Figure 2

Depositional and mineralogical characteristics of thrombolite. (a) Close-up view of thrombolite surface in the field, covered by a thin film of water. (b) Cross-section of thrombolite surface. (c) CLSM image of thrombolite surface with LBA staining. Observed phototrophs (yellow-red autofluorescence) are Leptolyngbya sp. (e.g., cyan arrows), Scytonema sp. (red arrow), and unicellular eukaryotic microalga Oocardium sp. (e.g., white arrows). Minerals (white reflected light) are present as peloids and fillings of acidic exopolymers (green fluorescence) secreted by Scytonema sp. (d) CLSM image of Coelosphaeriopsis sp. with LBA staining. (e) Transmitted (top) and cross-polarized (bottom) light images of a single peloid grain acquired from an 800 nm thin section. Rounded spaces inside of peloids are occupied by Coelosphaeriopsis sp. (f) Bright-field TEM image showing mineralized capsules of Coelosphaeriopsis sp. The NEXAFS analysis spot is indicated (spot 6). (g) Magnified region from (f). NEXAFS analysis spots (spots 7 and 8) and SAED patterns (spots 9 and 10) are indicated. (h) Ca 2p NEXAFS spectra. (i) Combined elemental map of the region indicated in (g). (j,k) SAED patterns of calcite crystals taken from (j) spots 9 and (k) 10. (l,m) Thin section images of thrombolite surfaces with the same microscopic field of view; (l) transmitted light image and (m) a composite of cross-polarized and fluorescence light images. Leptolyngbya filaments are recognized by yellow-green autofluorescence in (m). (n) Transmitted light image, as in (l), but showing the deeper part. Scale bars: (a,b) 1 cm; (c, l, m) 100 μm; (d,e) 5 μm; (f) 2 μm; (g) 500 nm; (n) 500 μm.