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Manganiferous rock varnish collected from Death Valley and Antarctica contains the smallest known terrestrial sedimentary deposits, with some layers only a few nanometers thick. Irregularities in these nanometer-scale layers are consistent with shrinking, cracking, and weathering of clay minerals. In the Death Valley rock varnish, very different High Resolution Transmission Electron Microscope (HRTEM) textures coexist that may be related to climatic change. HRTEM observations contradict previous microbial models of Mn-Fe enhancement, requiring a new three-step model of biomineralization and diagenesis for varnish formation.
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Krinsley et al. (1995) studied this question.
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