Teachers' notes - Dr Elizabeth Truswell, geologist

Antarctic work

One of the most interesting parts of your work has been to do with the material that has come off Antarctica and been preserved in the sea. When did you become interested in deep sea drilling?

My introduction to the deep sea drilling project came during a postdoctoral fellowship I had to the US. I was at Florida State University and they were heavily involved; they are in fact a repository for storage of some of the cores from the Ocean Drilling Program, or the Deep Sea Drilling Program as it was then. While I was there I became very interested in that program and I had the chance to go on one of their first cruises to Antarctica.

I was interested in the sea floor spreading history between Australia and Antarctica – particularly the way the modern marine environments around Antarctica had developed. South of the Antarctic Convergence, where there is a dramatic temperature change in the surface waters, the predominant biota is siliceous (silica forming organisms, diatoms, radiolarians); north of the Convergence we meet the calcareous organisms. And that boundary is easy to determine in the sedimentary record – I first became aware of it on the cruise to Antarctica. As we drilled holes I pinned up around my cabin the logs showing the distribution of these siliceous and calcareous organisms, and it became very apparent that this boundary was moving rapidly northwards through time as the climates changed.

The cold waters generated around Antarctica sink to the bottom and then they move northwards, right through to the north Atlantic. These are one of the major drivers of the ocean currents and of the distribution of heat on a global basis. This generation of Antarctic bottom water is an area that is even now not well understood and is the subject of much of the work of the Antarctic Cooperative Research Centre in Hobart.

By drilling holes in the sea floor you’ve been able to infer something about Antarctic floral history.

Almost any recent mud that one dredges up around Antarctica will contain pollen of a variety of ages. A lot of it is pollen of the more recent part of the Antarctic vegetation, of the Tertiary, pollen that’s very similar to much of the Australian Tertiary records. The record of the Antarctic muds is both intriguing and very frustrating. It gives us a kind of catalogue of the plants that once grew on Antarctica, but because the glacial action has been so strong, pollens of different ages tend to be mixed up together as the sediments have been churned up by the advance and retreat of ice shelves, so it’s very hard. While we now have quite a good record of what grew on Antarctica we have a much less clear understanding of when it grew there.

We do have a couple of sections where borehole information is better than average. A very recent cruise of the Ocean Drilling Program off Prydz Bay, east Antarctica, has given us a good section where we have a feel for the most recent Antarctic vegetation. The vegetation there was probably a fairly stunted version of much of the cool temperate Tasmanian vegetation, dominated by the southern beeches and some of the southern conifers, with a minor component of ferns and a few very interesting things like the sundews, the carnivorous plants that are very widespread in Australia. We know that they were there as part of that Antarctic vegetation.

At what time was this?

This is the Eocene period, so you’re looking at 40, 45 million years ago. It’s very hard from the information we have to know exactly when that vegetation was wiped out by increasing cold and by the growth of the ice cap. Our guess now is that it probably didn’t persist much beyond that. It may have persisted locally into the more recent period, the Oligocene, possibly even the Miocene, but we’re looking at an elimination of it probably around 20 million years ago, although there are some very controversial much younger beds there too.

But the other way we’ve been able to use the distribution of pollen around Antarctica is to pinpoint the rocks underneath the ice that might be the source of that pollen. And we were able to do this on one occasion where we had a good coverage of samples right across the Ross Sea. We were able to chart the percentages of pollen in those samples and find out that it was concentrated in a couple of great tongues on the eastern edge of the Ross Sea. Working with glaciologists from Cambridge, particularly David Drewry, we were able to pinpoint from those high pollen densities particular ice streams that were feeding down through the Ross Ice Shelf. We could look at the source of those ice streams and say ‘Well, somewhere here under the ice shelf we have Cretaceous and Tertiary beds that are being eroded.’