Integrating Charophyte Paleoecology and Geochemical Proxies in the La Pedrera de Meià Konservat-Lagerstätte (Southern Pyrenees, Spain): A Tool for better understanding fossil lacustrine settings

Charophytes, a group of green algae, are one of the main components found in continental carbonate microfacies. The combination of the palaeontologic (taxonomic and taphonomic) study with the sedimentologic analysis has demonstrates that the charophyte microfacies are highly diverse, representing a broad diversity of environments. However, the integration of these palaeoecological data with geochemical proxies (stable isotopes and elementary geochemistry) has not been poorly explored

Sixty-three thin sections from different levels were obtained along the 50 m-thick carbonate section of La Pedrera de MeiàKonservat-Lagerstätte (Southern Pyrenees, Spain). Two thin sections were obtained from charophyte-rich beds, which is demonstrated as useful to study charophytes in thin sections. A combined sedimentological study and taphonomic analysis of the fossils was performed to make palaeoecological inferences. The results were latter integrated with the palaeoenvironmental proxies, mainly elementary geochemistry and δ¹⁸O previously described in the same facies with the objective of refining the palaeoenvironmental reconstruction of the lake.

According to the palaeoecologial analysis, two communities are distinguished. Monospecific Echinochara sp.-Charaxis spicatus meadows solely occurred in temporary lakes formed during the early stages of the infilling. These meadows were associated to high Al, K, Fe and Ti concentrations, which suggests that the infilling of these lakes was associated to meteoric waters. The δ¹⁸O indicates intermittent periods of evaporation, which favoured the formation of cyanobacterial crusts around charophyte remains and the formation of mud-cracks visible in thin sections under the microscope.

The second charophyte assemblage coincides with the last stage of the lake infilling and was characterized by having two distinct communities, freshwater and brackish. The freshwater community as composed of mixed meadows with Atopochara-Clavatoraxis, Clavatoroidae utricles -Clavatoraxis, and Ascidiella-Favargerella plants while the meadows of brackish settings were composed of Porocharacean – Charaxis sp. and Echinochara sp.-Charaxis spicatus plants. This latter flora was associated to miliolids and broken dasycladaleans, suggesting brackish conditions. The Sr/Ba ratios are high suggesting marine influence, but REE+Y concentrations, and δ¹⁸O values previously reported for these lacustrine carbonates suggest predominantly freshwater conditions. These geochemical results suggest the establishment of dominantly freshwater environments with rare brackish to marine influence. This interpretation aligns with the presence of distinct brackish and freshwater charophyte assemblages, along with organisms exhibiting marine affinities, such as miliolids and dasycladaleans.

The present work demonstrates the value of cross-validation between geochemical and palaeontological and palaeoecological data of lacustrine settings that contain benthic organisms, such as charophytes and ostracods. In addition, this work also provides insights to understand the palaeoenvironmental conditions necessary for the thriving of these organisms in fossil lakes.