Geochemistry of coeval marine sediments in Mediterranean ODP cores and a land section: implications for sapropel formation models

The geochemistry of eastern Mediterranean sediments from the early Pleistocene insolation cycle i-156 was studied in the Vrica land section (Calabria, Italy) and in ODP Sites 967 (Eratosthenes Seamount) and 969 (Mediterranean Ridge). The ODP sediments are rich in carbonates and contain a sapropel with up to 20% organic carbon, whereas the Vrica sediments are rich in clays and contain a laminite with up to 1% organic carbon. Processes that affect land sections but not ODP sediments partly account for these differences. More desiccation and compaction of the land sections resulted in higher dry bulk densities. Subsurface weathering caused oxidation in sediments surrounding porous ash layers, resulting in lower organic carbon and total nitrogen contents, and lower C/N ratios and organic carbon isotopic values around those ash layers. Subaerial weathering only resulted in further desiccation and higher Na and K contents at the outcrop surface because of evaporation. Other geochemical differences between the ODP cores and the land section are interpreted in terms of different settings: the Vrica sediments have been deposited closer to land, in shallower water, with a higher sedimentation rate and in a more western part of the Mediterranean than the investigated ODP sediments. Thus, aluminium contents and organic carbon accumulation rates are higher, ‘terrestrial’ and redox-sensitive element to aluminium ratios as well as barium contents are lower, and organic carbon isotopic values are more negative in the sediments of Vrica than in the ODP cores. The formation of a laminite enriched in organic carbon at Vrica versus laminated sapropels at the ODP sites can, however, be explained by one model combining both higher productivity and better preservation during precession minima. The occurrence of an interval enriched in organic carbon around the anoxic laminite provides support for the hypothesis that anoxia is caused when a certain productivity threshold is exceeded. The inferred duration of the laminite is shorter than that of the sapropels, indicating that anoxia may have lasted longer in deep water (or in more eastern parts of the eastern Mediterranean) than in shallow environments (or in the western part of the eastern Mediterranean). This investigation shows that marine sediments in land sections may be directly comparable with their equivalents in marine cores. This presented comparison of organic-rich sediments in the well-constrained setting of the Mediterranean Neogene will enhance our understanding of older black shales in marine cores and land sections.