Optically Stimulated Luminescence (OSL) Dating - Taylor Valley

Ages03

Figure 1 Location of OSL samples in Taylor Valley.

We are dating proglacial fluvial terraces in Taylor Valley using OSL to determine the timing of glacial events following the Last Glacial Maximum (LGM) advance of the West Antarctic Ice Sheet (WAIS) into Taylor Valley, and to look at the evolution of soil properties along a chronosequence. By studying both relative soil ages and absolute soil ages we believe that a robust interpretation of the glacial history in Taylor Valley is possible.

There are about 100 fluvial terraces in Taylor Valley, ranging in elevation from sea level to 314 m. These terraces have been dated by 14C dating of buried algal layers, yielding ages ranging from 8,340 – 23,800 14C years BP (Stuiver et al. 1981, Hall et al. 2000).

What's wrong with the 14C dates?

Cold-dry environments tend to act like freezers, preserving organic carbon and recycling old material. It is well documented that algae in the Dry Valleys is readily transported by streams and wind, making issues of carbon recycling a major problem.  Potential recycling sources are described below:
  • RadioCarbon Reservoir Effect - Old carbon from glacial meltwater can be incorporated into growing algal mats.

  • Wind-Blow Algae - Algal mats desicate during the wintermonths and are blown away by strong katabatic winds from the Antarctic Plateau.

  • Water-Transported Algae - Upstream errosion can recycle old burried algae that is then incorporated into fluvial deposits.
Ages02

Figure 214C ages of fluvial terraces in Taylor Valley organized by the drainage or stream in which they were found (compiled from Hall et al. 2000).  Terraces at similar elevations, but different drainages, do not have similar ages, as would be expected from a valley-wide paleolake.







Fluvial Terraces

Ages01

Figure 3 A large fluvial terrace located at 80 m on
Crescent Stream (a), a soil profile with foreset beds
evident and no aeolian horizon (b), cross-bedded topsets
(c), and gravelly sand with an aeolian horizon evident (d).

Fluvial terraces in Taylor Valley have traditionally been termed ‘deltas’, but recent work with Ground Penetrating Radar suggests that although the surficial expression of these features appear deltaic, typically the internal stratigraphy is not. In light of these findings we believe it is more appropriate to use the more ambiguous term, fluvial sediment or terrace.

These fluvial features are thought to have formed when small ehpemeral streams from alpine glaciers flowed into paleo Lake Washburn.  14C dates of these deposits have been used to constrain lake levels, and by inference the retreat history of WAIS.  In contrast, we believe that the origin of these deposits is uncertain.  They may have formed in small ponds marginal to the WAIS ice lobe, as kame mounds, as deltas in a lake, or by downcutting older fluvial deposts.