Paleosalinity fluctuations and organic matter dynamics reveal coupled environmental and biotic changes in the Aptian Crato lake-lagoon system

Traditionally the Aptian Crato Formation of the Araripe Basin has been interpreted as a lacustrine system characterized by exceptional fossil preservation and pronounced hydrological variability. However, the relationships among paleosalinity fluctuations, organic matter dynamics, and biotic changes remain poorly understood. To investigate these interactions, we analyzed a ~10-m-thick interval (11–21 m) of the Pedreira Três Irmãos (PTI-01) section, Nova Olinda, northeastern Brazil, integrating total organic carbon (TOC), Rock-Eval pyrolysis, S/TOC ratios, carbonate carbon and oxygen isotopes, X-ray fluorescence geochemistry, sedimentological observations, and biotic events. Organic matter abundance and quality vary markedly throughout the studied interval, with relatively hydrogen-rich organic matter in the lower portion (HI up to ~612 mg HC/g TOC) and significant fluctuations higher in the interval. A major environmental transition occurs near 16 m, marked by abrupt increases in TOC and hydrocarbon-generating potential that coincide with pronounced variations in S/TOC and Sr/Ba ratios. These changes indicate recurrent fluctuations in paleosalinity and hydrological balance rather than a gradual trend toward increasingly saline conditions. Centimeter-scale gypsum layers independently support episodes of enhanced salinity and evaporative concentration, while a positive shift of approximately 1.7‰ in δ¹³Ccarb suggests concomitant changes in carbon cycling. The Hg/TOC ratios also display marked variability throughout the section, further highlighting dynamic biogeochemical conditions. Importantly, paleosalinity fluctuations are stratigraphically associated with successive occurrences of ascidians and calcareous dinoflagellates, whereas the occurrence of Pattersoncypris spp. and Sergipea varriverrucata provides additional biostratigraphic markers within the succession. The presence of organisms with marine affinities during some salinity pulses suggests that evaporation alone cannot fully explain the observed salinity changes and is consistent with episodic marine connectivity. The coupled variations in organic matter characteristics, carbon-isotope composition, paleosalinity proxies, and Hg/TOC demonstrate that the Crato lake-lagoon system experienced recurrent environmental and biogeochemical perturbations. These perturbations coincided with shifts in biotic distribution, highlighting a close relationship among hydrological variability, organic matter dynamics, and ecological change in the Aptian Crato lake-lagoon system and revealing the dynamic environmental conditions that characterized this renowned Konservat-Lagerstätte.