Investigating Lithologic Characteristics of Marine Magnetic Proxy Parameters

Abstract

'Investigating Lithologic Characteristics of Marine Magnetic Proxy Parameters' (Ph.D. thesis Christine Franke) The influence of lithology on (rock) magnetic proxy parameters in marine and paleoclimate research is the subject of this thesis. Variations of the magnetic as well as of the matrix mineral assemblage can influence the nature of magnetic signals and their recording process. A better mineralogical identification of the magnetic carriers, their formation and origin deliver information on transport pathways and material budgets enabling the reconstruction of the regional sedimentary system and its paleoclimatical history. The work presented in this thesis mainly concerns the Equatorial and South Atlantic. Chapter 2 demonstrates the influence of the matrix lithology on the signal of the relative paleointensity (RPI) by sedimentological and statistical means. Especially clay minerals such as illite and kaolinite are found to be highly correlative with the RPI. Possibilities how to correct for such lithology effects are proposed. Chapters 3, 4 and 5 introduce several novel techniques to identify and quantify the various magnetic fractions present in the sediment samples. Magnetic extraction was tested versus heavy liquid separation for their efficiencies. Merits and disadvantages of these methods are demonstrated by low-temperature (5 to 300 K) magnetic measurements. Magnetic particles were identified by scanning and transmission electron microscopic analyses. A lithological classification system for marine magnetic particles is established identifying several detrital terrestrial, marine and authigenic sources. Titanomagnetite and hemoilmenite of very similar elemental composition could be identified by electron backscatter diffraction at particle level. Sediments from a West-East transect of the Equatorial Atlantic were investigated in great detail to identify and quantify the various magnetic particle species and the paleoclimatic control of their distribution. The final chapters 6 and 7 presents two case studies from the western (Argentine Basin) and eastern (Cape Basin) Atlantic assessing the influence of early diagenetic processes on primary sedimentary records. In case of strongly reducing conditions primary magnetic carriers such as magnetite are depleted. Other magnetic carriers such as titanomagnetite and especially hemoilmenite have a better preservation potential and are relatively enriched. A previous unknown partial self-reversal behaviour of hemoilmenite was observed, which is indicative of such (paleo)environments. Another proxy parameter for marine paleoproductivity is barite content, which was assessed under the influence of reductive diagenesis. Bulk chemical measurements identified barite layers at and below the recent sulphate methane transition. Particle identification revealed that these enrichments consist of diagenetic barite preserved due to retarded dissolution enabling to model the kinetics of such processes.