GeoLog

ice core record

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

Wessel, P. and W. H. F. Smith, Free software helps map and display data, EOS Trans. AGU, 72, 441, 1991

Imaggeo on Mondays: Iceberg at midnight

Standing on the vast expanse of gleaming white sea ice of the Atka Bay, Michael Bock took this stunning picture of an Antarctic iceberg. The days, during the Antarctic summer, are never ending. Despite capturing the image at midnight, Michael was treated to hazy sunlight.

Icebergs at midnight. Credit: Michael Bock (distributed via imaggeo.egu.eu)

Icebergs at midnight. Credit: Michael Bock (distributed via imaggeo.egu.eu)

“Clearly visible [in the iceberg] are the annual snow accumulation layers which illustrate how the ice archive works.; as you look down the icy face, the ice gets older,” explains Michael. As more snow accumulates on the surface of the glacier, the underlying layers of snow are compressed by the weight from above, hence layers become thinner with increasing depth. On the ice shelf or on the Antarctic plateau these accumulation layers can only be seen when digging a snow pit. The obvious limitation of this is that only a few meters can be excavated with spades, limiting the observations one can carry out. Instead, to gain information about what happens deep within the ice pack, drill cores are usually used. Long cores of the layers of ice can be extracted , providing useful data. “One can drill into the ice (typically on the Antarctic plateau on ice divides or domes) reaching down to bedrock, with the retrieved ice core revealing long records of climatic history,” adds Michael. Deep ice cores can be more than 3000 m long. Depending on e.g. annual mean temperature and accumulation rate the age and resolution of these archives can vary greatly. Whilst this iceberg cannot be studied directly due to hazards associated with working underneath it does “serve as a beautiful visualisation of what we are searching for in ice core science”, explains Michael.

By Laura Roberts Artal and Michael Bock.

If you pre-register for the 2015 General Assembly (Vienna, 12 – 17 April), you can take part in our annual photo competition! From 1 February up until 1 March, every participant pre-registered for the General Assembly can submit up three original photos and one moving image related to the Earth, planetary, and space sciences in competition for free registration to next year’s General Assembly!  These can include fantastic field photos, a stunning shot of your favourite thin section, what you’ve captured out on holiday or under the electron microscope – if it’s geoscientific, it fits the bill. Find out more about how to take part at http://imaggeo.egu.eu/photo-contest/information/.

Geosciences Column: The Toba eruption probably did have a global effect after all

Almost everyone has heard of the Toba super-eruption, which took place on the island of Sumatra roughly 74,000 years ago, but the only evidence of tephra or tuff (volcanic fragments) from the eruption is in Asia, with nothing definite further afield. It has sometimes been thought that this huge eruption may have led to a volcanic winter, a period of at least several years of low temperatures following a large eruption. This is caused by the effect of enormous quantities of volcanic ash entering the atmosphere and reducing the sun’s penetration. Sulphides also help to reduce solar energy penetration and increase the Earth’s albedo (increasing the reflection of solar radiation and leading to cooler temperatures).

In a 2013 study, Anders Svensson and colleagues suggest that a link has now been made between the onset of Greenland Interstadials (GI) 19 and 20 and Antarctic Isotope Maxima (AIM) 19 and 20 for the Toba eruption. Stadials are periods of low temperatures, lasting less than a thousand years, during the warmer periods between ice ages. Conversely, interstadials are warmer periods lasting up to ten thousand years, occurring during an ice age but not lasting long enough to qualify as interglacial periods. The GI periods and AIMs are numbered according to a scale based on Dansgaard-Oeschger events. These are relatively short-lived climate fluctuations, which occurred 25 times during the last glacial period (covering 110,000 to 12,000 years ago) and are used to help date events.

The study’s claim is based on their matching of volcanic acidity spikes at both poles produced by increased sulphur compounds in the atmosphere following the Toba eruption, which have been matched to the existing dates based on Asian tephra records and the bipolar seesaw hypothesis. This hypothesis explains the thousand-year offset between temperature changes over the two poles during the last glacial period as being caused by a seesaw mechanism involving heat redistribution by the Atlantic Meridional Overturning Circulation (AMOC) system, whereby warm water moves north from southern waters, mixes with cooler water in Arctic regions, sinks, and returns south as deep bottom water.

The fluctuations in temperature associated with Dansgaard-Oeschger events over a few decades are reflected in Greenland ice cores but Antarctic ice cores show a picture of slower changes over hundreds to thousands of years, running out of phase with the Greenland records. A direct link was made by Carlo Barbante and colleagues in a 2006 paper. They used methane records from a Northern Greenland ice core and oxygen isotope records from the Antarctic Dronning Maud Land ice core to show direct coupling between warm events in the Antarctic and the duration of cold events in Greenland indicating their probable common origin in a reduction in AMOC.  An increase in freshwater entering the North Atlantic during warming would slow heat transport towards the north. This would lead to cooler surface air temperatures there and warmer temperatures in the southern waters and vice versa – taking several decades to pass from one hemisphere to the other, hence the name bipolar seesaw effect.

When volcanic gases, especially sulphurous gases, travel round the world in the atmosphere they form aerosols, which are trapped with air in precipitation at the poles in the form of bubbles. The bubbles are entombed at the depth at which firn (a type of rock-hard snow that looks like wet sugar) is compacted into ice. Succeeding precipitation and compaction over thousands of years provides evidence of atmospheric composition at different times, in trapped air bubbles, which can be analysed in ice cores. But the ages of both ice and gases are offset by 100-1000 years, depending on factors such as thickness of the firn, temperature and the presence of impurities in the ice. Ice cores are a bit like tree rings in that their thickness and other properties reflect climatic conditions at the time and differences can be counted as annual rings for the younger cores, before compaction makes it impossible to distinguish them.

Isotope data for Greenland and Antarctic ice cores over the past 140,000 years. (Credit: Leland McInnes)

Isotope data for Greenland and Antarctic ice cores over the past 140,000 years. (Credit: Leland McInnes)

Oxygen isotope and atmospheric methane signals in ice cores were available to link two separate records (NGRIP and EDML) for the period covering 80-123 thousand years ago.

Different gases can be used to date a particular part of an ice core and, as can be seen above, δ2H in Antarctica and δ18O in Greenland were used for dating purposes. 10Beryllium is found in the atmosphere and its levels change with solar activity and the Earth’s magnetic field. It is only found in the atmosphere for one or two years at a time so it can be used to help synchronise data from different ice cores more closely. A dating method using cosmogenic 10beryllium signals to match both Greenland and Antarctic ice cores to the Laschamp geomagnetic event (a short reversal of the Earth’s magnetic field). This helps pinpoint a particular date at around 41 thousand years ago, and provides a direct link between the ice-core horizons so that the data could be matched and signs of the eruption detected at the known eruption time.  With markers for 41,000 and 80,000 years ago synchronised for the two polar regions, the ice cores were examined for signs of sulphate acidity spikes indicating a major eruption.  The time scales were then combined for datasets from a number of ice cores (NGRIP, EDC, EDML and Vostok) to produce consistent scales for ice and gas records.

From the Greenland (NGRIP) and Antarctic (EDML) ice-core data, evidence of the Toba eruption was synchronised between them for approximately 2000 years around the known eruption time, using a pattern of bipolar volcanic spikes and the Greenland Ice Core Chronology 2005 defined for the NGRIP annual layer count, to compare with Antarctic data. They found evidence of large quantities of atmospheric sulphates (usually linked to volcanic eruptions) in both sets of data in the form of acidity spikes and linked these to the Toba eruption.

In fact, there are four bipolar acidity spikes within a few hundred years of the presumed Toba event, suggesting that there may have been several events, also confirmed by argon dating. Moreover, the authors found that the Toba eruption was linked to up to 4 acidity spikes occurring between 74.1 and 74.5 thousand years ago. These Toba events occurred at a time of rapid climate change from warm interstadial to cold stadial periods in Greenland and the equivalent Antarctic warming within 100 years, which perfectly agrees with the bipolar seesaw hypothesis.

Interestingly, another more recent study, showed that Lake Prespa in southeast Europe reached it’s lowest recorded level, or lowstand, at the time of the Toba eruption. The timing of the lowstand is dated at 73.6 ± 7.7 thousand years ago based on Electron Spin Resonance dating of shells. This short-lived lowstand also coincides with the onset of Greenland Stadial GS-20, with a possible link to the Toba eruption.

This new data and more accurate dating described in these studies, does tend to show that the Toba eruption (or series of eruptions) did, in fact have global impact.

By Gill Ewing, Freelance Science Writer

References:

EPICA Community Members: One-to-One coupling of glacial climate variability in Greenland and Antarctica, Nature, 444, 195-198, doi. 10.1038/nature05301, 2006.

Svensson, A., Bigler, M., Blunier, T., Clausen, H.B., Dahl-Jensen, D., Fischer, H., Fujita, S., Goto-Azuma, K., Johnsen, S.J., Kawamura, K., Kipfstuhl, S., Kohno, M., Perrenin, F., Popp, T., Rasmussen, S.O., Schwander, J., Seierstad, I., Severi, M., Steffensen, J.P., Udisti, R., Uemura, R., Vallelonga, P., Vinther, B.M., Wegner, A., Wilhelms, F. Winstrup, M.: Direct linking of Greenland and Antarctic ice cores at the Toba eruption (74 KaBP), Clim. Past, 9, 749-766, doi. 10.5194/cp-9-749-2013, 2013.

Wagner, B., Leng, M.J., Wilke, T., Böhhm, A., Panagiotopoulos, K., Vogel, H., Lacey, J.H., Zanchetta, G., Sulpizio, R.: Distinct lake level lowstand in Lake Prespa (SE Europe) at the time of the 74 (75) ka Toba eruption, Clim. Past, 10,  261-267, doi.10.5194/cp-10-261-2014, 2014.

Geosciences column: Playing back the Antarctic ice records

Satellites are keeping tabs on the state of Arctic and Antarctic sea ice, and have observed considerable declines in ice extent in many areas since records began, but what do we know of past sea ice extent?

Ice cores keep an excellent record of climate change, but until recently, ice cores have not been used to quantify patterns in past sea ice extent because few reliable compounds are preserved in the ice. While methanesulphonic acid (MSA) has been used in the past, it is an unstable compound and is easily remobilised after it has been deposited. The amount of sea-salt sodium deposited in brine pools or as high salinity crystals known as ‘frost flowers’ that form on the ice surface can also be used to identify changes in sea ice extent. These deposits are difficult to distinguish from larger sources of sea-salt sodium (aerosols and sea spray) though, making it a poor proxy.

Recent research published in Atmospheric Chemistry and Physics may provide an answer. An international team of geochemists have identified two stable indicators of sea ice extent in ice cores: the halogens bromine and iodine.

The oceans are considered to the be the main reservoir of bromine and iodine, but satellite data show these halogens also have a strong link with sea ice at the poles. Furthermore, recent research suggests that algae that grow under sea ice are big contributors to atmospheric iodine. Sea ice provides a substrate for algae to grow on during the spring. It is thin enough for light to penetrate, allowing the algae to photosynthesise, and also allows compounds produced by the algae to reach the atmosphere, including iodine and iodine oxide. These peaks in iodine oxide production are spotted by satellites during the spring.

The underside of Antarctic pack ice. The brown-green algae are an important food source for krill, as well as being a source of atmospheric iodine. (Credit: Kills and Marshall, 1995)

The underside of Antarctic pack ice. The brown-green algae are an important food source for krill, as well as being a source of atmospheric iodine. (Credit: Kills and Marshall, 1995)

During interglacials, the extent of sea ice is much smaller than during a glacial period. This means that the thin sea ice (where iodine is released into the atmosphere) is much closer to the coast. Air bubbles trapped in compacting continental snow preserve these atmospheric gases, which can be sampled in an ice core at a much much later date. Thus, when there is more iodine in the ice core, the extent of sea ice is small; when there is less, the thin edge of the ice sheet was much further from the coast.

Satellite measurements also show the amount of bromine oxide in the atmosphere peaks during the polar spring. This is because the increase in light level stimulates a series of photochemical reactions that convert bromine salts into bromine and bromine oxide, releasing it into the atmosphere. These events are known as bromine explosions and result in an increase in atmospheric bromine.

How bromine ions (bromide) makes it way from the ocean to the atmosphere and onto the surface of Antarctica (where it is later compacted by layers of snow, forming ice, and drilled to produce the Talos Dome ice core). Blue, red and green lines indicate aerosol-phase bromide, gas-phase bromide and hydrogen bromide, respectively. (Credit: Spolaor et al., 2013)

How bromine ions (bromide) makes it way from the ocean to the atmosphere and onto the surface of Antarctica (where it is later compacted by layers of snow, forming ice, and drilled to produce the Talos Dome ice core). Blue, red and green lines indicate aerosol-phase bromide, gas-phase bromide and hydrogen bromide, respectively (click for larger). (Credit: Spolaor et al., 2013)

Because the extent of sea ice is reduced during interglacials, bromine explosions must occur closer to the Antarctic coast, meaning more bromine will be trapped in the ice sheet during an interglacial period. Using sodium as a proxy for the amount of sea salt in the ice core, Andrea Spolaor and her team were able to work out how much of the bromine was in the atmosphere at the time. Since bromine explosions result in an an increase in atmospheric bromine, but have no effect on sodium, these events can be identified when the ratio of bromine to sodium in the ice core is high.

Now we can work out the extent of past sea ice, what lies ahead? The first part of the IPCC 5th Assessment Report released earlier today concerns the physical changes in the Earth’s climate and what we can expect in the future; the future of our changing planet will be discussed at this year’s Geosciences Information For Teachers workshop at EGU 2014, and you will be able to find a great discussion of climate change, its history and its impacts in the next issue of GeoQ – stay tuned!

By Sara Mynott, EGU Communications Officer

Reference:

Spolaor, A., Vallelonga, P., Plane, J. M. C., Kehrwald, N., Gabrieli, J., Varin, C., Turetta, C., Cozzi, G., Kumar, R., Boutron, C., and Barbante, C.: Halogen species record Antarctic sea ice extent over glacial–interglacial periods, Atmos. Chem. Phys., 13, 6623-6635, 2013.