15. Data Report: Petrographic, Cathodoluminescent, and Compositional Characteristics of Organogenic Dolomites from the Southwest African Margin1

Peir K. Pufahl2 and Gerold Wefer3

INTRODUCTION

The main objective of Leg 175 was to reconstruct the Late Neogene paleoceanographic history of the Benguela Current and associated upwelling regimes along the southwest African margin between 5 and 32S. This area is one of the great upwelling regions of the world and plays an important role in the global ocean-carbon cycle. It is characterized by organic-rich sediments that contain a high-resolution record of productivity history that is closely linked to changes in regional dynamics of circulation, mixing, and upwelling (Wefer et al., 1998). The Benguela Current region also provides an excellent setting to investigate the early diagenetic processes governing the formation of authigenic dolomite. Although unexpected, the discovery of widespread and pervasive dolomite horizons along the southwest African margin was one of the principal findings of Leg 175.

Dolomite precipitation within upwelling regimes is stimulated by the effects of high productivity and occurs early within the uppermost tens to hundreds of meters of sediment (Garrison et al., 1984; Hay, Sibuet, et al., 1984; Kastner et al., 1990; Kelts and McKenzie, 1982; Kulm et al., 1984; Lyle, Koizumi, Richter, et al., 1997; Middelburg et al., 1990; Pisciotto and Mahoney, 1981; Suess et al., 1988). Productivity must be sufficiently high to generate a favorable environment; the microbial respiration of sedimentary organic matter increases alkalinity and pH of pore waters and simultaneously alters Mg-Ca solution and crystal surface chemistries to promote dolomite precipitation (Baker and Burns, 1985; Baker and Kastner, 1981; Compton and Siever, 1986; Compton, 1988; Compton et al., 1994; Hardie, 1987; Mazzullo, 2000; Slaughter and Hill, 1990; Vasconcelos and McKenzie, 1997). These organogenic dolomites are distinguished from other types of authigenic carbonates by having a significant portion of their carbon derived from microbially degraded organic carbon (Froelich et al., 1979) and are thus characterized by 13C values that deviate strongly from those of normal marine carbonates (Friedman and Murata, 1979; Irwin et al., 1977; Murata et al., 1972; Pisciotto and Mahoney, 1981).

Here, we report the occurrence of organogenic dolomite horizons within organic-rich Neogene and Quaternary hemipelagic sediments from the southwest African margin. The purpose of this paper is to document the mineralogical, compositional, textural, and cathodoluminescent (CL) properties of dolomite horizons. It is not within the scope of this report to discuss horizon formation in detail. Aspects of horizon development based on the interpretation of these properties will be the focus of a subsequent paper.

1Pufahl, P.K., and Wefer, G., 2001. Data report: Petrographic, cathodoluminescent, and compositional characteristics of organogenic dolomites from the southwest African margin. In Wefer, G., Berger, W.H., and Richter, C. (Eds.), Proc. ODP, Sci. Results, 175 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/175_SR/chap_15/chap_15.htm>. [Cited YYYY-MM-DD]

2The Department of Earth and Ocean Sciences, The University of British Columbia, 6339 Stores Road, Vancouver BC V6T 1Z4, Canada. ppufahl@eos.ubc.ca

3Faculty of Earth Sciences (FB 5), University of Bremen, Postfach 33 04 40, 28334 Bremen, Federal Republic of Germany.

Initial receipt: 16 October 2000
Acceptance: 5 February 2001
Web publication: 5 June 2001
Ms 175SR-234

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