The Production And Consumption Of Halomethanes And Carbonyl Sulfide From Antarctic Tundra

Halomethanes,such as methyl chloride(CH3Cl),methyl bromide(CH3Br),chloroform(CHCl3),carbon tetrachloride(CCl4),are the main carriers of atmospheric chlorine and bromine,which can contribute halogen radicals to the stratosphere and catalyze ozone destruction.Carbonyl sulfide(COS)is the most abundant reduced sulfur compound in the atmosphere,which can produce sulfide aerosols to influence Earth's radiation balance.COS fluxes have also been used as a potential proxy to estimate ecosystem gross primary productivity(GPP).Over the years,a series of studies have been conducted over different marine and terrestrial ecosystems,to quantify their contributions to atmospheric halomethanes and COS,explore influencing factors and mechanisms.However,these studies are mainly confined to the mid-or low-latitudes,with very few studies at the high latitudes.Antarctic tundra is experiencing the most rapid warming globally,leading to the expansion of ice-free tundra.But no measurements on halomethanes and COS fluxes have ever been conducted over the 7.2×1010 m2 ice-free tundra in Antarctica.In addition,large numbers of penguins and seals populate on the Antarctic tundra,and significantly influence the biogeochemical cycles of halogen and sulfur though depositing excrements in their colonies,which may impact the exchange of volatile halocarbons and COS.Therefore,it is necessary to conduct such measurements to fill in the knowledge gap.In this dissertation,in situ static-chamber measurements of halomethanes and COS fluxes were conducted at normal upland tundra and sea animal(penguin,seal)colony tundra at Fildes Peninsula and Ardley Island in an Antarctic ecosystem to quantify their fluxes,analyze their influencing factors and evaluate their contributions to atmospheric compositions.Soil samples were also collected normal upland tundra,marsh tundra,penguin and seal colony tundra for laboratory-based incubations under different conditions,such as N2-anoxic,post-thermal sterilization,and freeze-thaw cycles(FTC),in order to get halomethanes and COS fluxes under different conditions,explore the production and degradation mechanisms,and get projected fluxes under global warming.The main results and conclusions included:(1)Methyl halides(CH3Cl and CH3Br)emissions and absorptions in coastal Antarctic tundraThe in situ observations showed that,normal upland tundra(UTS)was a source of CH3Cl due to the lichens and moss(46±11 nmol m-2 d-1),while penguin(-110±7 nmol m-2 d-1)and seal colony tundra(-52±2 nmol m-2 d-1)were CH3Cl sinks.Laboratory-based incubations showed that Antarctic tundra soil was a natural sink of CH3Cl and CH3Br,and the flux was-18.1?-2.8 pmol g-1 d-1 and-1.32?-0.24 pmol g-1 d-1,respectively.Upland tundra soil had higher absorption rates of CH3Cl and CH3Br in comparison to the soils in penguin and seal colonies and adjacent tundra.Penguins and seals pumped large amounts of excrements into their colony tundra,enhanced soil organic matter and halide contents,which facilitated the abiotic production of methyl halides and reduced the size of methyl halide sinks.Incubations under anoxic conditions and post thermal sterilization switched tundra soils from CH3C1 sinks to sources,suggesting that CH3Cl absorption was mediated by microbial metabolism,and the gross production was an abiotic process independent to oxygen.Overall,the uptake rates exceed the production rates,which make Antarctic tundra a sink of CH3Cl and CH3Br.The maximum methyl halide uptaking rates were observed from moist soil at 20%-35%GWC.Freeze-thaw cycle(FTC)incubations found that CH3Cl production rates attenuated in freeze periods and the uptaking rates increased in thaw periods over the cycles.Temperature-gradient incubation suggested that Antarctic warming will further amplify methyl halide sinks of Antarctic tundra.(2)Chloroform(CHCl3)emissions from coastal Antarctic tundraAntarctic tundra was a natural source of CHCl3(35 ±27 nmol m-2 d-1).Significantly different CHCl3 emission rates were observed across different tundra.In general,penguin colony tundra had higher CHCl3 emission rates(66±20 nmol m-2 d-1)in comparison to animal-free normal tundra.Laboratory-based incubations also showed that the highest CHCl3 emission rates(3.5-7.5 pmol g-1 d-1)occurred in tundra soils on the island populated with penguins.Manipulated incubations(anoxic incubation and post thermal sterilization)suggested that CHCl3 production was catalyzed by microbes(chloroperoxidase)and required oxygen.CHCl3 emission rates were also positively correlated with soil moisture,total organic carbon(TOC)and chloride contents(p<0.05),but negatively correlated with soil pH(p<0.05).Penguins accumulated large amounts of excrement(organic matter and halide contents)in their colony tundra,altered soil properties and microbial populations,which promoted microbial-mediated CHCl3 production.Temperature gradient and FTC incubations showed that CHCl3 emissions were suppressed by 70%in sub-zero(?)conditions.Taking the seasonal variation of CHCl3 emissions into account,Antarctic tundra is estimated to emit about 0.1 Gg CHCl3 per year,which is an important source for regional atmospheric CHCl3.(3)CCl4 uptake by coastal Antarctic tundra and its partial lifetime estimationCCl4 soil sink is not well quantified due to limited and scarce measurements on soil CCl4 fluxes from different ecosystems.This study found that Antarctic tundra was a natural sink of atmospheric CCl4.In situ static chamber measurements showed that mean CCl4 flux was-2.8±0.5 nmol m-2 d-1.No significant different was found across different types of tundra(p>0.05).Soil CCl4 fluxes did not show significant correlations with soil moisture,pH,TOC,total nitrogen or total phosphorous contents(p>0.05).Laboratory-based anoxic incubations showed that soil CCl4 fluxes was suppressed;post-thermal sterilization incubations showed that soil CCl4 sink was enhanced,which suggested that the degradation of CCl4 in tundra soil was an abiotic biogeochemical process and preferred oxic environment.Extrapolation indicated that Antarctic tundra absorbed about 12 metric tons of atmospheric CCl4 each year.Assuming CCl4 fluxes from this study is representative of global tundra ecosystems and taking other reported CCl4 soil sinks into account,CCl4 lifetime with respect to soil would be about 369 years,supporting the recent viewpoints that the atmospheric lifetime of CCl4 was underestimated.(4)The emissions and absorptions of COS from coastal Antarctic tundraAntarctic tundra was absorbing and producing COS simutanously.In situ static chamber measurements and laboratory-based incubations showed that normal upland tundra was a COS sink(-0.97±0.57 pmol m-2 s-1),seal(0.87 ± 0.69 pmol m-2 s-1)and penguin(1.35±0.38 pmol m-2 s-1)colony tundra were COS sources.COS fluxes were significantly correlated with soil moisture,TOC,total nitrogen,phosphorous and sulfur contents(p<0.05).Laboratory-based incubations showed that all types of tundra became COS sources under anoxic conditions or after soil thermal sterilizations,which suggested soil and plant may degrade atmospheric COS in a shared enzymatic pathway;soil COS production was an abiotic process preferred anoxic conditions.It was postulated that penguin and seal activities enhanced the contents of organic carbon and sulfur in tundra soils,which facilitated COS production.COS fluxes from sea animal colony tundra soil were positively correlated with soil temperature(p<0.05).Seal animal activities had also increase temperature sensitivity of soil COS fluxes(Q10=1.68).This study also found that soil COS flux may account for up to 50%of tundra ecosystem COS flux.Hence,soil COS flux should be corrected for when estimating the GPP of tundra ecosystem.