Atmospherically produced beryllium-10 in annually laminated late-glacial sediments of the North American Varve Chronology

• Published in: Geochronology
• Main Authors: G. Balco, B. D. DeJong, J. C. Ridge, P. R. Bierman, D. H. Rood
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2021
• Subjects: Geology; QE1-996.5; Stratigraphy; QE640-699; ice core; Ice Sheet
• Online Access: https://doi.org/10.5194/gchron-3-1-2021; https://doaj.org/article/e4603103377841e591235565434821ee

We attempt to synchronize the North American Varve Chronology (NAVC) with ice core and calendar year timescales by comparing records of atmospherically produced 10 Be fallout in the NAVC and in ice cores. The North American Varve Chronology (NAVC) is a sequence of 5659 varves deposited in a series of proglacial lakes adjacent to the southeast margin of the retreating Laurentide Ice Sheet between approximately 18 200 and 12 500 years before present. Because properties of NAVC varves are related to climate, the NAVC is also a climate proxy record with annual resolution, and our overall goal is to place the NAVC and ice core records on the same timescale to facilitate high-resolution correlation of climate proxy variations in both. Total 10 Be concentrations in NAVC sediments are within the range of those observed in other lacustrine records of 10 Be fallout, but 9 Be and 10 Be concentrations considered together show that the majority of 10 Be is present in glacial sediment when it enters the lake, and only a minority of total 10 Be derives from atmospheric fallout at the time of sediment deposition. Because of this, an initial experiment to determine whether or not 10 Be fallout variations were recorded in NAVC sediments by attempting to observe the characteristic 11-year solar cycle in short varve sections sampled at high resolution was inconclusive: short-period variations at the expected magnitude of this cycle were not distinguishable from measurement scatter. On the other hand, longer varve sequences sampled at decadal resolution display centennial-period variations in reconstructed 10 Be fallout that have similar properties as coeval 10 Be fallout variations recorded in ice core records. These are most prominent in glacial sections of the NAVC that were deposited in proglacial lakes and are suppressed in paraglacial sections of the NAVC that were deposited in lakes lacking direct glacial sediment input. We attribute this difference to the fact that buffering of 10 Be fallout by soil adsorption can filter out ...