Denniston co-authors paper that solves climate puzzle

Rhawn Denniston, geology professor at Cornell College and chair of the Environmental Studies program, is the co-author of a paper in the journal Nature Communications that solves a long-standing controversy about ancient climate change.

The paper, written by Matthew Lachniet from the University of Nevada-Las Vegas and co-authored by Denniston, along with Yemane Asmerom and Victor Polyak from the University of New Mexico, addresses the argument about how glaciers grow and melt over tens of thousands of years.

“The modern theory for the origin of ice ages holds that continental-sized glaciers, like the one that used to cover Canada, grow and melt over the scale of tens of thousands of years due to small changes in the way Earth moves around the Sun (so-called Milankovitch Cycles),” Denniston said.

This model was first proposed in the 19th century and in the 1970s, research on ocean sediment showed changes in glaciers at a pace consistent with the predictions from the Milankovitch theory. Controversy arose after a series of papers were published in the 1980s and 1990s, which analyzed minerals deposited by groundwater in the Nevada desert at a site called Devils Hole. These samples appeared to offer the first direct dating of the timing of ice ages over the past 500,000 years, and suggested that each of the last five ice ages ended ~15,000 years before the Milankovitch theory said they should have, a major challenge to understanding the drivers of global climate change. The Devils Hole record spurred a vigorous debate over the past 25 years, but never resulted in a satisfactory explanation.

“Our work has finally resolved the Devils Hole controversy,” Denniston said. “We precisely dated and then analyzed stalagmites (cave formations) from a series of caves located across southern Nevada, close to Devils Hole. When combined, these samples form a nearly continuous record spanning the last 170,000 years, thereby allowing the first direct test of the Devils Hole record. Our data show that ice ages in Nevada ended exactly when Milankovitch theory says they should have, demonstrating conclusively that the Devils Hole record does not, in fact, record global climatic change as it has been argued to have done, but instead reflects a suite of more regional climatic variables.

“In addition, our stalagmite record reveals a close connection global climate and the Southwest’s ice age mega-lakes, most of which are entirely gone today and which the Great Salt Lake is but a tiny remnant of. At the heart of the last ice age (20,000 years ago), one of these lakes—Lake Bonneville—was nearly comparable in size to modern day Lake Michigan, but disappeared when the last ice age ended 10,000 years ago. Our data show that reconstructions of Lake Bonneville water levels varied in time with changes in global climate, rather than moving in time with some unknown pacemaker.”

The paper was published in the May 2 edition of Nature Communications, and is available online.

The National Science Foundation has just given a grant to Denniston and his colleagues so that they can expand upon this research.