Preservation of sedimentary organic matter

The expansion of the oxygen minimum zone in the incipient North Atlantic during the Upper Cretaceous enhanced the preservation of sedimentary organic matter. Organic carbon values ranging from 1.7 to 13-7% and the proportions of unbound lipid to total lipid content (>70%) support this premise. A present point of contention is the origin of the organic matter-rich Upper Cretaceous deep-sea sediments. 

Deposition of organic-rich sediments further down the shelf and on to the continental slope and rise often occurs as a result of turbidite flows, redistributing organic-rich sediments from delta fronts or from further up the shelf and slope (Summerhayes 1983). While there is a certain amount of pelagic sedimentation, primary production decreases away from the coastline as nutrient levels decline, and detritus is largely recycled before it settles to the sea floor. However, this may not always have been so in the past, when the thermohaline circulation (Box 3.2) did not operate and there may have been widespread anoxia in bottom waters, aiding preservation of sedimentary organic matter (e.g. Cretaceous oceanic anoxic events Section 6.3.4). 

Meyers, P.A. (1994) Preservation of elemental and isotopic identification of sedimentary organic matter. Chem. Geol. 144, 289-302. 

The most direct field evidence that the extent of sedimentary organic matter preservation is affected by exposure to bottom-water oxygen comes from oxidation fronts in deepsea turbidites of various ages and depositional settings (Wilson et al., 1985 Weaver and Rothwell, 1987). One of these deposits in which the timing of the exposure to oxic and anoxic conditions is well documented, is the relict f-turbidite from the Madeira abyssal plain (MAP) —700 km offshore... 

Inevitably the molecular and isotopic signals preserved in sedimentary organic matter are biased. They tend to emphasize or overestimate contributions from those organisms that biosynthesize refactory compounds which survive better in the geological record. Thus, assessment of the relative importance of different biota based on quantification of their diagnostic constituents (58) in sediments may not accurately... 

Hedges, J.I., F.S. Hu, A.H. Devol, H.E. Hartnett, and R.G. Keil. 1999. Sedimentary organic matter preservation A test for selective oxic degradation. American Journal of Science 299 529-555. 

The best preservations of organic particles are found in quartz grains of syn-sedimentary origin. It is probable that the material was entrapped in amorphous silica which, upon dehydrating to solid opal, formed an incompressible matrix with minimal deformation. The resistance of opal and of the subsequently crystallized quartz provides a physical environment which preserves the structures and reduces the effects of heat and pressure over long periods, which otherwise lead to degassing and coalification of the organic matter. 

Pelet, R. (1983). Preservation and alteration of present-day sedimentary organic matter. In Advances in Organic Geochemistry, 1981 (M. Bjoroy et al., ed.). Wiley, New York, pp. 241-250. 

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