Other formats

    Adobe Portable Document Format file (facsimile images)   TEI XML file   ePub eBook file  

Connect

    mail icontwitter iconBlogspot iconrss icon

Victoria University Antarctic Research Expedition Science and Logistics Reports 2005-06: VUWAE 50

a. Context of research

a. Context of research

The 8 Ma relict ground ice in Beacon Valley, Antarctica has been the topic of much debate since Sugden and others reported it in 1995 (Sugden et al. 1995). There is little debate about the age of the volcanic ash, which dates the ice, but the emplacement of this ash and origin of the ice continues to be a matter of contention. The occurrence of relict ice is not unique to Beacon Valley, with a range of occurrences now reported throughout the Dry Valleys (Dickinson et al. 2003a). Such ice is likely to be much older than even the oldest ice in the present ice sheet, making it important because of the paleoenvironmental and paleobiological record it may contain. The ice also represents a terrestrial analogue for ice that is thought to exist on Mars. Thus, the overall aim of this ongoing project is to understand all aspects of relict ice in the Dry Valleys and the information that it can provide. The specific aim of this proposal is to obtain observations and materials for establishing the setting of these ice bodies and dating the length of time they have been in their present situation. The latter can be achieved by a new method that is based on the undisturbed accumulation of 10Be in the rock debris overlying the ice.

In addition to Beacon Valley, relict ice has now been confirmed to exist in Pearse, Columnar, Kennar, and Victoria Valleys, and is thought to be widespread throughout the Dry Valleys (Fig. 1). Studies associated with this project suggest that relict ground ice in the Dry Valleys can originate from either stranded remnants of glaciers (Marchant et al. 2002; Sugden et al. 1995) or buried glacial lakes (Hall et al. 2002; Kelly et al. 2002). Ground ice that is not relict may also result from a variety of in situ processes (Dickinson et al. 2003a; Dickinson and Rosen 1999; Dickinson and Rosen 2003). Relict ground ice appears to have been stranded in valley bottoms and is found as a continuum between two end-members: 1) massive clear ice with bubbles and trace amounts of debris to 2) highly deformed, debris-rich ice. The massive ice may derive from lake ice or ice cored moraines of wet-based glaciers pre-dating the present valley floors or ice cored moraines from the margins of the more recent cold-based glaciers. Highly deformed debris-rich ice may result from accumulated strain of multiple advances and retreats of cold-based glaciers (Fitzsimons et al. 1999). Clues to these origins may be determined from the chemistry and stable isotopic ratios of the ice as well as gas analysis of its bubbles. Such studies will need to be complemented with descriptions and analyses of the surrounding glacial sediments.

The age of the relict ice is critical to its value as a record of past climate. The difficulties in dating it are best exemplified by the numerous and conflicting ages, which range from 0.5 to 8Ma, published for Beacon Valley ice (Schafer et al. 2000; Stone et al. 2002; Sugden et al. 1995; Tschudi 2000; Ng et al. 2005). We have collected samples of relict ice and its overlying sediment for use in a new and evolving dating technique that involves the use of atmospherically-derived 10Be. This technique has been used with limited success for over a decade in dating soils from temperate areas. Recently, it has been applied to Antarctic soils (Graham et al. 1995; Graham et al. 2002) and sediments page 2 (Dickinson et al. 2003b) where it has produced logical and reasonable dates. However, the method needs further development and testing against a surface of known age. For this we have collected samples of Hart Ash between the Meserve and Hart glaciers in the Wright Valley. This ash has a radiometric date of 3.9 Ma.

Figure 1. Locations of valleys known to contain ancient ice.

Figure 1. Locations of valleys known to contain ancient ice.

Our approach to dating the surface of the relict ice uses 10Be produced in the upper atmosphere, which in Antarctica, accumulates in the salt components of the soil surface through snowmelt and evaporation (see method section for more detail). Atmospherically-derived 10Be differs in concentration by 1-2 orders of magnitude from the 10Be produced in situ that is used for surface exposure dating. In soils and sediments from three other areas in Antarctica (Dickinson et al. 2003b; Graham et al. 1995; Graham et al. 2002), we have page 3 assumed a closed system of 10Be accumulation. We also assume the 9Be/10Be ratio is fixed at the surface, and we think it is locked into clays and salts. We found that the 10Be/9Be ratio of the fines decreased systematically with depth to yield a decay age for the soil and sediment. The decay with depth suggests that Be is somehow infiltrating into the ground, by a process we do not fully understand. Nevertheless, the decay ages that we have obtained are reasonable for the soils and sediment analysed. We believe that this method should be applicable to dating the soils and ice-cemented sediments that overlie relict ice in the Dry Valleys, and indeed other parts of the Transantarctic Mountains.