Inorganic Carbon-14

Matt Baillie

3/25/04

HWR696T

Outline

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

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)

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

Temporal production variance

Temporal production variance

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

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

Getting 14C into groundwater

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

Ca

Cat14

0

14

ln8267t

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

Corrections

To correct the calculated 14C age, apply a correction factor, q:

Caq

Cat14

0

14

ln8267t

Corrections

Initial activity can be determined through the variations in atmospheric 14C through time

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

f

p

apparentreal

nn1

tt

SH(1,2)DICDIC

q2

SH2 mm

m

geo13

rech13

geo13

measDIC13

geo CδCδ

CδCδq

meas

4measCH DIC

CH2DICq

4 m

mm

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

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

soil14

DIC14

STAT Ca

Caq

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)

332

332ALK HCOCOH

HCO21COHq

mm

mm

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

final

rech

DIC

DICq

m

m

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

carb13

rech13

carb13

DIC13

Cδ CδCδ

CδCδq 13

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

meas

exchCOcarbmeasGF DIC

DICDICDICq 2

m

mmm

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))