Productivity-generated annual laminae in Mid-Pliocene sapropels deposited during precessionally forced periods of warmer Mediterranean climate.

Paleoproductivity estimates for a sequence of five mid-Pliocene Mediterranean sapropels illustrate the importance of insolation maxima in enhancing organic carbon accumulation. Well-laminated sapropelitic intervals in the Northern Apennines have been studied by a combination of sedimentological and micropaleontological analysis, detailed electron microscope description, bulk chemical composition, carbon-isotopic composition and elemental ratios. Each sapropel, formed during precessional minima, lasted 7.5 to 10 kyr, which is the same duration calculated for coeval counterparts in ODP sites in the eastern Mediterranean. The organic carbon mass accumulation rates of the studied sapropels show the same values of those calculated in the coeval Mediterranean sapropels, suggesting that the same productivity conditions were controlling sapropel formation in the whole Mediterranean, despite the differences in depositional setting and the strong variations in sedimentation rate. Mat-forming diatoms play an important role in increasing the settling velocity, allowing rapid sinking of organic matter and preventing bacterial remineralization in the water column. The consequent partial oxygen depletion at the seafloor increases preservation, which is therefore considered an effect, rather than a cause of the organic carbon accumulation.