18 April 2018

Cryospheric and Polar Processes Seminar: Gifford Miller

Cryospheric and Polar Processes Seminar
East Campus, RL-2, room 155, 11:00 a.m.

Most Northern Hemisphere glaciers achieved their maximum late Holocene dimensions during the anomalous cold of the Little Ice Age, receding in the early 20th Century. Rapid summer warming since the 1960s has resulted in accelerated mass loss throughout the Canadian Arctic. Snowline has been above 1900 m asl during most of the past two decades, and glaciers are losing mass at all elevations. However, whether increased melt has resulted in glacier dimensions smaller than previous warm times remains debated. Although glaciers are best known for their erosive capacity, sectors of cold Arctic glaciers that overlie flat terrain are often exceptional preservation agents, and as they melt back they expose intact landscapes, including rooted plants that have been entombed since killed by the expanding ice, and rocks that were not moved or eroded. We date newly exposed rooted plants by AMS, and measure in situ inventories in nearby rock surfaces to test the implications of the plant ages. We have 365 dates on recently deglaciated dead plants at elevations ranging from 500 to >1800 m asl. But, because ice caps do not grow and recede symmetrically, the dated plants span a wide range of ages. Where ice was thick (slow recession) or at low elevations, the plant dates are mostly from the last 2 ka (mean collection elevation ~1100 m asl; n=312). But for thin, high-elevation ice caps, where glacier dimensions are often mechanically limited by topography, the dates tend to be older. Fourteen plant samples returned ages averaging 9.3 ± 0.3 ka, supporting peak warmth in the earliest Holocene (12-9 ka), which has now been surpassed. And most interesting are 45 in situ plants from 30 unique ice caps that have ages >40 ka (mean elevation 1435 ± 150 m asl). To test the implication of these dates that summer warmth of the recent century exceeds any century during the Holocene we measured in situ in rock surfaces adjacent to nine of the -dead plants. Interpreting in situ in rocks is complicated by slow muogenic production through relatively thick ice, but most rocks revealed so little inventory that any exposure during the Holocene can be ruled out, consistent with the plant ages. Collectively, these data indicate that many landscapes exposed during recent summer warming were continuously ice-covered since the Last Interglaciation, and that contemporary warming is now causing some ice caps on Baffin Island to shrink to dimensions smaller than at any time in the past ~115 ka, including the HTM, when summer insolation was 9% higher than the past century.