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Inorganic Carbon-14

Inorganic Carbon-14. Matt Baillie 3/25/04 HWR696T. Outline. Production of 14 C Variance through time of 14 C production How to get 14 C into groundwater Complications and corrections Conclusions. Production in the atmosphere.

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Inorganic Carbon-14

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  1. Inorganic Carbon-14 Matt Baillie 3/25/04 HWR696T

  2. Outline • Production of 14C • Variance through time of 14C production • How to get 14C into groundwater • Complications and corrections • Conclusions

  3. Production in the atmosphere • 14C produced through secondary spallation reactions between neutrons and 14N atoms • 14C atoms then quickly combine with O2 to form 14CO2 • Subsurface production unimportant due to CO2 in soil From (Taylor, 2000)

  4. Temporal production variance • Variation in production of 14C in the atmosphere dependent on cosmic ray flux, which is in turn dependent on solar activity, geomagnetic field, etc. • Atmospheric production can be calibrated using dendrochronology, as well as U-Th dating of corals • Industrial age burning of fossil fuels has put a huge amount of “dead” carbon into the atmosphere, diluting atmospheric 14C • Atmospheric testing of nuclear weapons increased (up to double) the 14C in the atmosphere • Now approaching previous levels due to moratorium on atmospheric testing, as well as 14CO2 going mostly into the oceans

  5. Temporal production variance

  6. Temporal production variance

  7. Getting 14C into groundwater • 14CO2 incorporated into plants through photosynthesis, undergoing depletion • 14C is passed from plants to soil, and becomes slightly enriched due to the diffusion of 12CO2 into the atmosphere • Soil CO2 levels are 10-100 times greater than atmospheric CO2 levels, so absolute amounts of 14C are much higher in the soil than in the atmosphere

  8. Getting 14C into groundwater • In open system conditions (contact with the soil), 14C is replenished, and remains slightly enriched from soil levels • In closed system conditions, 14C is no longer replenished by the soil, and begins to decay away

  9. Getting 14C into groundwater

  10. Getting 14C into groundwater • Once the 14C is in closed system conditions and assuming no other processes affect it subsequently, the groundwater can be dated using the equation: where t is the mean residence time of the groundwater, at is the activity of the 14C at the time of sampling, and a0 is the initial activity of 14C

  11. Complications • What was the initial 14C activity in the atmosphere when the groundwater entered closed system conditions? • Carbonate dissolution introduces “dead” carbon into the groundwater, taking 14C-active carbon out of the groundwater • Matrix diffusion of 14C into dead-end pores decreases 14C in groundwater • Reduction of organics by sulphate adds 14C-free carbon to the groundwater • Geogenic (mantle/deep crust) 14C-free CO2 • Methanogenesis introduces “dead” carbon

  12. Corrections • To correct the calculated 14C age, apply a correction factor, q:

  13. Corrections • Initial activity can be determined through the variations in atmospheric 14C through time

  14. Corrections • Matrix diffusion: correction based on matrix porosity and fissure porosity in a dual-porosity aquifer • Sulphate reduction: stoichiometric correction • Geogenic CO2: δ13C correction • Methanogenesis: δ13C and stoichiometric correction

  15. Corrections • For carbonate dissolution, correction factors are more complicated, and there are therefore several different correction models that can be applied • Statistical correction • Alkalinity correction • Chemical mass-balance correction • δ13C mixing (δ13C model) • Fontes-Garnier model

  16. Carbonate corrections • Statistical correction • Simple geometric correction based on the type of aquifer system: • 0.65-0.75 for karst systems • 0.75-0.90 for sediments with fine-grained carbonate such as loess • 0.90-1.00 for crystalline rocks (from Vogel, 1970) • Can be estimated by: for any given recharge area • Limited in usefulness to waters found near the recharge area

  17. Carbonate corrections • Alkalinity correction • Correction based on the initial and final DIC concentrations (from Tamers, 1975) • Assumes fully closed system conditions, with no exchange between the groundwater and the soil CO2 during dissolution • Model is of “limited interest” (Clark and Fritz, 1997)

  18. Carbonate corrections • Chemical mass-balance correction • Closed-system model, with dissolution below the water table and no exchange with soil CO2 • Estimated by: • With mDICrech being estimable from the pH of the recharge area, and: mDICfinal = mDICrech+[mCa2++mMg2+-mSO42-+1/2(mNa++mK+-mCl-)] • Only useful in geochemically simple systems with no carbonate loss from the groundwater

  19. Carbonate corrections • δ13C mixing (δ13C model) • Uses 13C as a tracer, useful in open and closed systems. • First introduced by Pearson (1965) and Pearson and Hanshaw (1970), later modified to work at higher pH (7.5-10): • Enrichment factor chosen for the soil greatly affects groundwater age, and is based on pH in the recharge area; assumes that this pH was the same when the groundwater was originally recharged

  20. Carbonate corrections • Fontes-Garnier model (1979; 1981) • Calculates q based on both chemistry and δ13C values of groundwater • Uses Ca and Mg concentrations as a proxy for carbonate dissolution, as well as δ13C to partition the carbon into DIC that has exchanged with soil CO2 and that which has not • Does not take into account DIC sources aside from carbonate dissolution and soil CO2 exchange

  21. Conclusions • Inorganic 14C is a useful tool for determining mean residence time of groundwater IF: • Initial 14C activity is known • Recharge conditions can be determined • Conditions within the aquifer are somewhat known (in relation to carbonate dissolution) • Groundwater is not too old for the method to be useful (for all practical purposes, water must be at most 30,000 years in residence (Clark and Fritz, 1997))

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