The Magnetic View on the Marine Paleoenvironment:
Parameters, Techniques and Potentials of Rock Magnetic Studies
as a Key to Paleoclimatic and Paleoceanographic Changes
T. Frederichs*, U. Bleil, K. Daumler, T. von Dobeneck and A. M. Schmidt
Fachbereich Geowissenschaften, Universitat Bremen, Postfach 33 0440,
D-28334 Bremen, Germany
*correspondmg author (e-mail} .. thojred@geomarin.uni-bremen.de
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 SouthAtlantic 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 theAfrican and SouthAmerican 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 discrimiuated 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 domiuate 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 colunm. Resulting strong diagenetic effects
on the biomagnetic mineral component are traced in detail by high-resolution rock magnetic analyses
and transmission electron micrographs.
Introduction
Magnetic mineral inventories of marine sediments
reflect source materials, transport mechanisms and
diagenetic phenomena. Due to the particular physical properties of magnetic particles, their mineralogy, concentration and grain-size can be directly
determined from rapid bulk sample measurements.
Through space and time these records describe
varying paleoceanographic conditions related to
changes in lithosphere, hydrosphere and atmosphere (Fig. 1).
Following a suggestion by Thompson et a1.
(1980), this type of investigation, a relatively new
expansion of geophysics, is called Environmental
Magnetism. It was also the title of a milestone textbook by Thompson and Oldfield (1986) which provided a comprehensive summary of the broad range
of relevant rock magnetic methods to study marine,
limnic and fluvial sediments, soils and atmospheric
dust. More recent reviews were compiled by Lund
and Karlin (1990), Oldfield (1991), King and
From FISCHER G; WEFER G (eds), 1999, Use of Proxies in Paleoceanography: Examples from the South Atlantic. Springer-Verlag
Berlin Heidelberg, pp 575-599
Parameters, Techniques and Potentials of Rock Magnetic Studies
as a Key to Paleoclimatic and Paleoceanographic Changes
T. Frederichs*, U. Bleil, K. Daumler, T. von Dobeneck and A. M. Schmidt
Fachbereich Geowissenschaften, Universitat Bremen, Postfach 33 0440,
D-28334 Bremen, Germany
*correspondmg author (e-mail} .. thojred@geomarin.uni-bremen.de
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 SouthAtlantic 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 theAfrican and SouthAmerican 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 discrimiuated 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 domiuate 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 colunm. Resulting strong diagenetic effects
on the biomagnetic mineral component are traced in detail by high-resolution rock magnetic analyses
and transmission electron micrographs.
Introduction
Magnetic mineral inventories of marine sediments
reflect source materials, transport mechanisms and
diagenetic phenomena. Due to the particular physical properties of magnetic particles, their mineralogy, concentration and grain-size can be directly
determined from rapid bulk sample measurements.
Through space and time these records describe
varying paleoceanographic conditions related to
changes in lithosphere, hydrosphere and atmosphere (Fig. 1).
Following a suggestion by Thompson et a1.
(1980), this type of investigation, a relatively new
expansion of geophysics, is called Environmental
Magnetism. It was also the title of a milestone textbook by Thompson and Oldfield (1986) which provided a comprehensive summary of the broad range
of relevant rock magnetic methods to study marine,
limnic and fluvial sediments, soils and atmospheric
dust. More recent reviews were compiled by Lund
and Karlin (1990), Oldfield (1991), King and
From FISCHER G; WEFER G (eds), 1999, Use of Proxies in Paleoceanography: Examples from the South Atlantic. Springer-Verlag
Berlin Heidelberg, pp 575-599