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

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 saponi fication. 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 36pg/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.

Behavior of lipid compounds on laboratory heating of a Recent sediment.

“…However, experimental verification of the above hypothesis is required, since (1) the yields of hydrocarbons generated in heating experiments from fatty acids (Shimoyama and Johns, 1971) are low (less than 0.2%), (2) concentrations of fatty acids observed in a Tertiary core are low compared with hydrocarbons, and fatty acids do not decrease in sediment layers where bitumen generation is suggested (Johns, 1979); (3) heating experiments with Recent sediments did show an increase rather than decrease. in the concentrations of fatty acids (Harrison, 1976, Baedecker et al, 1977Kawamura and Ishiwatari, 1981). These observation seem to be inconsistent with the above hypothesis.…”

“…The same 1984a). The total concentration of w-hydroxy trends were obtained in a previous hearing ex acids increases slightly around 179'C, as seen periment (Kawamura and Ishiwatari, 1981). in Table 1.…”

“…The increase at 325°C is 46%. The decrease in fatty acid concentrations at lower temperatures has been found in our previous studies (Kawamura and Ishiwatari, 1981;1984c), where such a decrease was thought to be due to their entrap ment or incoporation into the geopolymers originally present in sediments or formed during heating through melanoidin-type reactions (Ka wamura and Ishiwatari, 1984c). The increase in the concentrations of extractable (unbound + bound) fatty acids at higher temperatures (> 150° C) will be discussed later.…”

“…(Kawamura and Ishiwatari, 1981;1984d). The extracts were separated by filtration (Whatman GF/C) and then concentrated to ca.…”

“…The results from laboratory heating experiments provide possible pathways of organic reactions in diagenesis. EXPERIMENTAL Lake Biwa surface sediment (oxic) was used in this heating experiment (Kawamura and Ishiwatari, 1981). This dark-gray sediment had an organic carbon content of 2.1 % (dry sedi ment) and a water content of 60% .…”