ABSTRACT

Leg 160 was the first in a two-leg program to investigate the tectonic and paleoceanographic history of the Mediterranean Sea. One focus of this leg was on accretionary and collisional processes associated with the convergent boundary between the African and European plates. The other focus was on the origin and paleoceanographic significance of sapropels, organic-rich layers that are intercalated in the Plio-Pleistocene sediments of the Mediterranean Basin.

One focus of drilling concerned collisional processes related to the tectonic history of the Eratosthenes Seamount. This is a crustal fragment in the process of collision with a convergent margin to the north, including Cyprus. A transect of four holes was drilled across the Eratosthenes Seamount, extending from the crestal area of the seamount, to its upper and lower slopes, and on the lower Cyprus margin. On the lower slope, an intact Upper Cretaceous and middle Eocene section was drilled, overlain by Plio-Quaternary deep-sea sediments. Shallow-water limestones, including corals and algae, and evidence of faulting and rapid subsidence, were recovered at the crestal site. Matrix-supported breccias, with a matrix of lower Pliocene microfossils, accumulated in rapidly deepening seas. Water depths increased during the early to late Pliocene, to a maximum depth today of around 2000 m. The Messinian(?) and Plio-Pleistocene sediments on the Cyprus margin record contrasting paleoenvironments and tectonic settings. The results from the Eratosthenes drilling are interpreted in terms of flexural faulting and collapse related to loading by an overriding Cyprus plate. The drilling has therefore documented a process of initial collision that is of fundamental importance to the interpretation of many mountain belts.

A further tectonic-related objective was the drilling of a transect of holes across the Milano and Napoli mud volcanoes. These are located on the northern margin of the Mediterranean Ridge accretionary complex in an area where the Mediterranean Ridge is believed to be thrust over a backstop of continental crust to the north. Drilling revealed that both volcanoes are well over 1 m.y. old and showed that much of the flank and crestal regions are composed of debris flows. These include clasts of mainly Miocene lithologies that were transported within the debris flows. The Milano volcano shows evidence for the existence of clathrates (solid methane hydrates) close to the seafloor. Gas (mainly methane) was detected at both mud volcanoes. Pore-fluid compositions indicate the presence of evaporites beneath. Drilling has thus shed important new light on the origin of the mud volcanoes, and will form a basis for future exploration.

More than 80 individual sapropels were recovered in distinctive packets and time intervals, which are separated by yellowish-brown, oxidized and carbonate-rich sediment. Individual beds in the packets were correlated between holes, separated by several hundred meters, and packets of sapropels could be matched between sites up to several hundred kilometers apart. Many individual sapropels are extraordinarily rich in organic carbon (up to 30% by weight and of predominantly marine origin) and display highly unusual magnetic properties. Sapropel occurrences mark periods when the Mediterranean catchment area experienced increased humidity and relatively high average temperatures. Once such conditions became established, they profoundly changed processes in the biologically active surface layer and at the seafloor. Our findings suggest that the development of deep-water anoxia in the eastern Mediterranean was an essential prerequisite for sapropel formation. The development of anoxia may be triggered by a reduction of deep-water formation, which is dependent on the salinities and temperatures in the surface layer. At the same time, sapropels indicate a dramatically increased carbon flux in the Eastern Mediterranean, which today is oligotrophic.

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