UNDERSTANDING THE SPATIAL VARIABILITY OF ENVIRONMENTAL-CHANGE IN DRYLANDS WITH ROCK-VARNISH MICROLAMINATIONS

A fundamental problem in the analysis of landforms and surface artifac ts is how to correlate the ages of non-contiguous surfaces. One soluti on to this problem may lie within the varnish coatings on desert rocks . When viewed with a light microscope in ultra-thin cross-sections, ro ck varnish reveals orange and black layers that record drier and wette r climates, respectively. Consistent patterns of alternating orange an d black microlaminae are evident in some 2900 rock-surface depressions in 420 ultra thin sections from 360 rocks in Death Valley and the sur rounding region. Microlaminae are organized into distinct layering uni ts that provide relative ages for geomorphic and archaeological surfac es. The largest uncertainty in developing calibrated chronologies for layering units is the inability to date specific layers; we resolve th is problem by correlating layering units with independent numerical ag es. Because rock varnishes are ubiquitous in deserts, their visual mic rolaminations have great potential as a tool to assess temporal and sp atial variations in dryland environments. This potential is illustrate d for alluvial-fan deposits in Death Valley, petroglyphs, and fault sc arps. One of the most surprising, if speculative, findings is that the ages of black laminations (wetter periods) in Death Valley coincide w ith the timings of iceberg armadas in the North Atlantic (Heinrich Eve nts).