Experimental burial 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 often 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 the shell of the bivalve Arctica islandica, the prismatic and nacreous layers of the hard tissue of the gastropod Haliotis ovina, and the skeleton of the coral 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 biocarbonate structural hard materials. Furthermore, we unveil that the diagenetic alteration of aragonitic biological hard tissues is a complex multi-step process where major changes occur even at the low temperature used in this study, 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 subsequently does the transformation of the aragonite into calcite occur. The kinetics of the alteration process is highly dependent on primary microstructural features of the aragonitic biomineral. While the skeleton of Porites sp. remains virtually unaltered for the entire duration of the conducted experiments, Haliotis ovina nacre undergoes extensive abiogenic aragonite precipitation. The outer and inner shell layers of Arctica islandica are significantly affected by aragonite transformation into calcite. This transformation is extensive for the prismatic shell layer of Haliotis ovina. Our results suggest that the majority of aragonitic fossil archives are overprinted, even those free of clear diagenetic alteration signs. This finding may have major implications for the use of these archives as geochemical proxies.