*The title of this blog comes from the title of Richard Alleys book on ice cores called ‘The Two-Mile time machine: Ice Cores, Abrupt Climate Change, and Our Future‘, which is a great read even years after its first publication.

The oldest continuous ice core provides records of temperatures and atmospheric composition extending 800,000 years back in time. This maximum age is tantalizingly close to the time when the rhythm of the glacial-interglacial cycles on Earth changed from having a periodicity of about 40 thousand years to one of about 100 thousand years (Figure 1), the cause of which remains unresolved. So, the quest is on to find an ice core with an interpretable continuous stratigraphy extending at least 1.5 million years into the past to throw some light on what was happening to Earth’s climate during this transition.

Figure 1: The record of Pleistocene (~2.58 million years to present) glacial and interglacial cycles revealed by sea floor oxygen isotope ratios, showing the recent period dominated by 100 thousand year glacial cycles with a large amplitude, compared to the 40 thousand year cycles with lower amplitude in the early Pleistocene, that can be related to variation in the Earth’s orbit around the sun. Data replotted from Lisieki and Raymo (2005), available at: http://lorrainelisiecki. com/LR04stack.txt, with annotations in approximate positions.

What conditions preserve a continuous record to ancient ice?

Obtaining the oldest possible continuous ice core records requires knowing where on the ice sheet we can find the best preservation of the greatest number of annual accumulation layers (Figure 2). To minimize the layer disturbance caused by horizontal ice flow, ice divides are usually best for ice core sites, because at these sites the snow is just being compressed downwards and at depth spreading out in a sort of radial fashion. That aside, the age at the base of an ice core is determined by a complex interplay of total ice thickness, surface snow accumulation rate, the rate at which annual layers are progressively compressed and thinned with depth as the ice flows outwards under its own weight, and the amount of older ice being removed by melting at the base of the ice caused by a combination of geothermal heat flux and the overburden pressure of the ice above. Clearly this is quite a lot of variables to get a handle on in a complex and remote environment.

Figure 2: A section of ice core showing the preserved annual stratigraphy. Ice core records can be analysed for time series records of air temperature, surface melt, snow accumulation, and atmospheric composition and aerosols. Photo credit: Jennie Hills.

Alternatively, it was shown in 2017 that very old ice can be found by drilling sideways into areas of so called blue ice, which is where very ancient ice is being upthrust from deep in the ice sheet against mountain ridges buried beneath the Antarctic ice sheet. This is well described in this cool article from that time: https://www.sciencemag.org/news/2017/08/record-shattering-27-million-year-old-ice-core-reveals-start-ice-ages. Unfortunately this type of coring has not yet yielded a continuous record as offered by a high quality vertical core, so the search for the longest possible climate record from a vertical ice core continues.

How can a vertical core site be chosen?

Suitable locations for obtaining the oldest continuous ice core records are most likely to be found near previously-cored east Antarctic domes, but in areas of thinner ice where there is no basal melting (Fischer at al., 2013). However, given the financial and logistical commitments of undertaking an Antarctic drilling program, detailed reconnaissance work is required to narrow down an optimal site, and maximize the chances of a coring effort being successful. This involves examining  geophysical and glaciological data. For example, Parrenin and others 2017 identified two potential areas that could offer ice at least 1.5 million years old near the Dome C EPICA drill site using data from a dense airborne radar survey (Figure 3a), which penetrates the ice and reveals the annual accumulation layer structure of the sub-surface. Identifiable layers spanning the last ~366 thousand years were assigned ages from the well-dated EPICA core. Below these layers that have ages assigned to them, a model of annual layer thinning over time was used to extrapolate the age from the radar layers to the base of the ice sheet, accounting for heat flow and basal melt. The most promising core site locations were identified as the two areas where the modelled 1.5 million year old former surface would be some distance above relatively invariant bedrock, where the stratigraphy is less likely to be disturbed (Figure 3b).

Figure 3: (a) Location of the surveyed area around the EPICA Dome C drilling site (red star) showing the bedrock elevation in the colour and the surface height contour lines in grey. Radar flight lines are shown in blue dots, with the transect shown in (b) highlighted in red. The labels AE indicate areas of potential old basal ice identified by a former study (Van Liefferinge and Pattyn, 2013). (b) Modeled age-depth profiles shown in colour with ice older than 1.5 Million years in white. Former surface layers (isochrones) are highlighted as while lines, and the bedrock surface in black. The locations of the Little Dome C Patch (LDCP) and North Patch (NP) are labelled approximately in both panels. Both panels are reproduced from Parrenin and others (2017).

Looking to the future

Extracting a continuous ice core stratigraphy of temperature and atmospheric composition extending back 1.5 million years is an exciting prospect as it could not only help solve the riddle of Earth’s changing glacial heartbeat, but also provide context for discontinuous records of ancient Antarctic ice.

The ‘Beyond EPICA-Oldest Ice’ project is now in Phase II, with funding confirmed for the drilling stage which followed the site exploration of Phase I. Press releases and news from spring and summer 2019 heralded the start of the drilling part of the project, and these articles in Nature and The Guardian give a good overview. Look out for updates on their website, and in the press, as if they are successful it will no doubt be big news!

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, doi:10.5194/tc-11-2427-2017, 2017.
• Fischer, H., Severinghaus, J., Brook, E., Wolff, E., Albert, M., Alemany, O., Arthern, R., Bentley, C., Blankenship, D., Chappellaz, J., Creyts, T., Dahl-Jensen, D., Dinn, M., Frezzotti, M., Fujita, S., Gallee, H., Hindmarsh, R., Hudspeth, D., Jugie, G., Kawamura, K., Lipenkov, V., Miller, H., Mulvaney, R., Parrenin, F., Pattyn, F., Ritz, C., Schwander, J., Steinhage, D., van Ommen, T. and Wilhelms, F. (2013) Where to find 1.5 million yr old ice for the IPICS “Oldest-Ice” ice core, Climate of the Past, 9 (6), pp. 2489-250, doi: 10.5194/cp-9-2489-2013, 2013.
• Van Liefferinge, B. and Pattyn, F.: Using ice-flow models to evaluate potential sites of million year-old ice in Antarctica, Clim. Past, 9(5), 2335–2345, doi:10.5194/cp-9-2335-2013, 2013.
• http://www.beyondepica.eu – The ‘Beyond EPICA-Oldest Ice’ consortium
• http://quantarctica.npolar.no – A free GIS package for Antarctica
• http://www.antarcticglaciers.org/glaciers-and-climate/ice-cores/ice-corebasics/– Ice core Basics from Antarctic Glaciers
• https://icecores.org/icecores/index.shtml – US National Ice Core Laboratory
• A recent Nature News article: https://www.nature.com/articles/d41586-019-03199-8