Organic geochemical and organic petrographic analyses of Deep Sea Drilling Project cores from the Eastern Atlantic and the Western Pacific have yielded data on the quantity, type and maturity of the sedimentary organic matter. Integrating these data with the sedimentology and paleoenvironment of the various sites has generated an understanding of mechanisms controlling the accumulation of hydrocarbon source rock type sediments in the deep sea. Terrestrially derived organic matter was found to be much more resistant to degradation under the sedimentary conditions in the deep sea environment than planktonic organic material. Only where massflow down the continental slope from an oxygen minimum zone at the shelf edge, in areas of high bioproductivity, produce organic rich sediments, oil or gas prone source rocks can be expected.
During the study of DeepSea Drilling Project cores from the continental slopes and rises of the eastern North Atlantic (DSDP Legs 47A, 47B and 5G) a number of processes have been observed to affect the accumulation of potential hydrocarbon source Tocks at this eastern passive margin of a major ocean. This paper is a synthesis of data from four sites concentrating on those processes which allow prediction of scenarios favourable for the accumulation of hydrocarbon source rocks. For comparison, preliminary results from Western Pacific sites (Japan Trench, DSDP Legs 56 ana-51) are included. The discussion is prefaced by a consideration of the general case of organic accumulation in open oceans.
The part of the carbon cycle operating in deep ocean waters and associated sediments, past and present, is poorly understood. Some understanding is necessary if the organic matter of deep Sea sediments is to be related via transport and depositional mechanisms to its source on land or in the sea. For example, the extent of biodegradation or reworking of organic matter by bottom fauna (benthos) and bacteria within the sediments of deep ocean basins is a point of contention, but it will determine the extent of preservation of easily biodegradable organic matter. Also poorly understood is the extent of seaward transport of terrestrial organic matter. To answer these and other questions, a carbon mass balance must be attempted.
Relating the types of organic matter found in deep ocean sediments to their sources implies a knowledge of transport and depositional mechanisms. At present this knowledge is in a state of qualitative speculation. McCave has discussed the vertical flux of particles in the ocean, while Meinhold quoting data from Bogdanow, quantitatively treats the cycling of organic matter in the world's oceans.
Some possibilities for the transport of organic matter are represented in Fig. 1(Available in full paper):
eolian transport followed by settling through the water body
sinking of autochthonous biomass (zoo and phytoplankton) produced in near surface waters
introduction of allochthonous suspended organic particles by rivers from a terrestrial source
turbid flow or slumping direct to deep sea sites
settling from suspension in ocean currents or settling from the nepheloid layer produced by deep water bottom currents.
