SMOW. δd = 8 δ 18 O δ 18 O. Craig 1961

Transcription

1 SMOW δd δd = 8 δ 18 O + 10 δ 18 O Craig 1961

2 " # 18 O "T $ / deg " #D "T $ 5.6 / deg "# 18 O "T $ / deg "#D "T $ 5.6 / deg Dansgaard 1964

3 -9-27 Little Am. 43 m C Wilkes 1168 m C Byrd 1530 m C S. Pole 2835 m C Vostok 3490 m -56 C δ 18 O

4 0-10 ANTARCTIC PRECIPITATION -20! 18 O T C Criss 1999

5 after Epstein et al. 1965

6 Isotope Studies of Glaciers & Ice Paleoclimatology Correlation of δ 18 O values with Temperature Correlation of δ 18 O values with annual precipitation (cm) Snow Stratigraphy Seasonal variations Ice Metamorphism Homogenization (reduction in seasonal amplitude) Attenuation Flow (thinning of layers)

7 Ice Core Studies: Greenland and Antarctica Clausen & Langway 1989

8 Top: coarse, granular firn Middle: ice showing annual layers Bottom: ditto with sand & silt Clear ice with air bubbles NASA/National Ice Core Lab

9 Amplitude ~ 15 ~ 7 ~ 4 ~ 0 Dansgaard et al. 1973

10 Johnsen et al 1972

11 VOSTOK Petit et al. 1997!D Age, ka

12 ICE CORES: Long duration climatic record- fossil meteoric ppt Cores to several km thick; ice thousands of years old Vostok: 78 28'S 'E Elev m (Lorius et al, 1985; Jouzel et al, p. 405) Modern annual: Tair = C Ppt. = 2.2 to 2.5 g/cm-y δdavg = -438 Thickness of ice sheet = 3.7 km 3623 m Ice Core! => fossil ice back to ~400 ka includes several glacial cycles! Large δ 18 O shifts with depth, w/lowest δ-values at last glacial maximum ~17 ka Vostok 6 T = 10 C Byrd 8 (-32 to -40) Dxs -> 0 Camp Century 12 (-29 to -41) Sawtooth type variations in detail Glacial maxima synchronous in N & S hemispheres Problems with Record

13 ICE CORES: Long duration climatic record- fossil meteoric ppt Cores to several km thick; ice thousands of years old Vostok: 78 28'S 'E Elev m (Lorius et al, 1985; Jouzel et al, p. 405) Modern annual: Tair = C Ppt. = 2.2 to 2.5 g/cm-y δdavg = -438 Thickness of ice sheet = 3.7 km 3623 m Ice Core! => fossil ice back to ~400 ka includes several glacial cycles! Large δ 18 O shifts with depth, w/lowest δ-values at last glacial maximum ~17 ka Vostok 6 T = 10 C Byrd 8 (-32 to -40) Dxs -> 0 Camp Century 12 (-29 to -41) Sawtooth type variations in detail Glacial maxima synchronous in N & S hemispheres Problems with Record Ice flows: different positions in core did not originate at same location Ice deeper down originated far away (to 200 Vostok), at higher altitudes Accumulation rate probably not constant over time Attenuation of thickness - timescale not simple => Ice Flow Models

14 VOSTOK Petit et al. 1997!D Age, ka

15 VOSTOK Petit et al Barnola et al !D CO 2, ppm Age, ka

16 VOSTOK Petit et al !D ! 18 O, approx V S 83 21W -62! 18 O AGE, ka Shackleton &Pisias 1985

17 Reexamine δ 18 O variations in carbonate cores Convoluted Record of Temperature & Ice Volume Variations Emiliani interpretation: ~6 C change in ocean surface temperatures; At least half of this change is now thought to be ice-volume effect Temperature changes in high latitude, continental temperatures are much greater than changes in ocean surface temperatures

18 Age Dating of Groundwater Applications: Groundwater Residence Time; Water mining How much water is available? How long will it last? Contaminant transport What is the source or risk of contamination? Climate change

19 Age Dating Methods 1. Seasonal Variations Stable Isotopes Tree Rings; Coral bands; Ice Cores 2. Natural radioactivity ( 3 H ), 14 C, 36 Cl,. 3. Inadvertent Tracers Tritium Chlorofluorocarbons Krypton-85 Dissolved Contaminants 4. Intentional Tracers Sulfur hexafluoride Noble Gases Dyes Variable accuracy but wide time range

20 Useful Isotopes for Age Dating of Groundwater Criss et al 2007

21 Cosmogenic Radionuclides: 3 H, 14 C, 36 Cl, 129 I 10 Be, 26 Al, 41 Ca Half Lives: 12.3 y 5730 a 301,000 a 15.7 Ma Trace Natural Abundances Production: 1 n + 14 N 3 H + 12 C 14 N(n,T) 12 C Decay: 3 H 3 He + β - + ν kev T 1/2 = 12.3 a Production: 1 n + 14 N 14 C + 1 H 14 N(n,p) 14 C Decay: 14 C 14 N + β - + ν MeV T 1/2 = 5730 a

22 Cosmogenic Radionuclides: Trace Natural Abundances Tritium abundance in young meteoric waters: < 25 TU Tritium Unit (T.U.) 1 T.U. = 1 3 H atom per hydrogen atoms = 7.1 dpm/liter = 3.2 pci/l 14 C modern plants = 15.3 dpm/g C (Anderson & Libby, 1951; pre bomb) ~1.3 atoms 14 C per trillion atoms C Measurements: Scintillation counting: 14 C; T AMS: 14 C, 36 Cl, 129 I 10 Be, 26 Al, 41 Ca

23 CAMS LLNL

24 N N o = e "#t T 12 = ln2 / " Measure Activity A with Scintillation counting A = "N so A A o = e #"t No depends on reactive and transport processes Source variations Dispersion/mixing/dilution Phase changes

25 Bravo Test 3/ Mt

26 4 3.5 Ottawa, Canada Hatteras, North Carolina 3 Log T.U YEAR Criss et al. 2007; Data from IAEA 2004

27 Log 10 TU decay Tritium in Precipitation Ottawa, Canada GNIP/IAEA YEAR

28 T - 3 He Method No = P +D = Surviving parent + radiogenic daughter T meas = e "#t T meas + 3 He tritogenic 3 He meas = 3 He trit + 3 He equil + 3 He excess + 3 He rad 4 He meas = 4 He equil + 4 He excess + 4 He rad

29

30 14 C Abundances in modern plants = 15.3 dpm/g C (Anderson & Libby, 1951; essentially pre-bomb ) Abundance variations Solar activity Magnetic field variations Isotopic fractionation (carbonate vs. tissue) Fossil fuel burning Bomb releases Compare results to tree rings; find ± 20% variations in 14 C in atmospheric CO 2 over centuries Groundwater 14 C ages: Mostly measured on DIC

31 Age-dating groundwater older than human occupation Radiocarbon ( ) 14 C 12 C ( C ) Age C C meas std 14 =! = fraction modern 8267 x ln(fmc) carbon or fmc (14 C/ 12 C) std is an oxalic acid whose radiocarbon abundance is equal to the abundance of atmospheric CO 2 in 1950 Radiocarbon dating typifies challenges in age-dating methods Where carbon comprises significant amount of aquifer matrix, water-rock rxn dominates over radioactive decay Volcanoes are another source of dissolved carbon absent in 14 C

32 60 50 Davis Municipal Wells 12, 16, 18 Criss & Davisson, 1996 Irrigation Well near Colusa, CA NO 3 (mg/l) YEAR Criss

33 Davis, CA Criss

34 NO 3 (mg/l) Davis Woodland Apparent 14 C Age Criss & Davisson 1996

35 Peligro Criss

36 Total Nitrogen Mississippi R. basin USGS

37 Inadvertent (Accidental) Tracers CFC s CFC-11 = CFCl 3 CFC-12 = CF 2 Cl 2 CFC-11:CFC-12 ratio is a function of time 85 Kr: T 1/2 = 10.7a fission product from fuel reprocessing VOC s: TCE trichloroethylene PCE tetrachloroethylene MTBE methyl tert-butyl ether (gasoline additive 1990 s-2006)

38 Artificial Tracers must be: Chemically suitable and harmless, even for potable supplies Conservative Water soluble supplies Measurable over a large dynamic range Inexpensive Common Tracers Sulfur hexafluoride Noble Gases (He, Xe) Dyes (Fluorescein ; Rhodamine ; etc)

39 Dye Tracing Many inert, water soluble, fluorescent dyes: Fluorescein Rhodamine WT Uranine Eosine Use of activated charcoal adsorbent packs and fluorometer Kentucky Division of Water Purpose: Establish hydraulic connectivity Determine travel time

40 Some Points to Consider When multiple dating techniques are used, their groundwater ages are normally discordant. In karst regions, the normal model assumption of isotropic permeability is poor, and the transport rate for contaminants etc. can be more than 100x faster than indicated by models.