The role of siliceous sponges in pre-Eocene marine Si cycle from the perspective of rock mineralogy

Abstract. The process of siliceous sponge dissolution during diagenesis was interpreted not only as an important part of marine Si cycle (comprising Si burial) but also as a significant mechanism of chert formation (so-called “chertification”; Maliva and Siever, 1989a). Both ideas were widely accepted by researches and are commonly used in geological studies. New research contradicts these seminal assumptions and indicates that in pre-Eocene marine Si cycle, although siliceous sponges were an important part of the ecosystems, did not play a controlling role in regulating dSi (= dissolved silicon) concentration in the porewater as well as in chert formation. The presented studies based on advanced mineralogical (XRD, EBSD; SEM-EDS) and microtextural (SEM) analysis of rocks and sponge remnants verify the role of siliceous sponges in the formation of Cretaceous siliceous rocks, by studying successions deposited in similar marine environments, which contain abundant fossils of siliceous sponges associated with cherts and authigenic silica polymorphs and those without them. For the first time, the mineralogical and microtextural transformations of siliceous sponge loose spicules/rigid skeletal networks, which led to their preservation as siliceous or pyrite/marcasite infillings and also in form of limonite coatings, are presented. The data presented here about the diagenesis of siliceous sponges skeletons opens the discussion on the usefulness of stable isotopic studies of δ³⁰Si in geological studies of fossils of silicifiers preserved as secondary silica polymorphs (opal-CT).