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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 53 (1969)

Issue: 2. (February)

First Page: 468

Last Page: 469

Title: Anaerobic Basin Sedimentation and Differential Preservation of Planktonic Foraminifera: ABSTRACT

Author(s): Wolfgang H. Berger

Article Type: Meeting abstract

Abstract:

In paleoclimatic reconstruction, fossil planktonic Foraminifera usually are related to ancient surface-water conditions. Processes on the ocean floor, however, are able to alter faunal composition drastically.

Ten core samples from various depths were taken in the Santa Barbara basin which is anaerobic below a sill depth of about 480 m. Sixteen samples were analyzed for their microfossil content, in four size fractions. In each size fraction there is a pronounced difference in composition between samples above and below sill depth. The anaerobic, black, laminated sediments obtained below sill depth contain numerous species and specimens, including very fragile forms such a Hastigerina digitata. Fragments are almost absent; pelagic gastropods and pelecypods (aragonite) are plentiful, as are thin-walled calcareous benthonic Foraminifera.

Just above sill depth, sediments are dark green and homogeneous, suggesting moderate aeration. The percentages of Globoquadrina dutertrei and of Globigerina pachyderma s.s. are much greater there than in the anaerobic samples. Fragments of both planktonic and benthonic Foraminifera are abundant. Thin-walled benthonic Foraminifera are much scarcer than in the deeper samples; aragonitic shells are virtually absent. Well above sill depth sediments are light green and homogeneous and appear well aerated. Both planktonic and benthonic foraminiferal numbers are greatly reduced and G. dutertrei strongly dominates all but the finest fraction. Fecal pellets are abundant.

There are several possible causes for the pronounced change in fossil fauna across the anaerobic-aerated boundary: (1) changes in supply, (2) mechanical destruction by benthonic organisms, and (3) chemical dissolution.

A change in supply from the overlying water is unlikely, because the effects of the aeration boundary do not extend into the productive zone. Mechanical destruction by benthonic animals is possible in the aerated zone. The siliceous skeletons of diatoms and Radiolaria, however, are much less affected by the aeration boundary than are calcareous fossils, which suggests that chemical dissolution is the most important factor. Two mechanisms may be responsible: (1) production of CO2 by the oxygenation of organic matter in the aerated zone, and (2) a more vigorous exchange of interstitial solutions with bottom water by burrowing activity. These two effects of bottom aeration should enhance calcite solution.

The study of the distribution of the resistant Foraminifera, such as G. dutertrei, G. pachyderma s.s., G.

End_Page 468------------------------------

truncatulinoides, and G. inflata, in ancient Californian sediments should lead to a better understanding of paleobasin oceanography by making it possible to determine sill depths and oxygen minimum distributions. Such studies have obvious applications in the determination of petroleum source rocks, because organic material is preserved in anaerobic conditions but destroyed in aerated sediments.

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Copyright 1997 American Association of Petroleum Geologists