Long-term experimental diagenesis of aragonitic biocarbonates: from organic matter loss to abiogenic calcite formation

Abstract. Carbonate biological hard tissues are valuable archives of environmental information. However, this information can be blurred or even completely lost as hard tissues undergo diagenetic alteration. This is more likely to occur in aragonitic skeletons because bioaragonite commonly transforms into calcite during diagenesis. For reliably using aragonitic skeletons as geochemical proxies, it is necessary to understand in depth the diagenetic alteration processes that they undergo. Several works have recently investigated the hydrothermal alteration of aragonitic hard tissues during short term experiments at high temperatures (T > 160 °C). In this study, we conduct long term (4 and 6 months) hydrothermal alteration experiments at 80 °C using burial-like fluids. We document and evaluate the changes undergone by the outer and inner layers of Arctica islandica shell, the prismatic and nacreous layers of Haliotis ovina shell, and the skeleton of Porites sp. combining a variety of analytical tools (X-ray diffraction, thermogravimetry analysis, laser confocal microscopy, scanning electron microscopy, electron backscatter diffraction and atomic force microscopy). We demonstrate that this approach is the most adequate to trace subtle, diagenetic alteration-related changes in aragonitic biocarbonates. Furthermore, we unveil that the diagenetic alteration of aragonitic hard tissues is a complex multi-step process where major changes occur even at the low temperature used in this study and well before any aragonite into calcite transformation takes place. Alteration starts with biopolymer decomposition and concomitant generation of secondary porosity. These processes are followed by abiogenic aragonite precipitation that partially or totally obliterates the secondary porosity. Only afterwards any transformation of aragonite into calcite takes place. The kinetics of the alteration is highly dependent on primary microstructural features of the aragonitic biomineral. While the skeleton of Porites sp. remains virtually unaltered within the time spam of the experiments, Haliotis ovina nacre undergoes extensive abiogenic aragonite precipitation, the outer and inner layers of Arctica islandica shell are significantly affected by aragonite transformation into calcite and this transformations extensive in the case of the prismatic layer of Haliotis ovina shell. Our results suggest that most aragonitic fossil archives may be overprinted, even those free of clear diagenetic alteration signs. This finding may have major implications for the use of these archives as geochemical proxies.