On Last Field Day, a South Pole Detour

The day at Hatcher Bluffs was exceptional. We flew the 240 miles from the South Pole and worked two moraines downstream of Strickland Nunatak and Hatcher Bluffs, near the head of the large Reedy Glacier. In many ways the work reminds me of beachcombing. I grew up in Minnesota on the north shore of Lake Superior and spent countless hours wandering the lake’s gravel beaches looking for agates and other interesting rocks. Occasionally I would find an unusual rock, like a garnet-bearing granitic gneiss, that must have been carried down from Canada by the Laurentide ice sheet during the last continental glaciation some tens of thousands of years ago.

Jeff Vervoort Blocky moraine at Strickland Nunatak.

Our work on the glacial moraines in the Transantarctic Mountains is much the same. The five of us fan out and scour the moraines, making a mental inventory and collecting what is interesting and different from the exposed bedrock. The two sites today were unusual in that the clasts in the moraines were completely different from each other despite coming from locations only 10 miles apart. This most certainly means the clasts were derived locally and reflect the difference in the exposed bedrock upstream at each site. These clasts were nonetheless useful because they were different from the usual local bedrock that we have been seeing at many other sites (Beacon sediments and Ferrar dolerites).

Jeff Vervoort Hauling samples over the blue ice back to the Twin Otter at Strickland Nunatak.

After dinner in the South Pole galley we flew back to CTAM at the end of a very long day — over the massive ice cap at the bottom of the world and the Transantarctic Mountains that serve effectively as a barrier to ice flow off the polar plateau. The slowly moving mass of ice that makes up the polar plateau begins to trickle through the small tributary glaciers but sweeps boldly into the broad outlet glaciers like the Beardmore, Nimrod and Byrd, and makes an impatient rush through the mountains toward the sea. By looking at the different surface features in the glacier surfaces, it is easy to get a sense of their relative velocities. When the glaciers funnel through the Transantarctic Mountains the ice is moving quite fast. Leigh Stearns of the University of Kansas estimates that the fast outlet glaciers like Byrd are moving up to 800 meters per year, or more than two meters a day on average.

Jeff Vervoort Ice at the edge of the polar plateau and the beginning of the Transantarctic Mountains.

Now that our fieldwork is complete, our plan is to pack up and fly to McMurdo tomorrow. Once there, we will have a day or two to organize all our gear and rock samples, pack up and head north. We have collected more than a ton of rock — about 350 different samples. This is a huge number — far more than we can analyze for age and isotopic composition, but with the screening protocol John mentioned in his last post, we will be able to target the most appropriate samples for full analysis.
My main contribution to this project is geochronology and radiogenic isotope geochemistry. The former is determining the age of rock samples. The latter uses radiogenic isotopes — so called because they are produced by decay from a radioactive parent — as tracers or “fingerprints” of the rock’s past history. We can use radiogenic isotopes, for example, to determine if a rock is derived from the Earth’s mantle or the crust, and roughly when this occurred. In conjunction with the age of the rocks, we can piece together when the continents formed and how they have evolved through Earth’s history.
We will be performing three different types of analytical measurements in our lab at Washington State University, all supported by National Science Foundation grants to John Goodge and myself: U-Pb zircon geochronology, Lu-Hf garnet geochronology and zircon Hf isotope geochemistry. Without getting wrapped up in the nitty-gritty of these techniques, each involves the fundamental process of the decay of unstable atoms (radioactive isotopes) to stable atoms (radiogenic isotopes). Because this process occurs at a constant rate, all radioactive and radiogenic isotopes contain the element of time. With some of these applications we can determine a geochronological age (e.g., uranium-lead isotopes in zircon; lutetium-hafnium isotopes in garnet), and in others we can use the radiogenic isotopes as a tracer to determine the origin of a rock (e.g., hafnium isotopes in zircon).

Clean lab at Washington State University with chromatographic columns for separating hafnium from dissolved garnet samples.

Jeff Vervoort Garnet-bearing gray gneiss.

Garnet crystals separated from a rock sample and observed under the microscope.

One interesting application that will be very helpful in this project is garnet geochronology. Garnet, in contrast to zircon, is primarily a metamorphic mineral that forms during orogenic or mountain building events. Garnets incorporate the element lutetium when they grow and largely exclude hafnium. By determining the hafnium isotopic composition and the lutetium/hafnium ratio we can determine the time at which the garnets in the rock formed. Garnet is very common in our glacial clasts and in the basement gneisses we collected near the Ascent Glacier camp. We will analyze them to get a different layer of information for our samples — namely when our samples went through periods of metamorphism or orogenesis.

Jeff Vervoort Mosaic of refrozen sea-ice blocks.

Many times over the past several weeks, when examining a new and interesting rock, I couldn’t wait to get the sample back to the lab to determine how old it is. As John mentioned in his last post, however, it will be some time before we will get our hands on these samples. After the samples reach McMurdo, they will have a long journey ahead of them by cargo ship. The last time we were in McMurdo, LC-130s, C-17s and even the A-319 Airbus were landing on the ice runway, along the route for the cargo ship. Now the ice runway has been moved to the shelf ice nearby (the “Pegasus” ice runway). Before a vessel can make it into the ice pier at McMurdo, an icebreaker will come in and clear a path. This is slated to start in mid-January.

Jeff Vervoort Moat-shaped moraine at the base of an icefall.

As my stay “on the ice” winds down, I have been reflecting on the trip. It has been an epic journey to a place I have long wanted to visit, and it has exceeded all expectations. Over the past month I have lived in a tent on a glacier at the bottom of the world; flown to extraordinary places in the Transantarctic Mountains in Twin Otters and helicopters; traveled by snowmobile across crevasse-scarred glaciers to view the complicated basement terranes; visited many field sites with fascinating geology; lived and worked in bitter katabatic winds; and enjoyed the peace and serenity of a brilliantly bright, clear, calm day. All of these things seemed so foreign and, perhaps, a little intimidating a few months ago.
As with all things, however, what once was exotic and foreign after time becomes very familiar and normal. It now seems normal to sleep in a tent with nothing between you and the ice but a thin camp pad. It now seems normal to jump in an airplane and fly for hundreds of miles over vast ice fields coursing through rugged mountains. This is not to say it becomes boring — far from it. But it is what you do on a daily basis; it is simply the way things are here.

Jeff Vervoort Dylan Taylor, John Goodge and Mark Fanning on top of Mount Sirius.

I also had time to reflect on our scientific work here. By all measures, it has been a huge success. We visited all of the field sites that we wanted to visit. We collected hundreds of samples that will keep us busy for some time. We have, regrettably, finished with the field aspect of the project; next up is the much more time-intensive and painstaking part. It will certainly not be as adventurous, but it is the most informative and — for me at least — the most scientifically satisfying. The nice thing about a project like this is that it will keep on giving. I’ll be able to mentally revisit this incredible setting a little bit with each analysis. At the same time, the changing atoms in the ice-borne bits of rock we’ve collected will help tell the fundamental narrative of the planet. Regardless of how the results turn out, it will be a fascinating story.