Rhagor - Opening our national collections

When Antarctica went into the deep freeze

[image: Sampling ancient mud cores in Tanzania]

A member of the team looking at the top layer of sediment deciding where best to sample to get different time intervals.

[image: Bringing up a core of mud from 34 million years ago]

Bringing up a core of mud from 34 million years ago

[image: a scientist describing and sampling the core]

After the core is brought up it is laid out for scientists to describe and take samples.

[image: Extreme close up of the 35 million year old foram: Cribrohantkenina inflata. discovered in the cores from Tanzania.]

Extreme close up of the 35 million year old foram: Cribrohantkenina inflata. discovered in the cores from Tanzania. More images of these intricate forams can be seen in the 'Up close with Nature' gallery.

Scientists from Amgueddfa Cymru - National Museum Wales and Cardiff University have found new evidence of past climate change, which helps solve some of the mystery surrounding the appearance of the vast ice-sheet in Antarctica 34 million years ago.

Antarctica hasn't always been covered with ice - the continent lay over the south pole without freezing over for almost 100 million years. Then, about 34 million years ago, a dramatic shift in climate happened at the boundary between the Eocene and Oligocene epochs. The warm greenhouse climate, stable since the extinction of the dinosaurs, became dramatically colder, creating an "ice-house" at the poles that has continued to the present day.

Global cooling

Many climate scientists are involved in trying to figure out what caused this climate shift. This should tell us more about how the climate responds to major controls like changes in the Earth's orbit around the sun, and the concentration of greenhouse gases in the atmosphere.

Past climate changes can be recorded by studying tiny microfossils in layers of deep sea mud. Up until now, scientists found that the oceans appear to have warmed up during this big climatic shift. Their studies suggested that warming seemed to coincide with ice-sheets appearing in both Antarctica and the Arctic. This conflicting evidence, of warming seas while ice-sheets grew, doesn't fit in with computer simulations of the climate at the time; the computer models don't show ice to be present in the Arctic."

Tanzania drilling project

The solution to this icy puzzle has come from a surprising place - Tanzania in East Africa. The Tanzania Drilling Project team, including scientists from Amgueddfa Cymru and Cardiff University, have been recovering cores of ancient mud deposited on the seafloor millions of years ago (which has since been geologically uplifted into land).

The Tanzanian cores are special because large thicknesses of mud were laid down over a relatively short time, meaning that climate changes through time are seen in great detail. Also, beautifully preserved microfossils are found in the cores.

The Tanzanian cores provide the first really clear picture of how sea-level fall fits in with the climate shift.

Setting the record straight

The chemistry of the Tanzanian microfossils has been used to construct records of temperature and ice volume over the interval of the big climate switch. These new records show that the world's oceans did cool as the ice-sheets appeared, and that the volume of ice would have fitted onto Antarctica. So the computer simulations of climate and the past climate data now match up.

The focus now is to look for evidence of the ultimate cause of this global cooling. The prime suspect is a gradual reduction of CO2 in the atmosphere, combined with a 'trigger' time when Earth's orbit around the sun made Antarctic summers cold enough for ice to remain frozen all year round.

How it works

The shell chemistry of pin-head sized animals called forams can tell us how ocean temperatures changed through time. Forams are great tools for studying climates of the past, which helps us learn about the uncertainties of our future greenhouse climate.

[image: 1). Forams take chemical elements from the ocean into their shells, using more magnesium at warmer temperatures.]

1). Forams take chemical elements from the ocean into their shells, using more magnesium at warmer temperatures.

[image: 2). Dead forams fall to the sea floor and build up in layers of mud over millions of years.]

2). Dead forams fall to the sea floor and build up in layers of mud over millions of years.

[image: 3). Today, going down through the mud layers is like going back in time. ]

3). Today, going down through the mud layers is like going back in time. If we can measure the magnesium content of forams going down through the mud, it gives us a record of how ocean temperature changed through time - more magnesium equals warmer temperature.

Further Reading

Lear, CH, Bailey, TR, Pearson, PN, Coxall, HK, Rosenthal, Y. Cooling and ice growth across the Eocene-Oligocene transition. Geology 36 (3), 251ÔŅĹ254. 2008.

http://www.gsajournals.org/perlserv/?request=get-abstract&doi=10.1130%2FG24584A.1

Article Date: 19 May 2008

2 comments

Trevor Bailey - Geology Department on 15 January 2010, 12:30 (Amgueddfa Cymru – National Museum Wales Staff)

Hi there,
Itís actually the same thing that has caused both poles to freeze over, a long-term global cooling of the climate that happened over the last 55 million years. Evidence from fossil plants, sediments left behind by ice, and the chemistry of ocean sediments, shows that this cooling was gradual and took place in several stages. The Antarctic continent froze before the Arctic Ocean as itís much easier for an ice sheet to build up on land. In the oceans salt lowers the freezing point of water to below 0°C, whereas snow (frozen freshwater) falling on land will remain frozen at 0°C.

A recent study using cores collected by the Tanzania Drilling Project reports estimates of the concentration of the greenhouse gas CO2 in the atmosphere at ~34 million years ago. These suggest that CO2 levels fell below a threshold amount which cooled the planet enough for snow to remain frozen year round on Antarctica and build up to form an ice sheet. It's a situation which is the opposite of the global warming we are currently experiencing.

We donít know what caused CO2 levels to fall in the first place, but two main geological processes have been proposed (which I don't have space to go in to here, sorry !):

  • Slower input of CO2 to the atmosphere from the Earth's mantle due to slower spreading at mid-ocean ridges.
  • Fast removal of CO2 from the atmosphere by the uplift and chemical weathering of silicate rocks.

At present Antarctica is still frozen as our climate is still cold enough. However future man-made global warming has the potential to melt it and cause about 60 metres of sea level rise.

Dr Trevor Bailey Ė Geology Department

Antartica and Artic on 13 January 2010, 10:02

Hey!! Um... i have a question... what caused antartica to freeze over (why is it still frozen), and how is this different than what caused the artic to freeze??

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Glossary

Epoch
An epoch is a division of geologic time. Epochs lasted tens of millions of years.

Microfossils
Fossils which can be examined only under a microscope.

Cores
Cores are cylindrical samples of rock or mud which are obtained from drilling on land or from the seafloor.

Forams
Short for Foraminifera, forams are microscopic animals which usually have a carbonate shell.

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