Field Notes: The Polar Field Services Newsletter

Learning About Greenland’s Ice Through Rocks

 

Jeremy Shakun collects sand.

Jeremy Shakun collects sand from a stream draining deglaciated terrain south of Narsarsauq. All photos Paul Bierman.

To understand past changes in Earth’s climate, scientists like geomorphologist, Paul Bierman (University of Vermont), must use a variety of information obtained from sources like tree rings, fossilized pollen, coral, ice cores, ocean sediments, and rocks. Bierman and colleagues Dylan Rood (Scottish Universities Environmental Research Centre and University of California, Santa Barbara), Jeremy Shakun (Boston University), Eric Portenga (University of Vermont), and Alice Nelson (University of Vermont) are examining unique geochemical properties of rocks collected from coastal Greenland and in marine sediments. This analysis will help the scientists reconstruct how the size of the Greenland ice sheet (GIS) changed over the last 6 million years. It will also yield clues about how the GIS might behave in a warming climate.

“Our goals are four-fold. We want to know when continental glaciations began on Greenland. We are also looking at how the extent of the Greenland ice sheet has changed through time. This research will help us understand how effective the GIS is at eroding and transporting material during times of ice sheet growth and compare sedimentation rates prior to continental glaciations,” explains Bierman, who runs UVM’s Cosmogenic Nuclide Laboratory.

Field Techniques

Bierman explains how they will analyze samples: “We use a combination of geochemical tools—Beryllium isotopes to examine rocks from land and in marine sands— to recreate several million years of Greenland ice sheet history. By using the oxygen isotope ratio of oxygen, we’ll characterize the shells of microfossils in the marine sediments as well so that we can better understand the timing of offshore deposition.”

Up and away.

Dylan Rood gives the up-and-away signal during lift of from the last sample site of the afternoon, 15km south of Narsarsauq.

Chemical Reactions

When cosmic rays enter earth’s atmosphere and intersect with Earth’s surface, they sometimes collide with atoms in rocks and change them. Oxygen atoms may be split resulting in a rare isotope of beryllium (Be10). Bierman analyzes the amount of Be10, along with a rare isotope of aluminum and several other elements, to better understand when and by how much the GIS grew and shrank.

“When the Greenland land surface is exposed during an ice sheet retreat, cosmic rays interact with rocks and Be10 forms,” says Bierman. “When the ice sheet expands again, glaciers push these rocks to the edge of the ice sheet where we collect them. Back in the lab we first determine where the rocks originated using maps and glacier flow models. Then, we extract Be10. Because of the radioactive properties of Be10, we can use it as a clock to tell us when and where the ice sheet was absent. We use the same method to examine quartz sand from the marine cores with the idea that rocks containing Be10 are eroded and pulverized into sand-sized particles by the GIS and eventually deposited on the continental shelf.”

Removing sediment from the ice margin

Jeremy Shakun and Eric Portenga removing sediment from the ice margin outside of Kanngerlussuaq.

Lab Work

In June, 2011, Bierman’s team collected nearly 60 rock samples (enough to fill two coolers) from near Kangerlussuaq and Narsarsuaq. In 2012, they worked in the area around Kulusuk in east Greenland.  Back in the lab, the team crushes samples using hammers, a jaw crusher, and a plate grinder until the detritus is less than one millimeter in size.  Using sieves, the scientists isolate sand-sized fraction which is then treated with hydrofluoric and nitric acid to dissolve everything but quartz.

Following another tedious series of acid baths and checks for sample purity, the quartz is dissolved and tiny quantities of Be and Al are extracted. Samples are dried and packed into stainless steel capsules about an inch long and half an inch wide.

Bierman and students then fly to the Accelerator Mass Spectrometry (AMS) at the Scottish Universities Environmental Research Centre to analyze samples for their exact amounts of Be10 and Al isotopes using a “football field-sized mass spectrometer” with the help of Dylan Rood, who operates the machine.

Making a plan

The field team makes a plan in an outwash stream near the ice margin.

Core Analysis

“We’ll use a similar method for analyzing marine core sediments – that said, it’s pretty out there in that no one has used this method on marine sediments before,” Bierman says. “We’ll also extract and date a species of the marine microfossil foraminifera, Neogloboquadrina pachyderma, to constrain the timing of ocean deposition and get a better handle on ice sheet behavior.”

Microfossils as Climate Indicators

 Pachyderma are also a good indicator of past climate.  When ocean temperatures are cool, the tiny shells form such that they coil to the left. When ocean temperatures are warm, the shell coils to the right. Cataloguing the amount of and coiling directions of pachyderma from the marine cores will help confirm periods of warmer and cooler climate and correlate data to known sea level and atmospheric carbon dioxide data.

During GIS Advancement

“We expect to see data that reflect a pulsing of material at different times. We should see more sediment and more Be10 during times of GIS advance when the ice sheet is scraping surface off material that was previously exposed and delivering it to the ocean,” says Bierman.

If the sediments do not contain any Be10, this would indicate a timer period in which the GIS was completely expanded and no surface rocks were exposed.

Studying Surf and Turf Together

“Looking at land and marine histories together will allow us to understand more about Earth’s climate through time better than if we looked at these data sets separately because we will have a continuous record of the last 6 million years of GIS behavior,” says Bierman. “This should allow us to speculate on how the GIS might react to a warming climate since the climate has been warmer in the past than it is now.”—Marcy Davis

 

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