How did plankton end up on top of Antarctica’s mountains?

Ars Technica How did plankton end up on top of Antarctica's mountains?

Enlarge / The Transantarctic Mountains.

Since 1984, researchers have been arguing about what to make of tiny ocean plankton shells sitting atop the Transantarctic Mountains. That is obviously not a place plankton would call home, so how did they get there? And what might their presence tell us about Antarctica's past?

The researchers who initially discovered the surprise diatom shells found them mixed in with glacially deposited sediment. They argued that the shells told a pretty big story. Around 3 million years ago, when these species of diatom were alive, we know there were several periods of climatic warmth. During these periods, the researchers said, the Antarctic ice sheets must have "collapsed" down to a much smaller size, opening basins in the interior of the continent which flooded with seawater. That is where these diatoms would have lived, piling up at the bottom of the seaway when they died.

When the climate cooled, the ice would have advanced back into these basins, scraping up diatoms and sediment and pushing it up against the Transantarctic Mountains—where it was perhaps lifted to even higher elevation by tectonic uplift of the landscape. This would require a much more dynamic ice sheet than most glaciologists expected, vulnerable to warmth that would drive serious melting and raise sea levels significantly.

This hypothesis proved controversial, and other researchers argued forcefully that other, less extraordinary explanations were possible. The debate formed two camps—the "stabilists" who favored alternate explanations that didn't involve large-scale retreats of Antarctic ice, and the "dynamicists."

Follow-up studies seemed to swing things toward the stabilists. The diatoms were actually concentrated in the surface layer of the glacial sediments, for example, implying that they had separately been deposited on top—perhaps carried by wind. And model simulations of Antarctica's ice didn't produce a massive retreat when fed the conditions of the Pliocene 3 million years ago.

A new study led by Northern Illinois University's Philip Scherer attempts to revisit the mystery of the stranded plankton, testing a sort of compromise hypothesis between the two camps. Even if the diatoms were blown up there on the wind, the stubborn fact remains that marine diatoms aren't really found anywhere else in Antarctica. You can find the occasional diatom in the Antarctic snow, but they are all continental species from sources far away (or the one place in Antarctica that isn't covered in ice—the McMurdo Dry Valleys). It's easy for wind to pick up dried-out diatoms, but it's not going to blow them straight out of ocean water. So how were marine diatoms ending up on the winds of 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 produces much more retreat than earlier efforts, better matching records of sea level from that time. The sensitive West Antarctic Ice Sheet basically disappears, but two important coastal sections of the East Antarctic Ice Sheet also retreat more than 500km inland across lowlands, which then become bays. Altogether, this raises global sea level by about 17 meters compared to the present day.

The mass of thick ice sheets actually depresses the Earth's crust downward beneath them, and the opposite happens when ice melts away—the land surface rebounds upward. The model simulation does this, too, and that's where the diatom story comes in.

During the retreat, these growing bays would be nutrient-rich plankton havens. But a few thousand years after the ice retreats into those new bays, some areas near the old coastline rebound above sea level, becoming exposed land. So the newly exposed land would probably be covered in sediment loaded up with marine diatom shells, all dried out and ready to be picked up by the wind.

Enlarge / The outline of a simulated Antarctic ice sheet shrunken by Pliocene warmth. Brown areas have rebounded above sea level several thousand years after the ice disappeared. Bright red dots mark locations in the Transantarctic Mountains where marine diatom shells have been found.

For the final step, the researchers ran a climate model simulation of the atmosphere over this changed Antarctic continent. While the winds at the surface mainly blew outward toward the coast (and down-slope), the pattern was different a few thousand meters up. The simulated upper level winds would send any diatoms lifted by storms straight toward the Transantarctic Mountains.

Put it all together and this seems to make a lot of sense. The "stabilists" can be right about the diatoms being blown up to the mountains, and the "dynamicists" can be right about the diatoms being linked to significant retreats of Antarctic ice. There isn't always a "middle way" between two sides of a scientific argument, but there might be in this case.

Apart from the pleasure of solving an interesting "cold case," this is actually important for our understanding of future sea level rise. We need all the clues we can get from Antarctica's past to help us predict how it will respond as we turn up the planet's thermostat.

Open Access at Nature Communications, 2016. DOI: 10.1038/ncomms12957  (About DOIs).

http://arstechnica.com/science/2016/09/how-did-plankton-end-up-on-top-of-antarcticas-mountains/

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