The Magnetic View on the Marine Paleoenvironment: Parameters, Techniques and Potentials of Rock Magnetic Studies as a Key to Paleoclimatic and Paleoceanographic Changes

Abstract

The eminent potential of Environmental Magnetism analytical techniques to delineate depositional regimes and climatic changes in the marine realm is reviewed and illustrated with results of three individual studies of sediment series from the South Atlantic Ocean. Rock magnetic properties related to the mineralogy, concentration, domain state and hence grain-size of the magnetic mineral assemblage are explained on grounds of physical principles and discussed as proxy parameters for terrigenous particle fluxes, bioproductivity and diagenetic redox conditions. With cluster analysis of rock magnetic parameters determined for a large collection of surface samples, the regional characteristics of recent depositional environments in the equatorial South Atlantic are established. Notably, the different input mechanisms of terrigenous material via fluvial transport by the Amazon and Congo Rivers at the African and South American continental margins are as clearly identified as the eolian transport from the Sahara and Sahel Zone into the central and eastern equatorial South Atlantic. Based on a detailed susceptibility log and measurements of various laboratory remanences, high-coercive hematite components and different magnetite grain-size fractions could quantitatively be discriminated in a late Quaternary sediment sequence from the central equatorial Atlantic. The data sets allow to assess variations in eolian influx from the Saharan dust plume and several redox events during the last 400 kyr can be recognized. While biogenic magnetite is generally of minor importance in pelagic deposits, it may completely dominate the sediment magnetic properties in high productive areas. An intense primary biologic productivity in surface waters of the Benguela upwelling center supplies a high flux of organic matter to the sea floor at the continental slope off Namibia and causes reducing conditions in the sediment column. Resulting strong diagenetic effects on the biomagnetic mineral component are traced in detail by high-resolution rock magnetic analyses and transmission electron micrographs.