Modeling Departures From Abiotic Expectations During the Calcium Carbonate Precipitation Process

83 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2007.Although many investigators have described factors responsible for the formation of calcium carbonate (CaCO3) in natural systems, quantifying the extent to which biological influences drive precipitation in carbonate depositional environments remains an unresolved problem. Thus, a quantitative understanding of biological-mineral-aqueous interactions is needed to determine the most influential aspects of these biogeochemical systems on precipitation. This investigation distinguishes mechanistic influences on CaCO3 mineralization and isotope fractionation by: performing kinetic experiments directly within a modern carbonate spring; determining the carbon and oxygen isotopic composition of aragonite and coexisting spring water; and modeling these experimental results to establish whether precipitation and isotopic fractionation are consistent with abiotic CaCO3 mineralization or better explained via alternative processes. At an aragonite saturation state (O) consistent with modern seawater (3.63 +/- 0.09), controlled in situ kinetic experiments show that: (1) the natural steady state aragonite precipitation rate is more than twice that determined when microbial biomass on the aragonite mineral surface is depleted by 0.2 mum filtration; and (2) inhibiting microbial viability with UV irradiation has no significant effect on the mean precipitation rate. Kinetic modeling of the CaCO3 precipitation process also reveals that microbial biomass catalyzes aragonite precipitation and can increase the empirical rate constant by more than an order of magnitude relative to filtered controls. Furthermore, aqueous and solid phase carbon isotope determinations demonstrate that previously documented abiotic fractionation mechanisms and equilibrium fractionation models, which account for temperature and speciation effects, are insufficient to explain isotope fractionation in carbonate depositional systems. These findings strongly suggest that microbial biomass influences the mechanism of aragonite precipitation and imply that changes in CaCO3 mineralization rates and carbon isotopic composition may be intimately linked with changes in local microbial biomass concentration throughout geologic history.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD