WHAT IS RECORDED IN YOUR BELEMNITE?

Carbonate skeletons of fossil marine organisms are widely used to reconstruct paleoenvironments. Specifically, the geochemistry of Jurassic and Cretaceous belemnite rostra is commonly applied to reconstruct paleoseawater properties. This is due to the assumption that belemnites, as member of the mollusc group, precipitated their rostra in equilibrium with the paleoenvironment. Secondly it was assumed that rostra represent originally dense low Mg calcite structures relatively stable against diagenetic alteration. More recently an increasing number of published data, show significant scatter in geochemical data when comparing belemnite rostra from the same stratigraphic level or within a single belemnite rostrum. This scatter is not explained by differential diagenetic overprint alone but also by vertical and horizontal migration patterns, seasonality, or changes in salinity. In order to test for an ultrastructural-related explanation for the observed scatter we employed a wide range of state-of-the-art analytical tools to well-preserved specimens. We found petrographic evidence that the primary, i.e. biogenic, ultrastructure of rostra of Megateuthis (Middle Jurassic), Belemnitella and Gonioteuthis (Late Cretaceous) was not a dense calcite structure, but contained primary porosity. The biogenic ultrastructure consists of a filigree framework of tetrahedrons with branches forming a honeycomb-like network. Data presented here suggest that these rostra yielded as much as 50 to 90% primary pore space. We propose that the pore space was originally filled with body fluid and/or organic compounds. Intra-rostral porosity was occluded subsequently by isopachous calcite cements of a non-biogenic origin. These may have been precipitated due to increased alkalinity related to the decay of organic matter. The resulting fabric represents a composite biogenic/abiogenic structure precipitated at different times and depths in the water column. We suggest that these findings have significance for those using belemnite rostra as archives of their paleoenvironment, for the reconstruction of belemnite paleoecology, and for the functional interpretation of belemnite rostra.