Alyssa R. AtwoodUC Berkeley, Geography Dept. · Georgia Tech, Earth and Atmos. Sci.
UC Berkeley, Geography Dept.
Georgia Tech, Earth and Atmos. Sci.
Skills and Expertise
Research Items (19)
We present a 3000-yr rainfall reconstruction from the Galápagos Islands that is based on paired biomarker records from the sediment of El Junco Lake. Located in the eastern equatorial Pacific, the climate of the Galápagos Islands is governed by movements of the Intertropical Convergence Zone (ITCZ) and the El Niño-Southern Oscillation (ENSO). We use a novel method for reconstructing past ENSO- and ITCZ-related rainfall changes through analysis of molecular and isotopic biomarker records representing several types of plants and algae that grow under differing climatic conditions. We propose that δD values of dinosterol, a sterol produced by dinoflagellates, record changes in mean rainfall in El Junco Lake, while δD values of C34 botryococcene, a hydrocarbon unique to the green alga Botryococcus braunii, record changes in rainfall associated with moderate-to-strong El Niño events. We use these proxies to infer changes in mean rainfall and El Niño-related rainfall over the past 3000 yr. During periods in which the inferred change in El Niño-related rainfall opposed the change in mean rainfall, we infer changes in the amount of ITCZ-related rainfall. Simulations with an idealized isotope hydrology model of El Junco Lake help illustrate the interpretation of these proxy reconstructions. Opposing changes in El Niño- and ITCZ-related rainfall appear to account for several of the largest inferred hydrologic changes in El Junco Lake. We propose that these reconstructions can be used to infer changes in frequency and/or intensity of El Niño events and changes in the position of the ITCZ in the eastern equatorial Pacific over the past 3000 yr. Comparison with El Junco Lake sediment grain size records indicates general agreement of inferred rainfall changes over the late Holocene.
A variety of lipid biomarkers were identified in sediments from El Junco Lake, Galápagos and their sources investigated for potential use in paleoclimate applications. A series of unusual sterols was also found, including 4α-methylgorgostanol, reported in only four species of dinoflagellates to date. We also tentatively assigned 22,23-methylene-4α-methyl-24-ethylcholest-5-en-3β-ol, the mass spectrum of which matched a sterol found in resting cysts of the dinoflagellate Peridinium umbonatum. In addition, we identified the novel sterol 4α,22,23,24-tetramethyl-5α-cholest-22E-en-3β-ol. Based on the unique sterol distribution, we hypothesize that a dinoflagellate from the genus Peridinium was the primary source of dinosterol and the novel sterols throughout the sediment record. The source specificity and abundance throughout the 3.7 m of recovered sediment make dinosterol an excellent target for hydrogen isotope analysis for use as a paleohydrological proxy in future studies. The abundant C30 and C32 1,20-diols and keto-ols, C29 9,10-diol and C29 1,9,10-triol likely derive from the ferns Azolla microphylla and Cyathea weatherbyana, while sources of the C30 1,16-diol and keto-ol, C32 1,18-diol and keto-ol, and the C30–C32n-alken-1-ols are likely limited to aquatic microalgae. Due to their source specificity, these diol, triol, keto-ol, and n-alkenol biomarkers present further tools for studying past environmental and climatic change.
We present two new methods for purifying dinosterol (4α,23,24-trimethyl-5α-cholest-22E-en-3β-ol) from sediments for the purpose of hydrogen isotope analysis via gas chromatography–isotope ratio mass spectrometry (GC–IRMS). The first method uses reversed phase-high performance liquid chromatography (RP-HPLC) to purify dinosterol from structurally similar 4α-methyl sterols that co-elute on GC analysis. Dinosterol purified from sedimentary sterol/alcohol fractions using this RP-HPLC method demonstrated an average yield of 80%. A very large isotope effect was observed during RP-HPLC purification, with a 560‰ range in δD value between the first and last 5% of a cholesterol standard, which is four times that during normal phase-HPLC (NP-HPLC) purification. However, we show that dinosterol recombined from 3–4 min of eluent during RP-HPLC purification yields highly reproducible and unbiased isotope values. Due to a larger isotope effect and lower sterol recovery during RP-HPLC, NP-HPLC purification is recommended for samples that do not contain 4α-methyl sterols that co-elute with dinosterol during GC. However, for samples that contain a variety of 4α-methyl sterols, RP-HPLC is more likely to yield baseline resolution of dinosterol. In the second method presented, RP-HPLC purification is preceded by NP-HPLC purification. Using this two step procedure, baseline resolution between dinosterol and all other compounds present was achieved for all samples with an average yield of 60% and, in many cases, dinosterol was purified from all other sedimentary lipids. For samples that contain a variety of 4α-methyl sterols and sitostanol concentration > 2× that of dinosterol, the two step purification method is recommended, as neither NP-HPLC or RP-HPLC alone is likely to yield baseline resolution of dinosterol.
Tropical rainfall patterns directly influence the subsistence lifestyle of more than a billion people and indirectly influence climate globally. Their seasonal changes are associated with the position of the Intertropical Convergence Zone (ITCZ) where deep convection causes heavy rainfall near 10°N in boreal summer and 3°N in winter. Dynamic controls on the ITCZ position are debated but paleoclimate evidence on and near continental Asia, Africa and the Americas suggests it has shifted substantially during the last millennium, reaching its southern-most position some time during the Little Ice Age (LIA, 1400-1850 AD). However, without records from the meteorological core of the ITCZ in the Pacific Ocean quantitative constraints on its position are lacking. We demonstrate with microbiological, molecular and hydrogen isotope evidence from lake, lagoon and bog sediments from islands across the tropical Pacific Ocean that the Pacific ITCZ was south of its modern position for most of the last millennium, by as much as 500 km during the LIA. A colder Northern Hemisphere at that time, possibly resulting from lower solar irradiance, may have driven the ITCZ south, implying small changes in Earth's radiation budget can profoundly impact tropical rainfall.
Water availability is fundamental to societies and ecosystems, but our understanding of variations in hydroclimate (including extreme events, flooding, and decadal periods of drought) is limited because of a paucity of modern instrumental observations that are distributed unevenly across the globe and only span parts of the 20th and 21st centuries. Such data coverage is insufficient for characterizing hydroclimate and its associated dynamics because of its multidecadal to centennial variability and highly regionalized spatial signature. High-resolution (seasonal to decadal) hydroclimatic proxies that span all or parts of the Common Era (CE) and paleoclimate simulations from climate models are therefore important tools for augmenting our understanding of hydroclimate variability. In particular, the comparison of the two sources of information is critical for addressing the uncertainties and limitations of both while enriching each of their interpretations. We review the principal proxy data available for hydroclimatic reconstructions over the CE and highlight the contemporary understanding of how these proxies are interpreted as hydroclimate indicators. We also review the available last-millennium simulations from fully coupled climate models and discuss several outstanding challenges associated with simulating hydroclimate variability and change over the CE. A specific review of simulated hydroclimatic changes forced by volcanic events is provided, as is a discussion of expected improvements in estimated radiative forcings, models, and their implementation in the future. Our review of hydroclimatic proxies and last-millennium model simulations is used as the basis for articulating a variety of considerations and best practices for how to perform proxy-model comparisons of CE hydroclimate. This discussion provides a framework for how best to evaluate hydroclimate variability and its associated dynamics using these comparisons and how they can better inform interpretations of both proxy data and model simulations. We subsequently explore means of using proxy-model comparisons to better constrain and characterize future hydroclimate risks. This is explored specifically in the context of several examples that demonstrate how proxy-model comparisons can be used to quantitatively constrain future hydroclimatic risks as estimated from climate model projections.
Large multi-decadal fluctuations of El Niño-Southern Oscillation (ENSO) variability simulated in a 4000-year pre-industrial control run of GFDL CM2.1 have received considerable attention due to implications for constraining the causes of past and future changes in ENSO. We evaluated the mechanisms of this low-frequency ENSO modulation through analysis of the extreme epochs of CM2.1 as well as through the use of a linearized intermediate-complexity model of the tropical Pacific, which produces reasonable emulations of observed ENSO variability. We demonstrate that the low-frequency ENSO modulation can be represented by the simplest model of a linear, stationary process, even in the highly nonlinear CM2.1. These results indicate that CM2.1’s ENSO modulation is driven by transient processes that operate at interannual or shorter time scales. Nonlinearities and/or multiplicative noise in CM2.1 likely exaggerate the ENSO modulation by contributing to the overly active ENSO variability. In contrast, simulations with the linear model suggest that intrinsically-generated tropical Pacific decadal mean state changes do not contribute to the extreme-ENSO epochs in CM2.1. Rather, these decadal mean state changes actually serve to damp the intrinsically-generated ENSO modulation, primarily by stabilizing the ENSO mode during strong-ENSO epochs. Like most coupled General Circulation Models, CM2.1 suffers from large biases in its ENSO simulation, including ENSO variance that is nearly twice that seen in the last 50 years of observations. We find that CM2.1’s overly strong ENSO variance directly contributes to its strong multi-decadal modulation through broadening the distribution of epochal variance, which increases like the square of the long-term variance. These results suggest that the true spectrum of unforced ENSO modulation is likely substantially narrower than that in CM2.1. However, relative changes in ENSO modulation are similar between CM2.1, the linear model tuned to CM2.1, and the linear model tuned to observations, underscoring previous findings that relative changes in ENSO variance can robustly be compared across models and observations.
Water availability is fundamental to societies and ecosystems, but our understanding of variations in hydroclimate (including extreme events, flooding, and decadal periods of drought) is limited because of a paucity of modern instrumental observations that are distributed unevenly across the globe and only span parts of the 20th and 21st centuries. Such data coverage is insufficient for characterizing hydroclimate and its associated dynamics because of its multidecadal-to-centennial variability and highly regionalized spatial signature. High-resolution (seasonal to decadal) hydroclimatic proxies that span all or parts of the Common Era (CE) and paleoclimate model simulations are therefore important tools for augmenting our understanding of hydroclimate variability. In particular, the comparison of the two sources of information is critical for addressing the uncertainties and limitations of both, while enriching each of their interpretations. We review the principal proxy data available for hydroclimatic reconstructions over the CE and highlight contemporary understanding of how these proxies are interpreted as hydroclimate indicators. We also review the available last-millennium simulations from fully-coupled climate models and discuss several outstanding challenges associated with simulating hydroclimate variability and change over the CE. A specific review of simulated hydroclimatic changes forced by volcanic events is provided, as well as a discussion of expected improvements in estimated forcings, models and their implementation in the future. Our review of hydroclimatic proxies and last-millennium model simulations is used as the basis for articulating a variety of considerations and best practices for how to perform proxy-model comparisons of CE hydroclimate. This discussion provides a framework for how best to evaluate hydroclimate variability and its associated dynamics using these comparisons, as well as how they can better inform interpretations of both proxy data and model simulations. We subsequently explore means of using proxy-model comparisons to better constrain and characterize future hydroclimate risks. This is explored specifically in the context of several examples that demonstrate how proxy-model comparisons can be used to quantitatively constrain future hydroclimatic risks as estimated from climate model projections.
The role of radiative forcings and climate feedbacks on global cooling over the last millennium is quantified in the CMIP5–PMIP3 transient climate model simulations. Changes in the global energy budget over the last millennium are decomposed into contributions from radiative forcings and climate feedbacks through the use of the approximate partial radiative perturbation method and radiative kernels. Global cooling occurs circa 1200–1850 CE in the multimodel ensemble mean with pronounced minima corresponding with volcanically active periods that are outside the range of natural variability. Analysis of the global energy budget during the last millennium indicates that Little Ice Age (LIA; 1600–1850 CE) cooling is largely driven by volcanic forcing (comprising an average of 65% of the total forcing among models), while contributions due to changes in land use (13%), greenhouse gas concentrations (12%), and insolation (10%) are substantially lower. The com- bination of these forcings directly contributes to 47% of the global cooling during the LIA, while the re- mainder of the cooling arises from the sum of the climate feedbacks. The dominant positive feedback is the water vapor feedback, which contributes 29% of the global cooling. Additional positive feedbacks include the surface albedo feedback (which contributes 7% of the global cooling and arises owing to high-latitude sea ice expansion and increased snow cover) and the lapse rate feedback (which contributes an additional 7% of the global cooling and arises owing to greater cooling near the surface than aloft in the middle and high latitudes).
The El Nino-Southern Oscillation (ENSO) drives the single largest interannual climate variability globally. However, how and why ENSO has changed in the past and how it may change in the future in association with changes in the mean state of the climate system are poorly understood. Climate modeling studies with General Circulation Models (GCMs) demonstrate that ENSO variability increases in response to a weakened AMOC associated with a large freshwater perturbation in the North Atlantic. We propose that the response of ENSO is highly sensitive to the tropical Pacific mean state biases present in GCMs and that the presence or absence of tropical Pacific mean state biases, as well as the magnitude of the AMOC reduction can lead to a wide range of ENSO responses. We demonstrate these principles using a linearized coupled atmosphere-ocean model of the tropical Pacific in addition to a series of fully coupled GCM simulations with the Community Earth System Model (CESM) in which freshwater perturbations were imposed to the North Atlantic. An identical set of experiments was also performed with CESM, but with the addition of heat flux corrections to reduce mean state biases in the tropics before the freshwater forcing was imposed. The processes responsible for the changes in ENSO variability due to the freshwater perturbation (with and without biases in the starting mean state) are quantified using an offline intermediate model of the tropical Pacific. Understanding the sensitivity of ENSO to such mean state changes and to biases in the mean state of the tropical Pacific in GCMs is critical for constraining past and future ENSO variability.
Well-preserved sediment from closed water bodies of atolls such as Lib Pond are rare opportunities to reconstruct the past regional climate, which pieced together across a latitude and longitude range identify the range of movement patterns of wider scale climate phenomena such as the Intertropical Convergence Zone (ITCZ) and El Niño Southern Oscillation (ENSO). We conducted the first physico-chemical survey of Lib Pond, a shallow, closed-water saline lake located on remote and difficult to access Lib Island in the Marshall Islands at 8° 18' 48.99″ N, 167 22' 51.90″ E in the Pacific Ocean, in July 2009. We performed a bathymetric survey, recorded salinity, dissolved oxygen, pH, and temperature profiles, monitored the tidal variability, and conducted a vegetation survey surrounding the lake. From bathymetric data we calculated the lake volume, which we used to estimate the lake's salt budget, and ultimately the residence time of water in the lake basin. We took a series of sediment cores from the lake, cores which indicate Lib Island's changing environment and climate. Radiocarbon measurements determined sediment age, and reveal significant mixing over the last 2 ka of deposition. We conclude that prior to 3 ka, Lib Island was an atoll with a central lagoon connected to the open ocean, which was then closed off from the open ocean to form the brackish system that exists today. We predict that the sediment accumulation in Lib Pond evident today will continue. As seawater is inhibited from exchanging with fresh water, Lib Pond will become a shallower lake with increasingly fresh water.
The Little Ice Age (LIA) is broadly defined as a time of cool Northern Hemisphere temperatures ~ 1350 – 1850 AD. Paleoclimate data suggest that changes in tropical rainfall patterns, including a southward shift of the Intertropical Convergence Zone (ITCZ), also occurred during this time. We investigate LIA climate change in the latest Coupled Model Intercomparison Project (CMIP5) General Circulation Models (GCMs). By comparing last millennium simulations, which are driven by the paleo record of natural and anthropogenic forcing, to unforced control runs of eight GCMs, we investigate whether LIA conditions occur in the CMIP5 last millennium runs and whether they can be attributed to external forcing. While the models generally simulate anomalous Northern Hemisphere cooling and expanded Arctic sea ice beginning ~ 1250 AD and progressing to 1850 AD, there is little agreement on the interhemispheric temperature gradient and on the behavior of the ITCZ during the last millennium. As tropical precipitation shifts are strongly anti-correlated with cross equatorial atmospheric energy transport in observations and in GCM simulations of global warming, we perform an energy budget analysis to evaluate the relationship between the position of the ITCZ and cross equatorial atmospheric energy transport in the last millennium simulations. Analysis of the energy budget also enables the changes in surface temperature, cross equatorial atmospheric energy transport, and position of the ITCZ to be attributed to the individual components of radiative forcing and climate feedbacks. This attribution analysis is used to investigate the range of temperature and ITCZ responses across models.
The climate dynamics of the tropical Pacific play a fundamental role in climate variability across the globe. Rainfall patterns in the tropical Pacific are largely governed by the position of the Intertropical Convergence Zone (ITCZ) and the El Niño/Southern Oscillation (ENSO). Robust reconstructions of past changes in these two climate phenomena have been elusive, as few paleoclimate records come from within the core regions of the ITCZ and ENSO but outside the complicating influence of continents. We present a Holocene rainfall record from the Galápagos Islands that is based on paired biomarker records from the sediment of El Junco Lake. Rainfall in this area is highly sensitive to both movements of the ITCZ and ENSO and is little affected by monsoons and other regional climate processes, allowing inference of changes in these large-scale climate phenomena from local rainfall patterns. We distinguish between ITCZ- and ENSO-related rainfall changes through analysis of biomarker distributions and hydrogen isotopes from several types of plants and algae that are thought to grow under differing climate conditions. These biomarker records show significant oscillations between wet and dry conditions throughout the Holocene and no evidence for multi-millennial scale changes in ENSO or the position of the ITCZ. The early Holocene (9 – 7 ka) is characterized by multi-centennial scale fluctuations in the position of the ITCZ and El Niño activity, while the mid-Holocene (6 – 4 ka) is characterized by predominately wetter conditions than modern and intermittent periods of a southward shifted ITCZ and weaker El Niño activity. The largest rainfall changes appear to have occurred over the last millennium even though the primary drivers of Earth’s climate, including orbital parameters and continental ice sheet extent, did not change significantly during this time. Changes in the tropical Pacific inferred from our records are concomitant with global-scale changes of the climate system and may have a common origin. In particular, periods of southward-displaced ITCZ and reduced El Niño activity appear to correspond to periods of increased North Atlantic drift ice and weakened monsoons, suggesting a possible global connection of these tropical climate changes.
Chloroform (trichloromethane, CHCl3) is the second largest carrier of natural chlorine in the troposphere after methyl chloride, contributing to the reactive chlorine burden in the troposphere and to ozone destruction in the stratosphere. Here we report CHCl3 flux measurements from coastal and interior tundra sites in northern Alaska, showing that the Arctic tundra can contribute substantial amounts of CHCl3 to the atmosphere. Emissions were measured during the 2005 and 2006 growing seasons over a range of vegetation types and hydrologic conditions, from wet sedge coastal to upland tussock tundra. Overall emissions averaged 45 nmol m-2 d-1, but fluxes were highly variable, ranging from
The Arctic tundra has been shown to be a potentially significant regional sink for methyl chloride (CH3Cl) and methyl bromide (CH3Br), although prior field studies were spatially and temporally limited, and did not include gross flux measurements. Here we compare net and gross CH3Cl and CH3Br fluxes in the northern coastal plain and continental interior. As expected, both regions were net sinks for CH3Cl and CH3Br. Gross uptake rates (−793 nmol CH3Cl m−2 day−1 and −20.3 nmol CH3Br m−2 day−1) were 20–240% greater than net fluxes, suggesting that the Arctic is an even greater sink than previously believed. Hydrology was the principal regulator of methyl halide flux, with an overall trend towards increasing methyl halide uptake with decreasing soil moisture. Water table depth was one of the best predictors of net and gross uptake, with uptake increasing proportionately with water table depth. In drier areas, gross uptake was very high, averaging −1201 nmol CH3Cl m−2 day−1 and −34.9 nmol CH3Br m−2 day−1; in flooded areas, gross uptake was significantly lower, averaging −61 nmol CH3Cl m−2 day−1 and −2.3 nmol CH3Br m−2 day−1. Net and gross uptake was greater in the continental interior than in the northern coastal plain, presumably due to drier inland conditions. Within certain microtopographic features (low- and high-centered polygons), uptake rates were positively correlated with soil temperature, indicating that temperature played a secondary role in methyl halide uptake. Incubations suggested that the inverse relationship between water content and methyl halide uptake was the result of mass transfer limitation in saturated soils, rather than because of reduced microbial activity under anaerobic conditions. These findings have potential regional significance, as the Arctic is expected to become warmer and drier due to anthropogenic climate forcing, potentially enhancing the Arctic sink for CH3Cl and CH3Br.
Temperate grasslands are believed to be a globally significant sink for methyl bromide (CH3Br) and perhaps methyl chloride (CH3Cl), compounds which lead to stratospheric ozone destruction. Fluxes of these compounds were measured at Konza Prairie, a tallgrass prairie in the Flint Hills of Kansas, during June 2006 and August 2007. A stable isotope tracer technique was used to distinguish between simultaneous production and oxidation processes, allowing the first gross flux measurements of CH3Cl and CH3Br from a tallgrass prairie. Observed gross uptake rates of CH3Cl and CH3Br were similar to what we previously observed from the shortgrass steppe in Colorado and annual grasslands in California, but much lower than reported fluxes from a grassland in northeastern North America. A water manipulation experiment was performed both under controlled laboratory conditions, as well as in the field, demonstrating that uptake rates of both CH3Cl and CH3Br were strongly affected by soil moisture. On the production side, new sources of methyl halides were identified in association with certain plant species. Fluxes of these halogenated trace gases were compared to environmental variables, such as air temperature and volumetric water content. Net fluxes of methyl iodide (CH3I), carbon tetrachloride (CCl4), and other halogenated volatile organic compounds (HVOCs), were also measured.
Arctic tundra may be a significant regional sink for CH3Cl and CH3Br, although the magnitude of this sink is poorly constrained because of the limited extent of field measurements. We sought to close this gap in knowledge by comparing gross and net fluxes of CH3Cl and CH3Br in the northern coastal plain and the continental interior. Net and gross fluxes were deconvoluted using stable isotope tracers. Net flux measurements indicated that both regions were net atmospheric sinks for CH3Cl and CH3Br, averaging -590 ± 107 nmol CH3Cl m-2 d-1 and -11.3 ± 2.6 nmol CH3Br m-2 d- 1. Gross uptake rates averaged -793 ± 128 nmol CH3Cl m-2 d-1 and -20.3 ± 2.9 nmol CH3Br m-2 d-1. Hydrology strongly influenced CH3Cl and CH3Br uptake, which varied as a function of hydrologic regime, soil volumetric water content and water table depth. The overall trend showed increasing CH3Cl and CH3Br uptake with decreasing soil moisture. Laboratory incubations suggested that this inverse relationship was the result of mass transfer limitation in wetter soils, rather than because of reduced microbial consumption under anaerobic conditions. Water table depth was one of the best predictors of net and gross uptake, with uptake rates increasing proportionately with water table depth. This finding has potential regional significance as changes in water table depth may alter the Arctic tundra sink for CH3Cl and CH3Br. We also observed CH3Cl and CH3Br uptake under anaerobic conditions suggesting that freshwater anaerobes may be a previously unidentified sink for methyl halides.
Temperate grasslands are considered to be a significant sink for CH3Br, although large uncertainties exist about the magnitude of this sink because of a paucity of field measurements. Here, we report the results of a combined field and laboratory study that investigated the effects of water, temperature, and plant community composition on CH3Cl and CH3Br fluxes in a semiarid temperate grassland. A novel stable isotope tracer technique was also employed to deconvolute simultaneous production and oxidation of CH3Cl and CH3Br. Net and gross fluxes were measured from different landforms (ridges, floodplains) and cover types (grass-dominated, shrub-dominated) to capture a representative range of hydrologic regimes, temperatures, and plant communities. In field experiments, net CH3Cl and CH3Br uptake was observed at all grass-dominated sites (−400±77 nmol CH3Cl m−2 day−1 and −3.4±0.9 nmol CH3Br m−2 day−1), while net CH3Cl emission (439±58 nmol CH3Cl m−2 day−1) was observed at sites dominated by the shrub Atriplex canescens, indicating that this plant is a strong CH3Cl producer. Gross CH3Cl and CH3Br oxidation were comparable with estimates from other dryland ecosystems (507±115 nmol CH3Cl m−2 day−1 and 9.1±2.2 nmol CH3Br m−2 day−1), although CH3Br oxidation rates were at least five times lower than those observed in more mesic temperate grasslands. We suggest that estimates of the temperate grassland CH3Br sink should be reduced by ≥19% (≥1.8 Gg yr−1) to account for the weaker sink strength of semiarid environments. Identification of A. canescens as a ‘new’ CH3Cl source may have important ramifications for the global atmospheric budget of CH3Cl, given the global distribution of this plant and its congeners and their widespread presence in many dryland ecosystems. Laboratory experiments revealed that soil water was the chief regulator of CH3Cl and CH3Br oxidation, while temperature had no observed effect between 14 and 26 °C. Oxidation rates rose most rapidly between 0.4% and 5% volumetric water content, suggesting that methyl halide-oxidizing bacteria respond strongly to small inputs of water under the very driest conditions. Soil drying and rewetting experiments did not appear to affect the oxidation of CH3Cl and CH3Br by soil microorganisms, which are presumably adapted to frequent wet/dry cycles.
Methane efflux from two lakes and several small ponds located on the North Slope of Alaska was estimated from measurements of ebullition and diffusion. This pilot study was performed over a several-day period during the months of July and August in 2006. Ebullition was measured using bubble traps constructed from 15-inch diameter, inverted funnels. Diffusion was estimated from measurements of dissolved CH4 profiles and use of a boundary layer diffusion model. Stable carbon isotope measurements were performed on CH4 in the collected bubble and water samples to assess methane production and oxidation in the lake sediment and water column. Transects were established on Cake Eater Lake, near Barrow, and on Fog 4 Lake, near the Toolik Lake Research Station. Five small ponds situated on the tundra surrounding Cake Eater Lake were also sampled for dissolved CH4 content. Cake Eater Lake is a large, flat-bottomed, thaw lake with a maximum depth of 1.3 meters along the transect. Fog 4 is a small, potential kettle lake with a maximum depth of approximately 5 meters. Bubble samples of 7 ml or more were found at 8.3% of the sites in Cake Eater Lake over a period of 24 hours with an average CH4 content of 40%. At Fog 4 Lake, 67% of the study sites produced greater than 7 ml of bubbles over 24 hours with an average CH4 content of approximately 55%. Diffusion appears to be a significant contributor to the total CH4 flux in the lakes and in the small ponds. The ponds appear to release more CH4 per unit area through diffusion than the nearby Cake Eater Lake.