Matthew S. Fantle

Evaluating the Ca isotope proxy

The use of Ca isotopes as a proxy for mass flux imbalances in the Ca cycle is evaluated critically. A compiled Ca isotope record for the last 45 Ma, derived from bulk nannofossil ooze and with a temporal resolution of ∼0.5 Ma, and an interpretation of the record are presented in the context of the global Ca cycle. This analysis, which assumes that nannofossil ooze records isotopic variations in seawater, indicates a dynamic Ca cycle in the Cenozoic. Such dynamic behavior has serious implications for the C cycle and suggests feedbacks between the Ca and C cycles to stabilize, or buffer, the oceanic carbon reservoir. Alternative applications of the Ca isotope proxy are investigated, using numerical models to determine the extent to which Ca isotopes are sensitive to other aspects of the Ca cycle; the results of the simulations are applied to specific cases in the Cenozoic. The simulations illustrate how variations in the global fractionation factor between calcium carbonate and seawater can produce trends similar to those observed when comparing previously published Ca isotopic compositions of marine barite to the nannofossil ooze record. The large drop in the δ44Ca value of bulk nannofossil ooze near the Eocene-Oligocene boundary can be reconciled in two ways, either as a substantial increase in weathering relative to sedimentation or as an indicator of changing depositional mode within the ocean. Though the preferred interpretation is not clear at present, it is evident that Ca isotopes stand to be a unique proxy for Ca cycling once the isotope systematics are elucidated.

Chemical weathering and lithologic controls of water chemistry in a high‐elevation river system: Clark's Fork of the Yellowstone River, Wyoming and Montana

Seasonal analyses of surface water geochemistry were conducted in the Clarks Fork of the Yellowstone watershed to determine whole-rock weathering rates. The Clarks Fork of the Yellowstone is a high-elevation catchment with distinct bedrock lithologies. Using dissolved solute concentrations and stream flow data, we calculated cation denudation rates of 119 g m−2 yr−1 (65,900 eq ha−1 yr−1) for carbonate-rich sedimentary rocks, 16.6 g m−2 yr−1 (8200 eq ha−1 yr−1) for andesitic volcanics, and 9.8 g m−2 yr−1(5300 eq ha−1 yr−1) for granitic gneisses. Ca/Na ratios indicate that chemical weathering of disseminated calcite in granitic rocks contributes to the total solute load in these subcatchments. Removal of this c(2100 eq ha−1 yr−1).

Isotopic approaches for quantifying the rates of marine burial diagenesis

[1] Diagenetic reactions in marine sediments have the potential to alter geochemical proxy records and affect the global carbon cycle over tens of thousands to millions of years. This article describes advances in the use of Ca, Sr, and U series isotopes in constraining carbonate recrystallization and silicate dissolution rates in marine systems. We specifically focus on recent efforts that interpret isotope variability in marine pore fluids using reactive transport models of varying complexity. Such studies suggest that calcite recrystallization rates are significant over time scales <1 Myr, approaching exchange rates of 0.4–1 Myr −1 . Over longer time scales, isotopic data point to continued exchange between calcite and coexisting pore fluid, though at lower rates than in young sediments. Extrapolating these recrystallization rates over tens of millions of years, we quantify the extent to which geochemical climate proxies such as Mg/Ca in calcite can be altered diagenetically. In some cases, such diagenetic effects significantly affect the interpretation of long‐term climatic trends, as well as determinations of absolute paleotemperatures. Silicate dissolution rates in siliclastic marine sediments appear to be similar in magnitude to dissolution rates in terrestrial environments (10 −7 –10 −6 yr −1 ). Such estimates suggest that silicate weathering in marine sediments may play an important role in the carbon cycle, at least over time scales approaching 400–500 kyr.

A model describing flowback chemistry changes with time after Marcellus Shale hydraulic fracturing

Between 2005 and 2014 in Pennsylvania, about 4000 Marcellus wells were drilled horizontally and hydraulically fractured for natural gas. During the flowback period after hydrofracturing, 2 to (7 to ) of brine returned to the surface from each horizontal well. This Na-Ca-Cl brine also contains minor radioactive elements, organic compounds, and metals such as Ba and Sr, and cannot by law be discharged untreated into surface waters. The salts increase in concentration to () in later flowback. To develop economic methods of brine disposal, the provenance of brine salts must be understood. Flowback volume generally corresponds to ∼10% to 20% of the injected water. Apparently, the remaining water imbibes into the shale. A mass balance calculation can explain all the salt in the flowback if 2% by volume of the shale initially contains water as capillary-bound or free Appalachian brine. In that case, only 0.1%–0.2% of the brine salt in the shale accessed by one well need be mobilized. Changing salt concentration in flowback can be explained using a model that describes diffusion of salt from brine into millimeter-wide hydrofractures spaced 1 per m (0.3 per ft) that are initially filled by dilute injection water. Although the production lifetimes of Marcellus wells remain unknown, the model predicts that brines will be produced and reach 80% of concentration of initial brines after ∼1 yr. Better understanding of this diffusion could (1) provide better long-term planning for brine disposal; and (2) constrain how the hydrofractures interact with the low-permeability shale matrix.

Metabolic diversity and ecological niches of Achromatium populations revealed with single-cell genomic sequencing.

Large, sulfur-cycling, calcite-precipitating bacteria in the genus Achromatium represent a significant proportion of bacterial communities near sediment-water interfaces at sites throughout the world. Our understanding of their potentially crucial roles in calcium, carbon, sulfur, nitrogen, and iron cycling is limited because they have not been cultured or sequenced using environmental genomics approaches to date. We utilized single-cell genomic sequencing to obtain one incomplete and two nearly complete draft genomes for Achromatium collected at Warm Mineral Springs (WMS), FL. Based on 16S rRNA gene sequences, the three cells represent distinct and relatively distant Achromatium populations (91–92% identity). The draft genomes encode key genes involved in sulfur and hydrogen oxidation; oxygen, nitrogen and polysulfide respiration; carbon and nitrogen fixation; organic carbon assimilation and storage; chemotaxis; twitching motility; antibiotic resistance; and membrane transport. Known genes for iron and manganese energy metabolism were not detected. The presence of pyrophosphatase and vacuolar (V)-type ATPases, which are generally rare in bacterial genomes, suggests a role for these enzymes in calcium transport, proton pumping, and/or energy generation in the membranes of calcite-containing inclusions.

Susceptibility of Goethite to Fe2+-Catalyzed Recrystallization over Time

Recent work has shown that iron oxides, such as goethite and hematite, may recrystallize in the presence of aqueous Fe2+ under anoxic conditions. This process, referred to as Fe2+-catalyzed recrystallization, can influence water quality by causing the incorporation/release of environmental contaminants and biological nutrients. Accounting for the effects of Fe2+-catalyzed recrystallization on water quality requires knowing the time scale over which recrystallization occurs. Here, we tested the hypothesis that nanoparticulate goethite becomes less susceptible to Fe2+-catalyzed recrystallization over time. We set up two batches of reactors in which 55Fe2+ tracer was added at two different time points and tracked the 55Fe partitioning in the aqueous and goethite phases over 60 days. Less 55Fe uptake occurred between 30 and 60 days than between 0 and 30 days, suggesting goethite recrystallization slowed with time. Fitting the data with a box model indicated that 17% of the goethite recrystallized after 30 days of reaction, and an additional 2% recrystallized between 30 and 60 days. The decreasing susceptibility of goethite to recrystallize as it reacted with aqueous Fe2+ suggested that recrystallization is likely only an important process over short time scales.