Triple oxygen (16O, 17O, 18O) and carbon (12C, 13C) isotope variations in bioapatite of small mammals – new insights concerning the reconstruction of palaeo‐CO2 concentrations and palaeotemperatures

Abstract

The ratios among the stable isotopes of oxygen (16O, 17O, 18O) and carbon (12C, 13C) in biogenic apatite of mammal teeth and bones have the potential to provide insight into the isotopic composition of the respective oxygen and carbon sources. The isotope ratios of these sources (e.g. drinking water, air oxygen, diet) are connected to specific environmental and climatic conditions, such as temperature, relative humidity, CO2 concentration, bioproductivity, palaeovegetation and vegetation density. Analysis of the oxygen and carbon isotope composition of diagenetically unaltered skeletal tissue of fossil mammals can provide evidence on these climatic and environmental parameters far back into Earth’s history. Furthermore, it is possible to receive information regarding the mode of life and specific behaviour of the examined taxa, e.g. migrational behaviour, birth seasonality, drinking behaviour, food preferences, resource partitioning and habitat use. From the early beginning of research in this specific field of biogeochemistry in the 1970s and 1980s until today, a large number of studies have been published which examine the oxygen and carbon isotope composition (18O/16O, 13C/12C) in tooth and bone material of (in most cases large) modern and fossil mammals. Less than two decades ago, an anomalous triple oxygen isotope composition (17O/16O, 18O/16O) has been observed in tropospheric oxygen, which is significantly different from nearly all other terrestrial materials. This anomaly in molecular oxygen is the result of non-mass-dependent fractionation due to photochemical processes in the stratosphere and is transferred to the troposphere by gas exchange. Its magnitude is a function of the atmospheric CO2 concentration and global bioproductivity. If it is possible to reconstruct the triple isotope composition of tropospheric oxygen to certain times in Earth’s history, conclusions can be drawn on the respective CO2 level or bioproductivity. However, geological archives, which are capable of this, are extremely scarce. As inhaled air oxygen is one of the most important oxygen sources in mammals, bioapatite of modern and fossil mammals is one of the few materials that have the potential to be such an archive. The present thesis evaluates variations in the triple oxygen isotope composition of bioapatite from modern mammals as well as its application potential as a new proxy for palaeo-CO2 reconstruction and as a tracer of diagenetically altered fossil skeletal tissue. Furthermore, interspecific, intraspecific and intraindividual variations of the oxygen and carbon isotope composition of bioapatite of modern rodents are investigated to reach a better interpretative background for respective analytical result from fossil representatives of this largest mammalian order. Chapter 2 evaluates the question, whether the anomalous isotope signature of tropospheric oxygen can be used as a tracer for diagenetic alteration of tooth and bone phosphate of fossil mammals. Therefore, the triple oxygen isotope composition of tooth enamel, dentine and to a minor extent also bone material of several individuals of Cenozoic small mammals (i.e. rodents) has been analysed separately. While all tooth enamel samples have a pronounced oxygen isotope anomaly within a range that is expected for diagenetically unaltered bioapatite of small mammals, all dentine samples have a considerably lower or no anomaly, which suggests isotopic exchange with diagenetic fluids. In chapter 3, the variations of the anomalous isotope signature of tropospheric oxygen in the skeletal tissue of modern mammals are evaluated. This has been conducted using two independent approaches: (1) By triple oxygen isotope analysis of bioapatite of modern mammals and (2) by developing a detailed mass balance model. The fraction of inhaled air oxygen in proportion to the other principal oxygen input sources is primarily a function of the specific metabolic rate, which largely scales with body mass. Therefore, samples of species that comprise a wide range of body masses, from a few g to several 1000 kg, were analysed, leading to the observation of an increasing anomalous oxygen isotope signature with decreasing body mass. Based on the obtained findings, it has been attempted to determine the magnitude of the anomalous oxygen isotope signature of the troposphere at different times within the Cenozoic era from Eocene, Oligocene and Miocene rodent tooth enamel, which gives insight into the respective CO2-levels. The theoretical mass balance model agrees well with the analytical data, and both indicate an increasing oxygen isotope anomaly in the bioapatite with decreasing body mass. The reconstructed CO2 concentrations generally agree with previous data obtained with other methods, but the associated error precludes resolving CO2 fluctuations in a range of a few 100 ppm. At the Palaeocene-Eocene boundary, one of the most remarkable environmental and climatic changes within the Cenozoic took place, which had its summit within the “Palaeocene Eocene Thermal Maximum” (PETM). This event was associated with a global negative carbon isotope excursion (CIE), whose source is still discussed controversially. Chapter 4 targets temperature change and CO2 fluctuations at the transition of both chronostratigraphic series. This is conducted with the help of a sample series of tooth enamel of the mammal genus Ectocion originating from the Clarks Fork Basin (Wyoming, USA), encompassing the respective time interval. The reconstructed temperature change is in good accordance with previous studies on the 18O/16O ratio from biogenic apatite within this period. The reconstructed CO2 levels also indicate that during the peak PETM phase a value of 1550 ppm was not exceeded. Thus, it is suggested that dissociation of marine methane clathrates has been the main source of the CIE. Chapter 5 investigates inter- and intraspecific as well as intraindividual variations of the carbonate oxygen, phosphate oxygen and the carbon isotope composition of seven different rodent species sampled from owl pellets of a single locality. The results are compared to similar studies on large mammals, and conclusions are drawn on the handling of sample material of small mammals when used for palaeoclimate reconstruction by means of stable isotope analysis. The variability in the oxygen and carbon isotope composition of the analysed tooth and bone material is not higher than that of many large mammals, supporting the relevance of rodents, which are considerably more abundant in the fossil record, for such studies. However, attention has to be paid to the fact that bioapatite-temperature calibrations derived from modern species need to be based exactly on the same skeletal tissue that is analysed from fossil material. This is important because of variations within the time intervals of the mineralisation of different teeth and bone material, in which significant differences of the isotope compositions could be observed, in particular regarding oxygen isotopes.