Short duration glacial advances: implications for cosmogenic dating chronologies
A/Prof. Duanne White1, Dr Reka Fulop2,3, Dr David Fink2, Dr Toshi Fujioka2,4, Mr Marcello Blaxell1, Mr Matt Jeromson1, Dr Alexandru Codilean3
1University Of Canberra, , Australia, 2ANSTO, , Australia, 3University of Wollongong, , Australia, 4Centro Nacional de Investigación sobre la Evolución Humana, CENIEH, , Spain
Cosmogenic nuclides are now the mainstay technique for constraining past ice advances in terrestrial environments. However, limited erosion of bedrock and recycling of older glacial sediments complicates the interpretation of dated records. Where younger deposits or striated bedrock surfaces contain nuclides inherited from previous periods of exposure, cosmogenic dates overestimate deglaciation by tens to hundreds of thousands of years. This is especially problematic in Antarctica, where low erosion rates and cold-based glaciation produce widespread inheritance in erratics or bedrock surfaces. Studies using long-lived nuclides such as Be-10 have identified sites where <5% of erratics are inheritance-free, making it logistically difficult to measure ice sheet retreat.
To address this issue, the short-lived nuclide in-situ C -14 is being applied in Antarctic deglaciation studies. The relatively recent deglaciation across most parts of Antarctica mean inheritance in erratics can be detected by discordant Be-10 and C-14 ages. Bedrock may also be dateable using C-14 if the inherited nuclides decay below measurement uncertainties while ice cover is present. However, both these approaches, particularly direct bedrock dating, make assumptions of the duration of the glacial advance.
To test these approaches we measured C-14 in bedrock surfaces from sites across East Antarctica between Enderby Land (48oE) and Vincennies Bay (107oE). At each site, the true deglaciation age was well established via Be-10 ages on erratics, and in some cases, coincident with the onset of biogenic sedimentation in lake and marine basins. We find that the assumption of a long-duration Last Glacial Maximum (LGM) is not widely met. The two inland sites appear to have been buried long enough to re-set the C-14 signal. However, at least four sites along the East Antarctic coastline displayed bedrock surfaces that were saturated with C-14, despite clearly having been covered by ice during the early Holocene.
These results mean that in-situ C-14 ages must be carefully interpreted, especially when used on bedrock. Bedrock surfaces that provide discrete ages can be used to indicate that deglaciation has occurred since the global Last Glacial Maximum. However, those with saturated surfaces provide equivocal results unless supporting evidence can support the expected duration of the LGM.
I am a quantitative geomorphologist and geochemist working to understand links between landscape and climate during the Quaternary period. I work largely in Australia, Antarctica and Sub-Antarctic Islands. I utilise a range of techniques but have a focus on applications of cosmogenic nuclides to constrain past ice sheet histories.