FIGURE 4. Association of organic matter with clays in the palaeosol.

a, Thin-section photograph of a sample from zone I. Seams of organic matter (black) typically occur in ferristilpnomelane-rich areas (light brown) in rock that is mostly composed of graded beds of aerosol quartz with minor calcite. b, Thin-section photograph of a calcrete sample from zone II. At least two generations of calcite are recognized: earlier calcite, rich in talc and organic C (grey); and later calcite, free of talc and organic C (white). This indicates repeated events of dissolution/precipitation of calcite. Seams of organic-C-rich clays, probably fossilized microbial mats, appear to have been distorted during the dissolution/precipitation events of calcite. c, A transmission electron microscope image of a portion of an organic-rich clay layer in the same sample of b. The black area is well-crystallized talc with a 9.45-Å lattice parameter. The white area with dark masses is amorphous graphite (organic matter) with a discontinuous and wavy layer structure. d, Concentration profiles of C, Mg and Fe, determined by an electron microprobe analyser, of a sample from the lowest part of zone I. Organic carbon occurs between quartz and chlorite grain boundaries and also within a chlorite-rich layer.