Experimental burial diagenesis of aragonitic biocarbonates: from organic matter loss to abiogenic calcite formation

[摘要] Carbonate biological hard tissues are valuable archives of environmental information. However, this information can be blurred or even completelylost as hard tissues undergo diagenetic alteration. This is more likely to occur in aragonitic skeletons because bioaragonite often transforms intocalcite during diagenesis. For reliably using aragonitic skeletons as geochemical proxies, it is necessary to understand in depth the diageneticalteration 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) hydrothermalalteration experiments at 80  ∘ C using burial-like fluids. We document and evaluate the changes undergone by the outer and innerlayers 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 forcemicroscopy). We demonstrate that this approach is the most adequate to trace subtle, diagenetic-alteration-related changes in aragoniticbiocarbonate 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 transformationtakes place. Alteration starts with biopolymer decomposition and concomitant generation of secondary porosity. These processes are followed byabiogenic aragonite precipitation that partially or totally obliterates the secondary porosity. Only subsequently does the transformation of thearagonite into calcite occur. The kinetics of the alteration process is highly dependent on primary microstructural features of the aragoniticbiomineral. 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 diageneticalteration signs. This finding may have major implications for the use of these archives as geochemical proxies.

[学科分类] 大气科学