The North American Varve Chronology (NAVC) is a nearly 6000-year-long sequence of annual sediment layers that was deposited in glacial lakes adjacent to the margin of the Laurentide Ice Sheet, which covered much of North America during the last ice age. At the time these sediments were deposited, between approximately 12,500 and 18,000 years ago, climate was warming and the Laurentide Ice Sheet was shrinking rapidly. The sediments that make up the NAVC are an annually resolved record of events that took place at the retreating ice margin, including the position and retreat rate of the ice margin, the flux of water derived from summer melting of the ice sheet, and large glacial-lake outburst floods that occurred periodically as ice-marginal lakes drained. The goal of this proposal is to understand the relationship of these events to climate changes that were occurring at the same time and are recorded in ice core records from Greenland, which is important because i) it will help to understand how large ice sheets responded to rapid climate warming, and ii) because this time period is a potential analog for the current period of rapid warming and likely shrinkage of glaciers and ice sheets. The obstacle to comparing the NAVC with Greenland climate records is that, although both records have been independently dated, the precision of these dates is a few hundred years, which is not precise enough to accurately determine which climate changes are related to which glacial events. This project will try to resolve this issue by comparing records of atmospherically-produced Beryllium-10 in both NAVC sediments and Greenland ice cores. Be-10 is a naturally occuring rare isotope of Beryllium that is created by cosmic-ray bombardment of the upper atmosphere and falls to earth in precipitation. Be-10 production changes through time due to changes in the Earth's magnetic field, and these changes occur everywhere on Earth at the same time. Be-10 is captured and preserved in sedimentary archives such as ice cores and lake sediments. Thus, Be-10 concentrations in the Greenland ice cores and in the NAVC lake sediments should show the same pattern of changes. By aligning these changes with each other, we can place the two records on a common timescale. Thus, even if we do not know the exact time that a particular climate change recorded in the Greenland ice cores took place, we will know exactly what ice-marginal events recorded in NAVC sediments were occurring at that time. We anticipate that we will be able to match these two records with a precision of several decades, which should be precise enough to determine the relationship between important climate changes and ice sheet change.
Technical The North American Varve Chronology (NAVC) is a 5659-year-long sequence of annually laminated sediments (?varves?) deposited in lakes adjacent to the retreating Laurentide Ice Sheet (LIS) in eastern North America. It is an annually resolved record of ice-marginal position, meltwater and sediment flux, and proglacial lake outburst floods between ca. 12,500 and 18,000 years ago. The goal of this project is to correlate this varve chronology with contemporaneous climate records derived from the Greenland ice cores. This is important because such a correlation would reveal the temporal relationship between terrestrial ice-marginal events and North Atlantic climate changes, which in turn would shed light on unresolved aspects of late-glacial ice sheet-climate interactions including, for example, i) the effect of proglacial lake outburst floods at the margin of the LIS on North Atlantic climate , ii) the effect of North Atlantic climate on surface ablation and retreat of the southeastern margin of the LIS, and iii) the apparent conflict between sustained cold climate in Greenland and significant ice margin retreat during this time period. The team proposes to correlate these two records at decadal precision, independently of the absolute dating of each of the records, by comparing records of atmospherically-produced (?meteoric?) Be-10 fallout in Greenland ice cores and NAVC sediments. A decadal-resolution record of Be-10 flux to the Greenland ice cores already exists. Here we propose to carry out corresponding Be-10 measurements in varved sediments of the NAVC to i) determine whether or not Be-10 concentrations in these sediments record changes in Be-10 fallout, and ii) if so, generate a record of Be-10 fallout flux comparable to the existing ice-core record so that the two can be correlated.
