T51 Looking to the Past to Understand the Future: Glacial Chronology and Process

Sunday, 22 October 2017
1:30 PM - 5:30 PM
Chelan 2

THE DEAD SPEAK: TRACKING THE CRYOSPHERIC RESPONSE TO CONTEMPORARY WARMING IN ARCTIC CANADA WITH ENTOMBED VEGETATION AND IN SITU 14C IN ADJACENT ROCKS

Paper number: T51-12
Program: Technical Sessions
Time: 4:20 PM – 4:40 PM

Authors

Abstract

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. However, increased summer warming since the 1960s has resulted in accelerated mass loss throughout the Canadian Arctic. Whether increased melt will result in glacier dimensions smaller than previous warm times remains debated. Although glaciers are best known for their erosive capacity, sectors of 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. We date newly exposed rooted plants by AMS 14C, and measure in situ 14C inventories in nearby rock surfaces to test the conclusions of the plant ages. We have 365 14C dates on emerging dead plants at elevations ranging from 500 to >1600 m asl, and 7 in situ 14C inventories on rocks adjacent to dated plants. Snowline has been above 1900 m asl during most of the past two decades. 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 limited by topography, the dates tend to be older. Thirteen samples collected between 1200 and 1400 m asl returned plant ages between 9.0 and 9.8 ka, supporting peak warmth in the earliest Holocene, which has now been surpassed. And most interesting 33 dates from 27 unique ice caps revealed rooted plants with 14C ages >40 ka (mean elevation 1440 m asl). Interpreting in situ 14C in rocks adjacent to plant collections is complicated by slow muogenic production through relatively thick ice, but most samples revealed so little inventory that any exposure during the Holocene can be ruled out, consistent with the 14C plant ages. Collectively, these data indicate that many landscapes exposed during recent summer warming were continuously ice-covered since the LIG. 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 at present.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-301977

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