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Saturday, June 28, 2014

A shift in ice ages long ago

Sou | 6:24 AM Go to the first of 10 comments. Add a comment

Today I came across an interesting article about ocean currents and ice ages, which was published in the early edition of Science on Thursday.

The paper is by Leopoldo Pena and Steven Goldstein and it's about what happened a very long time ago. Around 900,000 years ago, glaciations shifted from happening about once every 41,000 years to only once every 100,000 years. What the scientists describe was a major disruption to the thermohaline circulation between 950,000 years ago and 860,000 years ago. They say that the slowing, or perhaps cessation of deep ocean currents drew down a lot of CO2 from the atmosphere and stabilised the glacials at every 100,000 years.

From ScienceDaily.com:
In a new study in the journal Science, researchers found that the deep ocean currents that move heat around the globe stalled or even stopped, possibly due to expanding ice cover in the north. The slowing currents increased carbon dioxide storage in the ocean, leaving less in the atmosphere, which kept temperatures cold and kicked the climate system into a new phase of colder but less frequent ice ages, they hypothesize.
"The oceans started storing more carbon dioxide for a longer period of time," said Leopoldo Pena, the study's lead author, a paleoceanographer at Columbia University's Lamont-Doherty Earth Observatory. "Our evidence shows that the oceans played a major role in slowing the pace of ice ages and making them more severe."
The researchers reconstructed the past strength of earth's system of deep-ocean currents by sampling deep-sea sediments off the coast of South Africa, where powerful currents originating in the North Atlantic Ocean pass on their way to Antarctica. How vigorously those currents moved in the past can be inferred by how much North Atlantic water made it that far, as measured by isotope ratios of the element neodymium bearing the signature of North Atlantic seawater. Like a tape recorder, the shells of ancient plankton incorporate this seawater signal through time, allowing scientists to approximate when the currents grew stronger and weaker off South Africa.
They confirmed that over the last 1.2 million years, the conveyor-like currents strengthened during warm periods and weakened during ice ages, as previously thought. But they also discovered that at about 950,000 years ago, ocean circulation weakened significantly and stayed weak for 100,000 years; during that period the planet skipped an interglacial -- the warm interval between ice-ages--and when the system recovered it entered a new phase of longer, 100,000-year ice age cycles. After this turning point, the deep ocean currents remain weak during ice ages, and the ice ages themselves become colder, they find.
"Our discovery of such a major breakdown in the ocean circulation system was a big surprise," said study coauthor Steven Goldstein, a geochemist at Lamont-Doherty. "It allowed the ice sheets to grow when they should have melted, triggering the first 100,000-year cycle."

You can read the rest at ScienceDaily.com or, if you have a subscription, read the paper at Science.




Leopoldo D. Pena and Steven L. Goldstein. Thermohaline circulation crisis and impacts during the mid-Pleistocene transition. Science, 26 June 2014 DOI: 10.1126/science.1249770

10 comments:

numerobis said...

The worst thing about the paper copy of Science is that I read blurbs about the new cool findings on the web, then I have to wait 2-3 weeks before it arrives!

BBD said...

In a new study in the journal Science, researchers found that the deep ocean currents that move heat around the globe stalled or even stopped, possibly due to expanding ice cover in the north.

There's a school of thought that says the repeated scouring away of topsoil by the smaller ice sheets that formed prior to the mid-Pleistocene transition (the change from the '41ka world' to the '100ka world') gave rise to increasingly stable and larger ice sheets, grounded on bedrock not saturated soils. This meant a correspondingly greater albedo cooling effect and made it ever-more difficult for the modulation of NH insolation by the 41ka obliquity cycle to trigger a deglaciation.

And one day, the deglaciation trigger flipped from 41ka to the not-really-100ka combined precession/obliquity/eccentricity trigger dominant for the last ~750ka.

Pena and Goldstein's proposed mechanism is persuasive because it adds the extra push toward a colder climate in the form of CO2 drawdown initiated by altered ocean circulation.

_Jim said...

Right "Feel free to comment."

I submitted on the order of 3 or 4 comments already to a post and so far NONE have appeared.

riverat said...

That is interesting. Thanks for posting it. I always wondered why the timing of glacial periods changed so drastically and this may be the answer.

Sou said...

It might not seem like it, but sometimes I manage to grab a wink of sleep.

I'll add that the one-liner comments that I've seen from _Jim do not warrant the anguish he obviously feels at the delay.

Anonymous said...

You are right in that the scouring of soil (regolith) might have controlled ice sheet dynamics during this time. However, it has been shown that the extension of the ice sheets during the 41ka world was at least the same or even larger than during the 100ka world. However the ice sheets were significantly thinner (based on sea level records). I do not think the albedo from the ice sheets changed drastically as the coverage (i.e. reflective area) was almost identical

Jammy Dodger said...

Wasn't there an Anonymous recently who got agitated because you (Sou) had not responded to his question within 10 minutes?

I wonder where this sense of entitlement comes from?

I am amazed you can sleep when there is such important business to attend to.

numerobis said...

Related, in a copy I have received: http://www.sciencemag.org/content/344/6189/1244.abstract

Traces the evolution of outflow from the Mediterranean sea ~2-5 million years ago, which heavily affected the thermohaline circulation (the worldwide ocean currents) and mumble mumble something about glaciations. It might take me a while to really understand what it means.

numerobis said...

Oops, did I fail the captcha? I must be turning into a bot.

BBD said...

None of this makes much sense:

However, it has been shown that the extension of the ice sheets during the 41ka world was at least the same or even larger than during the 100ka world.

Did you mean "area extent"? By whom and when was this shown?

However the ice sheets were significantly thinner (based on sea level records). I do not think the albedo from the ice sheets changed drastically as the coverage (i.e. reflective area) was almost identical

You are saying that the area coverage of ice sheets during ~100ka glacials was the same as those that formed prior to the MPT? I would very much like to see a reference for this claim because I think it is mistaken.