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,2
1 Geology Department, University of Vermont, Burlington, Vermont 05405, USA
2 Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, California 94550, USA
Exceptionally high activities of in situ–produced 10Be and 26Al indicate that Australian bedrock surfaces are some of the most stable in the world. These cosmogenic nuclides demonstrate that Australian granite, exposed and sampled at 10 sites along a transect from the Eyre Peninsula (33°S) to the Kakadu area in the Northern Territory (14°S), has in some places eroded only decimeters over the past million years. However, nuclide activities are not saturated in any of the 61 exposed samples we analyzed, mandating that the surfaces we sampled are losing mass, albeit slowly, over time. Such mass loss indicates that inselbergs are dynamic landforms, confounding the assignment of discreet ages to inselberg surfaces. Nevertheless, the differential between slow rates of erosion measured on the very stable uppermost parts of these landforms and the more rapid rates measured below their flanks and below the adjacent grus-covered planes suggests that the forms of many large inselbergs, which stand meters to tens of meters high, must be at least Tertiary in age and possibly much older.
Activities of 10Be and 26Al in 61 exposed Australian samples range from 5.1 ± 0.1 x 106 to 5.3 ± 0.6 x 105 atoms·g–1 and 2.1 ± 0.1 x 107 to 3.4 ± 0.5 x 106 atoms·g–1, respectively (normalized to sea level, at latitudes of >60°, using only neutron scaling). Activities of 10Be and 26Al in seven shielded samples (3.5–13 m below the surface) are much lower, ranging from 3.4 ± 0.2 x 104 to 7.0 ± 0.1 x 104 atoms·g–1 and 1.6 ± 0.1 x 105 to 2.8 ± 0.2 x 105 atoms·g–1, respectively. 26Al/10Be ratios of exposed samples range from 3.89 ± 0.19 to 6.29 ± 0.82. If 2
uncertainty ranges and a nominal sea-level, high-latitude 10Be-production rate of 5.17 atoms·g–1·yr–1 are considered, ratio data for all but one surface sample are consistent with continuous surface or near-surface exposure and/or erosion. Rock surfaces farther north in Australia, where the climate is more humid, have lower nuclide activities; thus, we infer that surface stability is inversely related to mean annual precipitation.
From 61 exposed samples, we calculated maximum limiting, single-nuclide, model erosion rates of 0.3 ± 0.1 to 5.7 ± 1.0 m/m.y. and minimum limiting, single-nuclide, model exposure ages of 105 ± 16 to 1310 ± 190 ka (by assuming a sea-level, high-latitude 10Be-production rate of 5.17 atoms·g–1·yr–1). Seven shielded samples, in which the nuclide inventory is muon-generated, also have nuclide activities consistent with similarly low rates of erosion but over even longer time scales. It appears that the Eyre Peninsula inselbergs have been and are eroding so slowly that they may well be direct descendents of Cenozoic and perhaps even Mesozoic forms.
Key Words: Australia • cosmogenic • elements • erosion • inselbergs • weathering
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