650 years in an ice core

A 120 m ice core retrieved from Law Dome inland of Casey Station last October will provide AAD scientists with a 650 year record of the Earth’s climate. This record covers both natural and human induced changes in the Earth’s climate, in particular the natural climate phenomenon known as the ‘Little Ice Age’.

Historical records show that Northern Europe experienced the Little Ice Age between around 1400–1850 AD, but the extent to which it may have affected the global climate is not well understood. The sea ice that forms each year on the Southern Ocean around Antarctica may provide a clue, as the extent of the Antarctic sea ice cover varies in response to climate change. However, observations of sea ice extent are limited. Most research to date has used satellite data, ice edge records from whaling ships and even the observations of Captain Cook.

In November 2003 we published an article in Science magazine (Science 302:1203–1206) detailing the use of a ‘proxy’ record of sea ice extent, obtained through the analysis of methanesulphonic acid (MSA) in an ice core from Law Dome. This chemical is produced by certain species of algae associated with sea ice. The more sea ice there is, the more algae, and the more MSA in the ice core. MSA measurements on the ice core produced a 150 year record of sea ice extent. In order to extend the sea ice proxy into the Little Ice Age, however, we needed to obtain a core at least 500 years old.

Before going into the field to drill this ice core, there were a few considerations. Firstly, we needed to choose a drilling location on Law Dome that gave us low enough snow accumulation to get a 500 year record at around 100 m depth. If we chose the wrong site, we could have been drilling up to 500 m to obtain the 500 year record!

Secondly, we had a small 12-day window of opportunity to visit Law Dome and drill the ice core. This tight schedule required good planning, with a range of options, priorities and backups in case of poor weather and the ‘A’ (Antarctic) factor. With the help of the Operations Branch, Casey Station personnel and a lucky string of good weather, we were able to travel to the drill site, set up camp, assemble the drill, drill 120 m over eight days, disassemble the drill, and depart on schedule.

To work out when we had reached our target date in the ice core (more than 500 years) we used a technique known as ‘electrical conductivity measurement’ (ECM). Over the past 600 years there have been two gigantic volcanic events, namely Tambora in 1815 AD and Kuwae in approximately 1458 AD. Volcanic activity deposits acid sulphates in the ice core, which can be measured by ECM.

For the drilling season, we modified a laboratory-based ECM instrument that would be easy to use and quick to provide results in the field. The new system worked extremely well in its first field trial, producing an ECM trace within minutes of logging the ice core. It showed the trace corresponding to the Tambora eruption around the depth we expected to see it (32 m) and on day 12 we saw the trace containing the Kuwae eruption (95–120 m). We decided to keep drilling for the rest of day 12 and reached a depth of 120 m, which is dated at around 1350 AD – over 650 years old.

With the retrieval of the ice core completed, the fun part starts – ice core analysis. This analysis will help our team of climate scientists understand the natural changes in the Antarctic climate system through the Little Ice Age period, and to compare the natural warming at the termination of the Little Ice Age with recent warming. Stay tuned…

Mark Curran and Tas van Ommen
Ice, Oceans, Atmosphere and Climate Programme,
Australian Antarctic Division