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

Abstract

A multidisciplinary study of the upper Albian—Cenomanian portion of the Cismon section (Venetian region, northern Italy) was undertaken in order to characterize the cyclic alternations of carbonate-rich and carbonate-poor layers and to investigate their possible origin and cyclic patterns. Limestone semicouplets are characterized by abundant radiolarians and micarbs (micron-sized calcitic fragments), common planktonic foraminifera, strong bioturbation, good oxygenation as expressed by the Mn * and V/(V+Ni) parameters, high Si/Al ratio, low K/Al, in the absence 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 organic matter. The low Si/Al ratio and high K/Al ratio, the latter related to abundant feldspars, indicate that marlstones received a substantial contribution from a terrigenous source. The abundance of biogenic silica suggests that during limestone deposition surface waters were rather fertile due to an efficient recycling of nutrients from deeper waters. The enhanced fertility was coupled with an efficient current system at the welloxygenated sea floor which prevented the organic matter from accumulating. During marlstone deposition, surface waters were less fertile than in the limestone semicouplets, but still within the mesotrophic spectrum as indicated by the presence of fertility index species among calcareous nannofossils, whereas the bottom waters were oxygen-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 18O-depleted δ values. In the meantime, the water discharge supplied the nutrients to the surface waters necessary for the calcareous nannofossils to proliferate. The cyclic organization of limestone/marlstone couplets as revealed by power spectral analysis seems to represent climatic cycles. The alternating conditions are (1) efficient mixing, highly fertile surface and near-surface waters, and a well-oxygenated seafloor to (2) enhanced water runoff or decreased evaporation, or both, and oxygen-depleted bottom waters. We suggest that this deterministic sedimentary cyclicity could be explained by periodic orbital-climatic cycles, in tune with the Milankovitch cycles. Within this scenario, the latest Cenomanian Bonarelli Level is an exceptional event marked by a large positive δ 13C spike. It is characterized by high to very high fertility conditions in surface waters caused by increased upwelling that engendered high nutrient levels in near-surface waters. The enhanced production of organic matter was in excess for the oxygen available at the bottom floor, thus allowing the organic matter to accumulate in the sediments.