OPP 0435870 (R. RHEW) PROJECT FINDINGS – FINAL REPORT

Overview

Our SNACS research project, “ gas exchange in northern Alaskan coastal ecosystems”, sought to determine the relative importance of the Alaskan Arctic tundra in the atmospheric budgets of , especially methyl chloride (CH3Cl), methyl bromide (CH3Br), methyl iodide (CH3I), and chloroform (CHCl3). CH3Cl and CH3Br are the chief carriers of natural and to the stratosphere, where they catalyze the destruction of stratospheric . 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 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 measurements and laboratory studies. Along the way, we also made several advances in the understanding of (CH4) biogeochemistry on the tundra and in tundra lakes, which is critical given the potential feedback of this 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 / 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.

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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).

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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)

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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).

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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).

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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 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).

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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. While methyl halide fluxes are dominated by consumption in the aerobic active layer, methane fluxes are dominated by production under anaerobic conditions, with strong influences by plant ecology. Methane biogeochemistry research was led by collaborator Prof. J. von Fischer (Colorado State University), who joined the project in 2006. Using a new laser-based instrument to measure methane and carbon dioxide fluxes in real-time, we were able to make higher intensity CH4 flux measurements on the Arctic tundra than previously possible.

Methane emission and ecosystem respiration rates were measured at 163 points across the Barrow Environmental Observatory in July 2007, along with measurements of soil and vegetation properties. While ecosystem respiration showed a normal distribution, CH4 emissions were characterized by a log-normal distribution (Fig. 7).

Figure 7. CH4 and CO2 fluxes from the Arctic coastal tundra near Barrow in July 2007. (Figure from von Fischer & Rhew presentation at 2008 American Geophysical Union meeting, San Francisco, CA).

The greatest emissions were from tall, flooded Carex aquatilis sites, which disproportionately affected the overall flux. Thus, we conducted a follow-up study in August 2008 that focused on these high producing sites, hypothesizing that places with more primary production may provide more labile carbon to the anaerobic zones to produce more methane. Vegetation at sites were clipped to measure aboveground biomass, and both clear and opaque chambers were used to quantify gross primary production (GPP) and ecosystem respiration (ER).

Dark chamber methane emission rates were strongly correlated at a nearly 1:1 ratio to light chamber methane emission rates. This demonstrated the viability of using dark chambers, as conducted in the previous year.

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50% of the variance was explained by maximum vegetation height, while very little was explained by total biomass or GPP (Fig. 8). Small amounts of variance were explained by ER or % Carex coverage. This intriguing relationship is currently unexplained but points to a new direction of research: how above-ground plant height may correspond with root depths into the anaerobic methane producing zones.

Figure 8. CH4 fluxes in relation to vegetation height on Arctic coastal tundra near Barrow in August 2008. (Figure from von Fischer & Rhew presentation at 2008 American Geophysical Union meeting, San Francisco, CA).

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IV. Methane bubbling from tundra lakes: how fast and how much following thermokarst erosion?

Ebullition (bubbling) is a significant CH4 transport mechanism in Arctic lakes, but it is highly variable, both spatially and temporally, illustrating the need for new methods to quantify CH4 efflux from Arctic lakes on the landscape scale. Ebullition may be related to labile organic matter input. One lake we studied near Toolik Lake during a pilot project in 2006 showed CH4 emissions dominated by ebullition, which were spatially co-located with organic matter input from a stream and slumping of the lake shore. From this lake and from Cake Eater Lake in Barrow, we determined the carbon stable isotopic composition of CH4 of emitted bubbles was –69.2‰ for Cake Eater Lake in Barrow and –68.7‰ for Fog 4. Thus, acetate fermentation and CO2 reduction pathways both appear to be significant contributors to the CH4 production in lakes.

To better understand methane ebullition, experiments were necessary under controlled conditions. Hence, we designed, built and deployed incubation chambers that simulate the addition of different tundra layers to the bottom of Cake Eater Lake. These incubations chambers were placed in July 2007 and gas sampling continued in the non-frozen months through August 2008. Our results show large emissions of gases after only a week of incubation, but primarily from the active layer (Fig. 9). Meanwhile, the seasonally frozen active layer, the permafrost, and the control chamber incubations released orders of magnitude less. This trend continued through the summer of 2008.

1000.0 1 2 Active layer

100.0 3 P+C Ctrl 10.0 volume (m l) . 4

1.0

0.1

0.0 Cumulative CH 8/8 8/1 9/5 8/29 9/12 7/25 9/19 7/18 9/26 8/22 8/15

Figure 9. Cumulative CH4 volumes from different incubation chambers from Cake Eater Lake in 2007 until lake freeze-up in the end of September. (From Mazéas et al., in prep.)

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V. Additional research results

Our research provided several other benefits, including interesting side project results and professional development, which are described below.

A. Role of halocarbons during the spring ozone depletion events During the first LEADX experiment in April 2005, we collected several air samples from over an ice lead and over frozen tundra to evaluate concentrations of halocarbons. Bromoform, a compound that might be responsible for initiating bromine explosions, appears to be lower in concentration over a fresh ice lead than over the open snow-covered tundra. This lends further support to the idea that bromoform does not initiate the bromine explosion/ ozone depletion events. The presence of relatively high bromoform over the open tundra is unexplained. Other halocarbon concentrations, including those of the methyl halides, over the ice lead were not unusually high. Previous reports of high methyl bromide concentrations coinciding with low ozone concentrations were not observed here.

B. Results of Coastal Erosion on methane emissions Coastal erosion is an important mechanism on the Barrow peninsula, exposing carbon- rich sediments and depositing them into the ocean. We conducted a pilot outing to test the potential effect that this would have on enhancing methane emissions. We found that while recently eroded coastal sites emit methane on exposed surfaces, the magnitudes do not appear to be larger than typical saturated tundra soils.

C. Professional development of early career faculty members, post-doctoral researchers, graduate students, undergraduates and high school students This research supported the education and training of 3 graduate students, 5 undergraduates, and several high school students through outreach efforts. Two of the undergraduates are now in Ph.D. programs in earth/environmental sciences (Johnny Garcia at UC Davis and Alyssa Atwood at University of Washington). This research also supported the training and advancement of 2 post-doctoral researchers (Dr. Yit Arn Teh is now on the faculty of the University of St. Andrews in Scotland). Finally, it should be noted that this SNACS grant was critically important in the professional development of two early career faculty members: Prof. Robert Rhew and Prof. Joseph von Fischer. As trace gas biogeochemists with much collective expertise in temperate and tropical ecosystems, we believe that our interaction with the Arctic science research community has been a very productive and mutually beneficial experience.

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