סמינר החוג לגיאופיסיקה ולמדעים אטמוספריים ופלנטריים

Eli Tziperman, Harvard University

Snowball-Earth events: Biologically induced initiation, and coupled
ice-ocean flows on a frozen ocean

Absract:

The hypothesized complete freezing of the Earth during the Snowball Earth events of the Neoproterozoic Era (1,000 to 542 Myr) poses several interesting and unanswered questions.  We'll discuss ice flow over a frozen ocean, the surprisingly dynamic ocean circulation under a kilometer of ice, and the possibility that these events were initiated by bacteria in the deep ocean.

An ocean covered by thick ice and driven only by a very weak geothermal heat flux (0.1 watts/m^2) is shown to be characterized by strong zonal jets, an energetic turbulent eddy field and a dramatic meridional overturning circulation limited to very close to the equator.  We discuss the dynamics of the mean zonal jets and meridional circulations, and the relevant eddy-generating instability
mechanisms.

These Snowball Earth events 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, possibly evolutionary, to ocean microorganisms.  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 by either sulfate- or iron-reducing bacteria.  This can lead to changes in carbonate alkalinity and dissolved inorganic pool that efficiently lower the atmospheric CO2 concentration, plunging Earth into an ice age.