Sapropels occurrences are well-known in deep-water sediments from locations in the eastern Mediterranean extending from the Ionian Basin to the Levantine Basin. Drilling of deep-water post-Messinian sedimentary sequences by ODP Leg 161 has shown that occurrences of organic-carbon-rich sapropels and sapropel-like sediments continue westward into the Alboran Basin (Comas et al., 1996). Pliocene-Pleistocene sediments in the western Mediterranean typically contain little organic carbon (<0.2%), but multiple dark-colored layers in which TOC concentrations exceed 0.5% exist in Pleistocene parts of the sedimentary sequences in the Tyrrhenian Basin, on the Menorca Ridge, and in the basins of the Alboran Sea. A combination of Rock-Eval and C/N characterizations of the organic matter in these sapropels and sapropel-like layers shows that it is comprised of partially preserved marine material.
A feature common to the organic-carbon-rich sediments is that groups of these layers will occur at intervals of ~20 k.y. in parts of the sequences, but they will be absent in other parts. This characteristic indicates that the organic-carbon-rich layers represent a paleoceanographic setting different from today that recurred regularly in the past, yet one that depended on a complicated interplay of multiple factors and that may have been strongly influenced by local or regional paleoenvironments. The observed 20-k.y. spacing of the dark layers suggests that Milankovitch precessional cycles control the timing of their deposition. Precession of the Earth in its elliptical orbit around the sun causes the Earth-sun distance at summer and winter solstice—the times of greatest seasonal expression—to cycle between maxima and minima every 21 k.y. The Earth is now farthest from the sun at northern summer solstice, making northern hemisphere summers cooler than during the precessional minima when laminites/sapropels were often deposited.
Precessional climate changes that freshened the surface waters of the Mediterranean have been invoked as the underlying cause of the sapropels (Rossignol-Strick, 1985; Fontugne and Calvert, 1992; Sancetta, 1994). The basic premise is that summer-winter differences greater than at the present increased seasonality in the past, thereby causing greater precipitation and stronger winds in the Mediterranean region. These cyclic paleoclimatic changes evidently enhanced paleoproductivity, but to a different degree at each precessional minimum and at each location studied during Leg 161. When greatly enhanced, the delivery of marine organic matter to the seabottom was sufficiently large to exhaust the availability of dissolved oxygen in bottom waters, thereby temporarily establishing anoxic conditions at the seafloor and improving preservation of the magnified flux of organic matter to the sediments. A sapropel layer was deposited under these conditions. When not so greatly enhanced, the delivery of organic matter was not sufficient to create drawdown of bottom-water oxygen during the precessional minimum, and organic carbon was oxidized before or soon after incorporation into the sediment. No sapropel layer survived in the sedimentary record under the conditions of lesser productivity. The magnitude of paleoproduction of organic matter, therefore, was the primary factor involved in determining whether or not sapropels were deposited at the times of each precessional minima in the post-Messinian record of the Mediterranean, and local factors influenced the paleoclimate expressions of the Pliocene-Pleistocene precessional minima at the Leg 161 sites.