Variable marine productivity and high eolian input recorded by rhythmic black shales in Mid‐Cretaceous pelagic deposits from central Italy

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Abstract
Rhythmic interbedding of dark gray to black, laminated, calcareous shale (black shale) with lighter colored limestone, marlstone, or mudstone is a prominent feature of the Aptian‐Albian Scisti a Fucoidi in the Italian Apennines. Organic carbon contents of most black shales in the Scisti are less than 1.5%. Sedimentary structures and levels of bioturbation indicate that periods of poorly oxygenated benthic conditions alternated with more extended periods of well‐oxygenated benthic conditions during deposition of the Scisti. Geoochemical, isotopic, and visual studies of organic matter indicate variable marine productivity (generally low) and relatively high input of eolian dust during deposition of the Scisti black shales. One organic‐rich black shale near the middle of the formation may represent a brief period of stimulated marine fertility and high productivity coincident with benthic stagnation. Production of warm saline brines in evaporitic coastal settings, sinking of these brines into deepwater basinal settings, and development of stable salinity stratification is suggested as a causal linkage between arid climate, strong winds, and poorly oxygenated bottom waters in the Alpine‐Mediterranean Tethys during mid‐Cretaceous time.