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Variable epilimnion thickness; a possible cause for carbon and oxygen isotopic covariance in lacustrine carbonates


, : Variable epilimnion thickness; a possible cause for carbon and oxygen isotopic covariance in lacustrine carbonates. Boletim do Simposio sobre o Cretaceo do Brasil 5(Pages 295-300

Carbon and oxygen isotopic covariance in lacustrine carbonates has been reported as a remarkable feature in Quaternary and even more ancient lacustrine sequences. The cause for this phenomenon remains unclear, although the existence of a unique physical controlling parameter might be speculated. The main purpose of this contribution is to suggest a model which could satisfactorily explain the isotopic covariance in lacustrine carbonates. Examples from the Brazilian Lower Cretaceous and geolimnological literature could support this model. Here is emphasized the importance of an aspect of lake physics -- the epilimnion thickness -- to biological and chemical processes governing such isotopic covariance. Changes in the epilimnion thickness, related to variations in lake area and salinity, are thought to modify the rates of organic matter degradation through the water column and (super 12) C replenishment to the euphotic zone. Regarding the epilimnetic carbonate system, these changes would result in contrasting limnological conditions during lake level highstands and lowstands, producing the isotopic covariant behavior. Carbon and oxygen isotopic enrichments of carbonates in organic carbon-rich lithologies are proposed as an empirical test for the model. Deciphering the origin of isotopic covariance in lacustrine carbonates could provide some critical answers to the lacustrine stratigraphy, particularly those concerning correlations between climatically induced paleolimnological events and the resulting sedimentary record.

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