Differences In δ13C Values Research Articles

Fluid migration and vein formation during deformation and greenschist facies metamorphism at Ormiston Gorge, central Australia

ABSTRACTDuring the Alice Springs Orogeny, deformation at Ormiston Gorge, central Australia, occurred under lower‐ to middle‐greenschist facies conditions. Dolomites of the Bitter Springs Formation and quartzites. metagreywackes, and metapelites of the Heavitree Quartzite contain abundant early‐, syn‐, and post‐tectonic veins. However, though vein densities locally approach 15%, the distribution of veins and the oxygen isotope geochemistry of wallrocks and veins suggest that fluid movement was on a local scale. The Heavitree Quartzite contains quartz veins that, even along the main thrust plane, have similar δ18O values (13.5–16.9%o) to those of their wallrocks (13.6–16.9%o), with Δ18O(vein‐wallrock) values of ‐0.6 to 0.4%o. In contrast, the Bitter Springs Formation contains predominantly dolomite veins that have δ18O values of 23.4 to 27.7%o. These differences are observed even at the boundary between the Heavitree and Bitter Springs rocks, implying that significant fluid exchange between these rocks has not occurred, or that fluid flow was channelled through areas outside those sampled for this study. By contrast with the Heavitree Quartzite, δ18O values of wallrocks in individual samples of the Bitter Springs Formation are significantly higher (23.3–29.1%o) than those of the veins, with δ18O(vein‐wallrock) values up to ‐4%o (average of ‐2.1%o). These systematic differences in δ18O values most likely result from oxygen isotope fractionation caused by fluid immiscibility or disequilibrium dissolution. Smaller differences in δ13C values between some dolomite veins and wallrocks [δ13C(vein‐wallrock) up to ‐1.9%o, average of ‐0.5%o] are also explained by these processes. This study indicates that large volumes of veins may be produced by repeated fracturing and fluid migration within particular rock units, without involving large volumes of externally derived fluids.

Discrimination between12C and13C by marine plants.

The natural abundance13C/12C ratios (as δ13C) of organic matter of marine macroalgae from Fife and Angus (East Scotland) were measured for comparison with the species' ability to use CO2 and HCO 3- for photosynthesis, as deduced from previously published pH-drift measurements. There was a clear difference in δ13C values for species able or unable to use HCO 3- . Six species of Chlorophyta, 12 species of Phaeophyta and 8 species of Rhodophyta that the pH-drift data suggested could use HCO 3- had δ13C values in the range -8.81‰ to -22.55‰. A further 6 species of Rhodophyta which the pH-drift data suggested could only use CO2 had δ13C values in the range -29.90‰ to-34.51‰. One of these six species (Lomentaria articulata) is intertidal; the other five are subtidal and so have no access to atmospheric CO2 to complicate the analysis. For these species, calculations based on the measured δ13C of the algae, the δ13C of CO2 in seawater, and the known13C/12C discrimination of CO2 diffusion and RUBISCO carboxylation suggest that only 15-21% of the limitation to photosynthesisin situ results from CO2 diffusion from the bulk medium to the plastids; the remaining 79-85% is associated with carboxylation reactions (and, via feedback effects, down-stream processes). This analysis has been extended for one of these five species,Delesseria sanguinea, by incorporating data onin situ specific growth rates, respiratory rates measured in the laboratory, and applying Fick's law of diffusion to calculate a boundary layer thickness of 17-24 μm. This value is reasonable for aDelesseria sanguinea frondin situ. For HCO 3- -using marine macroalgae the range of δ13C values measured can be accommodated by a CO2 efflux from algal cells which range from 0.306 of the gross HCO 3- influx forEnteromorpha intestinalis (δ13C=-8.81‰) in a rockpool to 0.787 forChondrus crispus (δ13C=-22.55‰). The relatively high computed CO2 efflux for those HCO 3- -users with the more negative δ13C values implies a relatively high photon cost of C assimilation; the observed photon costs can be accommodated by assuming coupled, energy-independent inorganic carbon influx and efflux. The observed δ13C values are also interpreted in terms of water movement regimes and obtaining CO2 from the atmosphere. Published δ13C values for freshwater macrophytes were compared with the ability of the species to use CO2 and HCO 3- and again there was an apparent separation in δ13C values for these two groups. δ13C values obtained for marine macroalgae for which no pH-drift data are available permit predictions, as yet untested, as to whether they use predominantly CO2 or HCO 3.

Internal Standard Stable Carbon Isotope Ratio Method for Determination of C-4 Plant Sugars in Honey: Collaborative Study, and Evaluation of Improved Protein Preparation Procedure

Abstract A collaborative study of the internal standard isotope ratio method for measuring C-4 plant sugars in honey Is reported. For 5 Youden pairs of samples containing 2-14% C-4 sugars, analyzed by 9 laboratories, mean sr was 1.92% and SR was 2.19%. Improved performance parameters for determination of δ13C in honey are also presented. The method was adopted first action by AOAC International. Also presented Is an improved purification procedure for the honey protein required for the Internal standard method. The dialysis-precipitation procedure described is considerably less demanding of analyst's time and eliminates possible contamination of the preparation by Indigenous yeasts and molds. No difference In δ13C values was found between the collaboratively studied method and the method when used with the improved procedure to analyze 10 known pure honeys. The dialysis- precipitation procedure is recommended as a substitute for the repeated washing procedure described In the collaboratively studied method.

Carbon and nitrogen isotope ratios of mistletoes growing on nitrogen and non-nitrogen fixing hosts and on CAM plants in the Namib desert confirm partial heterotrophy.

Xylem-tapping mistletoe species growing on Mimosaccae, non-Mimosaceae and hosts performing Crassulacean acid metabolism (CAM) were studied along an aridity gradient in the Namib desert. °13C-values of mistletoes became more negative with decreasing nitrogen (N)-concentration in their leaves, while the host plants showed no such relationship. This might suggest that mistletoes regulate their water use efficiency according to the nitrogen supply from the host. However, further inspection of the data indicates that the relations of δ13C-values with leaf nitrogen in mistletoes may result from carbon input from the host. This is especially true for mistletoes growing on CAM plants which exhibit a very high δ13C-value, but show no evidence of CAM. It is calculated that about 60% of the carbon in mistletoes growing on C3 and on CAM hosts originated from the host. The hypothesis of Marshall and Ehleringer (1990) that xylem tapping mistletoes are also carbon parasites could explain the change in δ13C-values with N-supply and the difference in δ13C-values between mistletoes growing on C3 and CAM hosts.

Water status related photosynthesis and carbon isotope discrimination in species of the lichen genusPseudocyphellaria with green or blue-green photobionts and in photosymbiodemes.

Green lichens have been shown to attain positive net photosynthesis in the presence of water vapour while blue-green lichens require liquid water (Lange et al. 1986). This behaviour is confirmed not only for species with differing photobionts in the genusPseudocyphellaria but for green and blue-green photobionts in a single joined thallus (photosymbiodeme), with a single mycobiont, and also when adjacent as co-primary photobionts. The different response is therefore a property of the photobiont. The results are consistent with published photosynthesis/water content response curves. The minimum thallus water content for positive net photosynthesis appears to be much lower in green lichens (15% to 30%, related to dry weight) compared to blue-greens (85% to 100%). Since both types of lichen rehydrate to about 50% water content by water vapour uptake only green lichens will show positive net photosynthesis. It is proposed that the presence of sugar alcohols in green algae allow them to retain a liquid pool (concentrated solution) in their chloroplasts at low water potentials and even to reform it by water vapour uptake after being dried. The previously shown difference in δ13C values between blue-green and green lichens is also retained in a photosymbiodeme and must be photobiont determined. The wide range of δ13C values in lichens can be explained by a C3 carboxylation system and the various effects of different limiting processes for photosynthetic CO2 fixation. If carboxylation is rate limiting, there will be a strong discrimination of13CO2, at high internal CO2 partial pressure. The resulting very low δ13C values (-31 to-35‰) have been found only in green lichens which are able to photosynthesize at low thallus water content by equilibraiton with water vapour. When the liquid phase diffusion of CO2 becomes more and more rate limiting and the internal CO2 pressure decreases, the13C content of the photosynthates increases and less negative δ13C values results, as are found for blue-green lichens.

Use of stable-carbon isotope analysis to estimate marine and terrestrial protein content in gull diets

Stable-carbon isotope analyses of bone collagen of gulls (Larus spp.) were used to estimate the relative proportion of marine and terrestrial protein in the diets of gulls that are known to use both types of food sources. Mean δ13C values for Glaucous-winged Gulls (n = 23) collected at a Vancouver dump and for Western Gulls (n = 18) from Southeast Farallon Island were −15.0 ± 1.3 and −15.1 ± 0.5‰, respectively. No significant difference in δ13C values was found between adult male and female Western Gulls. Archaeological gull bones (n = 3) show a mean δ13C value of −13.6 ± 1.0‰ and support the assumed marine end point of −13.0‰. The range of terrestrial protein in the diets of coastal gulls was 0–61%. The narrow distribution of δ13C values for Western Gulls from the Southeast Farallon Island colony suggests that gull colonies may be calibrated so that changes in dependence on terrestrial protein can be monitored.