Topic 3 Organic Matter in Recent Sediments
The organic matter now found in ancient rocks was incorporated from the overlying water column at the time they were being deposited as sediments. In the oceans organic matter occurs in true solution and as particulate or colloidal material. The concentration of all three forms is highest near surface but decreases with depth for the first few hundred meters, and then remains approximately constant. Dissolved organic matter is more than an order or magnitude more abundant than particulate organic matter. As clay particles settle through the water they adsorb some organic matter from solution and this is then incorporated into the sediments. Particulate matter settles to the bottom and becomes part of the sediments, while the colloidal organic matter first flocculates and then settles (Figure 3.1). As the organic matter sinks through the water column bacteria and other organisms remove any of the organic compounds which they can metabolize, and at the water-sediment interface bottom feeders further screen the organic matter so that only relatively resistant organic material is incorporated into the sediments. The only exceptions to this are the organic compounds which are protected in some way. For example shells are held together by proteinaceous material which is protected from degradation by the enclosing carbonate.
Both the amount and type of the organic material reaching the sediments is controlled, at least in part, by the depositional environment and can be related to the productivity of the overlying waters, the grain size of the sediments, the physical conditions in the