Middle Miocene (Langhian) sapropel formation in the easternmost Mediterranean deep-water basin: Evidence from northern Cyprus

摘要

Mid-Miocene (Langhian; ～ 15.4 Ma) sapropels formed within the easternmost Mediterranean basin, now uplifted in northern Cyprus. These sapropels represent the oldest known sapropels in a predominantly marl succession. Six well-developed sapropels were studied. Strontium isotope dating of twelve samples gave a preferred age of ～ 15.4 Ma (Langhian); i.e. during the final phases of the Middle Miocene Climatic Optimum (MCO). The age of the best-preserved nannofossil assemblage (Langhian) is close to the strontium ages. The Langhian strontium ages are preferred over an alternative early Serravallian age for less well-preserved nannofossil assemblages. Total organic carbon contents in the sapropels reach maximum values of 3.9 wt.%. Relative to the host marls, the sapropels show enrichments in terrigenous-derived minerals and related major and trace elements. Sedimentological evidence indicates that the terrigenous sediments were eroded from the northern borderlands of the deep-water basin under warm, humid conditions. High fresh-water run-off from surrounding landmasses is likely to have promoted a low-salinity lid to the eastern Mediterranean deep-water basin. This, in turn, would have restricted deep-water ventilation and promoted widespread anoxia. Exceptionally high concentrations of chalcophile elements (e.g. Cu, Ni and Zn) are consistent with anoxic conditions. Abundant nutrient-rich fresh-water input is also likely to have stimulated siliceous productivity (although any siliceous microfossils did not survive diagenesis). A significant role for diagenesis in sapropel formation is indicated by the mobilisation of Ba from sapropels to marl directly beneath. Orbitally induced dry-wet oscillation, the mechanism invoked to explain the Pliocene to Holocene sapropels, apparently was already in place during the latest stages of the MCO when the Langhian sapropels accumulated. These sapropels accumulated immediately after the Middle Miocene closure of the Southern Neotethys when the Eastern Mediterranean Sea apparently became more sensitive to orbital cyclicity. The development of a semi-enclosed deep-water basin was, therefore, a prerequisite for sapropel formation.