Siliceous Crusts, Quartz Rinds and Biotic Weathering of Sandstones in the Cold Desert of Antarctica

Abstract

Sandstones of the Paleozoic Beacon Supergroup in the Dry Valley region of Antarctica (also known as the Ross Desert), undergo at least three chemical weathering processes. (1) Exudation of oxalate ± other chelators by endolithic microorganisms causes translocation of elements, producing distinctive Fe pigment patterns and accelerating mechanical weathering (exfoliation) in rocks colonized by endolithic organisms. (2) Formation of thin siliceous crusts (<0.1 mm) stabilizes rock surfaces. The siliceous crusts form by accumulation and in situ alteration of airborne dust composed of quartz, clays, and Fe oxy hydroxide s. Crust textures include petrographically amorphous lenses of silica, stained and laminated birefringent coatings, and chalcedonic protrusions in surface pits. (3) Silicification of porous quartz sandstones by growth of quartz in optical continuity with host grains produces impermeable rinds up to several cm thick. The rinds form by transport of silica toward subaerially-exposed surfaces due to wet/dry and warm/cold cycling of microenvironments. The resulting interstitial precipitates commonly trap pre-existing grain coatings in pores, thereby preserving biogenic pigment patterns in colonized rocks as trace fossils of endolithic microorganisms. The siliceous crusts and quartz rinds occur on colonized sandstones as well as on rocks without visible colonization. Calibration of their thicknesses on boulders of moraines of approximately known age permits age estimates of this assemblage of geological and biological weathering features. The time scales for all these processes are ˜ 104 to 106 years, suggesting that biological activity in the Antarctic cold desert is extremely low and that cold, arid conditions have persisted in the order of 2 million years.