Since 1984, researchers have argued over what to make of tiny ocean plankton shells that sit atop the Transantarctic Mountains. Of course, that’s not where plankton belong, so how did they get there? And what might their presence tell us about Antarctica’s past?
The researchers who initially discovered the surprising diatom shells found them mixed with glacier-deposited sediment. They argued that the grenades told quite a big story. About 3 million years ago, when these types of diatoms lived, we know that there were different periods of climatic heat. During these periods, the researchers said, the Antarctic ice sheets must have “collapsed” to a much smaller size, creating basins in the interior of the continent that are inundated with seawater. That’s where these diatoms would have lived, piling up on the bottom of the seaway as they died.
As the climate cooled, the ice would have pushed back into these basins, scraping up diatoms and sediment and pushing it up the Transantarctic Mountains—where it may have been lifted to an even higher level by tectonic uplift of the landscape. This would require a much more dynamic ice sheet than most glaciologists expected, vulnerable to heat that would severely melt and raise sea levels significantly.
This hypothesis proved controversial and other researchers strongly argued that other less extraordinary explanations were possible. The debate split into two camps: the ‘stabilists’ who favored alternative explanations that did not involve large-scale retreat of Antarctic ice, and the ‘dynamicists’.
Follow-up studies seemed to swing things in the direction of the stabilizers. For example, the diatoms were concentrated in the surface layer of the ice sediments, implying that they were deposited singly on top – perhaps carried by the wind. And model simulations of Antarctica’s ice caused no mass retreat when they fed Pliocene conditions 3 million years ago.
A new study led by Reed Scherer of Northern Illinois University seeks to re-examine the mystery of the stranded plankton and test some kind of compromise hypothesis between the two camps. Even if the diatoms there have been blown up by the wind, the persistent fact remains that marine diatoms are not found anywhere else in Antarctica. You can occasionally find diatoms in the Antarctic snow, but they are all continental species from sources far away (or the only place in Antarctica not covered by ice – the McMurdo Dry Valleys). The wind can easily pick up dried out diatoms, but it won’t blow them straight out of the ocean water. So how did marine diatoms end up on the winds 3 million years ago?
The first step in answering that question was to turn to the latest and greatest Antarctic ice sheet model simulation of the Pliocene warm periods. That model yields much more retraction than previous attempts, and is more consistent with sea level data from that time. The sensitive West Antarctic Ice Sheet is effectively disappearing, but two major coastal sections of the East Antarctic Ice Sheet also retreat more than 500 km inland across lowlands, which then become bays. All told, this raises global sea levels by about 17 meters compared to current times.
The mass of thick ice sheets actually pushes the Earth’s crust down, and the opposite happens when ice melts away: the land surface rebounds upwards. The model simulation does this too, and that’s where the diatom story comes into play.
During the retreat, these growing bays would be nutrient-rich plankton havens. But a few thousand years after the ice has receded into those new bays, some areas near the ancient shoreline rise above sea level again and become exposed land. So the newly exposed land would likely be covered in sediment full of marine diatom shells, all dried out and ready to be picked up by the wind.
For the final step, the researchers performed a climate model simulation of the atmosphere above this altered Antarctic continent. While the wind at the surface blew mainly outward towards the coast (and down), the pattern was different a few thousand meters higher. The simulated upper-level winds would send any diatoms lifted by storms straight to the Transantarctic Mountains.
Put it all together and this seems to make a lot of sense. The “stabilists” may be right about the diatoms being blown to the mountains, and the “dynamicists” may be right about linking the diatoms to significant Antarctic ice retreats. There isn’t always a “middle ground” between two sides of a scientific argument, but there could be in this case.
Aside from the fun of solving an interesting “cold case”, this is actually important to our understanding of future sea level rise. We need all the clues we can glean from Antarctica’s past to help us predict how it will react when we adjust the planet’s thermostat.
Open Access Nature communication2016. DOI: 10.1038/ncomms12957 (About DOIs).