Kinetic Isotope EffectsDelta NotationTo understand carbon isotope variations with time we must appreciate open system behavior of carbon isotopes in seawater. Many interesting natural systems are open such that reactants are constantly added and products constantly withdrawn.GEOL 342: Sedimentation and Stratigraphy20 April 2005Assoc. Prof. A. Jay KaufmanChemostratigraphyAlthough isotopes formed by radiometric decay are important in dating, some isotopesare stable in nature and these have well-defined ratios in the oceans and atmosphere. Physicaland biological processes, however, can redistribute these isotopes providing clues to ancientenvironments, climates, and biological communities. In recent years, chemical stratigraphy orchemostratigraphy has become an important tool for detailed correlation of widely separatedsedimentary deposits, especially for the Proterozoic Eon, which lacks a biostratigraphicframework.Stable isotope geochemistryStable isotope geochemistry is primarily concerned with variations in the isotope ratios ofH, C, N, O, Si, and S (and to a lesser degree to Li and B). These elements have severalcharacteristics in common:1. They have low atomic mass.2. The relative mass difference between rare (enriched) and abundant isotopes is large.3. They form chemical bonds that have a high degree of covalent character.4. The abundance of the rare isotope is sufficiently high to assure precise measurement ofisotope ratios by mass spectrometry.5. Generally the heavy isotope is concentrated in the solid phase in which it is more tightlybound.Isotopes – atoms whose nuclei contain the same number of protons but a different number ofneutrons (i.e., 12C contains 6 protons and 6 neutrons, but 13C contains 6 protons and 7 neutrons).12C = 98.89%13C = 1.11%16O = 99.76%18O = 0.21%There are about 300 stable isotopes in nature, while over 1200 unstable (or radiogenic)isotopes have been discovered thus far. Only 21 of the elements are “pure,” meaning that theyhave only one stable isotope; all others are mixtures of two or more.Small mass differences impart different physiochemical properties of molecules. Isotopeeffects are differences in chemical or physical properties which arise from variations in theatomic mass of an element (for example, evaporation and condensation of water). An isotopeeffect is not directly observable, but is inferred from its effect on isotopic abundances. Thepresence of an isotope effect leads to an isotopic fractionation. 1Equilibrium isotope effectsEquilibrium isotope effects (EIE) can be considered in terms of effects of atomic mass onbond enegy. When a light isotope is substituted for a heavy one, the nuclear charge and electronicdistributions remain the same, therefore the potential energy curve remains unchanged. A goodrule of thumb is that between two molecules the heavier isotope will accumulate in the one withthe strongest bonds. Kinetic Isotope EffectsKIEs occur due to differences in reaction rates of isotopic molecules. A normal KIE isone in which the species containing the lighter isotope reacts more rapidly. Kinetic isotopeeffects are intensely studied for the information that they can provide about mechanistic details ofreaction pathways (e.g., photosynthesis)Delta NotationBecause most isotopic measurements are differential measurements, and because theinteresting isotopic differences between natural samples occur at and beyond the third significantfigure of the isotope ratio, it is convenient to express isotope abundances in a differentialnotation.AXSTD = [(ARSample – ARSTD)/ARSTD]103 = (R/R)103The parameter AXSTD is referred to as the delta notation, and the units (parts per thousand) areassigned the ‰ and are called per mil.Common isotope standard reference materials include:2carbon – PDB (a reference calibrated to a Cretaceous-aged belemnite limestone from thePee Dee Formation of South Carolina)oxygen – PDB, SMOW (standard mean ocean water), SLAP (standard light Antarcticprecipitation)sulfur – CDT (Canyon Diablo triolite)nitrogen – airOxygen isotope chemostratigraphyIn 1947 Urey and Emiliani discovered that oxygen isotopes fractionate depending ontemperature. By examining the shells of foraminifera through the Pleistocene, they saw changesin the oceans that apparently were responding to temperatures changes associated with the iceages. Later work, however, showed that the more important effect on oxygen isotopes inseawater was the buildup of ice in continental glaciers.dD (l)-500-480-460-440-420-4000.0 5.0e+4 1.0e+5 1.5e+5 2.0e+5 2.5e+5 3.0e+5 3.5e+5 4.0e+5 4.5e+5atm. CO2 (ppmv)160180200220240260280300320Years before present0.0 5.0e+4 1.0e+5 1.5e+5 2.0e+5 2.5e+5 3.0e+5 3.5e+5 4.0e+5 4.5e+5d11B (l)24.424.825.225.626.026.426.8d18O (l)-3-2-10123Termination ITermination IIABCD65e5d5c5b5a432178BarbadosPapua New GuineaCarbon isotope chemostratigraphyBecause oxygen isotopes are so easily altered under the influence of diagenetic solutionsalternative methods in chemostratigraphy are commonly used to support correlations. Forexample, insofar as there is little carbon in most fluids, carbon isotopes in carbonate may morereadily reflect depositional rather than diagenetic processes. The carbon system is not controlledby ice volume or by temperature, but by processes and environments that affect the burial oforganic matter in sediments. Since carbon isotope distributions are primarily a reflection ofoceanographic and climatic changes on a global scale, strong temporal excursions can be goodtime markers.To understand carbon isotope variations with time we must appreciate open systembehavior of carbon isotopes in seawater. Many interesting natural systems are open such thatreactants are constantly added and products constantly withdrawn. 3At steady state the withdrawal of products equals the addition of reactants such that theamount of reactant in the system is constant, thus making isotopic calculation straightforward.The isotopic fractionation (A) between products is controlled by the isotope effect and therequirement that matter is neither created nor destroyed. This is the basis for understanding longterm changes in the carbon isotopic composition of seawater based on changing