Abstract Numerous field and laboratory studies over the past two decades claim that microbes catalyze nucleation and growth of dolomite at temperatures common in low-temperature geologic environments (25–60° C). However, a critical reexamination of the X-ray diffraction (XRD) data presented by these studies indicates that the laboratory products are not dolomite but rather a mixture of minerals, including very high-magnesium calcite (VHMC). Because VHMC can be “compositionally” indistinguishable from dolomite (i.e., 50 mol% MgCO 3 ), the positions of the principal (104) XRD reflection for VHMC and dolomite can be identical. Nevertheless, published XRD patterns of products derived from microbial experiments lack convincing evidence of cation ordering, which is a unique characteristic of carbonate minerals exhibiting R 3 (dolomite) symmetry. The lack of cation ordering in laboratory precipitates instead indicates that the products are VHMC, which possesses R 3 c (calcite) symmetry. Hence, previous laboratory studies have misidentified VHMC for dolomite. Despite the failure to synthesize dolomite in microbial experiments, the low-temperature laboratory results remain interesting. High-temperature (60–300°C) dolomitization experiments have long shown that ordered dolomite is invariably preceded by disordered VHMC precursors that recrystallize to dolomite over time. Although recrystallization from VHMC to ordered dolomite has not been documented in the low-temperature microbial experiments, it may be common in natural settings where higher surface temperatures and longer time periods overcome kinetic barriers to dolomite formation. Mineralogical arguments aside, petrological observations show that VHMC products from microbial laboratory experiments are dissimilar to both natural dolomites and high-temperature synthetic dolomites. First, the published microbial experiments produced VHMC or other carbonates as cements via direct precipitation from solution rather than by replacement of a CaCO 3 precursor, whereas the latter is demonstrated in high-temperature synthetic dolomites and inferred for most natural dolomites. Second, these precipitates tend to be spheroidal and/or dumbbell shaped, and as such they are fundamentally different from both the dominant rhombohedral form and the mimetic replacement textures observed in natural and high-temperature synthetic dolomites. Thus, the microbial products are not only mineralogically unlike natural dolomites, they also differ with respect to their mode of formation and their morphological characteristics.

Abstract The Yellowstone Geoecosystem ( Smith and Siegel, 2000 ; Morgan, 2007 ) comprises an amazing diversity of geologic features that control the character and distribution of geothermal features and soils of this landscape. The underlying bedrock geology of Yellowstone National Park (YNP) includes: Precambrian basement along the northern margin of the Park, with both low-grade metasedimentary rocks (biotite-andalusite-staurolite schists, quartzites and banded iron formations) and late Archean granitoid rocks. Phanerozoic sedimentary rocks in the northwestern corner of the Park in the southern Gallatin Range, that includes notably the Mississippian Madison Limestone near Mammoth Hot Springs, and Cretaceous shales and sandstones of the Kootenai, Eagle, Frontier and Harebell formations, south of Gardiner. The voluminous Eocene Absaroka Volcanics dominantly consisting of andesitic flows, tuffs, volcaniclastic rocks and hypabyssal intrusions. The iconic caldera-forming eruptive rocks of the Tertiary Huckleberry Ridge Tuff (2.1 Ma), Mesa Falls Tuff (1.2 Ma), and Lava Creek Tuff (0.61 Ma) of dominantly rhyolitic composition, and the post-caldera rhyolites (Christansen, 2001). The surficial geology of YNP ( Good and Pierce, 1996 ) additionally involves complex interactions of glacial geology ( Pierce, 1979 ) and active tectonics ( Pierce et al., 2007 ; Smith references). Tertiary volcanism and associated hydrothermal activity are directly related to the elevated hot spot that formed the regional Snake River Plain-Yellowstone system ( Smith and Siegel, 2000 ). Yellowstone contains the greatest number and diversity of geothermal systems on the planet, yielding an extensive array of extreme high-temperature environments, many of which are colonized by microorganisms that play either

Discoveries of Chicxulub impact ejecta of the Albion Formation in road cuts and quarries in southern Quintana Roo, México and Belize, broaden our understanding of ejecta depositional processes in large impacts. There are numerous new exposures of ejecta near the Río Hondo in Quintana Roo México, located at distances of 330–350 km from the center of the Chicxulub crater. A single ejecta exposure was discovered near Armenia in central Belize, 470 km from Chicxulub. The Albion Formation is composed of two lithostratigraphic units: the spheroid bed and diamictite bed, originally identified at Albion Island, Belize. The new spheroid bed exposures range from 2 to 5 m thick and are composed of altered glass fragments, accretionary lapilli, and pebble-sized carbonate clasts in a fine-grained calcite matrix. The base of the spheroid bed is exposed at Ramonal South in México and at Albion Island and Armenia in Belize, and at all three locations, the spheroid bed was deposited on a weathered karst land surface that had emerged in the Late Cretaceous. The new diamictite bed exposures are composed of altered glass fragments and carbonate clasts up to 9.0 × 3.2 m in size. In all but one of the new exposures, the diamictite bed extends to the surface with observed thicknesses up to 8 m. At Agua Dulce in México, the weathered top of the diamictite bed is overlain by thin-bedded Tertiary carbonates. No diamictite bed is found in Armenia, where the spheroid bed is overlain with a limestone conglomerate containing altered glass shards and shocked quartz. These discoveries indicate that ejecta are emplaced in large terrestrial impacts by at least two distinct flows: (1) an initial flow involving a volatile-rich cloud of fine debris similar to a volcanic pyroclastic flow, which extends >4.7 crater radii (the spheroid bed), and (2) a later flow of coarse debris that may not extend much beyond 3.6 crater radii (the diamictite bed). The former deposit we attribute to material entrained in the impact vapor plume, and the latter to the turbulent collapse of the ejecta curtain.