Unveiling the biogeochemical role of bat guano in shaping cave morphology in the Madingou region, Republic of the Congo

The research investigated cave deposits and stalagmites in two caves within the karst area of the Madingou region, Republic of the Congo, with a focus on biocorrosion features and guano deposits prevalent in these sites. A systematic inventory of biocorrosion features in the studied caves was complemented by geochemical analyses of sediments and corroded speleothems, providing valuable insights into the underlying processes. To monitor environmental conditions, a seasonally based approach was employed, involving temperature recording, punctual CO₂ measurements, and in-situ parameter monitoring during each sampling period. These efforts contribute to a detailed understanding of speleogenesis processes, particularly the influence of bat guano-induced corrosion.

Field studies revealed remarkable observations of cave morphology, highlighting intriguing biocorrosion features such as cupolas, guano-holes in the ground, and bell holes in the roof. These features closely resemble similar formations documented in other regions, including France and Slovakia. The observed elements have sparked reflections on their significance in the late morphological evolution of caves. Understanding the role of bat guano in shapingmorphology could shed light on the broader processes influencing cave evolution in various contexts.

Two samples were collected from the dark-colored superficial sediments, along with two core samples from monitored stalagmites and one sample from a corroded speleothem, all from two bordering caves in the Madingou region. These samples underwent elementary chemical analysis, revealing that the sediments contained 11% CaO and 24% P₂O₅, while the corroded speleothem exhibited 55% CaO and 47% P₂O₅. The high phosphate (P₂O₅) and calcium oxide (CaO) concentrations, confirm the direct impact of biocorrosion driven by bat guano on the evolution of cave features.

The process of biocorrosion is further supported by environmental parameters, including an elevated CO₂ concentration that vary between 480 and 500 ppm, higher than the atmospheric average (~400 ppm), and cave temperatures ranging between 19°C and 25°C. These conditions, which are ideal for microbial activity, accelerate the decomposition of bat guano and promote the formation of phosphoric acid and carbonic acid. These acids enhance carbonate dissolution and the precipitation of phosphate minerals, leading to significant chemical and structural alterations in the cave system over time. The impact of bat guano on cave evolution is also temporally contextualized by C-14 dating, which places the deposition of the superficial sediment layer at 1956 ± 28 BP. This dating aligns with a period of guano accumulation and biocorrosion activity, highlighting the longstanding influence of bat colonies on the cave's geochemical environment.

In summary, the high phosphate content within sediment and speleothems, carbonate corrosion, coupled with the radiocarbon-dated guano deposits, suggest that biocorrosion induced by bat guano decomposition has been a primary driver of recent cave morphology evolution in the Madingou region for at least two millennia. This biogeochemical activity reflects a dynamic interplay of biological and chemical processes under specific environmental conditions, resulting in both chemical modification and structural caves features reshaping.