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

• Main Authors: G. Balco, B. D. DeJong, J. C. Ridge, P. R. Bierman, D. H. Rood
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-01-01
• Series: Geochronology
• Online Access: https://gchron.copernicus.org/articles/3/1/2021/gchron-3-1-2021.pdf

<p>We attempt to synchronize the North American Varve Chronology (NAVC) with ice core and calendar year timescales by comparing records of atmospherically produced <span class="inline-formula">¹⁰</span>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 <span class="inline-formula">¹⁰</span>Be concentrations in NAVC sediments are within the range of those observed in other lacustrine records of <span class="inline-formula">¹⁰</span>Be fallout, but <span class="inline-formula">⁹</span>Be and <span class="inline-formula">¹⁰</span>Be concentrations considered together show that the majority of <span class="inline-formula">¹⁰</span>Be is present in glacial sediment when it enters the lake, and only a minority of total <span class="inline-formula">¹⁰</span>Be derives from atmospheric fallout at the time of sediment deposition. Because of this, an initial experiment to determine whether or not <span class="inline-formula">¹⁰</span>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 <span class="inline-formula">¹⁰</span>Be fallout that have similar properties as coeval <span class="inline-formula">¹⁰</span>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 <span class="inline-formula">¹⁰</span>Be fallout by soil adsorption can filter out short-period variations in an entirely deglaciated watershed, but such buffering cannot occur in the ablation zone of an ice sheet. This implies that proglacial lakes whose watershed is mostly glacial may effectively record <span class="inline-formula">¹⁰</span>Be fallout variations. We attempted to match centennial-period variations in reconstructed <span class="inline-formula">¹⁰</span>Be fallout flux from two segments of the NAVC with ice core fallout records. For both records, it is possible to obtain matches that result in acceptable correlation between NAVC and ice core <span class="inline-formula">¹⁰</span>Be fallout records, but the best-fitting matches for the two segments disagree, and only one of them is consistent with independent calendar year calibrations of the NAVC and therefore potentially valid. This leaves several remaining ambiguities in whether or not <span class="inline-formula">¹⁰</span>Be fallout variations can, in fact, be used for synchronizing NAVC and ice core timescales, but these could most likely be resolved by higher-resolution and replicate <span class="inline-formula">¹⁰</span>Be measurements on targeted sections of the NAVC.</p>