Triple Oxygen Isotopes of Cherts : Implications for the δ18O and Temperatures of Early Oceans
by Sukanya Sengupta
Date of Examination:2016-07-07
Date of issue:2017-06-19
Advisor:Prof. Dr. Andreas Pack
Referee:Prof. Dr. Andreas Pack
Referee:Prof. Dr. Jochen Hoefs
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Abstract
English
The temperature of Earth’s earliest oceans, in which life may have first originated, remains a controversial issue in Earth science. Over the last 50 years, many studies have attempted to apply the temperature dependent fractionation of oxygen isotopes 18O/16O to ancient marine chemical sediments in order to infer the temperatures of the ancient oceans. The δ18O composition of marine chemical sediments progressively decrease with increasing age leading to the suggestions that either the Precambrian oceans were also strongly depleted in 18O (~-13‰) but had the same temperature as modern day, or that they had the same δ18O value but were extremely hot (up to 80°C). A third opinion is that these chemical sediments are all diagenetically altered and unsuitable for paleoenvironmental studies. The current study introduces and demonstrates the use of a new additional parameter in resolving the classical problem – triple oxygen isotope composition of modern and ancient cherts. Mass dependent 18O/16O and 17O/16O fractionations result in resolvable unique curves in the triple oxygen isotope space, which may be used to identify individual equilibrium and kinetic processes. This thesis comprises, 1) a general introduction to the thesis; 2) a description of the general terminology and the technique for extraction as well as gas cleaning to obtain impurity free oxygen gas from cherts and silicates, suitable for high-precision δ17O analysis; 3) a geochemical mass balance model for δ17O and δ18O values of present and ancient seawater; 4) high-precision triple-oxygen isotope analyses and trace element analyses of cherts and amorphous silica samples – their implications for δ18O and temperature of Earth’s early oceans and 5) a general conclusion. This is followed by the Appendix, containing four manuscripts on which I am co-author. These are (I) manuscript entitled “Revealing the climate of snowball Earth from Δ17O systematics of hydrothermal rocks” by Herwartz et al. (2015); (II) manuscript entitled “A calibration of the triple oxygen isotope fractionation in the SiO2–H2O system and applications to natural samples” by Sharp et al. (2016); (III) manuscript entitled “The oxygen isotope composition of San Carlos olivine on the VSMOW2-SLAP2 scale” by Pack et al. (2016) and (IV) manuscript entitled “Tracing the oxygen isotope composition of the upper Earth’s atmosphere using cosmic spherules” by Pack et al. (2017). Chapter 1 is an introduction into the aim of the thesis and the discussion about low δ18O oceans vs. hot oceans. Chapter 2 first describes the terminology and data normalization methods used in the current study. A two-point calibration method of normalizing oxygen isotope data on the VSMOW2-SLAP2 scale is described and adopted. The next section of the chapter describes method that was used to extract oxygen from cherts, amorphous silica and silicates. After the oxygen gas is liberated from the sample, a critical step is cleaning it using a gas chromatography column to get rid of impurities like NF3, CF4, N2 and other trace gases in order to obtain pure oxygen gas. This pure gas is then suitable for analyses of triple oxygen isotopes composition it the dual-inlet mode of our mass spectrometer. Repeated measurements of the same gas over long periods of time yield high precision Δ'17O data (~10 ppm SD). Chapter 3 presents a geochemical mass balance model for the present and past triple oxygen isotope composition of the oceans. This chapter includes analyses of modern oceanic crust samples that have been altered by seawater at different temperatures. High-T: low-T alteration of oceanic crust is the main control on the present day oxygen isotope composition of seawater. Other fluxes that help maintain a steady-state oxygen isotope composition of seawater over time have also been discussed in the chapter. Finally, a projection is made on how these different processes could have varied in the past and resulted in a low δ18O and δ17O ocean, mainly via decreasing the ratio of high- to low-T oceanic crust alteration. This modeled trend provides the basis for the interpretation of the triple oxygen isotope data of our chert samples. Chapter 4 of this thesis presents high-precision triple oxygen isotope data of chert and amorphous silica (diatoms and sponges) samples from different geological locations, ages (Phanerozoic to Archean) and settings. The oxygen isotope data are used to revisit the relationship between temperature and 17O/16O, 18O/16O equilibrium fractionation in the silica-water system. Trace element analyses for most of the chert samples are also presented. This data along with the oxygen isotope data, are used to infer depositional and diagenetic histories of the individual chert samples with implications for the δ18O and temperature conditions of Precambrian ocean. The results reveal that many of the Precambrian samples are not in equilibrium with modern seawater composition at any temperatures; they also do not display equilibrium with an extreme light δ18O and light δ17O ocean. Most of the sample can be explained by equilibration with modern meteoric water or hydrothermal water –seawater mixtures. The current study, thus, provides an additional parameter for interpreting the oxygen isotope data of cherts and application to estimate Precambrian ocean δ18O and temperatures. The possibility of a hot Precambrian ocean remains, but equilibration with extremely light δ18O oceans (~-13‰) is excluded at least for most of the chert samples analyzed in this study. Chapter 5 is a chapter on the conclusions of the current study. Four manuscripts on which I am co-author have been included in the Appendix– (I) Herwartz et al. (2015), (II) Sharp et al. (2016) and (III) Pack et al. (2016). The contribution to the manuscript by Herwartz et al. (2015) is help with triple oxygen isotope analysis in the laboratory and writing some parts of the paper. For the manuscript by Sharp et al. (2016) I analyzed triple oxygen isotope values of diatom samples via laser fluorination using F2. My contribution to the third manuscript by Pack et al. (2016) is running oxygen extraction experiments from air, VSMOW2 and VSLAP2. My contribution to the fourth manuscript by Pack et al. (2017) is extracting and measuring air oxygen along with students (also as a part of their Bachelor thesis).
Keywords: oxygen+isotopes