GIRVANELLA OOIDS: EVIDENCE FOR MICROBIAL CONTRIBUTION TO THE OOID FORMATION

Ooids are small (usually less than 2 mm in diameter), coated carbonate grains that contain a nucleus surrounded by an external cortex. It is generally accepted that ooids form in agitated-water conditions, supplied with nuclei and supersaturated with calcium bicarbonate. Major controls on microstructures of ooid laminae are water chemistry and turbulence. However, microbial contribution to the ooid formation is rarely known. Here we show a new type of ooid, naming Girvanella ooids, from the Cambrian oolites of China. In these ooids, Girvanella that is thought to represent calcified sheath of a filamentous cyanobacterium, was well preserved in both the core and cortex, which provides direct evidence that microbes, most likely cyanobacteria, involved in the ooid formation and thus had a significant influence on the microstructures of the ooids. According to the microstructures, Girvanella ooids can be further subdivided into four types: (1) Girvanella spheroids, ooid-sized grains composed entirely of meshwork of Girvanella, with the cortex-nucleus boundary poorly defined; (2) Girvanella-core ooids, having well defined Girvanella nucleus that was probably a rounded fragment of the former cyanobacterial buildup; (3) Girvanella-cortex ooids, usually larger, whose cortices are characterized by alteration of thick, dark colored Girvanella laminae and thick, light colored micritic laminae, filaments of Girvanella arranged parallel to the laminar boundaries; (4) Crypto-Girvanella ooids, similar to the Girvanella-cortex ooids in microstructures of the cortex, comprising dark laminae and light laminae but lacking visible filamentous structures in dark layers, probably as a result of degradation prior to calcification.