Carbon isotopic composition of sterols in geochemical samples.
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Carbon isotopic composition of sterols in marine Holocene sediments, marine sinking particles, tree leaves, and soils were determined. δ13C values of algae-derived sterols such as 24-methylcholesta-5, 22-dien-3β-ol and dinosterol in the marine sediments range from −22.1 to −25.2‰, while those of 24-ethylcholest-5-en-3β-ol (24-ethylcholesterol) range from −22.6 to −24.3‰. We conclude that 24-ethylcholesterol in the marine sediments derives from marine algae, because their δ13C values are markedly different from those of the leaves of C3 (∼−29‰) and C4 (−14‰) plants and similar to those of the algal sterols.Keywords:
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Stable carbon isotope ratios (δ 13C values) and photosynthetic pigments in a 1000 a cypress (Sabina przewalskii) were investigated. The direct variations of δ 13C values were significant,with the highest δ 13C values in the north and the lower δ 13C values in the west.The changes of chlorophyll,carotoniod and flavoniod contents in tree leaves were similar to δ 13C variations.The results indicated that the latter was a long-term adaptation to local differences of light radiation,while the former was attributed to the effect of light radiation on photosynthesis and respiration.
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Photosynthetically active radiation
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C values in different organs are different. Tree leaves have the lowest, while tree trunks have the highest δ~ 13C value. δ~ 13C also varies with exposure, with a relatively lowest δ~ 13C value in the north. There is an inconsistent trend existing in wood δ~ 13C values of different height along the trunk, suggesting no carbon isotope discrimination during the period of the downward transportation of leaf's organic compounds. Irrespective of tree height and exposure, wood δ~ 13C values have become lower and lower for the past 60 years.
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We used a series of experiments to determine whether stable carbon isotope analysis of modern and fossil larval head capsules of chironomids allowed identification of their dietary carbon source. Our main focus was to assess whether carbon from naturally 13C-depleted methane-oxidizing bacteria (MOB) can be traced in chironomid cuticles using stable carbon isotope analysis. We first showed that a minimum sample weight of ~20 μg was required for our equipment to determine head capsule δ13C with a standard deviation of 0.5‰. Such a small minimum sample weight allows taxon-specific δ13C analyses at a precision sufficient to differentiate whether head capsules consist mainly of carbon derived from MOB or from other food sources commonly encountered in lake ecosystems. We then tested the effect of different chemical pre-treatments that are commonly used for sediment processing on δ13C measurements on head capsules. Processing with 10% KOH (2 h), 10% HCl (2 h), or 40% HF (18 h) showed no detectable effect on δ13C, whereas a combination of boiling, accelerated solvent extraction and heavy chemical oxidation resulted in a small (0.2‰) but statistically significant decrease in δ13C values. Using culturing experiments with MOB grown on 13C-labelled methane, we demonstrated that methanogenic carbon is transferred not only into the larval tissue, but also into chironomid head capsules. Taxon-specific δ13C of fossil chironomid head capsules from different lake sediments was analyzed. δ13C of head capsules generally ranged from −28 to −25.8‰, but in some instances we observed δ13C values as low as −36.9 to −31.5‰, suggesting that carbon from MOB is traceable in fossil and subfossil chironomid remains. We demonstrate that stable carbon isotope analyses of fossil chironomid head capsules can give insights into dietary links and carbon cycling in benthic food webs in the past and that the method has the potential to reconstruct the importance of MOB in the palaeo-diet of chironomid larvae and, indirectly, to infer past changes in methane flux at the sediment water interface in lakes.
Paleolimnology
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Isotope Analysis
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δ15N
TRACER
Isotopes of nitrogen
Isotope Analysis
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Proxy (statistics)
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Natural abundance
Isotopic ratio
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The carbon isotope compositions of 12 plant species leaves collected from the karst Mountainous area in Guizhou were analyzed.The characteristics of foliar carbon isotope compositions,the differences of carbon isotope compositions of interspecies and different habitat conditions,the changes of carbon isotope compositions with time and space were studied.The results showed that the foliar δ13C from the study area has an average of-28.14‰ and range of-26.98‰～-29.15‰.The mean value of δ13C is lower than that from other areas except for tropical rain area in China,and the change extent is narrower than that other areas.There are greater differences between species.The δ13C in Pterdium aquilinum var.latiusculum affected by habitat conditions is greater than Cyclosorus acuminatus,Rhus chinensis,Pyracantha fortureana and Coriatia sinica.The young leaves have higher δ13C values than the mature ones,but the change extents vary among species.The δ13C values increased with elevateel altitude both in small and large scales.However,the increasing extents of the δ13C values are different among species.
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