Contact: Ann Marie Menting, 617/353-2240 | firstname.lastname@example.org
(Boston, Mass.) — In research that promises to upend theories on Antarctic geology and usher in new thinking on climate change on the Earth, Associate Professor David Marchant of Boston University’s Earth Sciences Department has drawn from Antarctica’s Dry Valleys region a fresh understanding of the age and formation of its characteristic polygonal patterned ground. In doing so, Marchant also has estimated the glacier below this ground to be at least 8 million years old—formed during the Pliocene and Miocene epochs and, now, a fossil bank of pristine samples of ancient atmospheres.
Seeking to better understand the Earth’s climate throughout the ages, geomorphologist Marchant looks to the ground and glaciers of Antarctica’s Dry Valleys. This silent landscape, lying undisturbed for millennia, holds clues to changes in the Earth’s climate throughout time.
Prior to this research, Marchant had conducted isotopic analyses of volcanic ashes found in the valleys and had determined that dozens of the deposits were more than 10 million years old. One ash, dated at 8.1 million years, can be found in the valleys’ granite drift, a mud-rich matrix of unconsolidated dolerite and sandstone gravel and quartz grains deposited by the region’s dominant glacier millions of years ago. In Beacon Valley, an area of particular interest to Marchant, this drift bears a distinctive polygonal patterning, a craquelure that some scientists have theorized was caused by the convection of melt water from an underlying glacier they estimated to be more than 1 million years old.
The theory that convection drove this patterning process grew primarily from a model of glacier–ground interactions in the Northern Hemisphere. In the North, melt water saturates debris on top of glacial ice.
The Dry Valleys of Antarctica, however, are exceedingly dry and cold—too cold to allow for glacial melting or, Marchant reasoned, convection in debris overlying the glacier.
To test his hypothesis, Marchant undertook a geochemical analysis of the drift’s stratigraphy. Because of its texture, granite drift would reorganize if subjected to water moving from the glacier to the ground’s surface. Marchant checked for such reorganization using a dating technique known as cosmogenic 3He analysis, a method that measures the buildup over time of these nuclides within rocks near the Earth’s surface. If convection was at work, the nuclides would show a uniform presence from a polygon’s surface to the top of the underlying glacier. Marchant’s analysis, however, showed cosmogenic 3He concentrations steadily decreased with depth, indicating the stratigraphy of individual polygons was stable over time.
Having established the stability of the polygons, Marchant looked to the troughs that outline them for answers to the patterning process. Combining visual observations of stratigraphy and surface morphology of the drift with cosmogenic analyses of the troughs, Marchant modeled a dynamic process. This process begins with thermal-contraction cracking of the buried glacier, continues with sublimation that is hastened by a lag of gravel-sized rocks that forms in a polygon’s troughs, and terminates as wind-blown snow collects in these deep troughs and thermally insulates the underlying ice from further contraction.
Marchant’s cosmogenic nuclide measures indicate that, once established, the troughs can last up to 1 million years. In addition, he found that the rate of sublimation of the buried ice was between 5 m – 90 m per million years. Taken together, Marchant’s findings indicate that the drift and the glacier preserved beneath it have had a stable relationship for more than 8 million years.
“This discovery provides us a look back through time,” says Marchant. “The clues to atmospheric evolution may be found in the gases captured in this ancient ice.”
The Earth Sciences Department at Boston University offers an interdisciplinary study of the Earth as a system, with faculty conducting research in geophysics, hydrology, geology, image-analysis, geochemistry, sedimentology, and surface processes.
Boston University, with an enrollment of more than 29,000 in its 17 schools and colleges, is the fourth-largest independent university in the United States.