Experimental diagenesis of fatty acids in a sediment: Changes in their existence forms upon heating.

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Abstract
Heating experiments of a lacustrine sediment were conducted at 120, 150 and 198°C for 48hrs each for better understanding the behavior of fatty acids (FA's) in a sediment during early diagenesis. Two forms of FA's, “unbound FA's” extracted with organic solvents and “bound FA's” recovered by saponification of the pre-extracted sediments, were determined on unheated and heated sediments, with the following results: (1) The concentration of total (unbound + bound) lower molecular weight fatty acids (LFA's: C12 to C19) increased from 81μg/g dry sediment (unheated) to 95μg/g dry sediment at 198°C. The concentration of total higher molecular weight fatty acids (HFA's: C20 to C30) also increased from 36μg/g dry sediment (unheated) to 51μg/g dry sediment at 198°C. This fact suggests the presence of “tightly bound form” of FA's, i.e. FA's which can be recovered only after the thermal treatments of sediments. (2) Bound LFA's decreased rapidly on heating while unbound LFA's increased gradually. On the other hand, bound HFA's first decreased at 120°C and then increased at higher temperatures. Unbound HFA's show a similar changing pattern. The difference in the behavior on heating between bound LFA's and bound HFA's was interpreted in terms of their existence state which may be closely related with their origins.

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