Tracing The Hydrologic Cycle Groundwater origin Mixing Surface water g/w interaction Groundwater age Precipitation and climate Surface water studies With Environmental Isotopes

Environmental Isotopes Jan 14 Introduction to the environmental isotopes Jan 21 Tracing the water cycle- 18 O, 2 H Jan 28 Groundwater dating - 3 H Feb 4 Carbon cycle – 13 C, Radiocarbon Feb 11 Water cycle, carbon and climate – Veizer Feb 25 Nitrogen cycle – 15 N Mar 4 Water and carbon cycles on Mars - Fisher Mar 11 Crustal fluids – 18 O, D, 87 Sr, 129 I and 36 Cl Mar 18 Noble gases Mar 25 Selected topics - 6 Li, 10 Be, 11 B April 1 Presentations April 8 Presentations

Nucleosynthesis of the elements and isotopes

The Stable Environmental Isotopes Isotope Ratio % natural Reference abundance 2 H 2 H/ 1 H 0.015VSMOW 3 He 3 He/ 4 He Atmospheric He 13 C 13 C/ 12 C1.11VPDB 15 N 15 N/ 14 N0.366AIR N 2 18 O 18 O/ 16 O0.204VSMOW, VPDB 34 S 34 S/ 32 S4.21CDT 37 Cl 37 Cl/ 35 Cl24.23SMOC

Delta - permil:  - ‰ ‰ VSMOW

What is the relative enrichment or depletion of 18 O in crustal rocks (~0.204%) relative to VSMOW = 17.4‰ VSMOW  crustal rocks are enriched in 18 O by 17.4‰ or 1.7% relative to the standard VSMOW

Isotope Ratio Mass Spectrometry

Laser attenuation isotope analyser (Wavelength-Scanned Cavity Ring Down Spectroscopy – WS-CRDS) Laser absorption Reads fraction of heavy isotope bonds Direct reading of BOTH 18 O and D ratios Do it in the field!

Los Gatos – the original black box Laser attenuation isotope analyser (Wavelength-Scanned Cavity Ring Down Spectroscopy – WS-CRDS) and Picarro – nice small footprint

Laser attenuation isotope analyser (Wavelength-Scanned Cavity Ring Down Spectroscopy – WS-CRDS) Check out the sample requirements – 2 mL. Fill a tray of 100! – lots of good data.

Distribution of isotopes in nature Isotope fractionation during reactionIsotope fractionation during reaction Rayleigh distillation during reservoir depletionRayleigh distillation during reservoir depletion

Isotope fractionation, 

Physico-chemical fractionation

Isotope partitioning functions  = symmetry value m = mass of isotope E = the energy state summed from the zero-point to the energy of the dissociated molecule (J·mole –1 ) k = Boltzmann constant (gas constant per molecule) = n · · 10 –23 JK –1 T = thermodynamic temperature K

Diffusive fractionation v = molecular velocity (cm · s –1 ) k = Boltzmann constant (gas constant per molecule) = n · · 10 –23 JK –1 m = molecular mass (e.g · 10 –26 kg for 12 C 16 O 2 ) T = absolute temperature K

Diffusive Fractionation Diffusion in a vacuum Diffusion in air e.g. 13 C during CO 2 diffusion

Units Isotope Enrichment  Isotope difference in permil units between two reacting phases at equilibrium when  is small, then we can use:

Units Isotope Separation  Isotope difference in permil units between any two phases

For a water – vapor exchange at 25°C what is the  18 O of vapor, where: water  18 O w = 0.0 ‰ VSMOW

For a water – vapor exchange at 25°C what is the  18 O of vapor, where: water  18 O w = 0.0 ‰ VSMOW The fractionation factor (  ) is:  18 O w-v = The isotopic enrichment (  ):  18 O w-v = (  –1) ·10 3 = 9.3‰ and  18 O v-w = – 9.3‰

For a water – vapor exchange at 25°C what is the  18 O of vapor, where: water  18 O w = 0.0 ‰ VSMOW  18 O w-v = (  –1) ·10 3 = 9.3‰  18 O vapor =  18 O water –  18 O water-vapor = 0.0 – 9.3‰ = – 9.3‰ vapor  18 O v = –9.30‰ VSMOW

For most reactions in hydrogeology:  values are typically –50 to +50 ‰  values are close to 1 (0.98 to 1.02)  values are typically –20 to +20 ‰ Except for some extreme reactions and light isotopes... e.g. hydrogen gas produced from water is strongly depleted in 2 H and has a fractionation factor    H 2O-H2 = 3.76 at 25°C. What will be the  2 H value for H 2 produced from water with  2 H H2O = –75‰ at 25°C?

 2 H H2 = –754‰ VSMOW (but using ,  2 H H 2 = –75 – 2760 = –2835‰)

So, use the  simplification... when  is close to 1when  is close to 1 when the  -values are not too different from the reference (i.e. within a few tens of permil of 0)when the  -values are not too different from the reference (i.e. within a few tens of permil of 0)

Fractionation and Temperature ln  X-Y = aT –2 + bT –1 + c

Fractionation and Temperature

Fractionation - Other Systems