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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 42 (1958)

Issue: 5. (May)

First Page: 925

Last Page: 962

Title: Preservation of Chlorophyll Derivatives in Sediments Off Southern California

Author(s): Wilson L. Orr (2), K. O. Emery (2), John R. Grady (2)


Sediments from the sea floor off southern California have been examined for acetone-soluble chlorophyll derivatives which are regarded as the source material of the major porphyrins in crude oils. For quantitative estimation, the pigments in sediment extracts were calculated as pheophytin a which is believed to be the most abundant of several similar pigments present.

Approximately 120 surface sediment samples were examined: their sources ranged from beach and marsh through various sea-floor environments to the deep sea. Determinations were made for pheophytin, nitrogen, carbonate, and grain size. In addition, detailed analyses of seven cores were made. A dependence of pheophytin content on grain size is evident in surface samples and in cores. The pheophytin content of cores from nearshore basins decreases with depth of burial, the most abrupt change being near the mud-water interface. Deeper basins farther offshore show only slight decreases in pheophytin with depth of burial. Aside from the marsh deposit, the highest concentration of pheophytin occurs in the shallow basins that contain water having a low oxygen content.

The data indicate that the pheophytin content of sediments is almost independent of the production rate of chlorophyll-containing plankton and of dilution with sediments--other than coarse-grained turbidity-current deposits. Instead, the chief factor controlling its abundance appears to be the amount of decomposition undergone in settling through the water column and before burial in the sediment. The decomposition is controlled primarily by water depth, oxygen content of the water, and bottom topography.

The chlorophyll derivatives found in sediments are intermediate in structure between chlorophyll and the porphyrins found in petroleum. Magnesium has been lost but vanadium and nickel have not yet been introduced into the pigments. Further changes in the organic structure which must occur to convert these pigments into those found in petroleum involve simple reactions such as reduction of carbonyl groups, hydrogenation of carbon-carbon double bands, dehydrogenation, and decarboxylation. All of these changes may be expected to occur gradually in the sedimentary environment.

Chlorophyll derivatives in basin sediments are intermediate in quantity as well as structure between those of phytoplankton and those of petroleum. If as much as 1% of the pigments preserved in the surface zones of basin sediments are eventually converted to the porphyrins found in petroleum, the amounts are ample to furnish crude oils with a normal porphyrin content.

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