Sack Lunch Seminar (SLS)

Eli Tziperman (Harvard) - Biologically induced snowball initiation

The glaciations of the Neoproterozoic Era (1,000 to 542 Myr BP) were
preceded by dramatically light 13C isotopic excursions preserved in
pre-glacial deposits. Standard explanations of these excursions
involve remineralization of isotopically light organic matter and
imply strong enhancement of atmospheric CO2 greenhouse gas
concentration, apparently inconsistent with the glaciations that
followed. We propose a scenario in which the isotopic signal, as well
as the global glaciation, result from changes to the ocean biology,
possibly due to the evolution of microorganisms, specifically
eukaryotes. These changes may have lead to enhanced export of organic
matter from the upper ocean into anoxic sub-surface waters and
sediments. The organic matter undergoes anoxic remineralization at
depth via either sulfate- or iron-reducing bacteria. In both cases
this can lead to changes in carbonate alkalinity and dissolved
inorganic pool that efficiently lower the atmospheric CO2
concentration, possibly plunging Earth into an ice age. Time
permitting, we will also discuss the problem of two dimensional ice
flow over a snowball ocean, showing how the existence of semi-enclosed
seas may lead to strong sea-ice thickness gradients.