| Abstract: | The organic matter contents of most black shales are marine in origin. Marine organic matter usually has atomic C/N values of 5 to 8 (Meyers, 1994, Chem. Geol. 144:289-302), yet C/N values in black shales curiously range between 20 and 40 (Rau et al., 1987, Earth Planet. Sci. Lett. 82:269-279; Meyers, 1989, Mar. Petrol. Geol. 6:182-189), which are values typical of land-plant organic matter. Del 15N values of Neogene and Quaternary marine sediments usually range between 4 and 8 per mil but are -2 to 3 per mil in Cretaceous black shales (Rau et al., loc cit), which again mimic land-plant compositions.
The excursions in the nitrogen-based organic matter source proxies provide clues to the conditions leading to deposition of black shales. Both elevated production and improved preservation of marine organic matter were important. Marine productivity is usually limited by availability of dissolved nitrate, but if a mid-water anoxic zone expands upward into the photic zone, then nitrogen-fixing cyanobacteria can flourish. These organisms produce organic matter having a del 15N value close to atmospheric nitrogen (0 per mil). The C/N values of most modern marine sediments are similar to algal organic matter (5-8), but high values (10-20) are common under areas of elevated marine productivity (Twichell et al., 2002, Org. Geochem. In press). Their explanation involves selectively improved preservation of organic matter during its sinking in which preferential degradation of amino compounds occurs under suboxic water-column conditions (Van Mooy et al., 2002, Geochim. Cosmochim. Acta 66:457-465). The anoxic mid-water conditions needed for nitrogen-fixation are an amplication of the conditions needed to increase modern C/N values and may have led to the higher C/N values found in black shales. The high C/N and low del 15N values imply that the marine nitrogen cycle changed to increase availability of nitrogen to marine production of organic matter during times of formation of black shales. By analogy to Mediterranean sapropels, the change may have been accompanied by increased availability of phosphorus from increased land runoff during periods of wetter climate. |