New model of Si balance in the Late Cretaceous epicontinental European Basin

Abstract

The Si, which is an important element within the Earth's crust, mostly occurs as a monomeric silicic acid (DSi) in seawater and is utilised by marine organisms to build their skeletons made of opaline silica (BSi). It is also an essential component for many minerals formed in marine environments. The general model of the Si balance, mostly derived from studies of the current oceans, assumes Si input from the weathering of the Earth's crust, its flux in the water column under biological control, and the output via burial in marine sediments. In the Campanian-Maastrichtian (Late Cretaceous), the extremely high sea levels caused the reduction of the continents area and triggered the development of widely spread epicontinental seas. This, combined with extensive volcanic activity, spreading of the ocean floor, and active ocean ridges, elevated the overall Si concentration in the seawater, affected the proportion of Si input from terrestrial weathering and deep-sea fluxes, and influenced the development of the siliceous organisms. The model presented in this study reconstructs the Si balance in the Campanian-Maastrichtian European Basin based on geological records, characterizes the main sources of the DSi input, biological flux, and output via diagenesis. The data presented indicate the significant role of siliceous sponges in the biologically controlled part of the cycle and the DSi-rich inflow from the oceans, which were the principal sources of the (net) inputs of DSi. This data has linked the Si cycle to the bottom zone of the basin, which contrasts with the recent model in which the cycle is concentrated on the subsurface zone of the ocean. The data derived from petrographic studies of chalk and opoka lithofacies broadly show how the Si cycle had affected rock formation and highlighted the role of the mineralogical composition of the seabed mud in the output of DSi from the cycle. The sediment pathway of the DSi burial in the presented Cretaceous model was initiated by the early transformations of biogenic opal-A (BSi) into a more stable form of opal-CT a few centimetres below the seafloor during the early stages of diagenesis, impacting Si recycling. The new model of the Late Cretaceous Si cycle better recognises the role of siliceous sponges and diagenesis in the Si balance in modern environments and provides a paleoecological interpretation of the formation of chalk, opoka, and siliceous nodules during the Late Cretaceous in the European Basin. • In Late Cretaceous the main DSi input was by upwelling currents from deep oceans. • The siliceous sponges play a main role in DSi uptake from water column and its sink. • The Si recycling was limited by its fast burial and diagenetic transformations. • The Si cycle in Late Cretaceous epicontinental seas differs from the modern model.