Biologically Mediated Carbonate Precipitation

Earlier in this lecture (chapter 5), I pointed out that carbonate ooids are among the few carbonate particles that are though to be inorganically precipitated.

Photomicrograph of ooid limestone.  Grains are 0.5-1 mm in size.  Large grain in center shows well developed concentric calcite layers.

Ooids (photo in upper left shows hand specimen of ooid limestone) are the main form of inorganically precipitated carbonate and form limestones that look like layers of fish eggs. Ooids form in very shallow, warm water with strong  wave action (e.g. the Bahamas).  A small fragment of carbonate (e.g. a piece of shell material) is sloshed around by waves, calcite or aragonite is precipitated on this seed when it rests on the sediment surface, and then the precipitate is rounded and smoothed by wave action.  Repetition of this process leads to multiple concentric layers. This, in esscence is the conventional view of carbonate ooids genesis, but recent research indicates that organic matter and microorganisms may actually be involved in ooid formation.  Some information on that issue is provided below.

These pictures are from modern ooids that are currently forming in the Great Salt Lake of Utah.  The samples were recovered with all the organic matter (slime, mucus, bacteria) that typically is associated with these ooids.  The arrow in the left picture points out reddish areas, which are concentrations (pockets) of organic substances (mucus) that reacted to a histochemical stain.  The image at left shows one of these areas in enlargement, and the arrow points out a yellowish-green area at the base of these mucus pockets where small calcium carbonate crystals are in the process of formation.   The presence of this mucus (produced by bacteria) seems to support and localize calcite precipitation, suggesting biological mediation of carbonate precipitation.