ROLE OF CYANOBACTERIAL BIOFILMS IN THE GENESIS OF TRAVERTINE SPRING SPHERULITES

Spherulites may be defined as sub-spherical mineral grains that grow by accretion and which have a radially-organized internal fabric. In modern, carbonate-precipitating hot-spring environments, spherulites are a common feature of the distal apron environments of spring outflows. Spherulitic travertines have also been shown to be a common sub-facies of older deposits. In modern travertine springs at Mammoth Hot Springs (Yellowstone National Park) and at Thermopolis (Central Wyoming), sub-millimeter spherulites of calcite nucleate within exopolymer-rich cyanobacterial biofilms on the upper slopes of distal apron environments. In these springs, at temperatures <40 degrees C, the primary precipitate is calcite, which forms spherulites made up of elongate, radially-arrayed crystals. The maximum size of spherulites generally increases downslope, with larger (multi-mm) spherulites being a conspicuous component of the floor deposits of shallow microterracette ponds of distal slopes. The importance of microbiologically-induced precipitation in the origin of carbonate deposits is an active area of research, with a growing interest in the passive role that biopolymers play in controlling the nucleation of crystallites. In this study, I compare the nucleation and growth of calcite spherules from the aforementioned modern springs, over a range of temperature, salinity and pH. As noted, calcite spherules in these springs nucleate on exopolymer sheaths and within matrix exopolymers of cyanobacterial biofilms. With continued accretion of spherules, cyanobacterial sheaths, diatoms and other biological remains are often incorporated into crystallites, suggesting the potential for longer-term biosignature preservation. However, comparative studies of Miocene and Pleistocene-aged spherulitic travertines have revealed that bacterial microfossils are uncommon in diagenetically altered deposits. This observation is consistent with the oxidizing micro-environments that appear to dominate spherulitic environments of modern, travertine-precipitating springs during early diagenesis.