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Project Findings – Final Report OPP 0435870 (R. RHEW) PROJECT FINDINGS – FINAL REPORT Overview Our SNACS research project, “Halomethane gas exchange in northern Alaskan coastal ecosystems”, sought to determine the relative importance of the Alaskan Arctic tundra in the atmospheric budgets of halomethanes, especially methyl chloride (CH3Cl), methyl bromide (CH3Br), methyl iodide (CH3I), and chloroform (CHCl3). CH3Cl and CH3Br are the chief carriers of natural chlorine and bromine to the stratosphere, where they catalyze the destruction of stratospheric ozone. CH3Br is also a broad-spectrum agricultural and structural fumigant that is subject to international regulation due to its ozone depleting potential. A potential replacement for CH3Br is methyl iodide because of its similar efficacy against agricultural pests and its rapid photolysis in the atmosphere. In the lower atmosphere, CH3I is believed to be the dominant form of organic iodine and may influence aerosol formation and ozone loss in the boundary layer. Chloroform accounts for 1-2% of the natural chlorine load to the stratosphere and may also be important in understanding the cycling of other atmospheric constituents. We proposed to answer three important questions: 1) Are arctic coastal terrestrial ecosystems significant sources or sinks of atmospheric methyl halides or chloroform? 2) What are the environmental and biological controls on their fluxes? 3) Based on the identified factors controlling the fluxes of these compounds, how would climatic changes in the Arctic be expected to influence the overall fluxes? Through our research activities at Barrow and Toolik Lake between 2004-2008, we successfully addressed these questions through a unique dataset of flux measurements and laboratory studies. Along the way, we also made several advances in the understanding of methane (CH4) biogeochemistry on the tundra and in tundra lakes, which is critical given the potential feedback of this greenhouse gas to climate warming. Our research not only provided insights into the sources and sinks of trace gases in this changing ecosystem, but also points the direction for future important lines of research. To summarize our findings below, we discovered that the Alaskan Arctic tundra: 1) is a regionally important sink (not a source!) for CH3Cl and CH3Br, and that uptake rates are primarily controlled by hydrologic factors, with drier tundra showing faster uptake rates; 2) is a minor source of CH3I and is not likely to contribute to the springtime ozone depletion/ mercury deposition events; 3) emits chloroform (CHCl3) at surprisingly high rates, such that tundra may be a globally significant source of this compound; 4) emits the greenhouse gas methane (CH4) with a lognormal distribution, and that biology, hydrology, and geomorphology all influence emission rates; 5) can cause CH4 bubbling in Arctic lakes through the input of active layer (i.e. thermokarst erosion along shorelines), and that this source of organic matter is more important than thawed permafrost, at least on short time scales. This research has led to publications in Journal of Geophysical Research- Biogeosciences; Geophysical Research Letters and Global Change Biology. We have presented our research in 10 presentations at international conferences, including the American Geophysical Union meeting (2005, 2006, 2007, 2008), Integrated Land Ecosystem-Atmosphere Processes Study (iLEAPS) conference (2006), and the Ninth International Conference on Permafrost (2008). We expect 3 more publications to be forthcoming based on field data still undergoing analysis. Pg. 1 of 10 OPP 0435870 (R. RHEW) PROJECT FINDINGS – FINAL REPORT I. Tundra fluxes of methyl halides Coastal ecosystems in tropical and temperate latitudes are large sources of ozone- depleting methyl halides, but we discovered that the high latitude coastal tundra near Barrow, Alaska consumes CH3Br and CH3Cl at surprisingly high rates during the growing season. CH3Br and CH3Cl fluxes vary significantly with hydrologic conditions, with progressively higher net uptake rates observed with decreasing soil saturation (Fig. 1). In other words: the wetter the site, the smaller the uptake rate; the drier the site, the larger the uptake rate. Uptake rates of CH3Cl and CH3Br are therefore related to CH4 emissions, and hydrologic shifts in tundra will therefore lead to predictable patterns of methyl halide uptake. Even though the growing season at this high latitude site is brief, our measurements suggest that the Alaskan Arctic tundra is a regionally important net sink for these methyl halides (Rhew et al., 2007). Our measurements suggest that the seasonal uptake of these compounds may account for 10-20% of the seasonality observed in their concentrations. CH3I was emitted at all tundra sites, but emission rates were relatively small. Figure 1. Net fluxes of (a,b) CH3Cl, (c,d) CH3Br, (e,f) CH3I, and (g,h) CH4 in (left) June and (right) August 2005 field campaigns sorted by hydrologic regime (from Rhew et al., 2007). Pg. 2 of 10 OPP 0435870 (R. RHEW) PROJECT FINDINGS – FINAL REPORT After our 2005 field work, it was important to address three questions in 2006. First, are peak season (July) uptake rates even higher than our measured uptake rates in June and August? Second, are gross uptake rates much greater than net uptake rates (in other words, is there gross production at these sites, such that the gross uptake of methyl halides is even larger?). Third, how do uptake rates at drier inland tundra sites compare to the wet sedge tundra ecosystems of Barrow? Measurements from July 2006 showed larger net uptake rates than those measured in June and August of 2005 (Fig. 2). Applying a newly developed stable isotope tracer technique to separate net fluxes into gross production and consumption fluxes, we found that gross uptake rates were 20%-240% larger than the net uptake rates. (Teh et al., in press). We also compared uptake rates from coastal and interior tundra sites and found that uptake rates were even greater inland, presumably due to drier conditions. All three of these findings together strongly suggest that the Arctic tundra is an even greater sink than estimated in our earlier study. Figure 2. (a,b) Net fluxes, (c,d) gross production, and (e,f) gross uptake of CH3Cl and CH3Br for different hydrologic regimes, with data pooled from the Barrow Environmental Observatory and Toolik Lake (from Teh et al., in press) Pg. 3 of 10 OPP 0435870 (R. RHEW) PROJECT FINDINGS – FINAL REPORT In both of the above studies, we found that CH3Br and CH3Cl uptake fluxes are strongly correlated (Fig. 3). The CH3Cl: CH3Br molar uptake ratio is 44:1 to 49:1, similar to the ratios seen in other terrestrial ecosystems (shrublands, grasslands and boreal forests). This suggests that they are both taken up by similar, if not the same, mechanisms. Figure 3. (left) CH3Cl net fluxes regressed against CH3Br net fluxes for Barrow, 2005 (from Rhew et al., 2007). (right) gross CH3Cl uptake regressed against gross CH3Br uptake rates for Barrow and Toolik Lake, 2006, field data (from Teh et al., in press). Pg. 4 of 10 OPP 0435870 (R. RHEW) PROJECT FINDINGS – FINAL REPORT II. Tundra emissions of chloroform Chloroform (CHCl3) is the second largest carrier of natural chlorine in the troposphere after methyl chloride, contributing to the reactive chlorine burden the troposphere and to ozone destruction in the stratosphere. However, the major sources of this compound, especially natural terrestrial sources, are poorly characterized. The combined effort of 2005 and 2006 field measurements of CHCl3 from coastal and interior tundra sites show that the Arctic tundra can contribute substantial amounts of CHCl3 to the atmosphere. A rough extrapolation suggests that the tundra globally could account for 3.9 Gg CHCl3 per year, about 1.4% of the total estimated source to the atmosphere. Emission rates were widely variable, showing an approximately lognormal distribution (Fig. 4). Figure 4. Histograms showing the number of observed CHCl3 fluxes, binned by the range of values (BEO= Barrow Environmental Observatory; Toolik = Toolik Lake field site). The y-axis is on a natural logarithmic scale (from Rhew et al., 2008). CHCl3 fluxes did not show any patterns based on vegetation type or microtopography, but they did show significant differences based on hydrological regime. In particular, moist tundra showed a significantly higher mean flux than drained or dry tundra, with the flooded and wet tundra classifications in between (Fig. 5). Thus, fluxes did not follow a clear hydrologic gradient, as “moist tundra” was the intermediate soil moisture category where soils were saturated but water was not pooled at the surface. Figure 5. Box plots of tundra field CHCl3 flux measurements by hydrological regime. Uppercase letters indicate statistically significant differences between means of each hydrological regime (from Rhew et al., 2008). Pg. 5 of 10 OPP 0435870 (R. RHEW) PROJECT FINDINGS – FINAL REPORT Investigating the mechanisms of CHCl3 formation in the tundra is an exciting avenue for further research. This is because the formation of CHCl3 may be related to the chlorination of peat, which could be a reason why peat is recalcitrant to breakdown and hence an effective carbon sink. We conducted laboratory incubation experiments using cores of tundra peat and showed that emissions are not inhibited significantly under anaerobic conditions (Fig. 6). This was rather perplexing given that known biologic mechanisms were assumed to be aerobic. However, incubations that we have conducted since then suggest that the production of CHCl3 may in fact be abiotic. This would be a major step in our understanding of the biogeochemistry of this compound. Figure 6. Box plots of CHCl3 fluxes in laboratory soil core incubations by treatment (aerobic, anaerobic, flooded), including values from corresponding flux chambers (field) from which the soil cores were taken (from Rhew et al., 2008). Pg. 6 of 10 OPP 0435870 (R. RHEW) PROJECT FINDINGS – FINAL REPORT III. Methane fluxes from vegetated tundra Methyl halide and methane fluxes are both intricately related to hydrologic conditions on the tundra, but for very different reasons.
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