Samsara of organic carbon

Abstract

A summary is presented of results obtained in the study of the organic matter contained in sediments. The evolution of this organic matter with age is discussed and its importance is emphasized for the understanding of processes occurring during diagenesis. Organic Geochemistry is a recent Science: a well-documented review published in 1971' lists some 300 references, half a dozen of which refer to papers published before 1945. Reviews2 and monographs3 have put on record present results and the philosophy of the work, so that we can dispense with a balanced presentation. I shall only give a personal description of our present approach to organic geochemical problems, with no attempt at showing how much more the field owes to other groups, such as those of Calvin, of Eglinton, and of many petroleum companies. A global survey. Most argillaceous sediments studied so far contain some one or two per cent of organic carbon, be it a 'recent' sediment (say, from the Tertiary) or a 'very old' one (say, from the late Precambrian). A small mountain, 100 m high and 1 x 1 km large, may contain some 4 x 106 tons of organic carbon in its minerals; on its surface, if densely covered by vegetation, it may bear only some iO tons of carbon in living trees, plants, animals and micro-organisms: less than one thousandth of its fossil carbon. Such is the result of imperfect recycling. As organisms die, their organic matter decays, and their carbon returns to its most stable form in an oxygencontaining atmosphere: not dust, but carbon dioxide. This leads to the first, short lived, carbon cycle, followed in months or years. Recycling would be accomplished through re-use by photosynthesis, were it not for a flaw: this 'quick' cycle leaks into a second cycle, a long-lived one, followed in aeons. A small portion of organic matter escapes decay, and is buried with carbonates or silicates into sediments. It is preserved until orogenesis and erosion bring it back to the surface (Figure 1). One estimates the total mass of organic carbon accumulated in sediments at some 6 x 1015 tons—the mass of organic carbon present in living organisms at 3 x 1012 tons. Organic Chemists should therefore mostly study rocks! I shall restrict this report to our work on two lacustrine sediments, oil shales found near Strasbourg, in Messel (Germany) and Bouxwiller (France).