Isotopic Paleoclimatology

Abstract

Isotopic paleoclimatology originated from the seminal work of Harold C. Urey on both the experimental and theoretical aspects of isotope chemistry leading to and following his being awarded the Nobel Prize in 1934. In that year, he had already studied oxygen isotope ratios in stony meteorites and terrestrial rocks finding them to be constant to 2.5%. Later refinements gave permil accuracies then undreamed of and, after his second world war involvement as director of a program at Columbia intended to separate uranius isotopes, he treated the thermodynamics of isotopic equilibria and turned his attention to geochemistry and cosmic problems. After Nier constructed a mass spectrometer for measuring isotopic variations in gas samples with extraordinary accuracy, Urey further increased its precision and started to take the temperatures of Cretaceous belemnites by assessing the oxygen isotope fractionation between ocean water and the carbonate precipitated by the animals. This constitutes one of the greatest developments in the earth sciences and encompasses a wide spectrum of disciplines from biology through isotopic fractionation to the history of the Earth. At the University of Chicago, he created a world center of geochemistry where his colleagues included such luminaries as Willard Libby and Hans Suess and he published his chef d’oeuvre “The Planets: Their Origin and Development”. In 1958, Urey migrated to California where, at the Scripps Institution of Oceanography, he inaugurated a School of Chemistry at which the writer spent three years on paleotemperature analyses, applying prior researches of Heinz Lowenstam and Sam Epstein to Mesozoic belemnites using their paleotemperature scale and PDB standard. Subsequently, new scales were produced by Harmon Craig and others and attempts were made to derive them from two other mineral phases, namely biogenic silica-water and biogenic phosphate-water. These were successful, but the differences in the δ-values of carbonate, silica and phosphate in equilibrium with water were constant. Hence it proved impossible to use either silica or phosphate to assess the temperature and isotopic composition of water from measurements of δ¹⁸O values in pairs of biogenic carbonate, silica and phosphate. Further researches still progressing have accelerated the accumulation of additional knowledge about the climates of the past based on what some physicists initially commented on scornfully as “Urey’s plumbing” and which used to be discussed before he left us at the “Urey lunch table” beside the Pacific Ocean at La Jolla. As well as the isotopes of oxygen, those of hydrogen, carbon, nitrogen, sulphur, strontium, etc., have been examined and some of the results described in this chapter. The further back in time we go, the less reliable and scantier the evidence becomes, so approximately two-thirds of it deals with the isotopic paleoclimatology of the last 66.5 Ma. And, although the subject is discussed before that, indeed back to the Precambrian which ended almost 600 Ma ago, a caveat is needed regarding the limits of predictability in a chaotic solar system (see later). Important new topics are ventilated and these include state-of-the-art information apropos the global carbon cycle, the missing carbon problem, simulation of the ice age, modelling climatic response for the Pleistocene, modelling arrays, transformations, multivariate procedures and climate forcing, the quantifying of isotopic chrono-stratigraphy, ice-free oceans and geochemical changes in relation to mass extinctions. And some old topics are reviewed too, e.g. ice volume versus paleotemperatures, data from sediment cores in deep oceans and ice cores and extension of the technique from belemnites to other organisms such as brachiopods and foraminifera. Therefore, even taking into account necessary limitations of space, it is hoped that a useful bird’s eye view of isotopic paleoclimatology is presented. And while I cannot claim to have followed Urey’s maxim of “Treating the whole subject” adequately, I have tried my best within the constraints imposed remembering also his other statement that “this treatise as well as all others is surely incomplete and at most only partly true”.