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Movement of CO 2 from the atmosphere into land via photosynthesis and root respiration, the subsequent formation of bicarbonate in soil, and its storage in groundwater or precipitation as CaCO 3 in dryland soils are major processes in the global carbon cycle. Together, inorganic carbon as soil carbonate (∼940 PgC) and as bicarbonate in groundwater (∼1404 PgC) surpass soil organic carbon (∼1530 PgC) as the largest terrestrial pool of carbon. Yet, despite general agreement about its huge size as a carbon pool, controversy about the potential of inorganic carbon to sequester atmospheric CO 2 remains unresolved. We suggest that the controversy stems from the absence of a lexicon and propose a classification scheme that uses (1) calcium source illustrated by two widely recognized chemical reactions, and (2) the concept of carbonate “generations.” When calcium is derived from preexisting carbonate, an equilibrium reaction occurs that does not sequester carbon in soil carbonate but does sequester carbon in groundwater until bicarbonate precipitates as CaCO 3 . When calcium is derived from silicate minerals, a unidirectional reaction occurs that sequesters carbon in both soil carbonate and groundwater. The generations concept shows that carbon sequestration occurs only in the first generation when calcium is released directly from silicates. This classification not only enhances communication and provides a framework for quantifying amounts of fossil fuel carbon that can be sequestered within a geoengineering context, it provides more precise language for discussing the terrestrial carbon cycle through geologic time.

The phosphorus composition of banded-iron formations (BIFs) has been used as a proxy for Precambrian seawater composition and the paleoeredox state of Earth's surface environment. However, it is unclear whether the phosphorus in BIFs originally entered the sediment as a sorbed component of the iron oxyhydroxide particles, or whether it was incorporated into the biomass of marine phytoplankton. We conducted high-resolution mineral analyses and report here the first detection of an Fe(III) acetate salt, as well as nanocrystals of apatite in association with magnetite, in the 2.48 Ga Dales Gorge Member of the Brockman Iron Formation (a BIF), Hamersley, Western Australia. The clusters of apatite are similar in size and morphology to biogenic apatite crystals resulting from biomass decay in Phanerozoic marine sediments, while the formation of an Fe(III) acetate salt and magnetite not only implies the original presence of biomass in the BIF sediments, but also that organic carbon likely served as an electron donor during bacterial Fe(III) reduction. This study is important because it suggests that phytoplankton may have played a key role in the transfer of phosphorus (and other trace elements) from the photic zone to the seafloor.

To investigate the potential for the geologic storage of CO2 in saline sedimentary aquifers, 1600 t of CO2 were injected at 1500 m depth into a 24-m-thick sandstone section of the Frio Formation, a regional brine and oil reservoir in the U.S. Gulf Coast. Fluid samples obtained from the injection and observation wells before CO2 injection showed a Na-CaCl‐type brine with 93,000 mg/L total dissolved solids (TDS) at near saturation with CH4 at reservoir conditions. Following CO2 breakthrough, samples showed sharp drops in pH (6.5‐5.7), pronounced increases in alkalinity (100‐3000 mg/L as HCO3) and Fe (30‐1100 mg/L), and significant shifts in the isotopic compositions of H2O, dissolved inorganic carbon (DIC), and CH4. Geochemical modeling indicates that brine pH would have dropped lower but for the buffering by dissolution of carbonate and iron oxyhydroxides. This rapid dissolution of carbonate and other minerals could ultimately create pathways in the rock seals or well cements for CO2 and brine leakage. Dissolution of minerals, especially iron oxyhydroxides, could mobilize toxic trace metals and, where residual oil or suitable organics are present, the injected CO2 could also mobilize toxic organic compounds. Environmental impacts could be major if large brine volumes with mobilized toxic metals and organics migrated into potable groundwater. The d 18 O values for brine and CO2 samples indicate that supercritical CO2 comprises ;50% of pore-fluid volume ;6 mo after the end of injection. Postinjection sampling, coupled with geochemical modeling, indicates that the brine gradually will return to its preinjection composition.

The natural hydration of obsidian was first proposed as a dating technique for young geological and archaeological specimens by Friedman and Smith (1960), who noted that the thickness of the hydrated layer on obsidian artifacts increases with time. This approach is, however, sensitive to temperature and humidity under earth-surface conditions. This has made obsidian hydration dating more difficult, but potentially provides a unique tool for paleoclimatic reconstructions. In this paper we present the first successful application of this approach, based on combining laboratory-based experimental calibrations with archaeological samples from the Chalco site in the Basin of Mexico, dated using stratigraphically correlated 14 C results and measuring hydration depths by secondary ion mass spectrometry. The resultant data suggest, first, that this approach is viable, even given the existing uncertainties, and that a cooling trend occurred in the Basin of Mexico over the past 1450 yr, a result corroborated by other paleoclimatic data.

The characterization of intragrain mineralogical and isotopic zoning patterns provides the basis for estimating the duration of fluid-rock interaction associated with mineral replacement reactions. In the Rico, Colorado, hydrothermal system, oxygen isotope ratios in reaction rims on partially reacted plagioclase feldspar exhibit some of the largest gradients yet reported for individual grains (to 15%o). The extent of rim formation and accompanying isotopic exchange vary across the system as a function of temperature, fluid isotope composition, and the local fluid/rock ratio. Distal feldspars show narrow rims with 1 8 O enrichments relative to pristine feldspars. Feldspars intermediate or proximal to the system's center have wide reaction rims or are completely exchanged and show 180 depletions. Formation times of reaction rims and associated isotopic patterns have been estimated with a coupled reaction-diffusion model that suggests that hotter (∼250-350 °C) hydrothermal circulation was active for ∼100-300 k.y. in the center part of the system, perhaps only while the igneous heat engine was still magma. Cooler (∼150-200 °C) circulation was widespread, lasting for >1000 k.y.

An integrated lipid biomarker–carbon isotope approach reveals new insight to microbial methane oxidation in the Gulf of Mexico gas-hydrate system. Hydrate-bearing and hydrate-free sediments were collected from the Gulf of Mexico slope using a research submersible. Phospholipid fatty acids consist mainly of C16–C18 compounds, which are largely derived from bacteria. The phospholipid fatty acids suggest that total biomass is enhanced 11–30-fold in gas-hydrate–bearing sediment compared to hydrate-free sediment. Lipid biomarkers indicative of sulfate-reducing bacteria are strongly depleted in 13C (δ13C = −48‰ to −70‰) in the hydrate-bearing samples, suggesting that they are involved in the oxidation of methane (δ13C = −47‰ for thermogenic methane and −70‰ for biogenic methane). Isotopic properties of other biomarkers suggest that sulfur-oxidizing bacteria ( Beggiatoa ) may also contribute to the lipid pool in hydrate-bearing samples, which are characterized by less negative δ13C values (to −11.2‰). In the hydrate-free sample, fatty acid biomarkers have δ13C values of −27.6‰ to −39.6‰, indicating that crude oil (average ∼−27‰) or terrestrial organic carbon (average ∼−20‰) are the likely carbon sources. Our results provide the first lipid biomarker–stable isotope evidence that sulfate- reducing bacteria play an important role in anaerobic methane oxidation in the Gulf of Mexico gas hydrates. The coupled activities of methane-oxidizing and sulfate-reducing organisms contribute to the development of ecosystems in deep-sea environments and result in sequestration of carbon as buried organic carbon and authigenic carbonates. These have implications for studying climate change based on carbon budgets.