The Nature of Stromatolites: 3,500 Million Years of History and a Century of Research

Stromatolites are widely regarded as layered, early lithified, authigenic microbial structures – often domical or columnar in form – that developed at the sediment water interface in freshwater, marine and evaporitic environments (Fig. 1). In addition to this unusually wide environmental distribution, the exceptionally long geological record of stromatolites spans at least 3,500 million years (Ma) (Vologdin 1962; Hofmann 1969, 1973; Walter 1976a; Grotzinger and Knoll 1999; Riding and Awramik 2000). Most of these examples, together with those described by Kalkowsky (1908), are essentially originally carbonate in composition. More than a century of research has revealed many details of their diverse fabrics and complex history, but much still remains to be understood about stromatolites. This is not surprising considering their wide distribution in time and space; and it helps to account for a continuing problem with their definition. Kalkowsky (1908) considered stromatolites to be microbial sediments, but it has become increasingly difficult to maintain this view for all ancient examples, especially those more than ~1,000 Ma old. The aim of this article is to evaluate progress in understanding what stromatolites are, since they were first described in the 1800s. This makes it impossible to avoid the thorny problem of how they should be defined. The nature and definition of stromatolites have been persistent difficulties ever since Kalkowsky introduced the name. At first, the main question posed was “are stromatolites biogenic or abiogenic?” With time, the focus has shifted to whether all stromatolites are biogenic, or whether some are biogenic while others are abiogenic. Kalkowsky’s (1908) microbial interpretation of stromatolites was