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Geosciences Column: The best spots to hunt for ancient ice cores

Geosciences Column: The best spots to hunt for ancient ice cores

Where in the world can you find some of Earth’s oldest ice? That is the question a team of French and US scientists aimed to answer. They recently identified spots in East Antarctica that likely have the right conditions to harbor ice that formed 1.5 million years ago. Scientists hope that obtaining and analysing an undisturbed sample of ice this old will give them clues about Earth’s ancient climate.

The team published their findings in The Cryosphere, an open access journal of the European Geosciences Union (EGU).

Why study ancient ice?

When snow falls and covers an ice sheet, it forms a fluffy airy layer of frozen mass. Over time, this snowy layer is compacted into solid ice under the weight of new snowfall, trapping pockets of air, like amber trapping prehistoric insects. For today’s scientists, these air bubbles, some sealed off thousands to millions of years ago, are snapshots of what the Earth’s atmosphere looked like at the time these pockets were locked in ice. Researchers can tap into these bubbles to understand how the proportion of greenhouse gases in our atmosphere have changed throughout time.

As of now, the oldest ice archive available to scientists only goes back 800,000 years, according to the authors of the study. While pretty ancient, this ice record missed out on some major climate events in Earth’s recent history. Scientists are particularly interested in studying the time between 1.2 million years ago and 900,000 years ago, a period scientifically referred to as the mid-Pleistocene transition.

In the last few million years leading up to this transition, the Earth’s climate would experience a period of variation, from cold glacial periods to warmer periods, every 40,000 years. However, after the mid-Pleistocene transition, Earth’s climate cycle lengthened in time, with each period of variation occurring every 100,000 years.  

Currently, there isn’t a scientific consensus on the origin of this transition or what factors were involved. By examining old ice samples and studying the composition of the atmospheric gases present throughout this transition, scientist hope to paint a clearer picture of this influential time. “Locating a future 1.5 [million-year]-old ice drill site was identified as one of the main goals of the ice-core community,” wrote the authors of the study.  

The quest for old ice

Finding ice older than 800,000 years is difficult since the Earth’s deepest, oldest ice are the most at risk of melting due to the planet’s internal heat. Places where an ice sheet’s layers are very thick have an even greater risk of melting.

Mesh, bedrock dataset (Fretwell et al., 2013; Young et al., 2017) and basal melt rate (Passalacqua et al., 2017) used for the simulation. Credit: O. Passalacqua et al. 2018.

“If the ice thickness is too high the old ice at the bottom is getting so warm by geothermal heating that it is melted away,” said Hubertus Fischer, a climate physics researcher from the University of Bern in Switzerland not involved in the study, in an earlier EGU press release.

Last summer, a team of researchers from Princeton University announced that they had unearthed an ice core that dates back 2.7 million years, but the sample’s layers of ice aren’t in chronological order, with ice less than 800,000 years old intermingling with the older frozen strata. Rather than presenting a seamless record of Earth’s climate history, the core can only offer ‘climate snapshots.’

Finding the best of the rest

The authors of the recent The Cryosphere study used a series of criteria to guide their search for sites that likely could produce ice cores that are both old and undisturbed. They established that potential sites should of course contain ice as old as 1.5 million years, but also have a high enough resolution for scientists to study frequent changes in Earth’s climate.

Additionally, the researchers established that sites should not be prone to folding or wrinkling, as these kinds of disturbances can interfere with the order of ice layers.

Lastly, they noted that the bedrock on which the ice sheet sits should be higher than any nearby subglacial lakes, since the lake water could increase the risk of ice melt.

Magenta boxes A, B and C correspond to areas that could be considered as our best oldest-ice targets. Colored points locate possible drill sites. Credit: O. Passalacqua et al. 2018.

 

Using these criteria, the researchers evaluated one region of East Antarctica, the Dome C summit, which scientists in the past have considered a good candidate site for finding old ice. They ran three-dimensional ice flow simulations to locate parts of the region that are the most likely to contain ancient ice, based on their established parameters.

By narrowing down the list of eligible sites, the researchers were able to pinpoint regions just a few square kilometres in size where intact 1.5 million-year-old ice are very likely to be found, according to their models. Their results revealed that some promising areas are situated a little less than 40 kilometres southwest of the Dome C summit.

The researchers hope their new findings will bring scientists one step closer towards finding Earth’s ancient ice.

By Olivia Trani, EGU Communications Officer

Geosciences Column: The hunt for Antarctica’s oldest time capsule

Geosciences Column: The hunt for Antarctica’s oldest time capsule

The thick packs of ice that pepper high peak of the world’s mountains and stretch far across the poles make an unusual time capsule. As it forms, air bubbles are trapped in the ice, allowing scientists to peer into the composition of the Earth’s atmosphere long ago. Today’s Geosciences Column is brought to you by PhD researcher Ruth Amey, who writes about recently published research which reveals how a team of scientists might have found the oldest ice yet, which has important implications for our understanding of how Earth’s environment has changed over time.

Ice cores give us a slice through the past. By analysing the composition of ice and gas bubbles trapped within it, we can find out information about temperature, atmospheric conditions, deposition and even the magnetic field strength of the past.

This helps us to understand past conditions on the Earth, but currently the longest record is ~800,000 years (800 kyrs) old. One phenomenon scientists hope to understand better is a change in glaciation cycles. During the Mid-Pleistocene Transition, glaciation cycles changed from 40,000 year cycles related to the obliquity periodicity of the Earth’s orbit to longer, stronger 100,000 year cycles. Scientists of the ice-core community have their eyes on finding out why this change happened, and for this they need data from the onset of the change, between 1250 and 700 kyrs ago.

Which means we need much, much older ice.

A new study, published in EGU’s open access journal The Cryosphere has pinned down two locations where they think the base of Antarica’s ice sheet is significantly older. In fact they believe the ice could be as old as 1.5 million years, which would extend the current ice core record by ~700,000 years: nearly doubling it.

A Treasure hunt – using airborne radar and some simple models

The group, led by Frederic Parrenin at University of Grenoble Alpes, France, went on the hunt for the oldest ice East Antarctica could give them. The survival of ice is an interplay between many factors: the ice acts a little bit like a conveyor belt, being fed by accumulation, with the oldest information lost off the end by basal melting. This means areas of thinner ice, where there is less basal heating, often has a higher likelihood of the old, information-rich ice surviving.

Figure 2: A cross-section of ice in East Antarctica, from surface to bedrock, with colour bar showing the modelled ice age. The model identifies two patches of ice older than 1.5 Myr (shown in white): North Patch and Little Dome D Patch. Adapted from Figure 3 of Parrenin et al 2017.

Airborne radar can ‘see’ into the top three-quarters of the East Antarctica ice sheet. By identifying reflections within it, isochrones of ice of the same age can be traced. Parrenin’s group exploited an area in East Antarctica known as ‘Dome C’ with rich record of radar investigations. Using information derived from the radar, they then created a mathematical model, which balanced accumulation rate, heat flow and melting to give a simple 1-D ice flow model. This helps locate areas of accumulation and melting, which gives an indication of where ice might be the oldest, beyond the sight of the airborne radar. A nearby ice-core, EDC, also provided corroboration of their model.

X Marks the Spot

The team located two sites where they believe the ice to be older than 1.5 million years old, named Little Dome C and North Patch. And fortunately these sites are within a few tens of kilometres from the Concordia research facility, meaning drilling them is a real possibility.

This ancient ice could give vital insight into what happened in the Mid-Pleistocene Transition. What caused the new glaciation cycle onset? Was it a change in sea ice extent? A change in atmospheric dust? Decrease in carbon dioxide concentrations? Changes in the Earth’s orbit? The answers may well be locked in the ice.

By Ruth Amey, Postgraduate Researcher at the University of Leeds

 

References and Resources

Parrenin, F., Cavitte, M. G. P., Blankenship, D. D., Chappellaz, J., Fischer, H., Gagliardini, O., Masson-Delmotte, V., Passalacqua, O., Ritz, C., Roberts, J., Siegert, M. J., and Young, D. A.: Is there 1.5-million-year-old ice near Dome C, Antarctica?, The Cryosphere, 11, 2427-2437, https://doi.org/10.5194/tc-11-2427-2017, 2017

Berger, A., Li, X. S., and Loutre, M. F.: Modelling northern hemisphere ice volume over the last 3 Ma, Quaternary Sci. Rev., 18, 1–11, https://doi.org/10.1016/S0277-3791(98)00033-X, 1999

Imbrie, J. Z., Imbrie-Moore, A., and Lisiecki, L. E.: A phase-space model for Pleistocene ice volume, Earth Planet. Sc. Lett., 307, 94–102, https://doi.org/10.1016/j.epsl.2011.04.018, 2011

Jean Jouzel, Valérie Masson-Delmotte, Deep ice cores: the need for going back in time, In Quaternary Science Reviews, Volume 29, Issues 27–28, Pages 3683-3689, ISSN 0277-3791, https://doi.org/10.1016/j.quascirev.2010.10.002, 2010

Martínez-Garcia, A., Rosell-Melé, A., Jaccard, S. L., Geibert, W., Sigman, D. M., and Haug, G. H.: Southern Ocean dust-climate coupling over the past four million years, Nature, 476, 312–315, doi:10.1038/nature10310, 2011

Tziperman, E., and H. Gildor, On the mid-Pleistocene transition to 100-kyr glacial cycles and the asymmetry between glaciation and deglaciation times, Paleoceanography, 18(1), 1001, doi:10.1029/2001PA000627, 2003

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