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Hidden river once flowed under Antarctica

“Ice, by itself, is only capable of flowing at velocities of no more than tens of meters per year. That means the ice is being helped along. It’s sliding on water or mud or both.”

The West Antarctic ice sheet. Its ice drains to the sea via glaciers, and via ice streams that accelerate over distances of hundreds of kilometers. A new study focuses on the question of what causes the ice streams’ fast rate of flow. Image via NASA.

Rice University said on August 21, 2017 that its Antarctic researchers have discovered what they called “one of nature’s supreme ironies.” That is:

… on Earth’s driest, coldest continent, where surface water rarely exists, flowing liquid water below the ice appears to play a pivotal role in determining the fate of Antarctic ice streams.

What Antarctic scientists call ice streams are not liquid, flowing water. Instead, an ice stream is a wide corridor of noticeably fast flow within an ice sheet, that is, a wider mass of glacial ice. Antarctic ice streams flow at different rates, but surface observations show that a typical rate of flow might be hundreds of meters per year. The new study – led by Rice postdoctoral researcher Lauren Simkins – focuses on what might be happening under the ice streams. Simkins explained:

We … know that ice, by itself, is only capable of flowing at velocities of no more than tens of meters per year. That means the ice is being helped along. It’s sliding on water or mud or both.

Now there’s evidence for this idea, in these researchers’ discovery of a fossilized river system beneath the Ross Sea. The finding appeared online on August 21 in the peer-reviewed journal Nature Geoscience.

A map of ice streams in the West Antarctic Ice Sheet, via britannica.com.

Antarctica is covered by ice that’s more than 2 miles (3 km) thick in some places, and this ice is replenished each year by falling snow. So much of Antarctica’s ice is flowing seaward, and some of that seaward flow occurs in ice streams. If you’re standing on an ice stream, you can’t feel or see it move, but indeed it is moving. Gravity compresses the ice, and it moves under its own weight. Ice streams carry ice and sediment from the Antarctic interior to the surrounding ocean.

Even with the best modern instruments, the undersides of Antarctic ice streams can’t be observed directly. So it’s hard to know for sure what’s making them move so much faster than ice alone would be expected to move. The researchers at Rice University did a two-year analysis of sediment cores and precise seafloor maps covering 2,700 square miles (about 7,000 square km) of the western Ross Sea. The maps reveal that – only 15,000 years ago – the Ross Sea was covered by thick ice year-round; the ice later retreated hundreds of miles inland to its current location. The statement from the researchers said:

The maps, which were created from state-of-the-art sonar data collected by the National Science Foundation research vessel Nathaniel B. Palmer, revealed how the ice retreated during a period of global warming after Earth’s last ice age.

In several places, the maps show ancient water courses — not just a river system, but also the subglacial lakes that fed it.

Map of Antarctica, showing the Ross Sea. Some 15,000 years ago, this sea was ice-bound year-round. Now it’s still covered with ice for most of the year. Image via Wikimedia Commons.

The U.S. Antarctic Program’s research vessel Nathaniel B. Palmer has icebreaking capability and can operate year-round. State-of-the-art sonar data from this ship revealed the fossilized river system under the Ross Sea. Image via the National Science Foundation.

EarthSky asked Lauren Simkins how the new evidence of ancient water courses fits with the idea of possible flowing water under the ice streams today. She told us:

Water at the base of ice does influence how quickly ice flows; however, it all depends on the style of meltwater drainage. Does it happen in widespread sheet flows or in discrete channels, and for how long and how often are these subglacial ‘floods’ occurring?

So, it isn’t as simple as saying that all water drainage at the base causes faster flow.

The first step is to characterize these different styles either using observations from contemporary ice sheets or from paleo-ice sheets and then disentangle how they influence ice flow and retreat.

An example of a seafloor map – showing submarine topography – used by the Rice University oceanographers to identify the features found in their study. Image via L. Simkins/ Rice University.

She also said that – because there’s so little accessible information about how water presently flows beneath Antarctic ice – the fossilized river system offers a unique picture of how Antarctic water drains from subglacial lakes via rivers to the point where ice meets sea:

The contemporary observations we have of Antarctic hydrology are recent, spanning maybe a couple decades at best. This is the first observation of an extensive, uncovered, water-carved channel that is connected to both subglacial lakes on the upstream end and the ice margin on the downstream end. This gives a novel perspective on channelized drainage beneath Antarctic ice. We can track the drainage system all the way back to its source, these subglacial lakes, and then to its ultimate fate at the grounding line, where freshwater mixed with ocean water.

This schematic depicts a subglacial Antarctic river and overlying ice sheet. Black lines t1, t2 and t3 show where the ice sheet was grounded to the seafloor during pauses in ice retreat. Rice University researchers used such lines from precise maps of the Ross Sea floor to study how liquid water influenced the ice sheet during a period of its retreat starting about 15,000 years ago. Image via L. Prothro/ Rice University.

According to the statement from Rice, Simkins said meltwater builds up in subglacial lakes. First, intense pressures from the weight of ice causes some melting. In addition, Antarctica is home to dozens of volcanoes, which can heat ice from below. Simkins found at least 20 lakes in the fossil river system, along with evidence that water built up and drained from the lakes in episodic bursts rather than a steady stream. She worked with Rice co-author and volcanologist Helge Gonnermann to confirm that nearby volcanoes could have provided the necessary heat to feed the lakes.

Study co-author John Anderson, a Rice oceanographer and veteran of nearly 30 Antarctic research expeditions, said the size and scope of the fossilized river system could be an eye-opener for ice-sheet modelers who seek to simulate Antarctic water flow. For example, the maps show exactly how ice retreated across the channel-lake system. The retreating ice stream in the western Ross Sea made a U-turn to follow the course of an under-ice river. Simkins said that’s notable because:

It’s the only documented example on the Antarctic seafloor where a single ice stream completely reversed retreat direction, in this case to the south and then to the west and finally to the north, to follow a subglacial hydrological system.

Simkins and Anderson said the study might ultimately help other researchers better predict how today’s ice streams will behave and how much they’ll contribute to rising sea levels.

Read more about this study via Rice University

This is an old image, from 2001, showing an iceberg frozen in pack ice at the Ross Sea, Antarctica. Brocken Inaglory, who posted the image at Wikimedia Commons, said it was taken in a “very, very bad weather.” It’s a good illustration, though, of conditions sometimes endured by teams of scientists studying in Antarctica.

Bottom line: Using seafloor maps of Antarctica’s Ross Sea, researchers have discovered a long-dead river system that once flowed beneath Antarctica’s ice and influenced how ice streams melted after Earth’s last ice age.

Deborah Byrd

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