Aspartic acid, threonine, serine and other thermally unstableamino acids have been found in fine-grained elastic sediments of advancedgeologic age. The presence of these compounds in ancient sedimentsconflicts with experimental data determined for their simplethermal decomposition.
Recent and Late Miocene sediments and their humic acidextracts, known to contain essentially complete suites of amino acids,were heated with H2O in a bomb at temperatures up to 500°C in orderto compare the thermal decomposition characteristics of the sedimentaryamino compounds.
Most of the amino acids found in protein hydrolyzates areobtained from the Miocene rock in amounts 10 to 100 times less thanfrom the Recent sediment. The two unheated humic acids are rathersimilar despite their great age difference. The Miocene rock appearsuncontaminated by Recent carbon.
Yields of amino acids generally decline in the heated Recentsediment. Some amino compounds apparently increase with heating timein the Miocene rock.
Relative thermal stabilities of the amino acids in sedimentsare generally similar to those determined using pure aqueous solutions.The relative thermal stabilities of glutamic acid, glycine, and phenylalaninevary in the Recent sediment but are uniform in the Miocenerock.
Amino acids may occur in both proteins and humic complexesin the Recent sediment, while they are probably only present in stabilizedorganic substances in the Miocene rock. Thermal decomposition ofprotein amino acids may be affected by surface catalysis in the Recentsediment. The apparent activation energy for the decomposition ofalanine in this sediment is 8400 calories per mole. Yields of aminocompounds from the heated sediments are not affected by thermal decompositiononly.
Amino acids in sediments may only be useful for geothermometryin a very general way.
A better picture of the amino acid content of older sedimentaryrocks may be obtained if these sediments are heated in a bombwith H2O at temperatures around 150°C prior to HCl hydrolysis.
Leucine-isoleucine ratios may prove to be useful as indicatorsof amino acid sources or for evaluating the fractionation ofthese substances during diagenesis. Leucine-isoleucine ratios of theRecent and Miocene sediments and humic acids are identical. The humicacids may have a continental source.
The carbon-nitrogen and carbon-hydrogen ratios of sedimentsand humic acids increase with heating time and temperature. Ratioscomparable to those in some kerogens are found in the severely heatedMiocene sediment and humic acid.
