HR: 1340h
AN: B13A-1040    [Abstracts]
TI: Catalytic Generation Of Nanometer-sized Diamondoids From Thermal Decomposition Of Organic Matter In Modern Sediments
AU: * Wei, Z
EM: weizb@stanford.edu
AF: Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 United States
AU: Moldowan, M J
EM: molodwan@pangea.stanford.edu
AF: Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 United States
AU: Paytan, A
EM: apaytan@pangea.stanford.edu
AF: Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 United States
AB: Diamondoids are nanoparticles naturally occurring in petroleum and not known to be natural products. Experimental work of artificial maturation on modern sediments was completed to determine if diamondoids can be created from biosynthetic precursors (e.g., n-alkenes, fatty acids, highly branched isoprenoids, sterols, etc.) present in modern sediments. Addition of mineral catalysts (montmorillonite K10, acidic aluminosilicate and calcium carbonate) with distinctive properties to pyrolysis experiments conducted using organic-rich modern sediments can provide information on which type of catalyst is active and effective in the production of diamondoids. Trace amounts of diamondoids were detected in San Francisco Bay, Santa Barbara Basin, and Elkhorn Slough muds. These might have been contributed by slight contamination with unknown oil spills or oil seeps, as supported by the presence of both geological epimers (22S) and biological epimers (22R) for C31-C35 homohopanes in mud extracts. However, diamondoids were not detectable or possibly absent in Celestun Lagoon carbonate which is rich in microfossils. Various quantities of diamondoids were generated from the thermal alteration of organic matter in several modern sediments in the presence and absence of catalysts. It is likely that the formation of diamondoids is coincident with the occurrence of n-alkanes and other biomarkers since the thermal maturation of modern sediments produces a certain amount of diamondoids. Clay minerals usually behave as acidic catalysts in the transformation of organic matter to hydrocarbons. As expected, montmorillonite K10 and acidic aluminosilicate greatly facilitate the formation of diamondoids in modern sediments at 340°C. Rearrangements of certain strained polycyclic carbonium-ion intermediates at Lewis super-acid sites may be responsible for the relatively high abundance of diamondoids upon thermal stress in the presence of montmorillonite K10 or acidic aluminosilicate. In contrast, calcium carbonate shows an inhibition effect on the formation of diamondoids. Therefore, it is likely that these nanoparticle-like diamondoids are formed through a carbonium ion mechanism instead of an electron transfer mechanism.
DE: 0432 Contaminant and organic biogeochemistry (0792)
SC: Biogeosciences [B]
MN: Fall Meeting 2005