G. Brooks Avery

Rainwater dissolved organic carbon: concentrations and global flux.

Dissolved organic carbon (DOC) is a major component of both marine (23 μM) and continental (161 μM) rain, present in concentrations greater than nitric and sulfuric acids combined. Rain is a significant source of DOC to surface seawater (90 × 1012 g C yr−1), equivalent to the magnitude of river input of DOC to the open ocean and half the magnitude of carbon buried in marine sediments per year on a global scale. Current models of global carbon cycling focus primarily on inorganic forms of carbon and are unable to account for approximately 20% of the global carbon dioxide, suggesting a significant missing carbon sink. Quantification of the average DOC concentration in marine rain allows calculation of the global rainwater flux of DOC of 430 ± 150 × 1012 g C yr−1. When inorganic carbon is included, this rainwater carbon flux becomes 510 ± 170 × 1012 g C yr−1, which, although not the same carbon, is equivalent in magnitude to over one third of the missing carbon sink.

Effect of seasonal changes in the pathways of methanogenesis on the δ13C values of pore water methane in a Michigan peatland

The δ13C value of pore water methane produced in a Michigan peatland varied by 11‰ during the year. This isotopic shift resulted from large seasonal changes in the pathways of methane production. On the basis of mass balance calculations, the δ13C value of methane from CO2 reduction (average = −71.4 ± 1.8‰) was depleted in 13C compared to that produced from acetate (−44.4 ± 8.2‰). The dissolved methane at the site remained heavy (approximately −51‰) during most of the year. Tracer experiments using 14C-labeled CO2 indicated that during January 110 ± 25% of the methane was produced by CO2 reduction. Because of low-methane production rates during the winter, this 13C-depleted methane had only a slight effect on the isotopic composition of the methane pool. In early spring when peat temperatures and methane production rates increased, the δ13C value of the dissolved methane in shallow peat was influenced by the isotopically light methane and approached −61‰. Peat incubation experiments conducted at 15°C in May and June (when the peat reaches its maximum temperature) indicated that an average of 84 ± 9% of the methane production was from acetate and had an average δ13C value of −48.7 ± 5.6‰. Rising acetate concentrations during April-May (approaching 1 mmol L−1(mM)) followed by a rapid decrease in acetate concentrations during May-June reflected the shift toward methane production dominated by acetate fermentation. During this period, dissolved methane in shallow peat at the site returned to heavier values (approximately −51‰) similar to that produced in the incubation experiments.

Controls on methane production in a tidal freshwater estuary and a peatland: methane production via acetate fermentation and CO2 reduction

Rates of total methane production, acetate fermentation andCO2 reduction were compared for two different wetland sites. On aper-liter basis, sediments from the White Oak River estuary, a tidal freshwatersite in eastern North Carolina, had an annual methane production rate (53.3mM yr−1) an order of magnitude higher thanthat ofBuck Hollow Bog (5.5 mM yr−1), a peatlandinMichigan. Methane was produced in the White Oak River site on an annual basisbyboth acetate fermentation (72%) and CO2 reduction (28%) in a ratiotypical of freshwater methanogenic sites. Competition for acetate bynon-methanogenic microorganisms in Buck Hollow peat limited methane productionfrom acetate to only a few months a year, severely impacting annual methaneproduction rates. However, when acetate was available to the methanogens in thepeat during early spring, the percentage of methane production from acetatefermentation (84%) and CO2 reduction (16%) and rates of totalmethaneproduction were similar to those of the White Oak River sediments at the sametemperature. Rates of CO2 reduction and acetate fermentationconducted at both sites at various temperatures showed that Buck Hollow peatmethane production was also limited by a colder temperature regime as well asdifferences in the response of the CO2 reducing and aceticlasticmethanogens to temperature variations.

Controls on the stable carbon isotopic composition of biogenic methane produced in a tidal freshwater estaurine sediment

Abstract The δ13C value of methane in sediments from a tidal freshwater site in the White Oak River Estuary, North Carolina, exhibited a relatively small, but consistent, seasonal variation (∼3‰) with isotopically heavier values occurring during the warmer months (−66.1‰ summer, −69.2‰ winter). These isotopic shifts could have resulted from changes in: (1) isotopic compositions of precursor molecules; (2) kinetic isotope effects associated with methane production; or (3) pathways of methane production. Methane production rate and isotopic data from sediment incubation experiments and field measurements were used to determine the relative contributions of these factors to the observed seasonal variations. Although changes in δ13C values of biogenic methane are typically thought to result from changes in pathways of methane production, this study showed that a significant amount (36 ± 22%) of the seasonal variations between the δ13C value of methane produced in sediment incubation experiments could be attributed to changes in the δ13C value of the ΣCO2 pool. This was due to increased methane production rates and removal of 12CO2 with increasing temperature, a prevalent feature of methanogenic systems that may account for some of the frequently observed 13C enrichment in methane during warmer months. Combining the change in the δ13C value of the ΣCO2 pool with temperature-controlled changes in fractionation (α) resulting from kinetic isotope effects accounted for (53 ± 22%) of the 13C enrichment observed during summer sediment incubation experiments. Although large pathway changes were not observed in sediment incubation experiments, the remaining differences in δ13C values could have resulted from smaller, undetectable changes in the percentage of methane production from acetate (∼14%) and/or a shift in the δ13C values of methane produced from acetate (∼4‰).

Long-Term Temporal Variability in Hydrogen Peroxide Concentrations in Wilmington, North Carolina USA Rainwater

Measurements of hydrogen peroxide (H(2)O(2)), pH, dissolved organic carbon (DOC), and inorganic anions (chloride, nitrate, and sulfate) in rainwater were conducted on an event basis at a single site in Wilmington, NC for the past decade in a study that included over 600 individual rain events. Annual volume weighted average (VWA) H(2)O(2) concentrations were negatively correlated (p < 0.001) with annual VWA nonseasalt sulfate (NSS) concentrations in low pH (<5) rainwater. Under these conditions H(2)O(2) is the primary aqueous-phase oxidant of SO(2) in the atmosphere. We attribute the increase of H(2)O(2) to decreasing SO(2) emissions which has had the effect of reducing a major tropospheric sink for H(2)O(2). Annual VWA H(2)O(2) concentrations in low pH (<5) rains showed a significant increase over the time scale of this study, which represents the only long-term continuous data set of H(2)O(2) concentrations in wet deposition at a single location. This compositional change has important implications because H(2)O(2) is a source of highly reactive free radicals so its increase reflects a higher overall oxidation capacity of atmospheric waters. Also, because rainwater is an important mechanism by which H(2)O(2) is transported from the atmosphere to surface waters, greater wet deposition of H(2)O(2) could influence the redox chemistry of receiving watersheds which typically have concentrations 2-3 orders of magnitude lower than rainwater.

Controls on the redox potential of rainwater.

Hydrogen peroxide acting as a reductant affects the redox potential of rainwater collected at the Bermuda Atlantic Time Series Station, the South Island of New Zealand, the contiguous USA, and the primary study site in Wilmington, NC. Analytical measurements of both halves of redox couples for dissolved iron, mercury, and the nitrate-nitrite-ammonium system can predict the rainwater redox potential measured directly by a platinum electrode. Measurements of these redox couples along with the pH in rain yields pe⁻ between 8 and 11; the half reaction for hydrogen peroxide acting as a reductant using typical rainwater conditions of 15 μM H₂O₂ at pH 4.7 gives pe⁻ = 9.12, where pe⁻ = negative log of the activity of hydrated electrons. Of the six rainwater redox systems investigated, only manganese speciation appeared to be controlled by molecular oxygen (pe⁻ = 15.90). Copper redox speciation was consistent with superoxide acting as a reductant (pe⁻ = 2.7). The concentration of H₂O₂ in precipitation has more than doubled over the preceding decade due to a decrease in SO₂ emissions, which suggests the redox chemistry of rainwater is dynamic and changing, potentially altering the speciation of many organic compounds and trace metals in atmospheric waters.