Citation by P. Thompson Davis
The paper by Pendleton and co-authors is the most convincing report yet that current warming in the Arctic is the greatest since the Last Interglacial (MIS-5) ended about 115,000 years ago. The paper is one result from Simon Pendleton’s Ph.D. dissertation at the University of Colorado. The paper benefited greatly by Pendleton’s six co-authors, especially Giff Miller for his guidance of the overall project. Pedestal ice caps respond to rapid equilibrium line altitude (ELA) rise that exposes new landscapes of varying ages at different altitudes as opposed to cirque glaciers that are commonly influenced by variables such as snow avalanching and shielding from solar insolation. The highest ice caps have remained cold-based allowing preservation of entombed plants since their burial. Within 1 meter of receding margins from 30 different ice caps, 48 in situ tundra plants were collected for AMS 14C dating, thus the dead plants, primarily Polytrichum moss, only became exposed the same year that they were collected. Of the 48 measurements of plant 14C, 26 are “greater than ages” (radiocarbon dead), 19 calibrated to >40 ka, and three calibrated to 28-38 ka, each of these three with >40 ka replicates. Given that some recently exposed dead tundra plants may begin to regrow (“zombie mosses”) and then be re-entombed, in situ 14C measurements were made from adjacent rock samples to model cumulative exposure and burial history. Of the nine in situ 14C study sites with plant 14C ages >40 ka, only one in situ 14C rock sample showed the possibility of early Holocene exposure when solar insolation was near peak. Thus, based on the two types of 14C data presented in the paper and the isotopic temperature record from the NGRIP ice core in Greenland, about 115 ka was the most likely time that these pedestal ice caps last receded as much as they are receded today.
Response by Simon Pendleton
It is with a great sense of honor and, frankly, surprise, that I accept the 2023 QG&G Kirk Bryan Award on behalf of myself and my co-authors. The community of geomorphology and Quaternary Geology is filled with dedicated, creative, and supportive geologists all doing incredible work, and I am humbled to merely count myself amongst all of you. In truth, this paper is a reflection of that community; it was only possible through the coming together of a range of individuals, each with their unique sets of questions, approaches, and ideas. With that I want to begin by thanking all my co-authors for their input, expertise, and support.
As described in the citation, this work has involved many individuals over several decades to come to fruition. It took the chance collection of preserved plants nearly 60 years ago, some not insignificant improvements in radiocarbon dating, the invention of an entirely new surface dating technique (cosmogenic exposure dating), and the perseverance of individuals in the pursuit of understanding these landscapes and the climate secrets they hold. Of course, there was much hard work, including many hours of walking ice margins, and the processing of hundreds of preserved plants through Scott Lehman and John Southon’s labs. Not to mention three days tent-bound in a snowstorm on Baffin Island where Giff, Scott and I realized the need for independent surface exposure constraints. Here we had Nat Lifton to thank for providing those needed constraints through in situ cosmogenic radiocarbon. The final piece of the puzzle came through modeling ice cover histories, which was only possible with collaboration with Bob Anderson and Sarah Crump. In many ways, this paper encapsulates the theme of the Kirk Bryan award: the innovations made by others over past decades enabled our team to continue to advance the field and our understanding of these glacier-climate systems.
And as our glaciers and landscapes continue to change under a warming climate, it was only recently that I fully appreciated the irony this particular project. The irony that the warming of the climate - the very thing we are attempting to quantify and characterize – is revealing to us, through ice recession, the data we need to do just that. On one hand, the continued retreat of these ice margins reveals more preserved landscapes that can tell us much about past geomorphic, cryospheric, and climatic processes, but on the other hand, these newly exposed materials are ephemeral, and once they are gone, the record is lost forever. The recognition of this important work provided by this award further inspires me, especially as an early career geoscientist, to continue down this path, to continue working with this amazing community to advance this science.
This science would not be possible without the contributions and encouragement of so many. I want to express my gratitude to Dr. Thom Davis for spearheading my nomination and to Drs. Meredith Kelly, Mike Retelle, and Nicolás Young who all lent their support. I also owe thanks to my masters advisor Jason Briner, who not only steered me towards Giff and Baffin Island work, but also for helping me refine my science writing ability, a not insignificant task I assure you. I must also thank Giff Miller, who as my PhD advisor at the time was instrumental in guiding me through this project, and continues to mentor me to this day. Giff has also always underscored the value of developing a wide network of collaborators- without which multi-disciplinary work such as this would not be possible. Finally, my most sincere thanks to the Quaternary Geology and Geomorphology Division of GSA for this recognition.
This award does not come without mixed emotions, as I am sad that my late colleague and dear friend Dr. Sarah Crump is not here to celebrate with us. Not only was she a key contributor to this work but also a source of endless support professionally and personally. The Kirk Bryan award is given to a paper that advances the field, but in Sarah’s case, during her all too brief but shining career, she advanced the geosciences in her own way: through mentorship, inclusion, and community building in addition to her science. It is in that spirit of advancing the community that my co-authors and I are dedicating the monetary portion of this award to the Sarah Crump Fellowship to support the next generation of geoscientists in shining just as bright as Sarah.