Clues to a dramatic chapter of Earth's geological history

Date:
June 15, 2020
Source:
University of Rochester
Summary:
How could the planet be covered entirely in ice -- a state known as
'Snowball Earth'-- and still give rise to multicellular life? The
transition to such icy periods may not have been as abrupt as
previously thought, new research shows.



FULL STORY ========================================================================== Imagine Earth completely covered in ice. While it's hard to picture all of today's oceans and land masses obscured with glaciers, such an ice-covered version of the planet was not so far-fetched millions of years ago.


========================================================================== Lasting from approximately 1,000 to 540 million years ago, the dramatic
chapter is an important part of Earth's 4.5-billion-year history. Known
as the Neoproterozoic Era, the period of severe glaciation was a time
when multicellular organisms were beginning to diversify and spread
across the planet.

Many researchers posit that ice may have covered every surface of the
planet, stretching from the poles all the way to the hot tropics of
the equator -- a hypothesis known as "Snowball Earth." How was it
possible there was global ice -- even in the warmest areas of Earth? Researchers from the University of Rochester are shedding new light
on that question. By analyzing mineral data left by glaciers before
the onset of the Neoproterozoic Era, Scott MacLennan, a postdoctoral
research associate in the lab of Mauricio Ibanez-Mejia, an assistant
professor in the Department of Earth and Environmental Sciences, present
the first geological evidence that Earth may have had a cool climate
before Snowball Earth.

The study, published in Science Advances, provides important information
about a period of the planet's history that paved the way for the
development of complex life on Earth.



========================================================================== "This is a fascinating period, as these dramatic environmental changes
happened right as the first true animals were beginning to appear and
evolve on Earth," Ibanez-Mejia says.

WHAT CAUSED SNOWBALL EARTH? A critical aspect of understanding a
period of planetwide glaciation is determining what the climate was
like before Snowball Earth. Computer models indicate that a cool global
climate was necessary in order to initiate a Snowball Earth state, but
such a state has not been confirmed by geological evidence. Instead,
geological evidence has previously suggested that Earth had a warm and
ice-free climate immediately prior to the Neoproterozoic glaciation.

While scientists don't know the exact mechanisms that may have
caused Snowball Earth, they suspect that whatever they were, the
mechanisms involved a massive decrease in atmospheric carbon dioxide concentrations. There are several scenarios in which the atmospheric
carbon dioxide may have decreased. They include an increase in biomass
in the oceans, which may have taken carbon dioxide out of the atmosphere
and turned it into organic matter, or an increase in the weathering of
the continental crust, which also takes up carbon dioxide.

In order to determine whether these scenarios are feasible, however,
it's critical to know more about Earth's climate before the massive
glaciation events started.



==========================================================================
"If the Earth was very hot, it would mean the ocean was storing a lot
of heat, which would take a lot of time to get rid of in order to create
a Snowball Earth," MacLennan says.

UNLOCKING CLIMATE CLUES IN ROCKS Scientists can determine Earth's climate
at points in time by studying rocks that were deposited at different
times throughout Earth's history. MacLennan and his colleagues used
zircon dating methods to very precisely date glacial rocks found in
modern-day Virginia. Paleomagnetic data, which allows researchers to
determine where the continents were located thousands and even millions
of years ago, have established that Virginia was located in the middle
of a supercontinent within the tropics while the glacial rocks were
being deposited. The supercontinent later broke up into smaller parts.

The researchers discovered that the glacial rocks were actually deposited
30 million years before the first Snowball Earth. The observation was surprising because they had expected the glacial rocks to be related to
the Snowball Earth event. Instead, the discovery indicates that there
were glaciers in the tropics near the equator -- albeit at potentially
high altitudes -- even before Snowball Earth.

"The planet always gets colder away from the tropics and toward the poles because Earth receives most of its incoming sunlight at the equator,"
MacLennan says. "If there are glaciers in the tropics, the rest of the
planet must have also been very cold. This means that our previous vision
of a hot, humid world before the Snowball Earth is probably incorrect."
The potential trigger mechanism for the massive global cooling therefore
may not have been as extreme as some researchers believe; the planet
didn't immediately turn from a warm state to a frozen state but instead
appears to have experienced a more gradual cool-off into a Snowball
Earth state.

THE SURVIVAL OF LIFE IN THE NEOPROTEROZOIC ERA This research raises
interesting questions about what Earth was really like 800 to 700 million
years ago, before Snowball Earth events, during a time when interesting biological innovations were taking place as multicellular organisms were beginning to diversify.

"There have been a lot of questions about how multi- and single-cellular
life forms would survive the Snowball Earths, especially if there was
a rapid transition from a hot greenhouse world," MacLennan says. "Our
estimates for pre-Snowball climate imply the planet was probably colder
than the modern world, which means there may have been ample cold
environments at high latitude and altitude where organisms could have
adapted to these cold conditions."

========================================================================== Story Source: Materials provided by University_of_Rochester. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Scott A. MacLennan, Michael P. Eddy, Arthur J. Merschat, Akshay
K. Mehra,
Peter W. Crockford, Adam C. Maloof, C. Scott Southworth, Blair
Schoene.

Geologic evidence for an icehouse Earth before the Sturtian global
glaciation. Science Advances, 2020; 6 (24): eaay6647 DOI: 10.1126/
sciadv.aay6647 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200615140923.htm

--- up 20 weeks, 6 days, 2 hours, 34 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1337:3/111)