Biogenic Magnetite: Authigenesis and Diagenesis with Changing Redox Conditions in Lake Greifen, Switzerland
HAWTHORNE, TERESA B., and JUDITH A. MCKENZIE, Geology Institute, Zurich, Switzerland
A mineralogic, chemical, and rock-magnetic investigation was conducted in Lake Greifen, a moderate-sulfate containing eutrophic lake, in order to assess iron mineral diagenesis in a sedimentary sequence deposited during different tropic states and varying redox conditions. The sedimentary sequence is composed of 0.25 m of eutrophic anoxic varved sediments underlain by transitional laminated marls, deposited as the lake changed from oxic to dysaerobic conditions, and then a basal unit of pre-eutrophic bioturbated marls. Grain size, morphology, and composition of magnetite observed by TEM suggest that single-domain biogenic magnetite is currently produced within the upper anoxic varved sediments. Similarly characterized fine-grained magnetites observed in the pre-eutrophic sediments ind cate magnetite authigenesis prior to eutrophication. Low values of NRM, susceptibility, SIRM, and ARM in the anoxic varved sediments below the zone of magnetite production indicate the rapid destruction of magnetite. Within the transitional laminated sediments, dissolution of iron phases is documented by changes in NRM, SIRM, ARM, and ferrous chemistry together with SEM observation of pitted magnetite grains. Iron speciation analyses, interstitial water results, and the ARM:SIRM ratio indicate that reductive dissolution is progressive. These results suggest that (1) rapid destruction of fine-grained magnetite occurs in high productivity, low-sulfate containing anoxic sediments following dissolution of ferric phases; and (2) magnetite preservation reflects redox conditions that, in Lake G eifen, are determined by productivity. These findings differ from marine reductive diagenetic models in which magnetite dissolution is considered insignificant, or slow, in the absence of high sulfide concentrations.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)