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The quantity, type, and maturity of the organic matter were determined for 45 samples of continental rise sediments from the Neogene and Lower Cretaceous at DSDP Site 397, eastern North Atlantic. Kerogen concentrates were described by microscopy, and vitrinite reflectance values were determined (33 samples). The hydrocarbons in lipid extracts were analyzed by gas-chromatography (GC) and combined gas chromatography-mass spectrometry (GCMS) (12 samples). Allochthonous lower Miocene mudstones proved the most organic rich: both «-alkane distributions and kerogen microscopy indicated a mixed aquatic and terrestrial organic content. A dominance of phytane over pristane suggests a reducing environment of accumulation. These allochthonous-reduced sediments, slumping from the shelf edge to the continental rise, provide evidence for an oxygen-minimum layer during the Miocene in the Atlantic. The overlying organically lean autochthonous hemipelagic oozes and chalks contain predominantly terrestrial organic matter, a paradox which is discussed. The underlying Lower Cretaceous mudstones yield «-alkanes and kerogens typical of higher plants. Vitrinite reflectance and carbon preference index indicated that the section is immature, the trend from the former giving mean reflectance values of 0.45 per cent at 1450 meters. The GC-MS analysis of polycyclic hydrocarbons identified various steranes and triterpanes, as well as important amounts of their unsaturated analogs even under a sedimentary overburden of 1450 meters. An unusual C2g triterpane was found as a dominant component in one sample. INTRODUCTION MICROSCOPY Site 397 (DSDP Leg 47A) was drilled in the eastern North Atlantic, 110 km off Cape Bojador and south of the Canary Islands. The hole was drilled at the continental rise, in an area where canyons dissect the continental shelf edge. Sediments from Tertiary and Lower Cretaceous (Hauterivian) age were separated by a "V 100 m.y. unconformity. Sediments from the site were divided into five lithostratigraphic units (Site Report, this volume); lithostratigraphic Unit 4 was subdivided into five lithofacies (Fl to F5). These units are discussed at length in the Site Report (this volume). The majority of the 45 samples, collected from split cores using metal sampling tools, were stored frozen in "Teflon" sealed, aluminum containers until ready for laboratory processing. Some samples were taken from sediments extruded on deck by gas expanding in the plastic core liners. These were wrapped in aluminum foil and frozen. In both cases, great care was taken to remove sediment possibly contaminated during drilling or on-deck handling. Before burial affects the mineralogy of sedimentary rocks, the organic matter undergoes large-scale changes in both chemical and physical properties. One well-documented and relatively easily measured change is that of the increase in reflectivity of the small dispersed vitrinite-huminite particles present in many rocks (Raynaud and Robert, 1976). The types of organic particles present in a sediment can also be discerned with the aid of a microscope. In its simplest form, a distinction can be made between the generally amorphous fluorescent algal or planktonic remains of aquatic origin, and non-fluorescing and structured particles such as vitrinite and inertinite of higher plant (terrestrial) origin (Stach et al., 1975). Higher plants also produce structured fluorescing tissues such as spores, cuticle, and pollen. 'The term vitrinite used in bituminous coal terminology is synonymous with the term huminite used for brown coals or lignites; the boundary is about 0.5 per cent oil reflectance (Stach et al., 1975).

Organic Geochemistry of DSDP Leg 47A, Site 397 Eastern North Atlantic: Organic Petrography and Extractable Hydrocarbons