From the University of Texas at Austin
Scientists cast doubt on theory of what triggered Antarctic glaciation
A team of U.S. and U.K. scientists has found geologic evidence that casts doubt on one of the conventional explanations for how Antarctica’s ice sheet began forming. Ian Dalziel, research professor at The University of Texas at Austin’s Institute for Geophysics and professor in the Jackson School of Geosciences, and his colleagues report the findings today in an online edition of the journal Geology.
The Antarctic Circumpolar Current (ACC), an ocean current flowing clockwise around the entire continent, insulates Antarctica from warmer ocean water to the north, helping maintain the ice sheet. For several decades, scientists have surmised that the onset of a complete ACC played a critical role in the initial glaciation of the continent about 34 million years ago.
Now, rock samples from the central Scotia Sea near Antarctica reveal the remnants of a now-submerged volcanic arc that formed sometime before 28 million years ago and might have blocked the formation of the ACC until less than 12 million years ago. Hence, the onset of the ACC may not be related to the initial glaciation of Antarctica, but rather to the subsequent well-documented descent of the planet into a much colder “icehouse” glacial state.
“If you had sailed into the Scotia Sea 25 million years ago, you would have seen a scattering of volcanoes rising above the water,” says Dalziel. “They would have looked similar to the modern volcanic arc to the east, the South Sandwich Islands.”
Using multibeam sonar to map seafloor bathymetry, which is analogous to mapping the topography of the land surface, the team identified seafloor rises in the central Scotia Sea. They dredged the seafloor at various points on the rises and discovered volcanic rocks and sediments created from the weathering of volcanic rocks. These samples are distinct from normal ocean floor lavas and geochemically identical to the presently active South Sandwich Islands volcanic arc to the east of the Scotia Sea that today forms a barrier to the ACC, diverting it northward.
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  IMAGE: This is a reconstruction of the Scotia Sea area 25 million years ago, showing volcanoes of the ancestral South Sandwich arc (ASSA). They are now submerged, but were active at…Click here for more information. |
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Using a technique known as argon isotopic dating, the researchers found that the samples range in age from about 28 million years to about 12 million years. The team interpreted these results as evidence that an ancient volcanic arc, referred to as the ancestral South Sandwich arc (ASSA), was active in the region during that time and probably much earlier. Because the samples were taken from the current seafloor surface and volcanic material accumulates from the bottom up, the researchers infer that much older volcanic rock lies beneath.
Combined with models of how the seafloor sinks vertically with the passage of time, the team posits that the ASSA originally rose above sea level and would have blocked deep ocean currents such as the ACC.
Two other lines of evidence support the notion that the ACC didn’t begin until less than 12 million years ago. First, the northern Antarctic Peninsula and southern Patagonia didn’t become glaciated until less than approximately 12 million years ago. And second, certain species of microscopic creatures called dinoflagellates that thrive in cold polar water began appearing in sediments off southwestern Africa around 11.1 million years ago, suggesting colder water began reaching that part of the Atlantic Ocean.
The research team also includes Larry Lawver and Marcy Davis at The University of Texas at Austin’s Institute for Geophysics; Julian Pearce at Cardiff University (U.K.); P.F. Barker at the University of Birmingham (U.K.) (deceased); Alan Hastie at Cardiff University and the University of Edinburgh (U.K.); Dan Barfod at the Natural Environment Research Council’s Argon Research Facility (U.K.); and Hans-Werner Schenke at the Alfred Wegener Institute (Germany).
Support was provided by the U.S. National Science Foundation’s Office of Polar Programs, the U.K. Natural Environment Research Council, the Alfred Wegener Institute (Germany) and the British Antarctic Survey.
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I suppose the 3 month Antarctic winter with temperatures down to -80C had nothing to do with it.
On the evidence, vertical seafloor submergence affecting Southern Ocean circulation patterns seems the major precipitating factor in Antarctic glaciation. But long-term geophysics, plate tectonic dispositions, have most certainly contributed. In far-distant pre-Cambrian times, when all Earth’s continental landmasses fortuitously clustered in South Polar regions, a billion-year Ice Age persisted until Pangaea fragmented. Only when major components drifted north to equatorial regions did endless global winter (“Snowball Earth”) give way to the so-called Cambrian Explosion which fostered oxygen-rich Paleozoic environments.
This is clear evidence that Antarctica is geologically continuous with the South Sandwich Islands and, as such, belongs to Great Britain.
The Antarctic Circumpolar Current did start up at 34.6 million years ago as Australia/Tasmania separated from the continent. Antarctica rapidly glaciated over at this point in less than 100,000 years and glacial till/dropstones evidence indicates the glaciers extended right to the coast, partially onto the continental shelf at this time.
At about 26 million years ago, the Circumpolar Current between South America and Antarctica (this time) was disrupted again by the small cratons between them getting jostled around. The glaciers melted back by about half. The Current restarted at about 14 million years and Antarctica reglaciated at this time.
Here is a timeline of the events.
http://s22.postimg.org/804qp4xo1/Temp_Geography_45_Mys.png
There was a great new study released last week which took an in-depth look at the separation of Australia and India from Gondwana/Antarctica. There is a fascinating video of the timeline which shows there was enough separation, from Tasmania in particular, right around 34/35 million years ago (which allowed for enough deep ocean for a good current to start up around Antarctica).
http://www.sciencedirect.com/science/article/pii/S1342937X1300213X
Before that, the currents had to go around Australia/Indonesia as far north as 30S, almost into the tropics.
http://img35.imageshack.us/img35/8862/antoceancurrents35m.jpg
There is complex geology in the Drake Passage between South America and Antarctica. There are dozens of little continental plates/cratons which have moved around over time. They have alternately closed, then re-opened access to a Current through the Passage.
http://img25.imageshack.us/img25/8785/drakepassagerestricted.jpg
Long power-point animation of the continental drift of Antarctica from the University ofTexas Plates projects.
http://www.slideserve.com/natania/antarctica-keystone-of-gondwana
or
http://www.ig.utexas.edu/research/projects/plates/movies/akog.ppt
maybe its cold in antarctica
phlogiston, I thought that it also was clear evidence that we don’t own the Falklands, so I’d keep quiet about it.
“johnmarshall says:
July 12, 2013 at 5:29 am
I suppose the 3 month Antarctic winter with temperatures down to -80C had nothing to do with it.”
Actually…..no. When the Antarctic spring comes along with its -35C,ice melts. Or so I’ve been told. I seem to slightly remember from 40 years ago,that temps below -5C, H2O,whether sea or land, tends to start forming a solid molecular structure.
Do I need a /sarc?
The next step is drilling cores into the ASSA.
How may ASSA-holes will it take, I wonder?
I would hope that this site does not start down the path of showing the drilling of ASSA-holes to get to the bottom of the mystery of why the Antarctic is so frigid.
.
[The moderation team hopes that this exhausts the ASSA puns. —mod.]
There appears to be a big push in academia to attribute the onset of Antarctic glaciation to lowered CO2 rather than the formation of a deep Southern Ocean. But as so often happens, the stubborn facts of physical evidence spoil this beautiful theory. Antarctica started icing over at the Eocene/Oligocene boundary, when deep channels opened between that continent and South America and Australia.
But the new-found volcanic island arc, if real, does coincide with the partial deglaciation of Antarctica already noted in the Miocene. This graph of Cenozoic temperature is probably familiar to many readers:
http://theresilientearth.com/?q=content/case-doing-nothing-about-global-warming
“Now, rock samples from the central Scotia Sea near Antarctica reveal the remnants of a now-submerged volcanic arc that formed sometime before 28 million years ago and might have blocked the formation of the ACC until less than 12 million years ago…. .
“If you had sailed into the Scotia Sea 25 million years ago, you would have seen a scattering of volcanoes rising above the water,” says Dalziel. “They would have looked similar to the modern volcanic arc to the east, the South Sandwich Islands.”
In that case why don’t the South Sandwich Islands have any discernible effect on the ACC while this former arc 25 million years ago apparently blocked it.
southern Patagonia didn’t become glaciated until less than approximately 12 million years ago.
Of course that couldn’t possibly have something to do with the fact that the southern Andes didn’t exist before then, could it?
Actually I think this strengthens the theory that it was the inception of the ACC during the Early Oligocene that changed the Earth’s climate into an Icehouse State.
It has always been difficult to date the opening of the Drake passage, but if there was a major island arc forming there 28 MA ago, it must already have been in existence well before then (island arcs don’t form on land). And if that island arc was really extensive enough to appreciably impede the ACC for a while it would be a very neat explanation for the Mid Miocene warming 15-20 million years ago when Earth came partly out of the Ice-House state and parts of Antarctica was ice-free tundra (and that with a CO2-level about the same as today!).
…so if we want climate change, move some continents together or apart and/or change a few specified ocean currents. We could then choose warmer or colder…. 🙂 Maybe we know that but aren’t quite tectonic yet. Or just go from black to white roofs to affect climate. Maybe black in winter, then white in summer. Pineapples and kiwi for all.
More interesting to me is why a finger of Pacific Ocean floor was able to hold its ground here such that rather than be subducted itself it forced the advancing Atlantic, to be subducted under it to form the island arc in question. This process continues today as an active reversed Benioff zone can be found on the tip if the finger.
Aside: The Southern Ocean is an oceanographic and meteorological entity defined by the Antarctic gyre. It has no tectonic meaning.
The answer is probably that it was a short spreading arm. There is another of these strange fingers between North and South America. Ironically, this is another area much ballyhooed for causing the onset of glaciation.
Wild idea Friday:
Imagine our planet with the atmosphere and oceans but without continents. What would we have? I think there would be lovely gyres at both poles and everything else pretty much as we know it with an ITC, subtropical dead zones, etc.
Would there ever be ice? I personally think so because cold periods have occurred in many continental configurations. I sense a kahuna still at large.
Would there be a thermohaline circulation? Does it depend on ice?
Happy Friday…
Steveta. We (Britain) don’t ‘own’ the Falklands – we are the Sovereignty which exercises the authority on the Islands. In actual fact the Falklands governs itself. Proximity to another country, or even being on the same continental shelf does NOT indicate ownership – take a look at Google Earth with the seas stripped away! Argentina is itself a by-product of Spanish colonialism. Should Argentina hand themselves back to the Mapuche tribe? What date do you set as the base for ‘ownership’? Sorry to get side-tracked on a climate forum, but the ‘ownership’ of a landmass resting on a continental shelf is dependent – not on the shelf – but on politics, power, and purchase from many years ago. Governance cannot be retrospectively attributed arbitrarily, a date must be adhered to. The Falklands were barren islands, and claimed by several countries EVEN BEFORE ‘Argentina’ existed as a sovereign nation. Britain RE-established its rule in 1833, whereas Argentina did not become a federation of provinces until 1861. Argentina likes to gloss over this fact, yet it is this fact alone which means it NEVER belonged to the country that is Argentina. If you take Argentina’s rise to a state of power as 1776, then Spain has an equal right to ‘own’ the Falklands! But the country called Argentina did not exist until 1861 – 28 years AFTER Britain had claimed the islands. It really is as simple as that.
gymnosperm says:
July 12, 2013 at 9:58 am
Wild idea Friday:
Imagine our planet with the atmosphere and oceans but without continents. What would we have? I think there would be lovely gyres at both poles and everything else pretty much as we know it with an ITC, subtropical dead zones, etc.
Would there ever be ice?
Probably not, as the cold water at the poles would be constantly moving toward the equator, just as air masses do. 🙂
Surely a large landmass, if in the polar region, would freeze over anyway. Its precisely why the arctic melts the way it does and Antarctica doesn’t
Ya-but, was there not a consensus at one time against continental drift?
{ tadchem says:
July 12, 2013 at 6:55 am }
LOL Good one on a Friday.
Be skeptical. Ignore the mods.
Gary Pearse says:
July 12, 2013 at 11:43 am
Surely a large landmass, if in the polar region, would freeze over anyway. Its precisely why the arctic melts the way it does and Antarctica doesn’t
<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>
Yes, surely. It’s obvious to you, it’s obvious to me and to everyone else but these clowns who think that they can spin their research into something that it is not. This self-promotion game in science i.e., exaggerating the significance of the work, only serve to cast the work into a dubious light.
steveta-uk: wrong. Check up any up to date palaeogeographical atlas and you will find that the Falklands were once tucked up between southern Africa and Madagasgar. Sea floor spreading (continental drift/plate tectonics) then took the Falklands to their present position.
“Scientists cast doubt on theory of what triggered Antarctic glaciation”
It’s cold?
johnmarshall says:
July 12, 2013 at 5:29 am
I suppose the 3 month Antarctic winter with temperatures down to -80C had nothing to do with it.
I suppose the 3 month Antarctic winter is possible due to the ACC.
Else we would have rather something similar to the Arctic.