Are the Fractionation Corrections Correct: Are the Isotopic Shifts for 14C/12C Ratios in Physical Processes and Chemical Reactions Really Twice Those for 13C/12C?

Southon, John

doi:10.1017/s003382220003914x

Cited by 17 publications

(24 citation statements)

References 42 publications

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“…Several speleothem geochemical modeling studies have used a value of 2.3 for fractionation changes in 14 C/ 12 C vs. 13 C/ 12 C based on an attempt to determine the fractionation of 14 C relative to 13 C in photosynthesis (Saliege and Fontes, 1984). The experiment was probably compromised by contributions of fossil fuel and the value of 2.3 has no support from quantum theory (Southon, 2011).…”

Section: Soil Carbon Modelmentioning

confidence: 99%

Radiocarbon evidence for decomposition of aged organic matter in the vadose zone as the main source of speleothem carbon

Noronha

Johnson

Southon

et al. 2015

Quaternary Science Reviews

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a b s t r a c tSeveral recent studies have used records of the radiocarbon ( 14 C) bomb peak in speleothems to inversely model the soil a 14 CO 2 and the age distribution of soil organic material (SOM) above caves, in part to investigate the potential of speleothems as sensitive records of past SOM dynamics. The results of these modeling studies have suggested that soil CO 2 at karst sites is derived primarily from the decomposition of SOM with turnover times on the order of decades to centuries. This result is in stark contrast with observations of soil a 14 CO 2 at non-karst sites, which indicate that soil CO 2 is derived primarily from root respiration and decomposition of SOM with much shorter turnover times. This discrepancy suggests that SOM in karst settings may have a very different age distribution than sites that have been studied previously and/or that soil CO 2 is not the main source of speleothem carbon. To help resolve this discrepancy, we present an improved inverse model which we use to estimate the age of CO 2 above several caves. We also present results from a detailed case study of soil carbon dynamics at Heshang Cave, China. This work demonstrates that SOM in karst sites may be much older than SOM in non-karst soils that have been studied previously, but that CO 2 produced in the shallow soil zone is unlikely to be the main source of speleothem carbon. A review of the literature suggests that the most likely explanation for the aforementioned discrepancy is that decomposition of down-washed SOM in the vadose zone is the dominant source of speleothem carbon.

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Section: Soil Carbon Modelmentioning

confidence: 99%

Radiocarbon evidence for decomposition of aged organic matter in the vadose zone as the main source of speleothem carbon

Noronha

Johnson

Southon

et al. 2015

Quaternary Science Reviews

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“…The quotient of diffusion coefficients for a rare and common isotope is also the inverse of the fractionation factor, α, which is 1.0044 for 13 CO 2 diffusion through soil (Cerling et al, 1991), and is estimated to be approximately 1.0088 for 14 CO 2 (Southon, 2011). Using this relationship, we can simplify and discretize Eq.…”

Section: Surface Flux 14 Comentioning

confidence: 99%

“…FM was related to the sample 14 C / 12 C ratio following the derivation in Southon et al (2011), where it is shown that 14 C activity ≈ 14 C / 12 C. In the equation above [ 14 C / 12 C] S is the sample 14 C ratio, δ 13 C is the sample 13 C abundance in ‰ notation, which is used to normalize the 14 C ratio for mass-based fractionation to δ 13 C = −25 ‰, and 0.95 · [ 14 C / 12 C] OX1 is the normalized 14 C ratio of the oxalic acid I standard.…”

Section: Surface Flux 14 Comentioning

confidence: 99%

Biological and physical influences on soil 14CO2 seasonal dynamics in a temperate hardwood forest

et al. 2013

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Abstract. While radiocarbon ( 14 C) abundances in standing stocks of soil carbon have been used to evaluate rates of soil carbon turnover on timescales of several years to centuries, soil-respired 14 CO 2 measurements are an important tool for identifying more immediate responses to disturbance and climate change. Soil 14 CO 2 data, however, are often temporally sparse and could be interpreted better with more context for typical seasonal ranges and trends. We report on a semi-high-frequency sampling campaign to distinguish physical and biological drivers of soil 14 CO 2 at a temperate forest site in northern Wisconsin, USA. We sampled 14 CO 2 profiles every three weeks during snow-free months through 2012 in three intact plots and one trenched plot that excluded roots. Respired 14 CO 2 declined through the summer in intact plots, shifting from an older C composition that contained more bomb 14 C to a younger composition more closely resembling present 14 C levels in the atmosphere. In the trenched plot, respired 14 CO 2 was variable but remained comparatively higher than in intact plots, reflecting older bomb-enriched 14 C sources. Although respired 14 CO 2 from intact plots correlated with soil moisture, related analyses did not support a clear cause-and-effect relationship with moisture. The initial decrease in 14 CO 2 from spring to midsummer could be explained by increases in 14 Cdeplete root respiration; however, 14 CO 2 continued to decline in late summer after root activity decreased. We also investigated whether soil moisture impacted vertical partitioning of CO 2 production, but found this had little effect on respired 14 CO 2 because CO 2 contained modern bomb C at depth, even in the trenched plot. This surprising result contrasted with decades to centuries-old pre-bomb CO 2 produced in lab incubations of the same soils. Our results suggest that root-derived C and other recent C sources had dominant impacts on respired 14 CO 2 in situ, even at depth. We propose that 14 CO 2 may have declined through late summer in intact plots because of continued microbial turnover of root-derived C, following declines in root respiration. Our results agree with other studies showing declines in the 14 C content of soil respiration over the growing season, and suggest inputs of new photosynthates through roots are an important driver.

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“…Deviations in this value add only a small amount of error to resulting probable ages derived from 14 C dating (Wigley and Muller 1981). While Wigley and Muller (1981) concluded that variation in b from 2.0 would not significantly impact 14 C dates, Southon (2011) has recently suggested that the combination of anomalies in the literature and increasing accuracy and precision in 14 C dating necessitate a reappraisal of the assumed value of 2.0 (Southon 2011).…”

Section: Introductionmentioning

confidence: 99%

“…Given recent discussion regarding the fractionation ratio (Southon 2011) it may be worthwhile to investigate natural fractionation effects in isolation. The present article uses common models of carbon isotope fractionation used in plant biochemistry to model natural variation in carbon isotope fractionation during photosynthesis.…”

Section: Introductionmentioning

confidence: 99%

Using Models of Carbon Isotope Fractionation during Photosynthesis to Understand the Natural Fractionation Ratio

Drake¹

2014

Radiocarbon

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The fractionation correction b is used to correct for the fractionation of 14 C by using information from 13 C in samples. This value is assumed to have a value of 2, where the 14 C/ 12 C ratio is double that of the 13 C/ 12 C ratio. While natural and laboratory fractionation are usually not considered separately, this article explores the differential fractionation of 14 C and 13 C during the process of photosynthesis. Values of δ 13 C p can be used to calculate Δ 13 C p values, which in turn can be used to calculate Δ 14 C p , the discrimination against 14 CO 2 during photosynthesis. Models can then be built of Δ 14 C p /Δ 13 C p , an approximation for the natural fractionation ratio. This approximation suggests that for C 3 plants the ratio is ~1.90 and for C 4 plants the ratio is more variable. While error introduced by the natural fractionation is small, it is also possibly systematic, as b = 2.0 does not seem physiologically possible following these models of carbon fractionation during photosynthesis. The central aim of this study is to illustrate that b derives not from a natural constant, but rather from a variable natural process.

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Are the Fractionation Corrections Correct: Are the Isotopic Shifts for ¹⁴C/¹²C Ratios in Physical Processes and Chemical Reactions Really Twice Those for ¹³C/¹²C?

Cited by 17 publications

References 42 publications

Radiocarbon evidence for decomposition of aged organic matter in the vadose zone as the main source of speleothem carbon

Radiocarbon evidence for decomposition of aged organic matter in the vadose zone as the main source of speleothem carbon

Biological and physical influences on soil <sup>14</sup>CO<sub>2</sub> seasonal dynamics in a temperate hardwood forest

Using Models of Carbon Isotope Fractionation during Photosynthesis to Understand the Natural Fractionation Ratio

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Are the Fractionation Corrections Correct: Are the Isotopic Shifts for 14C/12C Ratios in Physical Processes and Chemical Reactions Really Twice Those for 13C/12C?

Cited by 17 publications

References 42 publications

Radiocarbon evidence for decomposition of aged organic matter in the vadose zone as the main source of speleothem carbon

Radiocarbon evidence for decomposition of aged organic matter in the vadose zone as the main source of speleothem carbon

Biological and physical influences on soil &lt;sup&gt;14&lt;/sup&gt;CO&lt;sub&gt;2&lt;/sub&gt; seasonal dynamics in a temperate hardwood forest

Using Models of Carbon Isotope Fractionation during Photosynthesis to Understand the Natural Fractionation Ratio

Contact Info

Product

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Are the Fractionation Corrections Correct: Are the Isotopic Shifts for ¹⁴C/¹²C Ratios in Physical Processes and Chemical Reactions Really Twice Those for ¹³C/¹²C?

Biological and physical influences on soil <sup>14</sup>CO<sub>2</sub> seasonal dynamics in a temperate hardwood forest