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

Abstract. We attempt to synchronize the North American Varve Chronology (NAVC) with the calendar year time scale by comparing records of atmospherically produced Be-10 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 18,200–12,500 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 time scale to facilitate high-resolution correlation of climate events. Total Be-10 concentrations in NAVC sediments are within the range of those observed in other lacustrine records of Be-10 fallout, but Be-9 and Be-10 concentrations considered together show that the majority of Be-10 is present in glacial sediment when it enters the lake, and only a minority of total Be-10 derives from atmospheric fallout at the time of sediment deposition. Because of this, an initial strategy to determine whether or not Be-10 fallout variations were recorded in NAVC sediments by attempting to observe the characteristic 11-year solar cycle in high-resolution sections of varve sequences was inconclusive: observed short-period variations at the expected magnitude of this cycle were not distinguishable from measurement scatter. On the other hand, we did observe centennial-period variations in Be-10 fallout that are replicated between separate varve sections and have similar magnitude and frequency as coeval Be-10 fallout variations recorded in ice core records. These are most prominent in glacial sections of the NAVC that were deposited in proglacial lakes, but are suppressed in paraglacial sections of the NAVC deposited in lakes lacking direct glacial sediment input, which leads us to conclude that proglacial lakes whose watershed likely includes a large portion of the ablation area of an ice sheet can effectively record Be-10 fallout. We matched observed centennial-scale Be-10 fallout variations in two segments of the NAVC to ice core Be-10 fallout records. Although the calibration of the NAVC to the calendar year time scale implied by these matches is similar to that proposed previously in independent calibrations based on radiocarbon data and correlation of climate events, matches for the two different segments disagree with each other and with the independent calibrations by 50–200 years. One of these matches is not consistent with independent evidence and is probably not valid, but the other is consistent with most, although not all, evidence and may be valid. This leaves several remaining ambiguities in whether or not Be-10 fallout variations can, in fact, be used for synchronizing NAVC and ice core timescales, but these could likely be resolved by higher-resolution and replicate Be-10 measurements on targeted sections of the NAVC.