Atmospheric 14C concentration

Rain and surface water dissolve small amounts of atmospheric C02. Significantly more C02 is added to water percolating through the soil layer, as soil air contains about 100 times more C02 as compared to free air. Soil C02 is produced by biological action such as root respiration and decay of plant material. This C02 was tagged by the atmospheric 14C concentrations, that is, about 100 pmc in pre nuclear bomb times (pre-1952) and up to 200 pmc in post-bomb years. 

D. (2001) Extremely large variations of atmospheric 14C concentration during the last glacial period. Science 292, 2453-2458. 

FIGURE 14.1 Atmospheric 14C concentration as reflected in tree rings and the bomb curve arising due to atmospheric nuclear weapon tests in the 1950s and 1960s. (Adapted from Palmblad, M. et at., J. Mass Spectrom., 40, 154, 2005. With permission.)... 

As partly mentioned before, natural and anthropogenic induced variations of the atmospheric C02 concentration and of the 14C/ 12C and 13C/12C ratios have been observed. For a quantitative discussion of these variations in relation to possible causes, models for the carbon cycle dynamics have been developed [21-25]. Compared to the noble gas radioisotopes 39Ar and 81Kr, for which we only have to consider a well mixed atmospheric reservoir, we have a much more complicated system for 14C. The C02 in the atmosphere exchanges with the carbon in the biosphere and with the... 

Despite the difficulty of interpreting 14C measurements on surface ocean water such measurements are of great interest. The net transport of excess 14C from the atmosphere to the sea depends on the difference between the 14C concentration in atmospheric C02 and that in the carbonate system at the sea surface. The decline in the atmospheric reservoir of excess 14C is therefore controlled by the 14C concentration at the sea surface. This in turn depends upon diffusion and advection into the deep sea. As the levels of excess 14C in the troposphere and the mixed layer of the sea begin to approach each other, mixing from the mixed layer of the sea into the deep sea will be the factor controlling the levels of excess 14C in the atmosphere. 

An anthropogenic addition of 14C into the atmosphere occurred with the nuclear bomb testing from 1952 to 1963, along with the introduction of bomb tritium. As a result, the 14C concentration also increased in plants (Fig. 11.2), in the soil C02, and in recharged groundwater. Values up to 200 pmc have been measured, but they decreased to about 120 pmc by 1987, and to about 110 pmc by 1995. 

Results of a hydrochemical study in basaltic aquifers of Hawaii (Hufen et al., 1974) are given in Table 11.2 and in a histogram in Fig. 11.4.This case study is of special importance because it deals with basaltic aquifers, in contrast to carbonate aquifers, to be discussed in the following section. The tritium values in the Hawaii study were 0.3-2.9 TU in 1974, indicating that most or all of the water had a pre-1952 age. The corresponding 14C concentrations were 85-97 pmc. The pre-1952 14C in the atmosphere was about 100 pmc. Hence, part of the waters reported in Table 11.2 maintained all their initial 14C, whereas others lost up to 15% of their initial value. This loss could occur for two reasons (1) interaction with 14C-devoid carbonate rocks (secondary in the basaltic terrain) and (2) aging of the water, reflected... 

Guidance to do with the location of H2S monitors on offshore facilities is provided by API RP 14C. Monitors are required when the atmospheric H2S concentration could be >50 ppm or where the H2S concentration in piping is >100 ppm. 

This the reverse of the process occurring in Equation 6.1, the decay of 14C, and results in a nearly constant concentration of 14C in the atmosphere. Any living organic synthesis on Earth, such as photosynthesis, will then capture the 14C and produce a 12C/14C ratio in living things to be fixed. When a tree is used for wood in an object such as a museum artefact then the 12C/14C ratio changes and the age of the sample can be calculated using Equation 6.5 as before. 

In contrast to the atmosphere, where much can be learned from a relatively small number of 14C sampling locations, the sea presents a formidable sampling problem. The sea does not achieve the nice vertical and zonal uniformity characteristic of the troposphere. The well-mixed surface layer of the sea, generally 50-100 meters deep but occasionally deeper, appears to achieve a uniform concentration but only vertically the levels of excess 14C in the mixed layer varies considerably from place to place. This arises partly from the upwelling of subsurface water of lower 14C content which perturbs the concentration of surface water in many areas of the ocean, adding to the difficulty of interpreting sea water 14C measurements. 

Since the industrial revolution of the early nineteenth century, large amounts of fossil fuels (oil, coal, gas) have been combusted, causing an increase of about 10% in the concentration of atmospheric C02. This added fossil C02 was devoid of 14C and, correspondingly, lowered the 14C 12C ratio in the air by about 10%. 

Answer 1.5 The temperature that prevailed at the base of the aerated zone at the time of recharge (via the concentration of the atmospheric noble gases) the depth of circulation or storage (via the water temperature) the types of rocks passed (via the chemical composition) the time the water stayed underground, i.e., its age (via the decay of tritium and 14C and the accumulation of radiogenic 4He and 40Argon). 

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