About 14,000 years ago, planet Earth was defrosting. Expansive ice sheets that covered most of continents were melting fast, signalling the end of the Ice Age. Temperatures climbed and more meltwater poured into the ocean. The transition between glacial and interglacial periods was swift, but the rate at which the ice sheets shrank during the swing has been somewhat of a mystery in the modern world.
(From Dalhousie University/ By Niecole Comeau and John Gosse) — Recently, an international team of researchers set out to decode that piece of history. John Gosse, a University Killam Professor in the Department of Earth Sciences, joined Brian Menounos at the University of Northern British Columbia and 12 additional experts.
Together they published a paper in Science that meticulously recalculated a great Ice Age deglaciation that occurred on the North American continent. The study provides new insight on ice sheets, warming and corresponding sea-level rise.
Unlocking secrets in the ice
The focus of their attention was the ancient Cordilleran Ice Sheet (CIS). It covered much of Western Canada throughout the Ice Age and was similar in size and water content as the Greenland Ice Sheet as it exists today. Previous attempts to nail down the timing of the CIS decay — when the ice margins receded — returned unusual results. The CIS is of particular interest to many scientists because unlocking its secrets could tell us how the ice sheet on Greenland might behave as global temperatures continue to rise.
“The total volume of water stored in the Greenland Ice Sheet corresponds to a global sea level rise of 7 metres,” says Dr. Gosse, a co-author on the study. “But a key question is: how fast can a grounded ice sheet melt during a period of rapid warming? By examining the CIS, Dr. Menounos and the rest of our team set out to find the answer.”
When the CIS melted, it uncovered the northernmost area of the Canadian Rocky Mountains. The previous radiocarbon dating data available suggested that CIS decay occurred 12,500 years ago. But global temperature and sea-level data from that period didn’t add up: warming began much earlier, so how was the ice still present?
“The lack of datable ancient organic materials in remote areas of the Rockies hampered previous attempts to determine more precise timing of receding ice sheet margins,” says Dr. Gosse. Carbon dating, he says, is based on the measurement of time since a plant or animal died and stopped exchanging carbon isotopes with the atmosphere.
Reshaping the coastline
A newer technique — terrestrial cosmogenic nuclide exposure dating, or surface exposure dating — determines when boulders underneath glaciers became exhumed, exposing them to cosmic rays. It doesn’t rely on whether or not animal and plant life was present, which it likely wasn’t on the tops of mountains covered by ice for tens of thousands of years. Dr. Gosse is the director of CRISDal, Canada’s only cosmic ray isotopes science laboratory. His involvement in this study helped the team determine that the CIS melted away 14,000 years ago, 1,500 years earlier than the story that older data told.
“The results reveal that the ice sheet decayed much more rapidly and earlier than previously determined, in a complex way partly because of the significant topography in the Canadian Rocky Mountains,” says Dr. Gosse. “By combining the new chronology with ice sheet numerical modeling, we could directly relate the demise of the ice sheet to the rapid global warming that happened immediately after the last glaciation.”
Modelling suggests that half of the massive CIS ice sheet melted away over the course of 500 years—a very short period, geologically speaking. In other words: it happened fast. That partial deglaciation corresponded to a 2.5-3 m rise in sea-level.
If history does indeed repeat itself…
Read the full article here: https://www.dal.ca/news/2017/12/04/learning-from-the-past–coastlines-and-the-ice-age.html