The late Ordovician glaciation was preceded by a fall in atmospheric carbon dioxide (from 7,000 ppm to 4,400 ppm). The dip is correlated with a burst of volcanic activity that deposited new silicate rocks, which draw CO2 out of the air as they erode. A major role of CO2 is implied by a 2009 paper. Atmospheric and oceanic CO2 levels may have fluctuated with the growth and decay of Gondwanan glaciation. Through the Late Ordovician, outgassing from major volcanism was balanced by heavy weathering of the uplifting Appalachian Mountains, which sequestered CO2. In the Hirnantian Stage the volcanism ceased, and the continued weathering caused a significant and rapid draw down of CO2. This coincides with the rapid and short ice age.