--> ABSTRACT: Carbon Cycling in Eutrophic Lakes: Models for Carbon Isotopic Excursions in Middle Ordovician Algal-Dominated (Gloeocapsamorpha) Organic Matter, by Judith A. McKenzie, David J. Hollander, and Kate Japy; #91022 (1989)

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Carbon Cycling in Eutrophic Lakes: Models for Carbon Isotopic Excursions in Middle Ordovician Algal-Dominated (Gloeocapsamorpha) Organic Matter

Judith A. McKenzie, David J. Hollander, Kate Japy

Eutrophic systems are environments conducive to the formation of organic carbon-rich source rocks. Carbon cycling and the development of seasonal carbon-isotope, surface to bottom water gradients in modern lakes can be used to evaluate changes in the carbon-isotope composition of ancient organic matter thought to have been produced under eutrophic conditions.

Studies in a temperate alkaline eutrophic lake, which undergoes complete circulation seasonally oxygenating the bottom waters, indicate that algal blooms are associated with a decrease in available CO2 and the organic matter tends to become isotopically heavier as the system converts to bicarbonate use (^Dgr13CDIC-POC = 18^pmil). Photosynthesis-respiration processes promote a transfer of 12C downward across the thermocline, whereby the dissolved inorganic carbon (DIC) of surface waters becomes isotopically heavier than the bottom waters. Similar studies in a subtropical, acidic, eutrophic lake, which undergoes incomplete winter circulation and remains anoxic year-round, demonstrate that the higher availability of CO2 during algal looms allows for a larger fractionation between the DIC and organic matter (^Dgr13CDIC-POC = 23^pmil). Under these extreme anoxic conditions, respiration processes, including methanogenesis, dominate photosynthesis, resulting in a general upward transfer of 12C whereby the DIC of the surface waters is isotopically lighter than the bottom waters. Thus, the ^dgr13C value of organic matter produced in the subtropical system is relatively more negative than in the temperate system.

These eutrophic lake models can be used to evaluate contrasting carbon-isotope excursions recorded in two Middle Ordovician organic carbon-rich formations of the east-central United States. The dominance in both formations of fossil blue-green algae (Gloeocapsamorpha) and very high TOC content suggests that production and preservation occurred under eutrophic conditions. The change from a negative to a positive excursion from the early to late Middle Ordovician could be interpreted as the result of increasing pH in conjunction with or as the result of a paleoenvironmental change from a warmer to cooler climate, leading to increased intensity of circulation in the eutrophic basins.

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