Siliceous sponges or seawater as a silicon source? Tracing silicon burial pathways of the Upper Cretaceous opal‐ CT deposits using δ

ABSTRACT

Classical models of formation of sedimentary siliceous rocks (flint, chert, opoka) commonly assume that the Cretaceous marine silicon (Si) cycle was primarily biologically controlled, broadly analogous to the modern ocean. In this framework, siliceous sponges are regarded as the principal source of dissolved silica (dSi) to sediment porewaters, driving precipitation of diagenetic silica polymorphs and formation of Cretaceous siliceous rocks. This study tests this assumption using a continuous succession of Upper Cretaceous carbonate–siliceous deposits, including opoka and marls, from the Central European Basin, deposited under dSi‐rich seawater conditions. Mineralogical and microtextural analyses were combined with Si isotope data obtained from isolated opal‐CT, a metastable paracrystalline silica polymorph, to trace porewater dSi sources during low‐temperature early diagenesis. The Si isotope data were interpreted using a conceptual Landmesser‐type diffusion model describing silica migration from source to precipitating mineral during early diagenesis, controlled by the dissolution and precipitation of metastable silica phases. Lithium isotope compositions provide additional constraints to distinguish detrital from authigenic clays and to evaluate their potential role in dSi uptake during clay authigenesis. Within the framework of the proposed conceptual model, the results suggest that dissolution of siliceous sponge skeletons was not the dominant source of porewater dSi in the studied deposits. Instead, the data support a model in which elevated seawater dSi concentrations and diffusion‐driven transfer of silica into porewaters exerted the primary control on opal‐CT precipitation. Clay minerals were predominantly detrital and did not significantly regulate the local diagenetic Si budget in the studied environment. These findings suggest that the biological control traditionally assigned to the pre‐Eocene Si cycle may warrant reconsideration and indicate that, unlike in the modern ocean, silica burial in Cretaceous carbonate–siliceous systems may have involved a substantial contribution from abiotic seawater–porewater diffusion and early diagenetic precipitation processes.