Despite a continuous increase in solar luminosity to the present, Earth’s glacial record appears to become more frequent,
though less severe, over geological time. At least two of the
three major Precambrian glacial intervals were exceptionally
intense, with solid evidence for widespread sea ice on or near
the equator, well within a “Snowball Earth” zone produced
by ice-albedo runaway in energy-balance models. The end
of the first unambiguously low-latitude glaciation, the early
Paleoproterozoic Makganyene event, is associated intimately
with the first solid evidence for global oxygenation, including the world’s largest sedimentary manganese deposit.
Subsequent low-latitude deglaciations during the Cryogenian
interval of the Neoproterozoic Era are also associated with
progressive oxidation, and these young Precambrian ice ages
coincide with the time when basal animal phyla were diversifying. However, specifically testing hypotheses of cause
and effect between Earth’s Neoproterozoic biosphere and
glaciation is complicated because large and rapid True Polar
Wander events appear to punctuate Neoproterozoic time and
may have episodically dominated earlier and later intervals
as well, rendering geographic reconstruction and age correlation challenging except for an exceptionally well-defined
global paleomagnetic database.
