Environmental magnetism of eastern Mediterranean sediments and aeolian dust flux changes over orbital timescales

Abstract

Magnetic techniques have been used widely in environmental studies for the past few decades. Environmental magnetism typically involves conventional room temperature measurements of bulk magnetic parameters and more advanced component-specific magnetic approaches. Conventional bulk parameters can provide continuous records of magnetic mineral concentration and/or particle size variations in sediments, respectively. However, such parameters are not necessarily well suited to identifying magnetic components within individual magnetic mineral assemblages. More advanced techniques, such as first-order reversal curve (FORC) diagrams and low-temperature (LT) magnetic measurements, can enable detailed discrimination of magnetic mineral assemblages. However, these measurements are time-consuming, so they cannot be used to develop high-resolution records. Furthermore, although these techniques have been used more frequently in recent years, much work remains to be done to unlock their full diagnostic power in environmental magnetism. In this thesis, I use marine sediments from the eastern Mediterranean Sea to assess bulk magnetic measurements, FORC diagrams, LT measurements, X-ray fluorescence core-scan elemental data, and transmission electron microscope (TEM) observations to investigate the benefits and limitations of conventional and advanced environmental magnetic techniques for sediments deposited under variable redox conditions, and to explore the use of LT magnetic properties for detecting different magnetic particle types in marine sediments. Eastern Mediterranean sediments were selected here because they contain complexly varying mixtures of detrital, biogenic, and diagenetically altered magnetic mineral assemblages that were deposited under varying oxic (organic-poor marls) to anoxic (organic-rich sapropels) conditions. I demonstrate in this work that conventional bulk magnetic parameters can be used to provide high-resolution records of environmental magnetic variations, while advanced measurements provide direct ground-truthing of mineral magnetic assemblages that enables calculation of magnetization contributions of different end members. Thus, a combination of conventional bulk parameters and advanced magnetic techniques can provide detailed records from which the meaning of environmental magnetic signals can be unlocked. In this work, I also demonstrate that LT studies enable clear identification of the extent of the presence of biogenic magnetite, superparamagnetic particles, and surficial maghemitization of magnetite particles. Furthermore, new high-resolution magnetic and planktic foraminiferal stable oxygen isotope (d18O) proxy records are presented together with published geochemical data from eastern Mediterranean sediments to discuss the causes of increased dust inputs from the Sahara Desert across the mid-Pleistocene Transition (MPT). After assessing hypotheses for increased Saharan dust inputs across the MPT, including increasing source-area aridity due to reduced precipitation, expanding dust source areas, and atmospheric CO2 reduction that led to decreased vegetation and soil cohesion, I find that increasing climate extremes boosted wind-blown dust production and emissions. This is important because the resultant aeolian aerosols play a major role in both the radiative balance of climate and biogeochemical cycles in areas where the dust settles. This work provides valuable insights into the application of conventional bulk and more advanced component-specific methods in identifying mineral magnetic assemblages in paleoceanographic and paleoenvironmental reconstructions, and in explaining increased Saharan dustiness across the MPT. Show more