Physics of Aquatic Systems II

Transcription

1 Physics of Aquatic ystems II 8. Dating young waters (shallow groundwater, lakes, upper ocean) Werner Aeschbach-Hertig Institute of Environmental Physics University of Heidelberg Contents of ession 8 Dating methods for young water (other than H & He) Chlorofluorocarbons (CFCs) ulfurhexafluoride ( ) Krypton-8 ( 8 Kr) Combined applications (multi-tracer studies): Examples from ocean, lakes, and groundwater Comparison of different ages Tracer age vs. true age, need for models Literature: Mook (), Vol., ch. ( H- He, 8 Kr); Cook and Herczeg (). Transient trace gases as dating tools Principles of transient trace gas dating methods Industrial activity has lead to increases of several trace gases in the atmosphere (CO, CH, CFCs, etc.) ome anthropogenic trace gases behave (nearly) conservative in water: potential hydrologic tracers! General concept for dating: olubility: Atmospheric increase increase in surface water Input: Unique time-dependent equilibrium concentration ignature of equilibration conserved in deep water sample Age = time of sampling time of equilibration As the H- He method, transient trace gases measure the time of isolation from the atmosphere atmospheric mixing ratio [pptv] atmospheric input air water solubility solubility equilibrium concentration [fmol/kg] dissolved input sample c air c diss CFCs CFCs CFC = Chlorofluorocarbons, also known as Freons (FCKW: Fluorchlorkohlenwassertoffe) F-: CCl F, F-: CCl F, F-: CCl FCClF Industrial use as refrigerants, sprays, cleaning/drying, foam blowing, heat transfer fluid Production since ~9, strong increase in atmosphere since ~9 mall natural production in volcanoes (?) Long-lived, strong ozone destroyers in stratosphere Production regulated by Montreal protocol in 987 (phase down) (Copenhagen amendment 99: CFC phase out by 99) table in oxic water, at least F- degrades under anoxic conditions Analysis: GC-ECD (Gas Chromatogr. - Electron Capture Detector)

2 CFCs in the atmosphere: measured data F- N CFCs in the atmosphere: complete input history F- F- from Walker et al.,, JGR : 8-9 F- N F- N from Walker et al.,, JGR : Pros and cons of CFCs Relatively simple, cheap, fast analysis everal compounds and ratios between them available Frequent contamination from non-atmospheric sources Degradation in anoxic water End of atmospheric growth due to production ban! = ulfurhexafluoride colorless, odorless, nonflammable, nontoxic, stable Used as electrical insulator in high-voltage switches/transformers Production since 9, strong increase in atmosphere since ~97 mall natural radiochemical production in fluorite minerals (?) Long-lived, most potent greenhouse gas (~' x CO ) Production and release controls with Kyoto protocol? Used as artificial tracer in hydrologic and gas exchange studies Analysis by GC-ECD 9 in the atmosphere: measured data time series measured in Heidelberg atmospheric mixing ratio [pptv] from Busenberg and Plummer,, WRR : -

3 Pros and cons of Relatively simple, cheap, fast analysis Continuously increasing input function Less non-atmospheric contamination than CFCs (?) table, conservative Very low concentrations ensitive to temperature (solubility) and excess air "Contamination" by natural (sub-surface produced) 8 Kr Radioactive noble gas isotope Half-life.7 yr, β - decay to 8 Rb Main source: Nuclear fuel reprocessing (ellafield, La Hague) Increase in atmosphere since ~9 Negligible natural background (cosmogenic production) Conservative (except radiodecay) Analysis by low-level counting (proportional gas counters) Very low concentrations / activities! 8 Kr input and dating Pros and cons of 8 Kr specific atmospheric activity [Bq m - air].. 8 Kr teady atmospheric increase ources (local enhancements) well-known Contamination unlikely Conservative Ratio 8 Kr/Kr: unaffected by excess air, temperature Complicated separation of Kr for counting Very low concentrations: large sample volumes needed! (gas from several hundred liters of water is collected) Multitracer studies I: Ocean E.g.: CFCs and H- He along -N section through N-Atlantic. Multitracer studies II: Caspian ea 7 8

4 Multitracer studies III: Locust Grove, Delmarva, UA Multitracer studies III: Locust Grove, Delmarva, UA from Ekwurzel et al., 99, WRR : from Busenberg and Plummer,, WRR : - Multitracer tudies IV: Töss Valley CFC contamination Multitracer tudies V: Lake Issyk-Kul Kyrgyzstan CFC = CFC tot CFC eq CFC ex (= non-atmospheric excess) Tracer ages in Lake Issyk-Kul Influence of mixing on transient gas tracer ages from Hofer et al.,, L&O 7:-. Why do the different tracer ages deviate from each other? -comp. mixing: effect depends on curvature (d c/dt ) of input fct. Linear increase of input: linear response of age on mixing : d c/dt > : ages tend to underestimate the true age CFCs in last ~ yr: d c/dt < : CFC ages overestimate true age

5 Non-linearity of the H- He age under mixing Tracer ages in Lake Issyk-Kul He [TU] τ = τ = τ = τ = τ = τ = τ = 7 age [yr] γ = H young / H old γ = / γ = γ = γ = Qualitative explanation of age deviations in Lake Issyk-Kul: CFCs: d c/dt < : apparent tracer age much too high : d c/dt > : tracer age slightly too low H- He: γ =, tracer age slightly too high τ = τ = H [TU]....8 fraction of the old component Age of mixture is biased towards the age of the H-rich component Issyk-Kul: H homogenous (γ = ): slightly overestimated ages Tracer ages in the Ocean ummary Good agreement between CFCs and H- He for low ages Older mixtures: H- He biased towards bomb peak age Maximum CFC age: ~ yr Transient trace gases: Dating based on atmospheric increase CFCs: most used, but: ~ flat input, contamination problems : relatively new, promising tracer for the next decades 8 Kr: ~ ideal tracer, but: hard to measure, needs large samples Multitracer studies Comparison and validation of different methods Differences between tracer ages due to mixing Apparent tracer ages true mean residence times Modeling needed for correct interpretation! 7 8