Isotopic Variability

Air N2 is well-mixed, and acts as a very large reservoir both factors buffer against much variability in the stable isotope ratios of Nj (Mariotti 1983). 

There are also indications that climate-related shifts in wood 5 C of 1 to 3%o have occurred in time such shifts have been observed at the Pleistocene-Holocene boundary (Becker et al. 1991 Van de Water et al. 1994), and at later (Epstein and Krishnamurthy 1990) and earlier times (Aucour et al. 1993 Leavitt and Danzer 1992). Other temporal variations in plants can be seasonal (Leavitt and Long 1991 Loader et al. 1995). 

Variability within plants and between individual plants from the same location, can be quite extensive (in the order of several %o), often as a result of differences in growing conditions (light intensity, humidity), that can vary over a very short distance (Saurer et al. 1995). Systematic differences seem to occur between life forms such as trees, shmbs, cushion plants, etc. (Tieszen 1991 Tieszen and Fagre 1993a Valentini et al. 1995, 1992), and between different botanical groups (for example, deciduous versus coniferous trees Leavitt and Newberry 1992 Ramesh et al. 1986 Stuiver and Braziimas 1987). 

Variability in Plants, Nitrogen Uptake and Environmental Factors 

Plants take up inorganic nitrogen, either as atmospheric Nj, through symbiotic nitrogen fixation in legumes such as pulses, some shrubs and trees. 

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As a result, in the Holocene a clear pattern of climate-induced variation in 5 C values of plant material and in bone collagen exists across Emope (Van Klinken et al. 1994). Figures 3.1 and 3.2 show that wood and collagen values are closely correlated, demonstrating that the trend is passed on in the food chain. The trend of enrichment from northwestern to southern Europe correlates very strongly with the climatic pattern across Europe the climatic isotopic variability is on the order of 2 to 4%o. 

It is not the purpose of this paper to described human isotopic variability in Europe in detail (this will be done elsewhere Richards, in prep), but to... 

Leavitt, S.W. and Long, A. 1991 Seasonal stable-carbon isotope variability in tree rings possible palaeoenvironmental signals. Chemical Geology (Isotope Geoscience Section) 87 59-70. 

Tieszen, L.L. and Fagre, T. 1993a Carbon isotopic variability in modem and archaeological maize. 

What PCA is actually doing in this case in terms of processes is rather inappropriate. There is a consensus for explaining mantle isotopic variability as mixing geochemical... 

Figure 6. Summary of ranges in isotopic compositions for natural terrestrial samples as discussed in this volume. Isotopic variability in extraterrestrial samples is often greater. Isotopic compositions reported as 5 values in units of per mil (%o), based on isotopic ratios and reference standards as used in this volume (Table 1). Note that the range of isotopic compositions for Li is much greater than the scale used in the diagram, where 5 Li values vary from -21 to +54. In many cases, relatively large isotopic fractionations occur during redox reactions (see Chapter 3), and the common oxidation states in near-surface natural environments are listed on the left.

Inclusions of the CV3 led to the search for isotopic signatures of individual nucleosynthetic processes, or at least for components closer to the original signature than average solar compositions. They have also begun to demonstrate the isotopic variability of matter emerging from these processes in agreement with astrophysical and astronomical expectations. The principal features of inclusions are an up to 4% 0 enriched reservoir in the early solar system, variations in a component produced in a nuclear neutron-rich statistical equilibrium, and variations in the contribution of p- and r-process products to the heavy elements. 

Because of its isotopic variability, background must be reduced at any cost. The matrix of samples and standards must be reduced by appropriate chemistry to trace amounts, typically to a total concentration far smaller than the element to be analyzed. This requirement is most critical when the mass bias is inferred not internally from the sample itself, but externally from bracketing standards or from a different element used for isotopic normalization. Even the most dilute heavy species may drastically affect mass discrimination. 

Tomascak PB, Banner JL (1998) Lithium isotope hydrogeochemistry a multi-collector ICP-MS study. Geochemical Perspectives on Environmental Processes New Theoretical and Analytical Approaches to Sources, Transport, and Bioavailability of Trace Elements, St. Louis, Conf Abst Tomascak PB, Langmuir CH (1999) Lithium isotope variability in MORB. EOS Trans, Am Geophys Union 80 F1086-1087... 

At this point, nothing is known about Cu and Zn isotopic variability in seawater. Zn is very depleted in surface waters because it behaves as a nutrient. A substantial amormt of Zn isotope data is available for sediments. Marechal et al. (2000) found that the 5 Zn values of clay minerals from different environments (Paleozoic shales, including a black shale, Mediterranean sapropels. Pacific and Atlantic sediments, a eolian dust) fall within a narrow range (0.17-0.35%o) centered around the magmatic values and therefore reflect the Zn isotope composition... 

The data described above demonstrate a substantial fractionation of isotopes for both Cu and Zn. Zinc isotopic variability is particularly noticeable in the marine environment the 8 Zn... 

Ehrlich et al. (submitted) measured Cu isotopic fractionation between aqueous Cu(II) and covellite between 2 and 40°C (Fig. 10). The temperature-dependent isotope fraction is fairly large 3%o) and hints at a redox control of Cu isotopic variability in abiotic systems. Marechal and Sheppard (2002) conducted experiments at 30 and 50°C between malachite and a chloride solution for Cu isotope fractionation and between smithsonite and a nitrate solution for Zn. They found that, in this temperature range, Cu in malachite is 0.2 to 0.4%o lighter than in the chloride solution. Replacing the chloride by nitrate ion reduces fractionation which indicates that the coordination of the Cu ion dictates isotopic fractionation. In contrast, Zn isotope fractionation between smithsonite and fluid is extremely small (<0.1%o). 

Although equilibrium fractionations have been documented for some transition metal (i.e., Fe), they should be small and may be overwhelmed by kinetic fractionations in low-temperature and biological systems (Schauble 2004). For any transition metal, it remains to be demonstrated that biological effects dominate the natural isotope variability. 

Because of the isotopic variability and the high cosmic abundance of oxygen, oxygen isotopes are very useful for meteorite classification. Below the condensation temperature of silicates and above the condensation temperature of ices, approximately 25% of the oxygen in the solar nebula is predicted to have occurred in condensed solids, with the remainder in gaseous molecules. Chondrites provide samples of the condensed oxygen in the early solar system. 

The Late glazes are isotopically. much more variable than the Early glazes, which may indicate an increase in the number of sources of lead used by Mesopotamian potters. The increased isotopic variability could indicate that... 

It is essential to understand the isotopic variability within authigenic minerals that form the basis of terrestrial paleoclimate records and between such samples that are isochronous within an outcrop of a stratigraphic formation. Except for variability studies in biogenic materials (teeth, bone, shells) (e.g., Kohn et al. 2002), we know of no paleoaltimetry studies that have examined this natural variability in authigenic or pedogenic proxy materials. Such variability studies should be conducted before any quantitative estimates of paleoelevation can be made. 

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