Four Missouri stalagmites yield consistent overlapping records of oxygen and carbon isotopic changes and provide a climate and vegetation history with submillennial resolution from 75 to 25 thousand years ago (ka). The thorium-230–dated records reveal that between 75 and 55 ka, the midcontinental climate oscillated on millennial time scales between cold and warm, and vegetation alternated among forest, savanna, and prairie. Temperatures were highest and prairie vegetation peaked between 59 and 55 ka. Climate cooled and forest replaced grassland at 55 ka, when global ice sheets began to build during the early part of Marine Oxygen Isotope Stage 3.

High-precision uranium-thorium mass spectrometric chronology and ¹⁸ O- ¹³ C isotopic analysis of speleothem calcite from Cold Water Cave in northeast Iowa have been used to chart mid-Holocene climate change. Significant shifts in δ ¹⁸ O and δ ¹³ C isotopic values coincide with well-documented Holocene vegetation changes. Temperature estimates based on ¹⁸ O/ ¹⁶ O ratios suggest that the climate warmed rapidly by about 3δC at 5900 years before present and then cooled by 4δC at 3600 years before present. Initiation of a gradual increase in δ ¹³ C at 5900 years before present suggests that turnover of the forest soil biomass was slow and that equilibrium with prairie vegetation was not attained by 3600 years before present.

Significance: Abundant in the geologic record, but scarce in modern environments below 50 ° C, the mineral dolomite is used to interpret ancient fluid chemistry, paleotemperature, and is a major hydrocarbon reservoir rock. Because laboratory synthesis of abiotic dolomite had been unsuccessful, chemical mechanisms for precipitation are poorly constrained, and limit interpretations of its occurrence. Here we report the abiotic synthesis of dolomite at 25 ° C, and demonstrate that carboxylated surfaces on organic matter catalyze precipitation through complexation between carboxyl groups and Mg ²⁺ , removing water to make Mg ²⁺ available for dolomite precipitation. This mechanism is consistent with dolomite formation in depositional environments rich in organic matter. Our experimental protocol provides opportunities for calibrating conditions of low-temperature dolomite formation throughout the geologic record.

For many decades researchers have attempted to clarify the factors controlling the precipitation of calcium carbonate polymorphs and the incorporation of Mg into calcite precipitating in natural systems. In this study a novel experimental approach delineates the role of instantaneous supersaturation and solution Mg/Ca ratio in calcium carbonate mineralogy and composition. The results of this study show that in solutions with different Mg/Ca ratios, the transition between the aragonite and the calcite + aragonite precipitation fields is controlled by a combination of the saturation state of the solution with respect to CaCO3 and the Mg/Ca ratio in solution. The data show that for increasing solution Mg/Ca ratios, progressively higher supersaturation level is required for calcite precipitation. The predominance of aragonite at high solution Mg/Ca and low supersaturations is attributed to a relative decrease in calcite growth rates as a result of increasing incorporation of Mg2+ in the calcite lattice at higher solution Mg/Ca ratios. As suggested by abundant experimental work, increasing solution Mg/Ca decreases the growth rate of calcite while aragonite growth rates stay unaffected. Our experiments do demonstrate that despite decreased growth rate the percentage of MgCO3 in calcite increases with increasing solution Mg/Ca ratio. As calcite growth rates decrease, aragonite growth rates stay constant and it becomes the dominant mineral phase in solutions with high Mg/Ca ratio and low supersaturations. These findings are relevant to present-day mineralogical distributions in carbonate systems and might be the key to elucidate the environmental conditions responsible for mineralogic changes through geologic time. The observed dependence of calcium carbonate mineralogy on solution composition and supersaturation suggests that carbonate mineralogy is controlled by kinetics of growth.

In Lake Matano, Indonesia, the world's largest known ferruginous basin, more than 50% of authigenic organic matter is degraded through methanogenesis, despite high abundances of Fe (hydr)oxides in the lake sediments. Biogenic CH₄ accumulates to high concentrations (up to 1.4 mmol L⁻¹) in the anoxic bottom waters, which contain a total of 7.4 × 10⁵ tons of CH₄. Profiles of dissolved inorganic carbon (ΣCO₂) and carbon isotopes (δ¹³C) show that CH₄ is oxidized in the vicinity of the persistent pycnocline and that some of this CH₄ is likely oxidized anaerobically. The dearth of NO₃⁻ and SO₄²⁻ in Lake Matano waters suggests that anaerobic methane oxidation may be coupled to the reduction of Fe (and/or Mn) (hydr)oxides. Thermodynamic considerations reveal that CH₄ oxidation coupled to Fe(III) or Mn(III/IV) reduction would yield sufficient free energy to support microbial growth at the substrate levels present in Lake Matano. Flux calculations imply that Fe and Mn must be recycled several times directly within the water column to balance the upward flux of CH₄. 16S gene cloning identified methanogens in the anoxic water column, and these methanogens belong to groups capable of both acetoclastic and hydrogenotrophic methanogenesis. We find that methane is important in C cycling, even in this very Fe-rich environment. Such Fe-rich environments are rare on Earth today, but they are analogous to conditions in the ferruginous oceans thought to prevail during much of the Archean Eon. By analogy, methanogens and methanotrophs could have formed an important part of the Archean Ocean ecosystem.