Every rock has a story, and some contain long histories of changing landscapes. This summer, a team of geologists from Victoria University of Wellington Te Herenga Waka spent days walking along Antarctica’s mountain slopes to collect rocks that had been dropped by glaciers.
“The glaciers are doing a lot of the work for us,” says Jamey Stutz, of the university’s Antarctic Research Centre. “They are plucking rocks from their bed, incorporating them into their flow and dropping them off when they ultimately melt out, either next to the mountain or out to sea.”
Known as “erratics”, these rocks represent the geology buried beneath Antarctica’s massive ice sheets, but the team’s primary interest is in deciphering the story about how glaciers change shape as they flow from the continent’s interior. For this, the team is hunting for “perchies” – rocks that are perched on the side of mountains in such a delicate position that only glaciers could have deposited them there. Essentially, they are marker stones, tracing how far up the mountain a glacier once reached.
To figure out how long ago the perchies fell out of the glacier, the team uses a method known as cosmogenic dating. When a rock is exposed, it is blasted by high-energy cosmic rays that have been ejected by supernovae, says the university’s Kevin Norton. “These cosmic particles smash into the atoms in the rock and transform them into new, different atoms. The longer a rock sits at the surface, the more these novel atoms build up and we can pull them out and count them and tell how long a rock has been exposed.”
Wringing this information from a rock is an involved process. First, it has to be crushed into particles about 1mm in size. This rock dust then has to be separated into fractions to isolate quartz crystals, which are cleaned, dissolved in acid and evaporated until “all you’re left with in the bottom of a teflon beaker is a little bit of black scum – and that’s beryllium”.
More specifically, it’s an isotope of beryllium (chemically the same but heavier because of an extra neutron), which acts like clockwork.
This summer, the team went on a rock hunt on Minna Bluff, a nine-million-year-old volcanic land tongue with a hook, not far from Scott Base, which forces glaciers flowing past to make a 120-degree turn to continue out to sea. They returned with a sparkly, mica-rich lump of gneiss and rounded, bullet-shaped cobbles delivered by the Byrd glacier and easily spotted on the otherwise volcanic surface. They then collected bucketfuls of rocks from the Byrd’s southern side.
Together with rocks collected elsewhere and the help of the cosmogenic chemical chronometers, the team has shown that glaciers like the Byrd experienced a period of rapid thinning some 8000-6000 years ago. We know from ice cores and cave deposits that the peak of the last ice age was about 20,000 years ago, but to understand what happened around the start of the deglaciation, the team will need to scale even higher mountains that “stick up high enough near dynamic-enough glaciers”, Norton says.
One of the possible explanations for the more recent glacial thinning lies in the shape of the ocean floor. If a glacier is resting on a ridge of bedrock below sea level that has a backward slope, it loses its foothold once the grounding line rolls over the high point. “There’s runaway ice loss until the grounding lines get back onto sound footing.”
Stutz says this makes the rocks’ stories relevant to understanding the wider West Antarctic Ice Sheet, the smaller of the continent’s two icy blankets, which rests partly below sea level and is vulnerable to this process. Stutz is also the assistant director of an international rock depository at Ohio State University and hopes the collective efforts of Antarctic rock hunters will help unravel how and why the continent’s glaciers change.
