and the hydrosphere. Figure I8 summarizes Earth's entire paleoclimate history, and Figure I9 shows the better-known Phanerozoic Eon, with carbon, strontium and sulfur isotopic ratios that are linked to major climate changes. Figure I10 shows an anti-correlation between atmospheric CO 2 levels and d 18 O values (proxy for oceanic temperature), which tracks the latitude of ice-rafted glacial debris. The Cryogenian Period of Neoproterozoic time (about 750–580 Ma) contains rocks deposited in two or more severe Icehouse intervals (Harland, 1964; Knoll, 2000). Laminated cap carbonates with depleted d 13 C ratios are found on top of glacial marine diamictites in many successions (Kauffman et al., 1997). The sharp juxtaposition of icehouse versus greenhouse deposits has led some to suggest that rapid and extreme climate changes took place in Neoproterozoic time. The Snowball Earth hypothesis proposes that during these Neoproterozoic glaciations, the world ocean froze over. The cap carbonates are thought to have been deposited during a subsequent alkali-nity event, caused by rapid warming and supersaturation of sea water on shallow continental shelves (Hoffman et al.