Aragonite-to-aragonite diagenesis and its bearing on carbonate archives and proxy interpretation

• Applicant: Professor Dr. Adrian Immenhauser
• Subject Area: Mineralogy, Petrology and Geochemistry ; Geology
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 541132158

Aragonite is abundantly formed in Earth's surface environments. The volumetrically perhaps most significant manifestations are aragonite biominerals in marine ecosystems, predominantly reefs, carbonate platforms, and abiogenic aragonite cement. Aragonite is relevant for sedimentology, carbonate diagenesis, biomineralisation, material science, archaeology or metamorphic petrology. Over the past century, outstanding research has focussed on the mineral aragonite, which has significance as an archive (speleothems, scleractinian corals, molluscs etc.) of Earth's more recent climate variations and processes. In this sense, it is surprising that research focussing on early diagenetic aragonite-to-aragonite alteration has been sporadic at best. We here make the case that (biogenic) aragonites may undergo early diagenetic recrystallisation to a secondary aragonite fabric that is often organic-matter lean and characterised by its coarser fabric. This feature is observed at aragonite-to-calcite reaction fronts but is also a product of aragonite stabilisation. The limited amount of data available thus far is based on observing natural aragonites but is now tested and verified in alteration experiments in the laboratory. Do we know if aragonite-aragonite recrystallisation is common in natural environments or limited to specific diagenetic boundary conditions? No, we do not. Do we know what the impact of aragonite-aragonite recrystallisation on proxy data is? No, this is essentially unknown and may range from none to significant. If geochemical recalibration occurs, it will be most significant for those concerned with dating analyses or spatially very highly resolved geochemical sampling techniques. It seems clear that the transitional stages in aragonite-to-calcite neomorphism are underexplored as most previous research has focused on the educt (primary aragonite) and the product (stable calcite). In the context of the research proposed here, we apply a combination of sophisticated experimental approaches and investigate naturally altered (early diagenetic) field samples. This approach is a prominent feature of this project and goes beyond most previous studies based on either field or experimental work. We will analyze geochemical proxy data (C, O, coupled delta47 - delta48 clumped isotopes and elemental ratios) to explore the significance of aragonite-aragonite diagenesis on the geochemical archive of these materials and build a more refined aragonite-to-calcite neo-morphism model. We aim to establish a protocol to recognize secondary aragonite in natural and experimentally altered samples. We intend to explore how hydrogeochemistry and organic matter in the crystal lattice enhance or inhibit aragonite-aragonite diagenesis. The research proposed here has significance for a wide range of disciplines in carbonate research and has the potential to close important gaps in our understanding of carbonate diagenesis under Earth-surface conditions.

• DFG Programme: Research Grants
• International Connection: Austria
• Cooperation Partner: Professor Dr. Martin Dietzel