Late neogene laminated and opal-rich facies from the Mediterranean region: Geochemical evidence for mechanisms of formation

The mechanisms for the formation of laminated and opal-rich sediments from southern Italy (Bianco and Vrica) and southern Sicily (Gela) are investigated using a geochemical approach. The Vrica laminates are relatively rich in organic carbon while the Gela laminites are not. Carbon/nitrogen ratios, carbon isotopic composition and lipid content of the organic matter at Vrica indicate that the laminates represent periods of increased input of terrestrial organic matter and enhanced preservation. Based on oxygen isotopic studies of planktonic foraminifera, a basin stratification model is presented to explain the formation of laminites at both Vrica and Gela. Biogenic silica and carbonate content, and the carbon isotopic composition of benthic foraminifera suggest that the Bianco diatomites represent periods of increased productivity. The isotopic composition of planktonic foraminifera from Bianco indicates that the increased productivity was a result of basin eutrophication brought on by continental runoff. The low organic carbon content of the Bianco diatomites is attributed to continuous bottom water ventilation. The results of laminites studies indicate that they are not always rich in organic matter, as generally assumed. Moreover the formation of organic-rich laminites can be the result of enhanced preservation, not increased productivity. Results from Bianco indicate that preservation plays a primary role in controlling the organic carbon content of sediments. \ua9 1988