Microorganisms have long been thought to impact CaCO 3 precipitation, but determining the extent of their infl uence on sediment formation has been hampered by our inability to obtain direct experimental evidence about mineral formation processes in natural environments. We address this problem by conducting kinetic experiments within a modern terrestrial carbonate spring to determine aragonite precipitation rates and to quantify the relative infl uences of aragonite saturation state (Ω a ), microbial biomass concentration and microbial viability on CaCO 3 mineralization in advection-dominated transport regimes. At an Ω a value consistent with modern seawater (3.63 ± 0.09), our 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 µm fi ltration; and (2) inhibiting microbial viability with ultraviolet (UV) irradiation has no signifi cant effect on the mean precipitation rate. Furthermore, our modeling of the CaCO 3 precipitation process, which uses the empirical crystal growth rate expression and additional in situ kinetic measurements, reveals that reducing biomass concentrations by 45% can decrease the empirical rate constant by more than an order of magnitude. These fi ndings strongly suggest that microorganisms catalyze CaCO 3 precipitation in advection-dominated systems and imply that changes in carbonate mineralization rates may have resulted from changes in local microbial biomass concentration throughout geologic history.

Peat deposits can concentrate chalcophilic metals such as Zn and Cd by biogeochemical processes; as a result, there is a possibility that the solubility, mobility, and bioavailability of these metals could increase when such deposits are drained and cropped, initiating oxidation of organic matter and sulfides under aerobic conditions. We use spectroscopic, chemical, and bioassay approaches to characterize high Zn (88-15,800 mg kg(-1)), Cd (0.55-83.0 mg kg(-1)), and S (3.52-9.54 g kg(-1)) peat soils collected from locations in New York State and Ontario that overlie Silurian-age metal-enriched dolomite bedrock. Total and KNO3-extractable trace metals were determined by ICP emission spectrometry, and labile Cd and Zn were measured in the KNO3 extracts by anodic stripping voltammetry. A greenhouse bioassay with maize and canola was conducted to determine the bioavailability and toxicity of the soil Zn and Cd. The electronic oxidation states of sulfur in the peat soils were determined by X-ray absorption near edge spectroscopy (XANES) and Zn and S distribution in soil particles by energy-dispersive X-ray absorption (EDX) spectroscopy. Sulfur-XANES analyses show that a high percentage (35-45%) of the total soil S exists in the most reduced electronic oxidation states (such as sulfides and thiols), while <5% exists in the most oxidized forms (such as sulfate). EDX analyses indicate a microscopic elemental association between Zn and S in these soils. Despite the EDX evidence of close association between Zn and S in soil particles, conventional X-ray diffraction analyses of the bulk soils did not detect a mineral phase of sphalerite (ZnS) in any of the soils. The distribution coefficients (Kd) for Zn and Cd increased with soil pH and indicated stronger Cd retention than Zn in the peats. The results of the bioassaytests showed that most of the high-Zn soils were very phytotoxic, with plant shoot Zn levels exceeding 400 mg kg(-1). Conversely, Cd concentrations in the plant shoots were generally below 2 mg kg(-1), showing a tendency toward low Cd phytoavailability relative to Zn. The information gained from the spectroscopic analyses (S-XANES and EDX) was used to explain the macroscopic observations (Cd and Zn Kd values and phytoavailability data) in these peat soils; we conclude that sulfur biogeochemical cycling may play an important role in Zn and Cd retention in these organic soils.

An extensive data set of the physical and chemical attributes of two modern hot springs in the Mammoth Hot Springs complex of Yellowstone National Park, Wyoming, U.S.A., yields a strong correlation between travertine depositional facies and the temperature, pH, and flux of the hot-spring water from which the travertine precipitated. Because advection dominates in these hot-spring drainage systems, we quantify variability between and within springs in order to construct a hydrologic model that defines the primary flow path in the context of key macroscopic travertine accumulation patterns. This model, based on 343 in situ triplicate measurements, provides the basis for the use of travertine facies models to quantitatively reconstruct hot-spring aqueous temperature, pH, and flux solely from precipitated travertine. As an example reconstruction, we deduce that previously described Pleistocene apron and channel facies travertine quarry deposits from central Italy precipitated from hot-spring waters with a pH of 6.86 6 0.19 and a temperature of 65.4 6 3.6uC.

No abstract