Understanding the spatial variability of environmental change in drylands with rock varnish microlaminations

A fundamental problem in the analysis of landforms and surface artifacts is how to correlate the ages of non-contiguous surfaces. One solution to this problem may lie within the varnish coatings on desert rocks. When viewed with a light microscope in ultra-thin cross-sections, rock varnish reveals orange and black layers that record drier and wetter climates, respectively. Consistent patterns of alternating orange and black microlaminae are evident in some 2900 rock-surface depressions in 420 ultra thin sections from 360 rocks in Death Valley and the surrounding region. Microlaminae are organized into distinct layering units that provide relative ages for geomorphic and archaeological surfaces. The largest uncertainty in developing calibrated chronologies for layering units is the inability to date specific layers; we resolve this problem by correlating layering units with independent numerical ages. Because rock varnishes are ubiquitous in deserts, their visual microlaminations have great potential as a tool to assess temporal and spatial variations in dryland environments. This potential is illustrated for alluvial-fan deposits in Death Valley, petroglyphs, and fault scarps. One of the most surprising, if speculative, findings is that the ages of black laminations (wetter periods) in Death Valley coincide with the timings of iceberg armadas in the North Atlantic (Heinrich Events).