10-01-2025
Charophyte grasses in a Cretaceous lake and their relation with water chemistry described
Geological and paleontological analysis techniques describe the evolution of a Cretaceous lake at the La Pedrera de Meià site.
The study reveals how environmental variations shaped the charophyte communities of the fossil lake more than 125 million years ago.
A team formed by researchers from the Faculty of Earth Sciences of the University of Barcelona and the Biodiversity Research Institute (IRBio), the Autonomous University of Barcelona and the Conca Dellà Museum has described the charophyte meadows and their relationship with the water chemistry of the La Pedrera de Meià paleontological site (Montsec, Pyrenees). The study was carried out based on the analysis of thin layers of limestone (layers 0.03 millimeters thick) that were subsequently observed under a microscope. Although the observation of limestone sheets often does not allow the identification of fossils at the species level, it does give a very accurate idea of the algae that were present in the lakes.
Charophyte grasses and water chemistry
The observation of the sheets has allowed the description of two different communities. The first formed solely by plants with fruiting of Echinochara sp. and thallus of Charaxis spicatus, and was formed during the first phases of filling the lake, in small temporary bases. The high concentrations of elements such as iron, titanium or potassium indicate that these bases were filled by meteoric waters, that is, coming from torrents that reached the lake. The oxygen isotopes indicate moments of evaporation, which is also reflected in the formation of desiccation cracks that are observed under the microscope.
The final stage of lake filling presents two charophyte communities, one freshwater and one brackish. The freshwater community was composed of very diverse, even diverse, grasses with three types of plants, one with Atopochara fruiting bodies and Clavatoraxis thalli, Ascidiella fruiting bodies associated with Favargerella thalli and Clavatoracies fruiting bodies associated with Clavatoracies thalli. The saltwater community was dominated by Charaxis thalli that possibly belonged to Porocaracies, a family that dominated these environments in the Early Cretaceous, and to a lesser extent by Echinochara sp. fruiting bodies and Charaxis spicatus thalli. Foraminifera and marine algae have been found associated with this second community, which supports a certain salinity. The geochemical analysis of these rocks indicates that there was an inflow of brackish water rich in strontium, but this was low compared to the inflow of fresh water, since other elements such as gallium or rare earths present typical values of fresh water.
Getting to know a fossil lake
The study has allowed a better understanding of the conditions of the lake that is today known as “La Pedrera de Meià”. Paleontological studies in La Pedrera de Meià date back to the beginning of the 20th century and during this time, the site has become a Barremià paleontological site (130–126 million years old) of global interest, because some of the oldest flowering plants in the world, Montsechia vidalli, have been found, as well as hundreds of animal remains including fish, amphibians, reptiles and even birds. This study has focused on micropaleontology and how the conditions of the lake have changed throughout its history and how this has affected the algal communities. It also proposes a new methodology to study charophyte remains and better understand fossil lakes, by combining the study of charophytes and geochemistry. Finally, it is worth noting that the study has been published in open access in the journal Facies, thanks to the agreement between the University of Barcelona and Springer: this initiative makes scientific research available to everyone.
Reference article:
Pérez-Cano, J., Gil-Delgado, A., Oms, O. et al. Paleoenvironmental constraints influencing charophyte communities in an Early Cretaceous Karstic Lake, La Pedrera de Meià Konservat-Lagerstätte (Southern Pyrenees): combining geochemical and palaeoecological analyses. Facies 71, 4 (2025). https://doi.org/10.1007/s10347-024-00696-9