Rates of Quaternary Ice Movement Across the Vestfold Hills from Exposure Age Dating.Entry ID: ASAC_831
Abstract: The data set consist of in-situ cosmogenic Be-10 and Al-26 surface exposure ages for subglacial erratics in the Vestfold Hills.
Samples were collected during the 1994/95 summer season and processed at the research School of Earth Sciences, ANU. Al-26/Al-27 and Be-10/Be-9 ratios were measured using the 14UD acellerator mass spectrometer at the Research School of Physical Sciences and ... Engineering, ANU.
The Earth is continually being bombarded by high-energy cosmic rays that originate predominantly from super nova explosions within our galaxy. Interactions between these high energy cosmic rays and the Earth's atmosphere creates secondary cosmic rays, including neutrons and muons. When reaching the Earth's surface these high energy particles can penetrate meters into rock and sediment. Nuclear interactions between neutrons and muons and minerals such as quartz, calcite, K-feldspar, and olivine, produce long-lived terrestrial cosmogenic nuclides (TCNs) such as Be-10, Al-26 and Cl-36. The production rates of these TCNs are almost unimaginably small - a few atoms per gram of rock per year, however using accelerator mass spectrometry (AMS) we can detect and count the cosmogenic isotopes down to levels of a few thousand atoms per gram (parts per billion of parts per billion!).
The build-up of cosmogenic isotopes through time provides us with a way to measure the length of time since a rock surface became exposed to cosmic radiation. For example, the Vestfold Hills were buried under the East Antarctic ice sheet and shielded from cosmic rays by the ice. But when the ice margin retreated the Vestfold Hills became exposed to cosmic rays and TCNs began to accumulate in quartz crystals of glacial erratics left behind by the retreating ice. By measuring the Be-10 and Al-26 concentration in these erratics it has been possible to determine when the last ice retreat took place.
Apart from providing a history of ice retreat across the Vestfold Hills, this study also highlighted the need for very careful sample selection. The production of TCNs in a rock surface decreases roughly exponentially with depth, with very little production occurring below about 2 meters. If a surface has been exposed to cosmic rays and is subsequently covered by ice that erodes 2 or more meters of rock from the surface the accumulated TCNs would be removed. However, if the ice does not erode 2 or more meters of rock than the surface has an inherited TCN concentration after the ice retreats.
Striated bedrock surfaces are often considered as classic indicators of how erosive ice is. However, unpublished data from striated bedrock surfaces in the Vestfold Hills yielded much older ages than the deglaciation ages obtained from erratics, indicating that glacial erosion of the bedrock sites was not sufficient to remove the cosmogenic nuclide inventory inherited from prior exposure periods. This cosmogenic nuclide inheritance does however indicate that the sampled bedrock surfaces had previously been exposed to cosmic rays, i.e. the continental ice margin has repeatedly advanced and retreated across the Vestfold Hills.
Importantly, if the aim is to obtain deglaciation ages from glacial landscapes or deposits, every effort should be made to sample glacial erratics rather than striated bedrock.
Astract from the referenced paper:
Cosmogenic nuclide measurements on two subglacially derived erratics from the Vestfold Hills, East Antarctica indicate retreat of the continental ice margin from a position at least 5 km west of its present location around 12-9 ka. These ages are similar to C-14 dates on the oldest organic sediment in lakes around the Hills, suggesting that biological colonization of lake basins coincided with glacial retreat. Whether this indicates recession of ice that extended across the entire Vestfold Hills, or an amelioration of climate that simply thawed lakes and melted a much less extensive ice cover cannot be established from the results so far. recession appears to have continued recently, with results from a third sample indicating emergence of a small nunatak, Ultima Bluff, within the past 2 ka.
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Start Date: 1994-09-30Stop Date: 1995-03-31
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DIF Creation Date: 2001-08-06
Last DIF Revision Date: 2013-05-07