STROMATOLITES: A 3.5 BILLION YEAR ICHNOLOGIC RECORD

Stromatolites are unique trace fossils that record both the affect of microbes on the local environment and the effects of environmental influences on microbial ecosystems. Like all trace fossils, whether the product of animals or plants, stromatolites should be regarded as the preservation of behavior filtered through taphonomy and diagenesis.

The ichnologic record of stromatolites—and other microbial-sediment interactions—dates back 3.5 billion years and we can observe stromatolites forming today. However, clear understanding of the behaviors responsible for stromatolite formation remains elusive. Part of the confusion rests with the multiple pathways through which stromatolites form. The four most widely cited paths are as follows: 1) Trapping Model. Bacteria, primarily cyanobacteria, trap and bind mobile grains. 2) Precipitating Model. Stromatolites form through inorganic precipitation of cements on organic mats or adjacent to organic mats. 3) Stabilizing Model. Microbes spread out of the seafloor, stabilizing the sediment beneath the mats. 4) Calcifying Model. Microbes undergo in situ precipitation of minerals in the organic casings or in extracellular polymeric substances. Recognizing which model is active for the formation of the studied stromatolite is critical before addressing more complex questions about the relative roles of environment and biota in stromatolite construction.

Although stromatolites are not usually thought of as trace fossils, there are many aspects of the study of stromatolites that are in parallel with ichnology. Stromatolites record ecologic interactions not preservation of body fossils as originally thought. Furthermore, the organisms responsible for formation are rarely preserved. Second, there is not a one-to-one relationship between organisms and unique stromatolite types. Rather, there is generally a strong environmental/sedimentologic control. Third, stromatolites can be used for facies identification similar to Seilacherian ichnofacies. Finally, work has shown that there are some secular constraints on stromatolite formation, leading credence to their biostratigraphic utility.