Download Dates and dynamics: Snowball Earth comes of age

Geodaze 2015, Tucson, AZ
Dates and dynamics: Snowball Earth comes of age
Paul F. Hoffman
Dept of Earth & Planetary Sciences, Harvard University, Cambridge, MA
School of Earth & Ocean Sciences, University of Victoria, Victoria, BC
Abstract
Snowball Earth is a theoretically well-defined climate state in which the entire ocean and
most continental areas are covered by dynamic glaciers, driven by sublimation in the equatorial
zone of the sea glacier. Ice albedo instability is the trigger and deglaciation is self-induced, due
to the steady rise of atmospheric CO2 from normal volcanic outgassing in the absence of sinks
for carbon on the frozen planetary surface. Snowball Earth is invoked for two Cryogenian glacial
epochs when dynamic ice sheets are known from geological evidence to have existed on all
continents including low-latitude carbonate platforms. The snowball hypothesis makes three sets
of falsifiable predictions: (1) Cryogenian glacial epochs were long-lived (millions to tens of
millions of years), and both the onsets and terminations were globally synchronous at low
paleolatitudes; (2) deglaciations occurred under extreme (105 ppmv) atmospheric CO2 radiative
forcing and consequent ocean acidification, and were followed by very high surface temperatures
and intense weathering; and (3) the biosphere was irreversibly impacted. The first two prediction
sets and arguably the third are now strongly supported by observations unavailable when the
hypothesis was proposed. At the same time, climate models suggest that the snowball
atmosphere, cryosphere, ocean and lithosphere were more dynamic and unstable than initially
assumed, consistent with geological evidence. Many aspects of the phenomenon are counterintuitive, meaning that insights from modeling are essential to avoid testing false predictions.
The ultimate cause of Cryogenian refrigeration appears to have been basalt weathering following
emplacement of the 0.72-Ga Franklin large igneous province across paleoequatorial (present
Arctic) Laurentia during the breakup of supercontinent Rodinia. Biomarkers tentatively imply a
Cryogenian origin for multicellularity in animals and for the dominance of green algae among
eukaryotic primary producers in Phanerozoic oceans. Snowball Earth is a good example of why
we study old rocks: to discover things we never would if we only studied young rocks.
Paul Hoffman is a sedimentary and tectonic geologist with extensive field experience in
northern Canada and southern Africa. His best known papers are United plates of America,
the birth of a craton (1988), Did the breakout of Laurentia turn Gondwanaland inside-out?
(1991) and A Neoproterozoic snowball Earth (1998).