Thermal, geological and biological processes shape the internal fabric and fluorescence of amber from La Cumbre, Dominican Republic

Dominican amber from the La Cumbre deposit exhibits a variety of colors, ranging from the more common yellow and red to those showing distinctive blue and bluish-green daylight-induced fluorescence. The study aimed to investigate its internal fabric and geochemical composition using stereoscopic and scanning electron microscopy, CHNS elemental analysis, and gas chromatography-mass spectrometry to discuss the possible factors that might have contributed to the exceptional properties of this amber. The results revealed striking microstructural features such as (i) a more or less regular network of three-dimensional domains, resembling desiccation structures, with interstices filled with clays and carbonates (resembling the desiccation pattern), (ii) a foam-like texture, with embedded mineral inclusions of Fe, Zn, Ti, and Cu. These features suggest that thermal processes, likely linked to the volcanic activity and/or local forest fires, contributed to the resins' transformation during the fossilization. The fluorescent blue and green hues are typically restricted to the superficial fossilized tree resin layers of variable thickness (from several to several dozen mm) and well correlate with the structural irregularities rather than uniform chemical markers. However, the presence of perylene in one blue specimen and clerodane-type biomarkers may suggest some impact of microbial or fungal activity on the coloration phenomena. The green amber displays unusual chemical signatures, including elevated oxygen content and uncyclized hydrocarbons, hinting at less mature polymerization processes. These findings point to a complex interaction of geological and biological factors shaped by environmental conditions during and after resin exudation in La Cumbre. Possibly volcanism, local fires, and also biogenic degradation by fungi contributed to the present shape, color, internal fabric, and fluorescent properties of Dominican amber from the La Cumbre deposit.