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In modern marine environments, preservation of organic carbon in sediments is influenced by several variables: (1) primary production; (2) water depth and bottom-water oxygen content; and (3) bulk sedimentation rate--higher sedimentation rates enhance organic carbon accumulation rates. High organic carbon values and accumulation rates are found today under highly productive upwelling zones where intense oxygen-minimum zones (O2 < 0.5 mL/L) impinge on upper continental slopes or outer shelves. However, high organic carbon concentrations are also found in the abyssal sediments of the anoxic Black Sea, where surface productivity is relatively low. Therefore, when examining occurrences of ancient marine "black shales," we must be able to distinguish the relative effects of the variables discussed above. The globally widespread "black shales" of some periods in the geologic past cannot be simply explained by patterns of upwelling alone; other, perhaps unusual, conditions must have contributed to their origin.
The Cretaceous is one period during which, at times, marine "black shales" were much more globally widespread than today. These intervals of time have perhaps unfortunately been termed "oceanic anoxic events," but the basic concept of an "anoxic event" involves a time envelope on the order of 106 years, during which organic carbon burial appears to have been more widespread in a variety of marine environments than at other times. However, regional or interbasinal differences in the timing, amounts, and types of organic carbon preserved are apparent during a single "anoxic event," and "anoxic events" of different ages differ in overall character. Therefore, no single model can explain the origin of these widespread episodes of organic carbon deposition. There are common asso iations between "anoxic events"; they tend to occur during global marine transgressions, and they are marked by warmer, more equable global climates.
Three main "anoxic events" occurred during the Cretaceous. These were of late Barremian-mid-Albian age (peak at Aptian-Albian boundary), mid-Cenomanian-early Turonian age (peak at Cenomanian-Turonian boundary), and late Coniacian-early Campanian age. During portions of the first two "anoxic events," organic carbon burial in pelagic and hemipelagic environments may have exceeded 10 times that of today, as indicated by calculations of accumulation rates and model calculations from secular ^dgr13C curves. The amount and types of organic carbon preserved in strata of different basins suggest that expansion and intensification of deep-water oxygen deficits were responsible for organic carbon preservation in many settings. This may have been partly due to the decreased solubility of oxygen in warm, saline deep-water masses. The feedback between sea level and development of surface and deep-water masses was an important factor. During the Aptian-Albian episode, overall surface-water productivity appears to have been low, and burial of terrestrial organic carbon in marine environments was significant. Enhanced marine surface productivity may only have been important during the relatively brief Cenomanian-Turonian episode.
It is important that we understand the nature of these "anoxic events" so that predictive models of organic contents and types can be constructed and utilized in frontier areas of hydrocarbon exploration, among other reasons.
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