AAAS: "Life may have survived far north of equator during 'Snowball Earth'" "More than 600 M years ago, planet was frozen from pole to pole, covered in half-km-thick ice sheets that darkened every ocean." New geochemical evidence from ancient rocks suggests zones of open ocean may have been present north of the Tropic of Cancer. When temperatures drop, planetary ice caps expand, reflecting more sunlight + creating further cooling. "If the ice manages to creep to roughly 30° to 40° latitude—about where North Africa and the continental United States are today—the global climate enters a runaway freezing cycle and glaciers end up covering the entire planet within a few hundred years." The geologic record indicates at least 2 of these episodes have occurred, the more recent one — the Marinoan Ice Age — was from 654 to 635 million yrs ago. "To study conditions during this period, Xiao and his colleagues examined a thin layer of dark shale found in Shennnongjia National Forest in south China that dates to the Marinoan."   The shale was chock full of various algal fossils but also nitrogen compounds, suggesting oxygenated waters were present where N2 + O2 were able to freely interact. But Paul Hoffman, a geologist at Harvard University who pioneered the Snowball Earth hypothesis, argues  it's likelier the fossils were from algae in shallow polls of freshwater atop glaciers. More data needed.   #snowballearth    #paleogeology