Preservação excepcional de insetos fósseis da Formação Crato (Cretáceo Inicial, Brasil) por meio de técnicas paleométricas

Abstract

Fossils from the Crato Formation are part of a Konservat-Lagerstätten deposit and are central to debates on the paleobiological and geochemical processes governing fossilization. The Crato Formation (Aptian, Araripe Basin, NE Brazil) represents one of these exceptional deposits, with three-dimensional preservation of insects in laminated limestones, alternating between beige and grey limestone. This study investigated the chemical and mineralogical composition of insect fossils from grey limestone (GL) through a paleometric approach—Raman spectroscopy, X-ray fluorescence (XRF), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), and ultrathin lamella preparation by Focused Ion Beam – Scanning Transmission Electron Microscopy (FIB-STEM). Results reveal the recurrent presence of framboidal pyrite in the fossil cuticle, whereas internal compartments are enriched in iron sulfates, mainly jarosites. The zoned distribution of these minerals is interpreted as diagenetic microenvironments with spatial variation in pH and redox conditions: pyrite precipitated in buffered peripheral zones, while more acidic internal regions favored jarosite formation. These patterns complement the previous taphonomic model, which limited pyritization to beige limestones, and demonstrate that different mineralizing regimes can coexist in GL. We propose a new taphonomic model for Crato Formation insects, in which internal geochemical compartmentalization and microbial activity define mineral preservation gradients at micro- to nanoscale.