The use of stable carbon isotopes as a means of studying energy flow is increasing in ecology and paleo- ecology. However, secondary fractionation and turnover of stable isotopes in animals are poorly understood pro- cesses. This study shows that tissues of the gerbil (Meriones unguienlatus) have different 613C values when equilibrated on corn (C4) or wheat (C~) diets with constant 13c/1ac contents. Lipids were depleted 3.0%o and hair was enriched 1.0%o relative to the C4 diet. Tissue 6t3C values were ranked hair > brain > muscle > liver > fat. After changing the gerbils to a wheat (C3) diet, isotope ratios of the tissues shifted in the direction of the 6a3C value of the new diet. The rate at which carbon derived from the corn diet was replaced by carbon derived from the wheat diet was ade- quately described by a negative exponential decay model for all tissues examined. More metabolically active tissues such as liver and fat had more rapid turnover rates than less metabolically active tissues such as hair. The half-life for carbon ranged from 6.4 days in liver to 47.5 days in hair. The results of this study have important implications for the use of 6~3C values as indicators of animal diet. Both fractionation and turnover of stable carbon isotopes in animal tissues may obscure the relative contributions of isotopically distinct dietary components (such as C3 vs. C,, or marine vs. terrestrial) if an animal's diet varies through time. These complications deserve attention in any study using stable isotope ratios of animal tissue as dietary indicators and might be minimized by analysis of several tissues or products covering a range of turnover times.

Fractionation and turnover of stable carbon isotopes in animal tissues: Implications for δ13C analysis of diet

Stable isotopes in tree rings.

Lignin, elemental, and stable carbon isotope compositions are reported for local plants and for coarse (>63 µm) and fine (<63 µm) suspended particulate materials collected along a 1,950-km reach of the lower Amazon River during four contrasting stages of the 1982–1983 hydrograph. Fluxes of chemically recognizable lignin in the two size classes generally parallel each other along the mainstem with the fine fraction usually predominating. Particulate organic matter transported in the coarse size fraction of the mainstem and its major tributaries is composed of recently formed and well preserved tree leaf debris along with some wood. Organic matter in the fine size fraction is comparatively old, degraded, and rich in immobilized nitrogen and derives primarily from soils. C-4 grasses, which are abundant in the mainstem floodplain (varzea), are not major components of either the coarse or fine particulate material in the river.

Particulate organic matter in both size fractions is introduced largely from upstream sources within the Rio Solimoes and Rio Madeira drainage basins. Most of this organic matter is unreactive and is transported conservatively with mineral particles along the Amazon mainstem. However, some downstream compositional trends are seen in both size fractions which reflect the addition or exchange of highly degraded, 13C-depleted, and lignin-poor organic materials from lower basin sources.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1986.31.4.0717

Compositions and fluxes of particulate organic material in the Amazon River1

Comparison of 14C (carbon-14) in archived (pre-1963) and contemporary soils taken along an elevation gradient in the Sierra Nevada, California, demonstrates rapid (7 to 65 years) turnover for 50 to 90 percent of carbon in the upper 20 centimeters of soil (A horizon soil carbon). Carbon turnover times increased with elevation (decreasing temperature) along the Sierra transect. This trend was consistent with results from other locations, which indicates that temperature is a dominant control of soil carbon dynamics. When extrapolated to large regions, the observed relation between carbon turnover and temperature suggests that soils should act as significant sources or sinks of atmospheric carbon dioxide in response to global temperature changes.

/pdf/rapid-exchange-between-soil-carbon-and-atmospheric-carbon-ac5d0kl97r.pdf

Rapid exchange between soil carbon and atmospheric carbon dioxide driven by temperature change

The magnitude and timing of the response of the soil carbon reservoir to changes in land use or climate is a large source of uncertainty in global carbon cycle models. One method of assessing soil carbon dynamics, based on modeling the observed increase of 14C in organic matter pools during the 30 years since atmospheric weapons testing ended, is described in this paper. Differences in the inventory and residence time of carbon are observed in organic matter from soils representing tropical (Amazon Basin, Brazil) and temperate (western slope of the Sierra Nevada mountains, California) forest ecosystems. The majority of the organic carbon in the upper 22 cm of the tropical soil (7.1 kgC m−2) has residence times of 10 years or less, with a minor component of very refractory carbon. The estimated annual flux of carbon into and out of the soil organic matter in this horizon of the mineral soil, based on modeling of the 14C data, is between 1.9 and 5.5 kgC m−2 yr−1. In contrast, organic matter in the temperate soil over a similar depth interval (0-23 cm; 5.2 kgC m−2), is made up of approximately equal amounts of carbon with residence times of 10, 100, and 1000 years. The estimated annual flux of carbon into and out of this soil is 0.22 to 0.45 kgC m−2 yr−1. Rapid turnover of organic matter with density <1.6 - 2.0 g cm−3contributes a major component of the annual flux of carbon into and out of both soil types. Hydrolysis of mineral soil organic matter of density < 1.6-2.0 g cm−3 removed 14C-enriched components from the temperate soil but had no effect on the 14C content of the residue in 0 - 22 cm layer of the tropical soil. The results presented here show that carbon cycle models which treat soil carbon dynamics as a single reservoir with a turnover rate based on radiocarbon measurements of bulk soil organic matter underestimate the annual fluxes of organic matter through the soil organic matter pool, particularly in tropical regions.

/pdf/comparison-of-carbon-dynamics-in-tropical-and-temperate-2s9clnfsne.pdf

Comparison of carbon dynamics in tropical and temperate soils using radiocarbon measurements

Conventional wet or dry combustions are associated with the formation of nitrogen oxides which even in minute amounts cause undesirable effects in proportional counters. Simultaneous microdeterminations of total carbon and carbon-l4 may be performed by oxidation with cupric oxide in sealed Vycor tubes at 850 deg C. Masganese dioxide is added io provide free oxygen and cupric chloride displaces carbonyic whenever basic metals are present. The carbon determination is as precise as it is in other types of oxidaiion used for carbon radioassay and the carbon dioxide is very pure as judged by its counting characteristics. (auth)

B. J. Corcoran

Papers

Sealed Tube Combustions for Determination of Carbon- 14 and Total Carbon