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Abstract

Changing sea level is a major factor in the pattern of enrichment of organic carbon in marginal and epicontinental seas. Organic-carbon-rich facies accumulate preferentially during major transgressive episodes. Rising sea level promotes retention of nutrients in marginal seas through several possible mechanisms, leading to higher organic production and/or eutrophic conditions. Transgressive seas also create circumstances that lead to seasonally or longer-term enhanced water-column stratification and development of anoxia in combination with eutrophism. Finally, rising sea level promotes nearshore trapping of terrigenous clastic material, creating condensed intervals that are characterized by enrichments in organic carbon. The interplay of these mechanisms is illustrated by integrated studies of the Holocene Black Sea, the Cenomanian–Turonian of the U.S. Western Interior Basin, and strata of the Middle to Late Devonian Appalachian Basin.

Anoxia and high productivity developed in the Holocene Black Sea around 7.6 ka, leading to deposition of a sapropel with up to 20 wt. % organic carbon. The development of eutrophic conditions coincided with rising sea level and overflow of saline waters from the Mediterranean Sea. Trapping of river-derived nutrients in the Black Sea behind the shallow sill to the Mediterranean and the high freshwater flux to the Black Sea Basin during climatic warming are additional causes of eutrophication associated with the global mid-Holocene transgression. Organic-carbon contents increase towards the basin center because of lower clastic dilution and focusing of organic-carbon transport from margins to center.

Repeated transgressive–regressive episodes in the Cenomanian–Turonian led to progressive flooding of the Western Interior Basin of North America, culminating in deposition of maximum highstand intervals in the early Turonian. High organic-carbon contents characterize transgressive episodes at multiple temporal scales as indicated by sequence stratigraphic analysis. The major enrichments of organic carbon in the Cenomanian Graneros, Lincoln, and Hartland shales during transgression are interpreted as reflecting enhanced stratification under salinity to thermally stratified conditions in a silled basin characterized by high fluvial input. River-derived nutrients were responsible for higher production of organic matter preserved under anoxic conditions that resulted, in part, from enhanced water-column stratification. Episodes of organic-carbon enrichment in the Bridge Creek Limestone and Fairport Chalk occur during rising sea level and highstands and are interpreted as representing eutrophication caused by the entrainment of nutrient-rich, oxygen-poor waters of an oxygen-minimum zone that impinged on the southern basin sill. These waters were tapped only during transgressive episodes that exceeded about 75–100 m water depth over the sill. The combination of eutrophication and decreased clastic dilution associated with the transgressive episodes led to maxima of organic-carbon contents (to 8 wt. %) that were highest in distal portions of the basin.

Rising sea level, in combination with tectonic subsidence, also profoundly influenced the pattern of organic enrichment in Middle to Late Devonian strata of the Appalachian Basin. Sediment starvation during transgressions led to organic enrichment in shales, which, together with seasonal benthic anoxia beneath thermally stratified waters, enhanced remineralization of nutrients from sedimented organic matter. These nutrients fueled a “eutrophication pump” that may have augmented an already rising nutrient inventory resulting from the evolution of vascular land plants and concomitant increases in the flux of land-derived nutrients.

Comparison of data sets among these three intervals of organic enrichment, widely separated in time, clearly illustrates the linked roles of sedimentation, nutrient supply, primary production, and microbial metabolism, with change in relative sea level acting as a master variable influencing each set of processes.

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