Fossilization in Geopark Araripe studied through X-ray diffraction, scanning microscopy and thermogravimetric analysis

The Geopark Araripe, located in Northeastern Brazil, is the first UNESCO Natural Park in the South hemisphere and a world-famous fossil deposit of the Early Cretaceous period (approximately 120 million years). Fossilized fish fauna in Geopark Araripe is found inside of sedimentary rocks in three-dimensional forms. In the present study sedimentary rocks and fossil fish Rhacolepis bucalis have been carefully analysed by means of X-ray powder diffraction, scanning electron microscopy and termogravimetric analysis. Mineralogical composition of the fossil fish was explained in terms of facts occurred at the initial stages of the opening of the South Atlantic and the oceanic hydrothermal phenomena (black smoker'', white smoker’’ and warm-water events). The occurrence of organic substance was, for the first time, evaluated in collapsed internal elements (intestinal and muscles) by termogravimetric analysis.

💡 Research Summary

The Araripe Geopark in northeastern Brazil, a UNESCO Natural Park, preserves an exceptional Early Cretaceous (≈120 Ma) fish fauna, notably the species Rhacolepis bucalis. This study applied three complementary physical‑chemical techniques—X‑ray powder diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA)—to a fossil specimen and the surrounding sedimentary concretion (coded N1Q1) in order to elucidate the mineralogical pathways and the presence of residual organic matter.

XRD measurements were performed on a Rigaku DMAXB diffractometer in Bragg‑Brentano geometry. Qualitative phase identification using Philips X’Pert HighScore software and the International Centre for Diffraction Data (ICDD) database revealed that the bulk of the fossil, the collapsed internal elements (intestine and muscle remnants), and the surrounding rock share a dominant diffraction pattern corresponding to pure calcium carbonate (calcite, ICDD 85‑1108). Crystals observed in the body cavity displayed two distinct morphologies: transparent prismatic crystals identified as calcite and white cubic crystals identified as barium sulfate (BaSO₄, ICDD 76‑0213). A weaker peak near 2θ ≈ 32° was detected only in the fossil scales and internal elements, matching hydroxyapatite (Ca₅(PO₄)₃(OH), ICDD 86‑0740). The presence of hydroxyapatite is interpreted as biologically derived phosphate from fish bone and scale tissue, whereas BaSO₄ is attributed to hydrothermal input and does not participate in the primary mineralization of the organism.

SEM imaging (Philips XL30) confirmed the crystal habits: prismatic calcite crystals ranged from 10–30 µm in length, while cubic BaSO₄ crystals measured 20–50 µm. Both crystal types were uniformly distributed throughout the fossil cavity, indicating simultaneous infiltration of mineral‑laden waters during early diagenesis.

TGA (TA Instruments SDT 2960, heating rate 10 °C min⁻¹, N₂ flow 100 mL min⁻¹) showed a major weight loss of ~40 % between 650 °C and 810 °C for both the sedimentary matrix and the internal elements, corresponding to the decomposition of CaCO₃ and hydroxyapatite. Notably, the internal‑element sample exhibited an additional 9 % weight loss between 360 °C and 470 °C, which the authors attribute to the volatilization of residual organic material (proteins, lipids). This is the first direct thermogravimetric evidence of preserved organic matter within the collapsed internal structures of an Araripe fossil.

The authors link these mineralogical observations to the geodynamic context of the early South Atlantic opening. They propose that catastrophic rifting and associated seafloor hydrothermal activity (“black smoker”, “white smoker”, and warm‑water vent) dramatically increased dissolved Ca²⁺ and CO₃²⁻ concentrations in bottom waters. This chemically enriched milieu facilitated rapid permineralization of the fish’s soft tissues by calcite precipitation. Gas production by early bacterial decay likely ruptured the body wall, providing pathways for BaSO₄ to infiltrate the cavity. Concurrently, localized pH elevation from ammonia release may have promoted microcrystalline calcite deposition. The hydroxyapatite detected is considered a relic of the original biological phosphate, not a hydrothermal product.

In conclusion, the fossilization of Rhacolepis bucalis in the Romualdo Member of the Santana Formation represents a rare confluence of specific water chemistry, rapid mineral infiltration, and microbial activity. The preservation of organic residues opens avenues for molecular paleontology, including the potential recovery of ancient DNA or protein fragments, which could illuminate micro‑ and macro‑evolutionary processes at the molecular level. The study underscores the importance of integrating mineralogical, microscopic, and thermal analyses to reconstruct fossilization pathways in exceptional Lagerstätten.