Classic Cambrian thrombolite reveals a 2.5 Ga history of ministromatolites and their ‘ghost’ clots

Stromatolite and thrombolite are two major categories of microbial carbonate, distinguished by macrofabric: laminated in stromatolite and clotted in thrombolite. Since these categories are defined at hand-specimen scale, their distinction is inherently resolution-dependent. Re-study of Aitken’s (1967) typical thrombolite, from the Cambrian of Canada, shows that—in thin-section—the individual microbial carbonate clots can be seen to be delicately laminated. Cambrian thrombolites with similar indistinctly laminated clots occur in China and the USA. Similar, but generally larger and distinctly laminated, ministromatolites (sma ll stromatolites, ≤20 mm wide) can be common in the Proterozoic. They appear to be generally scarce in the Phanerozoic but occur locally in Holocene freshwater streams and lakes. Clots in present-day near-sea domes and columns at Lake Clifton, Australia, widely regarded as thrombolite analogs and formed by calcified cyanobacterial colonies, closely resemble Aitken’s Cambrian e xample; their initial lamination can be obliterated during early diagenesis. These observations emphasize that the distinction between thrombolite and stromatolite is based on human perception at hand-specimen scale. We propose that fluctuating decline in marine microbial calcification during the Proterozoic–Phanerozoic transformed distinctly laminated ministromatolites into smaller forms whose lamination is too delicate to be evident to the naked eye—ministromatolite ‘ghosts’—and that these form the clots in Aitken’s typical Cambrian thrombolite. Proterozoic ministromatolites, Cambrian thrombolites, and their present-day non-marine analogs, reflect a ~2.5 Ga history of fluctuating marine calcification in microbial, most likely cyanobacterial, colonies.