BORATE SOIL CONCENTRATIONS AS AN EXPOSURE AGE CHRONOMETER IN ANTARCTIC BLUE ICE MORAINES

Reliably dated sedimentary deposits from glacier and ice sheet settings represent a critical resource for understanding past glacial behavior in polar regions. Cosmogenic isotopes are frequently used to assess sediment exposure ages, but often produce heterogeneous exposure age results. Reasons for this heterogeneity include: inheritance, variable shielding by snow, periglacial processes, and collapse of ice-cored moraines during ablation and melt. Because of these complexities, multiple independent methods are useful for calculating definitive exposure ages.

We measured the concentrations of water soluble salts across a transect of progressively older blue ice moraine sediment in the Transantarctic Mountains. This site is in a high-elevation, hyper-arid environment, where liquid water is scare and ephemeral, allowing atmospherically deposited salts to remain on the landscape. Samples were collected from the top 5 cm of the soil profile, just below the zone of wind deflation.

Independently, cosmogenic exposure dating was previously carried out along the same transect, revealing progressively older ages away from active ice to a maximum of ~0.5 Ma. Along this transect, borate development has a near perfect linear correlation to exposure age (R² >0.99). Other salt species also correlate with cosmogenic exposure age, but the relationship is weaker (R² ~0.8) and is logarithmic for nitrate and sulfate. These differences likely originate from the acidity of N and S aerosols, which must react with fresh mineral surfaces before salts can form, and from variable mineral sources of halides. Borate, by contrast, sources from non-reactive particulate aerosols, lacks a major mineral source, and is consistent with constant deposition rates.

The highly linear nature of borate accumulation with time allows for its use as a proxy to verify or complete an exposure age record in cold, hyper-arid landscapes. Salt content can be quickly and cheaply measured and can therefore be assessed over a large number of samples. While long-term borate deposition rates are not established, the method can be employed where it is calibrated by independent measurements. It can be used to model exposure age where cosmogenic nuclide data provide heterogeneous results or to extend an exposure age analysis over greater portions of a landscape.