--> ABSTRACT: Evolution of Organic Matter in a Eutrophic Lake: Transition from Biological Material to Kerogen and Implications for Origin of Lacustrine Type 1 Oils, by David J. Hollander, Mireille Vandenbroucke, Francoise H. Behar, and Judith A. McKenzie; #91022 (1989)

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Evolution of Organic Matter in a Eutrophic Lake: Transition from Biological Material to Kerogen and Implications for Origin of Lacustrine Type 1 Oils

David J. Hollander, Mireille Vandenbroucke, Francoise H. Behar, Judith A. McKenzie

A detailed study of the optically determined amorphous organic matter (type 1) in the water column and the sediments from eutrophic Lake Greifen provide a model for determining the pathway by which lacustrine algae are incorporated into sediments and later transformed into type 1 kerogens.

Mass balancing of organic fractions (fulvic acids, humic acids, and stable residues (proto-kerogens)) in combination with infrared spectroscopy and gas chromatographic studies of these fractions suggest that two processes in the formation of kerogens are proceeding simultaneously: (1) selective decomposition of the structurally and functionally unique fulvic acids and (2) polycondensation/polymerization of structurally and functionally related fractions in the lacustrine algae leading to the formation of complex humic acids and finally to the stable residue phase.

During the last 100 years, Lake Greifen has undergone major increases in productivity and oxygen utilization as a result of increased nutrient influx. This progressive eutrophication is clearly seen in the sediment cores, which show a transition from oxic marls to anoxic sapropels. Two filtered samples of biological material allow us to characterize the initial elemental atomic ratios of the autochthonous organic matter (H/C = 1.76, O/C = 0.30). Elemental ratios of sedimented residues across the oxic/anoxic transition vary dramatically, with H/C values ranging from 0.97 to 1.53 and O/C values from 0.28 to 0.15. Hydrogen indices of stable residues range from 395 mg HC/g organic carbon in the oxic sediments to 888 in the anoxic sediments. The values of elemental ratios and hydrogen indi es of stable residues from the anoxic sediments are within the ranges published for type 1 lacustrine source material, whereas stable residues in the oxic sediments fall within ranges for type 2 (marine) or type 3 (terrestrial) source material. These results suggest that the accumulation of type 1 lacustrine algal organic matter is restricted to severely eutrophic lake environments. In other words, the rare occurrence of type 1 organic carbon-rich deposits in the sedimentary record is probably related to the fact that lacustrine algae are highly labile and their preservation requires rapid burial in shallow environments characterized by severely oxygen-depleted water masses.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.