Mid-Pliocene warm climate and annual primary productivity peaks recorded in sapropel deposition

摘要

The Mid-Pliocene deposition in the Northern Apennines includes finely laminated sapropelitic horizons (insolation-cycles i-292 to i-282; 3.058 to 2.983 Myr) intervening within the slope-basin deposition of the foredeep, during a late highstand dated to the Globorotalia bononiensis zone. An integrated sedimentological, geochemical and micropaleontological study of the sapropel-bearing succession allowed the estimation of the time involved in the sapropel layer deposition, and to define the origin of the organic matter and the contribution of the primary productivity. The calculated average sedimentation rate of the section, including the intervening bioturbated beds, was about 0.4 mm yr(-1)-similar to the rest of the pelitic deposition during the whole Pliocene. Within individual sapropels, recurrently-spaced 0.3 to 0.6 mm thick laminae consisting of calcareous and/or siliceous plankton tests, alternating with pyrite- and organic-rich mudstones, were interpreted as the record of annual variation in the supply. We could thus calculate the duration of individual sapropels by simply counting the number of couplets occurring in the layers: these spanned between 7500 and 10 000 yr. The kerogen content of the sapropels is >= 65 % marine origin, whereas the same component in the intervening bioturbated intervals is similar to 10%. This supports the hypothesis that the sapropel deposition is linked to significant primary productivity peaks. In contrast, changes in the terrigenous supply or strong disoxia were not recorded at the bottom. Taking the nutrient supply as constant, productivity peaks could then be forced mainly by variations in the intensity of the insolation. The latter, is linked to the precession minima and has a wide oscillation; this oscillation occurred even at the end of the Mid-Pliocene warm climate, which was mainly characterized by a progressive climate deterioration that heralded the Northern Hemisphere glaciation.