Age dating desert varnish
Beyond the arid realm, non-riverine chemical films on rocks are dull and black in colour and attracted little attention, being presumed to be vaguely ‘organic’.
In seasonally flooded tropical riverine environments, vitreous silica-rich films of brown to purple-black colour are sometimes present on exposed bedrock potholes (Tricart 1972) and point bar cobbles, which may be cemented together to form a resistant carapace.
Limestones, for example, typically do not have varnish because they are too water-soluble and therefore do not provide a stable surface for varnish to form.A micro-environment p H above 7.5 is inhospitable for manganese-concentrating microbes.In such conditions, orange varnishes develop, poor in manganese (Mn) but rich in iron (Fe).Clay, then, acts as a substrate to catch additional substances that chemically react together when the rock reaches high temperatures in the desert sun. An important characteristic of black desert varnish is that it has an unusually high concentration of manganese.Manganese is relatively rare in the Earth's crust, making up only 0.12% of its weight.
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Although there are many inherent assumptions and potential limitations, cation-ratio dating has been verified on relative age-sequences from a Death Valley debris cone, Negev Desert talus flatirons, and prehistoric lake levels at Searles Lake in California.