FORMATION OF ARAGONITE CEMENT BY NANNOBACTERIA IN THE GREAT-SALT-LAKE, UTAH

Brine-shrimp egg cases in growth cavities in modern stromatolites in t he Great Salt Lake, Utah, are replaced by aragonite and cemented toget her by aragonite cement. The fabric of the cement changes dramatically as the distance from the egg case increases. The cement within 50 to 70 mu m of the egg case exhibits a random fabric of 10 to 20 mu m equa nt crystals, The surface of the cement is covered by bead-like bumps, 0.1 mu m in diameter, interpreted as nannobacteria, Overlying the rand om, ''beaded'' fabric with a relatively abrupt transition are epitaxia l, prismatic aragonite crystals that have smooth crystal surfaces lack ing bead-like bodies. The smooth-surfaced prismatic aragonite crystals are interpreted to be ''normal'' abiotic precipitates, whereas the '' beaded'' microspar is interpreted to result from biotic processes, whe re the nannobacteria serve as catalysts for creation of the cement. A population explosion of bacteria occurs as the organic material of egg case rots, which alters the microchemical environment and induces a r apid precipitation of aragonite, enclosing tens of thousands of nannob acteria, As the organic material is destroyed, reproduction of bacteri a slows and epitaxial, prismatic aragonite crystals nucleate and grow abiotically on the structureless, ''biotic'' layer.