ORBITALLY INDUCED LIMESTONE MARLSTONE RHYTHMS IN THE ALBIAN-CENOMANIAN CISMON SECTION (VENETIAN REGION, NORTHERN ITALY) - SEDIMENTOLOGY, CALCAREOUS AND SILICEOUS PLANKTON DISTRIBUTION, ELEMENTAL AND ISOTOPE GEOCHEMISTRY/

A multidisciplinary study of the upper Albian-Cenomanian portion of th e Cismon section (Venetian region, northern Italy) was undertaken in o rder to characterize the cyclic alternations of carbonate-rich and car bonate-poor layers and to investigate their possible origin and cyclic patterns. Limestone semicouplets are characterized by abundant radiol arians and micarbs (micron-sized calcitic fragments), common planktoni c foraminifera, strong bioturbation, good oxygenation as expressed by the Mn and V/(V+Ni) parameters, high Si/Al ratio, low K/Al, in the ab sence of pyrite and organic matter. The marlstone semicouplets are, on the contrary, frequently laminated, rich in pyrite and organic matter with common fish remains in the absence of radiolarians. The Mn and V/(V+Ni) parameters indicate that the marlstone deposition occurred in dysoxic to anoxic conditions, which allowed the preservation of organ ic matter. The low Si/Al ratio and high K/Al ratio, the latter related to abundant feldspars, indicate that marlstones received a substantia l contribution from a terrigenous source. The abundance of biogenic si lica suggests that during limestone deposition surface waters were rat her fertile due to an efficient recycling of nutrients from deeper wat ers. The enhanced fertility was coupled with an efficient current syst em at the well-oxygenated sea floor which prevented the organic matter from accumulating. During marlstone deposition, surface waters were l ess fertile than in the limestone semicouplets, but still within the m esotrophic spectrum as indicated by the presence of fertility index sp ecies among calcareous nannofossils, whereas the bottom waters were ox ygen-depleted. To accommodate fertility at the surface with stagnation at the bottom the scenario implies an enhanced stratification caused by an increase in freshwater runoff monitored by O-18-depleted delta v alues. In the meantime, the water discharge supplied the nutrients to the surface waters necessary for the calcareous nannofossils to prolif erate. The cyclic organization of limestone/marlstone couplets as reve aled by power spectral analysis seems to represent climatic cycles. Th e alternating conditions are (1) efficient mixing, highly fertile surf ace and near-surface waters, and a well-oxygenated seafloor to (2) enh anced water runoff or decreased evaporation, or both, and oxygen-deple ted bottom waters. We suggest that this deterministic sedimentary cycl icity could be explained by periodic orbital-climatic cycles, in tune with the Milankovitch cycles. Within this scenario, the latest Cenoman ian Bonarelli Level is an exceptional event marked by a large positive delta(13)C spike. It is characterized by high to very high fertility conditions in surface waters caused by increased upwelling that engend ered high nutrient levels in near-surface waters. The enhanced product ion of organic matter was in excess for the oxygen available at the bo ttom floor, thus allowing the organic matter to accumulate in the sedi ments.