Distribution, Diversity and Preservation of Shallow-Water Foraminifera in the Extremely Organic-Rich Mud Flats of French Guiana

This study investigates the modern distribution, diversity, composition, and extreme preservation challenges of shallow-water benthic foraminifera along the nearshore marine mud flats of French Guiana. This coastal environment is overwhelmingly controlled by the vast, high-flux discharge of sediment and organic matter (OM) from the Amazon River system, creating a highly stressed habitat for calcifying organisms. Quantitative analysis reveals a severely constrained and moderately diverse fauna. The community structure is heavily skewed towards extremely small species (e.g., Ammonia, Elphidium), and a few robust calcareous taxa (e.g., Eponides). The dominance of stress-tolerant genera is consistent with a highly turbid, high-organic, and potentially low-oxygen environment.

The primary finding is the exceptionally poor preservation of calcareous foraminiferal shells, acting as a powerful taphonomic filter. This dissolution is a direct consequence of the extremely high rates of OM decomposition within the muddy sediments. Microbial breakdown (remineralization) of the abundant Amazon-derived OM rapidly consumes oxygen and generates large quantities of carbon dioxide (CO₂). This CO₂ increases the concentration of carbonic acid (H₂CO₃) in the sediment pore waters, leading to a significant pH decrease. The resulting undersaturation (Ω < 1) with respect to CaCO₃ minerals (calcite and aragonite) triggers the rapid chemical dissolution of the foraminiferal tests. The poor buffering capacity of the fine-grained, terrigenous muds exacerbates this effect. In addition to the completely preserved foraminifera, almost all samples contain an impressive number of hardened, brown molds (ˈSteinkerneˈ) that capture the original shape of the foraminifera's chamber arrangement, as well as the space once occupied by the living cell. Here, the original calcium carbonate shell is dissolved by acidic waters over time. What remains is a three-dimensional "negative" of the shell's interior.

The sedimentary environment of the French Guiana mud flats represents an end-member environment where near-complete post-mortem dissolution of the calcareous fraction severely biases the fossil record and acts as an effective taphonomic filter. The observed foraminiferal census, dominated by Ammonia, Elphidium and a very few robust taxa, therefore represents a highly-biased reflection of the original living community. This has significant implications for paleoenvironmental reconstructions based on foraminifera in similar high-organic-flux, tropical deltaic systems, highlighting the need to account for dissolution-driven loss of the calcareous fraction.