IN-SITU 14C AND 36CL: A TOOL FOR ANALYZING COMPLEX EXPOSURE AND BURIAL HISTORIES OF GLACIAL LANDSCAPES

In-situ cosmogenic nuclides can be used to determine ages of glacial landforms if glacial erosion removes previously accumulated nuclides and resets the clock. This is often the case in temperate and alpine glacial settings, where ice is warm-based. But under conditions of cold-based ice, common in the Arctic, such removal is often incomplete, which leads to inheritance of cosmogenic nuclides from previous exposure episodes. As a result, an apparent age calculated from a long-lived cosmogenic nuclide, such as ³⁶Cl or ¹⁰Be, will be incorrect in two ways. First, it will overestimate the age of the last deglaciation because some fraction of the measured nuclide inventory is inherited from previous exposure (of unknown duration). Second, it will underestimate the age of the penultimate deglaciation because of the burial (of unknown duration) by ice before the last deglaciation and the resulting hiatus in cosmogenic production. In principle, the duration of the two exposure episodes and of the intervening burial episode can be determined with two or more cosmogenic radionuclides with different half-lives. These nuclides must be selected according to the expected age range of the surfaces. Late Quaternary surfaces require a combination of one long-lived nuclide, such as ³⁶Cl (half-life of 301 ky) or ¹⁰Be (1.5 My), with the short-lived ¹⁴C (5730 y). We describe an approach that uses in-situ ³⁶Cl and ¹⁴C, discuss its application to unravelling complex histories of glacial landforms from the Canadian arctic, and explore how this pair of nuclides could be used in the analysis of inventories of long-lived nuclides measured in other glacial landforms.