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The magnitude of the terrestrial soil sink for atmospheric carbon tetrachloride (CCl4) remains poorly constrained, with the estimated uncertainty range of CCl4 partial lifetimes between ~110 and 910 years. Field observations are sparse, and there are uncertainties in extrapolating these results to the global scale. Here we add to the published CCl4 fluxes with additional field measurements, and we employ a land cover classification scheme based on Advanced Very High Resolution Radiometer measurements that align more closely with the measurement sites to reevaluate the global CCl4 soil sink. We calculate an updated partial lifetime of CCl4 with respect to the soil sink to be 375 (288–536) years, which is 50 to 90% longer than the most recently published best estimates of the soil sink partial lifetime (195 and 245 years). This translates into a longer overall atmospheric lifetime estimate, which is more consistent with the observed atmospheric concentration trend and interhemispheric gradient.

Static flux chamber measurements of CCl4 uptake by soils in boreal, subtropical and tropical forests have been used to reassess the sink strength for this ozone depleting chemical. Happell and Roche (Geophys. Res. Lett. 30(2), 1088–1091, 2003) used flux estimates from soil concentration gradients to calculate a partial CCl4 atmospheric lifetime (τsoil) of 90 years. More recently, it is has been assumed that a better estimate of τsoil is 195 years (Montzka et al. 2011). In the work here, the rate of CCl4 uptake was calculated from 453 flux chamber measurements using an exponential fit to the chamber CCl4 concentration change with time. This analysis indicated that the flux rate estimate in Happell and Roche (Geophys. Res. Lett. 30(2) 1088–1091, 2003) was overestimated by 2.75, yielding a new estimate of τsoil for CCl4 of 245 years. Significant correlations of CCl4 uptake to temperature, soil moisture, or time of year were not observed. This work provides additional evidence that CCl4 uptake by soils is a common process and needs to be considered when developing an atmospheric budget for this compound.

The International Halocarbons in Air Comparison Experiment (IHALACE) was conducted to document relationships between calibration scales among various laboratories that measure atmospheric greenhouse and ozone depleting gases. This study included trace gases such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs), as well as nitrous oxide, methane, sulfur hexafluoride, very short-lived halocompounds, and carbonyl sulfide. Many of these gases are present in the unpolluted atmosphere at pmol mol−1 (parts per trillion) or nmol mol−1 (parts per billion) levels. Six stainless steel cylinders containing natural and modified natural air samples were circulated among 19 laboratories. Results from this experiment reveal relatively good agreement (within a few percent) among commonly used calibration scales. Scale relationships for some gases, such as CFC-12 and CCl4, were found to be consistent with those derived from estimates of global mean mole fractions, while others, such as halon-1211 and CH3Br, revealed discrepancies. The transfer of calibration scales among laboratories was problematic in many cases, meaning that measurements tied to a particular scale may not, in fact, be compatible. Large scale transfer errors were observed for CH3CCl3 (10–100%) and CCl4 (2–30%), while much smaller scale transfer errors (< 1%) were observed for halon-1211, HCFC-22, and HCFC-142b. These results reveal substantial improvements in calibration over previous comparisons. However, there is room for improvement in communication and coordination of calibration activities with respect to the measurement of halogenated and related trace gases.

Carbon tetrachloride (CCl4) is an ozone-depleting substance, which is controlled by the Montreal Protocol and for which the atmospheric abundance is decreasing. However, the current observed rate of this decrease is known to be slower than expected based on reported CCl4 emissions and its estimated overall atmospheric lifetime. Here we use a three-dimensional (3-D) chemical transport model to investigate the impact on its predicted decay of uncertainties in the rates at which CCl4 is removed from the atmosphere by photolysis, by ocean uptake and by degradation in soils. The largest sink is atmospheric photolysis (74 % of total), but a reported 10 % uncertainty in its combined photolysis cross section and quantum yield has only a modest impact on the modelled rate of CCl4 decay. This is partly due to the limiting effect of the rate of transport of CCl4 from the main tropospheric reservoir to the stratosphere, where photolytic loss occurs. The model suggests large interannual variability in the magnitude of this stratospheric photolysis sink caused by variations in transport. The impact of uncertainty in the minor soil sink (9 % of total) is also relatively small. In contrast, the model shows that uncertainty in ocean loss (17 % of total) has the largest impact on modelled CCl4 decay due to its sizeable contribution to CCl4 loss and large lifetime uncertainty range (147 to 241 years). With an assumed CCl4 emission rate of 39 Gg year−1, the reference simulation with the best estimate of loss processes still underestimates the observed CCl4 (overestimates the decay) over the past 2 decades but to a smaller extent than previous studies. Changes to the rate of CCl4 loss processes, in line with known uncertainties, could bring the model into agreement with in situ surface and remote-sensing measurements, as could an increase in emissions to around 47 Gg year−1. Further progress in constraining the CCl4 budget is partly limited by systematic biases between observational datasets. For example, surface observations from the National Oceanic and Atmospheric Administration (NOAA) network are larger than from the Advanced Global Atmospheric Gases Experiment (AGAGE) network but have shown a steeper decreasing trend over the past 2 decades. These differences imply a difference in emissions which is significant relative to uncertainties in the magnitudes of the CCl4 sinks.

Atmospheric concentrations of carbon tetrachloride (CCl 4 ) were removed by bulk aerobic soils from tropical, subtropical, and boreal environments. Removal was observed in all tested soil types, indicating that the process was widespread. The flux measured in field chamber experiments was 0.24 ± 0.10 nmol CCl 4 (m 2 day) −1 (average ± standard deviation [SD]; n = 282). Removal of CCl 4 and removal of methane (CH 4 ) were compared to explore whether the two processes were linked. Removal of both gases was halted in laboratory samples that were autoclaved, dry heated, or incubated in the presence of mercuric chloride (HgCl 2 ). In marl soils, treatment with antibiotics such as tetracycline and streptomycin caused partial inhibition of CCl 4 (50%) and CH 4 (76%) removal, but removal was not affected in soils treated with nystatin or myxothiazol. These data indicated that bacteria contributed to the soil removal of CCl 4 and that microeukaryotes may not have played a significant role. Amendments of methanol, acetate, and succinate to soil samples enhanced CCl 4 removal by 59%, 293%, and 72%, respectively. Additions of a variety of inhibitors and substrates indicated that nitrification, methanogenesis, or biological reduction of nitrate, nitrous oxide, or sulfate (e.g., occurring in possible anoxic microzones) did not play a significant role in the removal of CCl 4 . Methyl fluoride inhibited removal of CH 4 but not CCl 4 , indicating that CH 4 and CCl 4 removals were not directly linked. Furthermore, CCl 4 removal was not affected in soils amended with copper sulfate or methane, supporting the results with MeF and suggesting that the observed CCl 4 removal was not significantly mediated by methanotrophs.

The formation of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) significantly contributes to the total uptake and storage of anthropogenic gases, such as CO2 and chlorofluorocarbons (CFCs), within the world’s oceans. SAMW and AAIW formation rates in the South Pacific are quantified based on CFC-12 inventories using hydrographic data from WOCE, CLIVAR, and data collected in the austral winter of 2005. This study documents the first wintertime observations of CFC-11 and CFC-12 saturations with respect to the 2005 atmosphere in the formation region of the southeast Pacific for SAMW and AAIW. SAMW is 94% and 95% saturated for CFC-11 and CFC-12, respectively, and AAIW is 60% saturated for both CFC-11 and CFC-12. SAMW is defined from the Subantarctic Front to the equator between potential densities 26.80–27.06 kg m � 3 , and AAIW is

The apparent CFC-11, -12 and -113 ages of Upper Floridan Aquifer water discharged from 31 springs located in Florida and Georgia ranged from 11 to 44 years when samples were collected in 2002 and 2003. Apparent 3 H/ 3 He ages in these springs ranged from 12 to 66 years. Some of the springs sampled did not yield valid CFC ages because one or more of the CFCs were contaminated by non-atmospheric sources. Of the 31 springs sampled, six were contaminated with all three CFCs and nine were contaminated with one or two CFCs. Of the remaining 16 springs, the CFC distributions of four could be modeled assuming a single source of water, and 11 were best modeled by assuming two sources of water, with one of the water sources >60 years old. The CFC and 3 H/ 3 He apparent ages and the simple mixing models applied to these ages suggest that past impacts to the water quality of water recharging the sampled springs may take anywhere from 0 to ∼60 years or more to appear in the discharging spring water. In 27 springs where both 3 H/ 3 He ages and CFC ages were available, five springs gave similar results between the two techniques, while in the other 22 cases the 3 H/ 3 He apparent ages were 8–40 years greater than the CFC ages. Large excesses of 4 He were observed in many of the springs, consistent with a source of older water. This older water may also carry an additional and unaccounted for source of 3 He, which may be responsible for the greater 3 H/ 3 He ages relative to the CFC ages. We believe that the large excess 3 He and 4 He values and apparent age differences are related to regional climate variations because our samples were obtained at the end of a 4-year drought.

Poor agreement between 3 H/ 3 He ages and CFC-11 and CFC-12 ages suggests that CFCs may not be conservative tracers in the Everglades National Park. 3 H/ 3 He ages were used to calculate the expected concentration of CFC-11 and CFC-12 in groundwater from wells 2 to 73 m deep. The expected concentrations of CFCs were compared to the measured concentrations and plots of the % CFC-12 and CFC-11 remaining offered no evidence that significant CFC removal was occurring in the groundwater at depths ≥2 m, suggesting that CFC removal occurs at shallower depths. Except where CFC contamination was suspected, CFC-11, CFC-12 and CFC-113 concentrations in fresh surface water were nearly always below solubility equilibrium with the atmosphere. Measurements of CFC-11, CFC-12 and CFC-113 in pore water indicate a 50–90% decrease in concentration 5 cm below the groundwater–surface water (GW–SW) interface. In the same 5 cm interval CH 4 concentrations increased by 300–1000%. This suggested that CFCs were removed at the GW–SW interface, possibly by methane-producing bacteria. CFC derived recharge ages should therefore be viewed with caution when recharging water percolates through anoxic methanogenic sediments.

Measured concentrations of relatively nonreactive, anthropogenic halocarbon tracers (CFC-11, CFC-12, CFC-113) were used to infer the time since recharge, or age, of groundwater collected from the Upper Glacial and Magothy Aquifers underlying Brookhaven National Laboratory on Long Island, NY. On the basis of the reconstructed historical atmospheric concentrations of CCl4, the initial CCl4 concentration for the precipitation that recharged the aquifer was estimated as a function of age. Correlation of measured and estimated initial CCl4 concentrations within the aquifer, over inferred ages of 0−50 yr, suggested that CCl4 was being removed in situ with a half-life of 14 ± 4 yr. Groundwater samples collected at the water table had CCl4 concentrations that were ≤50% of equilibrium with contemporary atmospheric concentrations, suggesting that removal was also significant in the unsaturated zone. Soil gas profiles confirmed that atmospheric CCl4 was being removed from the unsaturated zone, with only ∼25% of the initial CCl4 being present in the gas phase at a depth of 30 cm, and with no evidence for removal of CFC-11, CFC-12, or CFC-113. A time−series of soil gas profiles col lected before and after a major rainfall event indicated that most removal occurred in the top 15 cm of soil. The flux of CCl4 into the soil was estimated to be ∼8600 ± 5100 pmol m-2 d-1, and removal of CCl4 in soils therefore has the potential to significantly affect the global atmospheric lifetime of this compound. The observed degradation in bulk aerobic environments raises questions concerning the conventional wisdom that CCl4 is degraded significantly only within reducing environ ments.

Gas phase standards, containing CCl3F (CFC-11), CCl2F2 (CFC-12), CCl2FCClF2 (CFC-113), CH3CCl3 and CCl4 in the pmol mol−1 range, were prepared gravimetrically in dry N2. The standards were prepared with the relative ratios of the various compounds close to those found in seawater, to facilitate calibration of oceanic halocarbon measurements. The standards made in this study were intercalibrated against an atmospheric-ratio-based standard that was analyzed at both Scripps Institution of Oceanography (SIO) and NOAA's Climate Monitoring and Diagnostics Laboratory (CMDL). A total of 21 standards was made, with the concentrations of the above halocarbons reported on both the CMDL and SIO 1993 scales. These standards are currently being used by ten different research groups making oceanic halocarbon measurements. The average systematic deviation of all compounds in our standards was generally ± 5% or less from the SIO 1993 and CMDL scales. Deviations for CH3CCl3 between our standards and the CMDL scale were slightly larger, with our standards, on average, approximately 10% lower.

The International Halocarbons in Air Comparison Experiment (IHALACE) was conducted to document relationships between calibration scales among various laboratories that measure atmospheric greenhouse and ozone depleting gases. Six stainless steel cylinders containing natural and modified natural air samples were circulated among 19 laboratories. Results from this experiment reveal relatively good agreement among commonly used calibration scales for a number of trace gases present in the unpolluted atmosphere at pmol mol−1 (parts per trillion) levels, such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs). Some scale relationships were found to be consistent with those derived from bi-lateral experiments or from analysis of atmospheric data, while others revealed discrepancies. The transfer of calibration scales among laboratories was found to be problematic in many cases, meaning that measurements tied to a common scale may not, in fact, be compatible. These results reveal substantial improvements in calibration over previous comparisons. However there is room for improvement in communication and coordination of calibration activities with respect to the measurement of halogenated and related trace gases.

CH 3 I was measured in open ocean waters during two cruises to the tropical Atlantic Ocean and a late fall cruise to the Greenland and Norwegian Seas (GSNS). In warm, tropical surface waters subject to high solar irradiance, average CH 3 I saturation anomalies were positive (1.5-7.7 pmol kg -1 ), indicating a sea-to-air flux. This contrasted with negative saturation anomalies (-0.65 ± 0.02 pmol kg -1 ) measured in cold surface waters of the open ocean GSNS subject to low-light. High latitude oceans may therefore be a significant sink for atmospheric CH 3 I during the fall and winter. The locations and/or seasons where samples were analyzed were all characterized by relatively low biological production and the CH 3 I saturation anomaly along 19°S decreased from 7.7 ± 0.6 to 3.4 ± 0.4 pmol kg -1 when entering a more productive upwelling zone. Taken together these observations suggest a chemical, as opposed to biological, production mechanism for this compound in the open ocean. Within the open ocean of the GSNS, multiple linear regression between the observed CH 3 I saturation anomaly and variables including light intensity, water temperature, CFC-11 saturation (indicator of gas exchange and deep mixing), and distance from the Norwegian Coastal Current (indicator of coastal or southem sources) showed that light intensity was the only significant predictor, explaining 79 % of the variance. Photochemical production may, therefore be dominant source of CH 3 I within the open ocean and this may have important implications for the large-scale, seasonal cycling of iodine between the ocean and the atmosphere.

Emission rates from the standing water of swamps to the atmosphere calculated from literature-derived exchange coefficients were greater than measured diffusive fluxes of CH,. In north Florida mixed hardwood swamp experiments performed in the absence of ebullition, the calculated flux of CH, was 3.6 +2.6 (n = 22) times larger than the measured flux. In an Everglades cypress dome, calculated CH, fluxes were 3.lk2.5 (n = 3) times greater than measured fluxes. New mass-transfer coefficients at zero windspeed were calculated for CH, emission from the stagnant swamp water in the mixed hardwood swamps (0.75 f0.54 cm h-l, n = 22) and in the cypress dome (0.83kO.51 cm h-l, n = 3). These values were significantly less than the calculated mass-transfer coefficient for CH, (1.7 cm h-l) determined in another study. We hypothesize that insoluble organic surface films physically, chemically, or biologically inhibited CH, transfer from the floodwater to the atmosphere. Consistent with these findings we also observed greater retardation of 13CH, efflux relative to 12CH, efllux in swamp experiments relative to those conducted in the laboratory. Methane emitted from standing water was depleted in 13C by 0.8+0.3% (n = 16), 3.6+3.2% (n = 12), and 2.6f0.8(?” (n = 3) relative to the isotopic composition of dissolved CH, in the laboratory, mixed hardwood swamps, and cypress dome.

This study reports the first measurements of the [delta][sup 13]C of CH[sub 4] emitted from seasonally flooded swamp forests in the southeastern United States. The seasonally averaged [delta][sup 13]C of CH[sub 4] emitted from a north Florida swamp forest located in the St. Marks National Wildlife Refuge was -52.7 [+-] 6.11[per thousand] (error is [+-] one standard deviation throughout, n = 28), a value [sup 13]C-enriched, relative to typical wetland emissions. In an Everglades cypress dome, the average [delta][sup 13]C of emitted CH[sub 4] was -52.5 [+-] 6.7 [per thousand] (n = 3). Consistent with attenuation of CH[sub 4] emission by CH[sub 4] oxidation in these environments, CH[sub 4] emitted via diffusion from the St. Marks swamp forest was enriched in [sup 13]C by 6.4 [+-] 5.8 [per thousand] (n = 28) and D by 57 [+-] 36 [per thousand] (n = 6) relative to sedimentary CH[sub 4]. Emission experiments, performed in situ with inhibitors of aerobic CH[sub 4] oxidizing bacteria, were used to calculate the fractionation factors ([alpha]) for stable carbon and hydrogen isotopes of CH[sub 4] undergoing transport and oxidation. Values ranged from 1.003 to 1.021 and 1.050 to 1.129, respectively. The best estimates for carbon and hydrogenmore » [alpha] values were 1.020 and 1.068, respectively. The [delta] values of produced (sedimentary) CH[sub 4] were relatively constant in the St. Marks subtropical swamp forest. Additionally, because the transport of CH[sub 4] to the atmosphere was dominated by molecular diffusion, variations in the magnitude of CH[sub 4] oxidation appeared to be the primary factor controlling the [delta] values of emitted CH[sub 4]. This contrasts with systems dominated by bubble ebullition, where variations in CH[sub 4] production mechanisms have been hypothesized to be the primary factor controlling the [delta] values of emitted CH[sub 4].« less

Water level controlled gas emissions from North Florida swamp forests. Under flooded conditions, CO2 and CH4 were the principle carbon gases transported to the atmosphere by bubble ebullition and molecular diffusion. The respective emission rates were for CO2, 29.3 ± 16.4 (13% by means of ebullition, 87% by means of diffusion, error is ± 1 standard deviation throughout) and for CH4, 2.16 ± 2.24 (45% by means of ebullition, 55% by means of diffusion) mol m−2 yr−1. Methane emissions were significantly attenuated by CH4 oxidation which occurred primarily at the sediment-water interface. Forty-six ± 22 % (n=19) of the belowground CH4 diffusing to this interface was oxidized before it could escape to the atmosphere. Under dry conditions, CO2 was the principle carbon gas released and atmospheric CH4 was consumed by microbes in the soil. The respective rates were 101.2 ± 26.80 and −0.015 ± 0.005 mol mr−2 yr−1. A carbon budget for the degradation of soil organic matter was developed for a swamp forest site under flooded and dry conditions. Assuming that live root respiration accounted for 67% (value determined in a swamp forest and is at the upper range of literature values) of the total CO2 emissions (given above), we calculate that under flooded conditions carbon remineralization proceeded at a total rate of 11.9 mol C m−2 yr−1. Forty-nine percent of the remineralization was by means of nonmethanogenic processes which produce CO2; the balance was by means of methanogenic processes, which produce both CH4 and CO2. Under dry conditions, remineralization was dominated by aerobic processes at a rate of 33.7 mol C m−2 yr−1. Carbon inputs to the soil occurred by aboveground and belowground production. Aboveground litter production contributed 25.6 mol C m−2 yr−1. If belowground production contributed an equal amount, then over the course of this study organic carbon accumulated in the soils at rates of 39.3 and 17.5 mol C m−2 yr−1 under flooded and dry conditions, respectively. If root respiration accounted for only 6% (lowest value in literature) of the total CO2 emissions, organic carbon would accumulate in the soil at a rate of 21.6 mol C m−2 d−1 under flooded conditions and be lost from the soil at a rate of 43.8 mol C m−2 d−1 under dry conditions.

Rates of methane emission associated with Florida Typha domingensis Pers. and Typha latifolia L. peaked in the early daylight hours and were two to four times higher than the relatively constant rates observed in the afternoon and night. Factors associated with methane emission peaks were increasing solar illumination which drives pressurized bulk flow ventilation in T. domingensis and T. latifolia, opening of stomata, and decreasing concentrations of methane within plant stems. Cladium jamaicense Crantz, which employs diffusive gas exchange, did not exhibit diurnal variations in methane emission rates although stomatal conductance varied diurnally. Within the Florida Everglades methane emission rates were higher in T. domingensis areas (143±19 mg CH4 m−2 day−1) than in C. jamaicense areas (45±15 mg CH4 m−2 day−1). These elevated rates were related to the higher above ground biomass and production in T. domingensis areas relative to C. jamaicense, which suggests that quantitative differences in plant biomass and production rather than qualitative differences between these plant species may control methane emissions. Methane emission was 2.7±1.4% of net daily ecosystem production (NEP) in a T. domingensis area and 14±5.8% and 3.4±4.2% of NEP in two C. jamaicense areas.

The concentrations of tritium ( 3 H) and helium isotopes ( 3 He and 4 He) were used as tracers of groundwater flow in the surficial aquifer system (SAS) beneath Everglades National Park (ENP), south Florida. From ages determined by 3 H/ 3 He dating techniques, groundwater within the upper 28 m originated within the last 30 years. Below 28 m, waters originated prior to 30 years before present with evidence of mixing at the interface. Interannual variation of the 3 H/ 3 He ages within the upper 28 m was significant throughout the 3 year investigation, corresponding with varying hydrologic conditions. In the region of Taylor Slough Bridge, younger groundwater was consistently detected below older groundwater in the Biscayne Aquifer, suggesting preferential flow to the lower part of the aquifer. An increase in He with depth in the SAS indicated that radiogenic 4 He produced in the underlying Hawthorn Group migrates into the SAS by diffusion. Higher Δ 4 He values in brackish groundwaters compared to fresh waters from similar depths suggested a possible enhanced vertical transport of 4 He in the seawater mixing zone. Groundwater salinity measurements indicated the presence of a wide (6-28 km) seawater mixing zone. Comparison of groundwater levels with surface water levels in this zone indicated the potential for brackish groundwater discharge to the overlying Everglades surface water.

[1] Analysis of soil gas from the world's major biomes indicates that carbon tetrachloride (CCl4) uptake by soils is a ubiquitous process. The global flux of CCl4 from the atmosphere to soils was estimated to be 27 ± 21 kilotons per year (Kt yr−1), corresponding to a partial atmospheric lifetime of 90 years (yr). The total lifetime of CCl4 in the atmosphere, taking into account the stratospheric, oceanic and soil sinks is estimated to be 20 yr. The recently identified oceanic and soil sinks of atmospheric CCl4 suggest that current emission estimates of CCl4 may be underestimated by ∼60%.

Data collected as a part of this study from 1968–2002 and data from other studies from 1950–1967 show that themaximum atmospheric concentration of tritium gas (HT) occurred in the early to mid 1970s, which corresponds to theera of frequent, large underground nuclear tests. These data clearly show that the major source of HT to the atmospherebetween 1962 and the early 1990s was the underground testing of nuclear weapons. Samples collected at both ourAlaska and Miami stations clearly show marked increases in the autumn of each year between 1970 and 1975 that wereassociated with large underground explosions conducted by the former Soviet Union at the Novaya Zemlya test site.Other significant sources of HT include accidental releases of HT used in the manufacture and maintenance of nuclearweapons stockpiles, the reprocessing of spent nuclear fuel rods and emissions from nuclear power plants. Since theearly 1990s, when underground testing largely ceased, emission estimates from our data agree very well with UnitedNations estimates of worldwide releases from fuel reprocessing and nuclear power plants, suggesting that the nuclearpower industry is now the major source of HT to the atmosphere. DOI: 10.1111/j.1600-0889.2004.00103.x

The stable carbon isotopic composition of CH4 is used to study the processes that affect it during transport through plants from sediment to the atmosphere. The enhancement of CH4 flux from Cladium and Eleocharis over the flux from open water or clipped sites indicated that these plants served as gas conduits between the sediments and the atmosphere. Lowering of the water table below the sediment surface caused an Everglades sawgrass marsh to shift from emission of CH4 to consumption of atmospheric CH4. Cladium transported gases passively mainly via molecular diffusion and/or effusion instead of actively via bulk flow. Stable isotropic data gave no evidence that CH4 oxidation was occurring in the rhizosphere of Cladium. Both CH4 stable carbon isotope and flux data indicated a lack of CH4 oxidation at the sediment-water interface in Everglades marl soils and its presence in peat soils where 40 to 92 percent of the flux across the sediment-water interface was oxidized.

Methane flux was linearly correlated with plant biomass (r = 0.97, n = 6 and r = 0.95, n = 8) at two locations in a Florida Everglades Cladium marsh. One location, which had burned 4 months previously, exhibited a greater increase in methane flux as a function of biomass relative to sites at an unburned location. However, methane flux data from both sites fit a single regression (r = 0.94, n = 14) when plotted against net CO2 exchange suggesting that either methanogenesis in Everglades marl sediments is fueled by root exudation below ground, or that factors which enhance photosynthetic production and plant growth are also correlated with methane production and flux in this oligotrophic environment. The data presented are the first to show a direct relationship between spatial variability in plant biomass, net ecosystem production, and methane emission in a natural wetland.