Controlling for anthropogenically induced atmospheric variation in stable carbon isotope studies

Abstract

Increased use of stable isotope analysis to examine food-web dynamics, migration, transfer of nutrients, and behavior will likely result in expansion of stable isotope studies investigating human-induced global changes. Recent elevation of atmospheric CO2 concentration, related primarily to fossil fuel combustion, has reduced atmospheric CO2 delta13C (13C/12C), and this change in isotopic baseline has, in turn, reduced plant and animal tissue delta13C of terrestrial and aquatic organisms. Such depletion in CO2 delta13C and its effects on tissue delta13C may introduce bias into delta13C investigations, and if this variation is not controlled, may confound interpretation of results obtained from tissue samples collected over a temporal span. To control for this source of variation, we used a high-precision record of atmospheric CO2 delta13C from ice cores and direct atmospheric measurements to model modern change in CO2 delta13C. From this model, we estimated a correction factor that controls for atmospheric change; this correction reduces bias associated with changes in atmospheric isotopic baseline and facilitates comparison of tissue delta13C collected over multiple years. To exemplify the importance of accounting for atmospheric CO2 delta13C depletion, we applied the correction to a dataset of collagen delta13C obtained from mountain lion (Puma concolor) bone samples collected in California between 1893 and 1995. Before correction, in three of four ecoregions collagen delta13C decreased significantly concurrent with depletion of atmospheric CO2 delta13C (n > or = 32, P < or = 0.01). Application of the correction to collagen delta13C data removed trends from regions demonstrating significant declines, and measurement error associated with the correction did not add substantial variation to adjusted estimates. Controlling for long-term atmospheric variation and correcting tissue samples for changes in isotopic baseline facilitate analysis of samples that span a large temporal range.

Stable isotope series from elephant ivory reveal lifetime histories of a true dietary generalist

“…Gaps (n ¼ 6) within series were filled by taking the median of the three adjacent cells on either side. d 13 C series were corrected for twentieth-century decreases in atmospheric d 13 CO 2 using the exponential model described in an earlier paper [43].…”

Changing Carbon Isotope Ratio of Atmospheric Carbon Dioxide: Implications For Food Authentication

“…Martin et al (25) present maple syrup data from 1992 to 1995 that show an average decline of 0.099 ( 0.024%/year. During this period δ 13 C-(AtmCO 2 ) declined by 0.030%/year (13), which is within the 95% confidence interval of this subset of the maple syrup data set. Pang et al (29) present a summary of 20,888 analyses of Chinese commercial honeys from 1998 to 2004.…”

“…For the period covered by maple syrup samples in the study to ca. 1.0% (13). Shifts of this magnitude are commonly seen in the literature for plant materials over similar time periods.…”

Wheatear molt and assignment tests: ongoing lessons in using stable isotopes to infer origins

“…from Cayuga Lake had d 13 C lower than -20%, reflecting the enriched values of macrophytes in that lake (Post 2002). Similarly, mountain lions (Puma concolor) in northern California without access to marine food resources had more enriched d 13 C values (Long et al 2005), even after accounting for discrimination associated with bone collagen. Individual red squirrels (Tamiasciurus hudsonicus) in southeast Alaska had values between -20 and -23% (Ben- David et al 1997), which would translate into more enriched values in a specialized predator.…”