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

Abstract

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.

Pb Removal Efficiency by Calcium Carbonates: Biogenic versus Abiogenic Materials

“…In the time length of the experiments conducted, the second type of porosity can form only through the dissolution of water-soluble biopolymeric components exposed on the surface of the biomineral to interaction with the aqueous solution. Not all biopolymers are soluble and biopolymer degradation is a very slow process at low temperatures. ,, Since biocarbonate dissolution and cerussite precipitation are concomitant processes, the impact of this newly formed porosity through biopolymer dissolution on the overall Pb scavenging process will largely be modulated by the characteristics of the precipitated cerussite layer.…”

“…The similarity of the S BET of the pristine samples and the thermally treated ones indicates that the presence of the polymeric component does not lead to an overestimation of the specific area of the pristine samples. The slightly higher surface area of the thermally treated samples can be explained by the generation of porosity during biopolymer degradation . Part of this porosity can also be destroyed during the thermal treatment due to recrystallization and abutting of the crystal units.…”

“…The slightly higher surface area of the thermally treated samples can be explained by the generation of porosity during biopolymer degradation. 69 Part of this porosity can also be destroyed during the thermal treatment due to recrystallization and abutting of the crystal units. The average biopolymer contents of BIO-ARG and BIO-CAL samples, as determined by thermogravimetric analysis (TGA) in a Mettler Toledo TGA/SDTA 851 thermal analyzer in an oxygen atmosphere, are 9.8 and 1.7 wt %, respectively.…”

“…Not all biopolymers are soluble and biopolymer degradation is a very slow process at low temperatures. 69,103,104 Since biocarbonate dissolution and cerussite precipitation are concomitant processes, the impact of this newly formed porosity through biopolymer dissolution on the overall Pb scavenging process will largely be modulated by the characteristics of the precipitated cerussite layer. Lead Scavenging Capacity of Biogenic Calcium Carbonates: BIO-CAL versus BIO-ARG.…”

Formation of Strontianite and Witherite Cohesive Layers on Calcite Surfaces for Building Stone Conservation

“…Aragonite and calcite type structures share structural features since both consist of layers of hexagonally arranged calcium atoms alternating with also hexagonally arranged CO 3 layers . The similarities between both types of structures increase the possibility for the development of epitactic relationships between crystals with aragonite-type and calcite-type structures. − Strontianite and witherite have molar volumes that are 5.6 and 24.1% larger than the molar volume of calcite, respectively. Therefore, we hypothesize that the replacement of a micrometric outermost layer of the calcite crystals by either strontianite or witherite may result in the formation of cohesive layers, constituting a potentially effective treatment for protecting limestone and marble surfaces from interaction with the alteration vectors.…”