We discuss the physical and enzymatic bases of carbone isotope discrimination during photosynthesis, noting how knowledge of discrimination can be used to provide additional insight into photosynthetic metabolism and the environmental influences on that process

Carbon Isotope Discrimination and Photosynthesis

The influence of diet on the distribution of nitrogen isotopes in animals was investigated by analyzing animals grown in the laboratory on diets of constant nitrogen isotopic composition. 
The isotopic composition of the nitrogen in an animal reflects the nitrogen isotopic composition of its diet. The δ^(15)N values of the whole bodies of animals are usually more positive than those of their diets. Different individuals of a species raised on the same diet can have significantly different δ^(15)N values. The variability of the relationship between the δ^(15)N values of animals and their diets is greater for different species raised on the same diet than for the same species raised on different diets. Different tissues of mice are also enriched in ^(15)N relative to the diet, with the difference between the δ^(15)N values of a tissue and the diet depending on both the kind of tissue and the diet involved. The δ^(15)N values of collagen and chitin, biochemical components that are often preserved in fossil animal remains, are also related to the δ^(15)N value of the diet. 
The dependence of the δ^(15)N values of whole animals and their tissues and biochemical components on the δ^(15)N value of diet indicates that the isotopic composition of animal nitrogen can be used to obtain information about an animal's diet if its potential food sources had different δ^(15)N values. The nitrogen isotopic method of dietary analysis probably can be used to estimate the relative use of legumes vs non-legumes or of aquatic vs terrestrial organisms as food sources for extant and fossil animals. However, the method probably will not be applicable in those modern ecosystems in which the use of chemical fertilizers has influenced the distribution of nitrogen isotopes in food sources. 
The isotopic method of dietary analysis was used to reconstruct changes in the diet of the human population that occupied the Tehuacan Valley of Mexico over a 7000 yr span. Variations in the δ^(15)C and δ^(15)N values of bone collagen suggest that C_4 and/or CAM plants (presumably mostly corn) and legumes (presumably mostly beans) were introduced into the diet much earlier than suggested by conventional archaeological analysis.

Influence of Diet On the Distribtion of Nitrogen Isotopes in Animals

Global silicate weathering and CO2 consumption rates deduced from the chemistry of large rivers

The International Atomic Energy Agency (IAEA), in cooperation with the World Meteorological Organization (WMO), has been conducting a world-wide survey of hydrogen (H/'H) and oxygen (O/O) isotope composition of monthly precipitation since 1961 At present, 72 IAEA/WMO network stations are in operation Another 82 stations belonging to national organizations continue to send their results to the IAEA for publication The paper focuses on basic features of spatial and temporal distribution of deuterium and O in global precipitation, as derived from the IAEA/WMO isotope database The internal structure and basic characteristics of this database are discussed in some detail The existing phenomenological relationships between observed stable isotope composition of precipitation and various climate-related parameters such as local surface air temperature and amount of precipitation are reviewed and critically assessed Attempts are presented towards revealing interannual fluctuations in the accumulated isotope records and relating them to changes of precipitation amount and the surface air temperature over the past 30 years

Isotopic Patterns in Modern Global Precipitation

The second edition of The Biomarker Guide is a fully updated and expanded version of this essential reference. Now in two volumes, it provides a comprehensive account of the role that biomarker technology plays both in petroleum exploration and in understanding Earth history and processes. Biomarkers and Isotopes in the Environment and Human History details the origins of biomarkers and introduces basic chemical principles relevant to their study. It discusses analytical techniques, and applications of biomarkers to environmental and archaeological problems. The Biomarker Guide is an invaluable resource for geologists, petroleum geochemists, biogeochemists, environmental scientists and archaeologists.

The Biomarker Guide

Products and distinguishing criteria of bacterial and thermochemical sulfate reduction

Large H2S accumulations in deep carbonate reservoirs have been attributed to thermochemical sulphate reduction. Although organic matter has been invoked as the reducing agent, individual reactants in specific environments have hitherto not been identified. Carbon isotope data reveal that light hydrocarbon gases have been effective reducing agents in some Palaeozoic foothill reservoirs of western Canada.

Chemical and isotopic evidence of thermochemical sulphate reduction by light hydrocarbon gases in deep carbonate reservoirs

The origin and the fate of sulphate in groundwater has concerned hydrogeologists for many decades. With increasing industrialisation throughout this century, the differentiation between natural and ‘man-made’sulphate became important. A particular challenge is evaluating potential and existing additions of the latter to groundwater and drinking water supplies. On regional and even global scales, sulphate as a component of ‘acid rain’captured scientific and public awareness in the 1970s and 1980s. Subsequently, intensive research programs on the effects of atmospheric sulphate deposition on terrestrial and aquatic ecosystems were initiated. Today, more than two-thirds of the sulphate in atmospheric deposition in heavily industrialised regions of the northern hemisphere is of anthropogenic origin (Section 7.3.2.4). The rate and the degree to which these increased sulphate inputs have led to acidification of surface water and groundwater has been a matter of considerable scientific debate in the past two decades.

Sulphur and Oxygen Isotopes in Sulphate

Abiogenic reduction of sulfate by acetic acid has been conducted at temperatures ranging from 241 to 340°C. The sulfides produced and the remaining sulfates have been chemically and isotopically quantified. The reaction of the aqueous sulfate reduction obey first-order kinetics. The rate constants are strongly dependent on temperature. The temperature dependence of the reaction rate (min-1) is given by the following rate law: log k=-10.3×103/T+15.1. The activation energy for the reaction of sulfate reduction is 208 kJ mol-1. The remaining sulfates display distinct sulfur isotopic variations under a closed system due to the kinetic isotope effect. The kinetic isotope fractionation factor can be expressed as: 103(α-1)=3.32×106/T2-4.19. The kinetic isotope effects during abiogenic reduction of the sulfate may depend on the atomic hydrogen produced, independent of the reducing agents.

https://www.jstage.jst.go.jp/article/geochemj1966/24/1/24_1_21/_pdf

The role of organic acid in the abiogenic reduction of sulfate and the sulfur isotope effect

The total carbon δ13C values of two C3 halophytes,Salicornia europaea L. ssp.rubra (Nels.) Breitung andPuccinellia muttalliana (Schultes) Hitch., native to inland saline areas of Alberta, Canada, were determined for plants grown under controlled conditions of supplied NaCl in the nutrient solution, and for plants found growing in the field. Field specimens were collected along line transects which ran from areas of high salinity to areas of low salinity across the pattern of species zonation. The δ13C value of the two species seemed to reflect the water potential of the soil (ψ
 w
 soil
 ) as measured arbitrarily at a depth of 10 cm, becoming less negative as the ψ
 w
 soil
 decreased. Over a linear distance of 5.55 m,S. europaea spp.rubra showed a shift of +5.3‰ as the ψ
 w
 soil
 went from-25x102 kPa to a minimum of-73x102 kPa. ForP. nuttalliana, the δ13C values differed by 3.4‰ over a distance of 7.45 m where the maximum difference in ψ
 w
 soil
 was 12.7x102 kPa. However, δ13C values ofP. nuttalliana only roughly reflected the spatial trends in ψ
 w
 soil
 at the time of collection. In the growth chamber, the δ13C value ofS. europaea ssp.rubra changed by a maximum of +8.0‰ when the solute potential of the nutrient solution (ψ
 w
 soil
 ) was dropped from-0.25x102 kPa to-64.25x102 kPa; while the δ13C value ofP. nuttalliana changed by a maximum of +10.8‰ when the ψ
 w
 soil
 was dropped from-0.25x102 kPa to-40.25x102 kPa. Linear regression analyses indicated that the δ13C values of both species were strongly correlated (P<0.2%) with ψ
 w
 soil
 . The observed shifts in δ12C may represent changes in the mode of photosynthetic CO2 fixation. However, a number of other explanations, some of which are discussed in the text, are also possible. A proper ecophysiological interpretation of such shifts in δ13C values of C3 plants awaits a better understanding of the isotope fractionation mechanisms involved.

H. Roy Krouse

Papers

Products and distinguishing criteria of bacterial and thermochemical sulfate reduction

Chemical and isotopic evidence of thermochemical sulphate reduction by light hydrocarbon gases in deep carbonate reservoirs

Sulphur and Oxygen Isotopes in Sulphate

The role of organic acid in the abiogenic reduction of sulfate and the sulfur isotope effect

Shifts in carbon isotope ratios of two C3 halophytes under natural and artificial conditions.