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- Isotope fractionation (3) (entfernen)
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In most chemical reactions, stable isotopes are fractionated in a mass-dependent manner, yielding correlated isotope ratios in elements with three or more stable isotopes. The proportionality between isotope ratios is set by the triple isotope fractionation exponent θ that can be determined precisely for, e.g., sulfur and oxygen by IRMS, but not for metal(loid) elements due to the lower precision of MC-ICP-MS analysis and smaller isotopic variations. Here, using Mg as a test case, we compute a complete metrologically robust uncertainty budget for apparent θ values and, with reference to this, present a new measurement Approach that reduces uncertainty on θ values by 30%. This approach, namely, direct educt-product bracketing (sample−sample bracketing), allows apparent θ values of metal(loid) isotopes to be determined precisely enough to distinguish slopes in three-isotope space. For the example of Mg, we assess appropriate quality Control standards for interference-to-signal ratios and Report apparent θ values of carbonate−seawater pairs. We determined apparent θ values for marine biogenic carbonates, where the foraminifera Globorotalia menardii yields 0.514 ± 0.005 (2 SD), the coral Porites, 0.515 ± 0.006 (2 SD), and two specimens of the giant clam Tridacna gigas, 0.508 ± 0.007 (2 SD) and 0.509 ± 0.006 (2 SD), documenting differences in the uptake pathway of Mg among marine calcifiers. The capability to measure apparent θ values more precisely adds a new dimension to metal(loid) δ values, with the potential to allow us to resolve different modes of fractionation in industrial and natural processes.
Various plant compartments of a single bell pepper plant were studied to verify the variability of boron isotope composition in plants and to identify possible intra-plant isotope fractionation. Boron mass fractions varied from 9.8 mg/kg in the fruits to 70.0 mg/kg in the leaves. Boron (B) isotope ratios reported as δ11B ranged from -11.0 to +16.0 (U ≤ 1.9, k=2) and showed a distinct trend to heavier δ11B values the higher the plant compartments were located in the plant. A fractionation of Δ11Bleaf-roots = 27 existed in the studied bell pepper plant, which represents about about 1/3 of the overall natural boron isotope variation (ca. 80). Two simultaneous operating processes are a possible explanation for the observed systematic intra-plant δ11B variation: 1) B is fixed in cell walls in its tetrahedral form (borate), which preferentially incorporates the light B isotope and the remaining xylem sap gets enriched in the heavy B isotope and 2) certain transporter preferentially transport the trigonal 11B-enriched boric acid molecule and thereby the heavy 11B towards young plant compartments which were situated distal of the roots and typically high in the plant. Consequently, an enrichment of the heavy 11B isotope in the upper young plant parts located at the top of the plant could explain the observed isotope systematic. The identification and understanding of the processes generating systematic intra-plant δ11B variations will potentially enable the use of B isotope for plant metabolism studies.
Knowledge about feeding behavior is essential to determine trophic interactions and reconstruct predator-prey relationships in modern and past foodwebs. Traditionally, nitrogen isotopes (δ15N) of collagen are used to quantify the ingestion of animal protein, however collagen usually does not preserve over geological time scales. To infer the diet of extinct vertebrates from fossil material more resistant dietary proxies such as Ca and Mg isotopes are needed, which are major elements in the bioapatite of bones and teeth. Magnesium is a bio-essential element that replaces calcium in the bioapatite lattice and bone and enamel δ26Mg values of extant mammals increase systematically along the foodchain [1, 2].
The existing δ26Mg data, however, is scarce, in particular with respect to carnivores, thus still limiting the capability of this dietary proxy to reliably determine trophic differences between plant- and animal-feeders. To better constrain trophic-level effects recorded in Mg isotopes, we analyzed δ26Mg of bioapatite from modern mammals with a focus on faunivores, both carnivores and as yet unexplored insectivores.
The trophic level effect of δ26Mg is influenced by the geological substrate which causes isotope variability in δ26Mg of faunal remains between different ecosystems [1, 2]. Therefore, as first-order proxy for sample provenance and to assess potential influences of the bedrock substrate of the animals´ habitats on δ26Mg of bones and teeth, we measured the 87Sr/86Sr on the same specimens. This information will enable us to refine trophic level effects and determine whether δ26Mg can be used to distinguish different faunivores isotopically. This will be of paramount importance for dietary reconstructions of trophic niches in fossil foodwebs.
[1] Martin et al. (2014) Geochmica Cosmochimica Acta 130, 12-20. [2] Martin et al. (2015) Proceedings of the National Academy of Sciences 112, 430-435.