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Darby is Tracing Climate Cycles with Help of $1.2 Million Microprobe

Darby at controls of electron probe microanalyzer

The electron probe microanalyzer that anchors its own compact laboratory in Old Dominion University's Oceanography and Physical Sciences Building looks something like a photocopier, and anyone other than a scientist might underestimate what it can do. For ODU geological oceanographer Dennis Darby, the instrument has opened new research vistas, and he expects from it nothing less than major revelations about global climate change.

One of the most expensive research instruments on the ODU campus, the $1.2 million microprobe is enabling Darby to sift through climate clues dating back thousands of years. His goal is to chart natural climate cycles, and to determine how those cycles may be better understood separate from random climate changes that scientists believe are caused by man-made atmospheric pollution.

For a couple of decades, Darby has been working to piece together evidence of climate cycles that goes back 100,000 or so years in the Arctic, a region that has a surprisingly large impact on climate elsewhere in the world, and especially in the Northern Hemisphere.

He does his detective work by analyzing sediments, mostly from core samples that have been collected when researchers drill a hollow tube into the floor of the Arctic Ocean or nearby seas.

That's where state-of-the-art microprobe analysis comes in. For nearly two decades prior to the new instrument's arrival at ODU in 2009, Darby used an "ancient" probe that was difficult to operate and not as precise in its analysis as the new one. Just how ancient was the old instrument? When Penn State donated it to him in the early 1990s, it had already put in many years of service.

Darby's work is possible because of an iron-grain chemical fingerprinting technique he developed that enables him to determine the landmass where the grains originated. A speck of sand, for example, may be from a particular peninsula of Greenland, yet be found in a sea-bottom core sample many hundreds of miles away. This provides evidence about winds and currents-and therefore the overall weather patterns-that brought the grain to its resting place.

The top of a cylindrical core sample is made up of recently deposited sediments on the sea bottom; deeper down in the sample are sediments from times past. There are numerous ways to date a grain of sand depending on where it shows up in the vertical sample. Darby has worked with some samples that were deep enough to have sediments dating back more than 40 million years.

Analysis of core samples, therefore, can help identify long-standing climate patterns. These revelations are of heightened importance now because of concerns about climate change and global warming.

Even if natural cycles are responsible for some recent warming trends, this doesn't let humans off the hook for polluting the atmosphere, Darby said. Human influence may combine with natural cycles to increase global warming. Still worse, the possibility exists that his project will reveal that the frozen North should be in a natural cooling cycle now, but that man-made influences are causing it to become warmer instead.

The focus on the Arctic by climate scientists has guaranteed Darby a steady stream of research grants, one of them being the $500,000 contributed by the National Science Foundation (NSF) in 2009 to assist in the university's purchase of the new microprobe. His latest NSF grant - $433,000 for a three-year project extending through July of 2014 - is for analysis of core samples from the bottom of the Denmark Strait off Greenland. The project is titled "Do Holocene Variations in Arctic Sea Ice and Greenland Icebergs Drifting Through Denmark Strait Reflect Natural Cycles?"

The Denmark Strait is outside the Arctic but directly in the path of ice drifting from the Arctic and east Greenland. It is an area where cold water and floating ice from the polar region come into contact with warmer waters that are pushed north by global ocean circulation. The melting there of sea ice or icebergs releases sediments trapped in the ice. The sediments sink to the bottom and build up layer after layer on the sea bottom.

By analyzing samples from that sea bottom, Darby can determine how much imported sediments from polar ice has been deposited in the Denmark Strait, where the sediments came from and any weather patterns that caused this to happen.

Currently, a melt-back of the Greenland Ice Sheet (covering roughly 80 percent of the surface of Greenland) seems to be speeding up and the calving, or breaking off, of icebergs is increasing. Similar melting trends have also been reported in the Arctic Ocean.

"We've all seen stories about the rapid demise of the Greenland Ice Sheet. This is representing about .7 millimeter of the sea level rise of the last decade," Darby explained. "That's a good portion of the global sea level rise that we've seen - about 25 percent of it."

He said the Greenland Ice Sheet has probably been shrinking for the last 100 to 200 years. "But it has been speeding up recently and we didn't expect it to speed up so quickly. It looks like it could become an even bigger player in sea level rise, and there is a lot of concern in this area."

Another concern has to do with the creation of deep-ocean water off Greenland where polar currents run into warmer currents from the south. Just north of Iceland, deep-water is created where the same current involved in the Gulf Stream off our coast becomes cold and salty enough to sink. This acts like a "pump" that sustains global ocean circulation. Without this there would be a much weaker Gulf Stream as we know it today to warm the eastern United States before it pushes north and east across the Atlantic Ocean to also warm the British Isles and Scandinavia.

This sinking - to a depth of a mile or more - pulls warmer surface water northward and pushes deep, cold water southward, helping to maintain the global conveyor belt of ocean circulation.

Any Arctic warming or melting of Greenland ice affects this deep-water creation process by diluting the salty Gulf Stream near Iceland and slowing its descent thus weakening the global circulation and changing the climate in any region whose weather is influenced by the circulation.

"Climate change could have a big negative impact on economic growth," Darby said. "Anything that changes the weather is not good for the economy. We need to be prepared. It is my hope that we stop arguing about whether our burning fossil fuels causes climate change and start focusing on how to deal with it. Because it's real."

Darby's research is not directly involved in weighing human contributions to climate change, such as increases in carbon dioxide in the atmosphere brought on by combustion. "We're looking for natural conditions that are helping to cause this global warming and sea level rise. There seems to be a natural pacing to climate change. If you don't know what changes are naturally occurring over the long haul, you don't know how to deal with conditions over the short term."

Recent research by Darby has produced evidence of 1,500-year cycles in Arctic oscillations, which are broad swirls of winds and currents that switch directions and produce periods of cooling interchanged with periods of warming.

Evidence of this climate-cycle pacing is there only in broad terms, however; the pacing is difficult to pin down precisely in time. Darby's current NSF project in the Denmark Strait is designed to zero in on 50-100 year resolution for the cycles, to the point that he can determine what the natural influences are right now on Arctic/Greenland weather, and what influences we can expect in the near future.

"Is there a consistent, significant pacing that will help with our predictions?" he asks in explaining the work.

The NSF also has funded collaborative work on this project by John Andrews and Anne Jennings of the University of Colorado, two prominent paleoclimate experts. Darby's results also will be used by a project team led by Grant Bigg of the University of Sheffield in England, which is modeling iceberg calving variation around Greenland through the 20th century.

For now, the electron microprobe, which is made by the Cameca company, is performing its analysis of grains from Denmark Strait core samples, running almost continuously, day and night. "In the 27 months we've been using the instrument, we have done almost 100,000 grain analyses. In the previous nearly 18 years with the old machine, we did only 150,000 grain analyses. This new one is a beautiful instrument," Darby said.

An electron microprobe works similarly to a scanning electron microscope. A sample is bombarded with an electron beam, and X-rays are emitted at wavelengths characteristic to the elements being analyzed. This enables the prevalent elements present within small sample volumes (typically 10-30 cubic micrometers or less) to be determined.

This article was posted on: October 6, 2011

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