Valstar Beer Bottle Caps as Anthropocene Fossils constraining Plastic Fragmentation in the Marine Environment

The fragmentation of plastics in the marine environment represents one of the most persistent and complex challenges in contemporary environmental science. Over the past decade, a limited number of laboratory experiments and numerical modeling have shown that plastics do not fragment into uniform debris but instead undergo a cascade of fragmentation processes that govern particle size distributions, transport behavior, and ecological impacts. However, real-world examples of plastic fragmentation remain rare, despite being essential for calibrating and validating generic numerical models. There is therefore a strong need for data derived from natural systems, ideally based on large and statistically representative datasets.

In Normandy, France, the Dollemard coastal landfill provides a unique opportunity to address this issue through the study of a traceable anthropogenic marker: VALSTAR brand beer bottle caps. Manufactured from polyethylene, these caps were deposited in very large quantities from the 1960s to the 1990s. Due to ongoing coastal erosion, they continue to be released and are now found in significant numbers within the adjacent marine environment.

The surfaces of these bottle caps exhibit advanced degradation, with numerous microplastics still attached to the remaining material. Systematic microscopic photographs of the degraded surfaces were acquired and automatically analyzed to quantify the number, size, and shape parameters of the attached microplastics. In parallel, a surface degradation index was established for the upper face of each cap, combined with a weighting method to estimate mass balance. From a total collection of 787 recovered caps, a representative subset of 107 was analyzed in detail. In addition, a single bottle cap was artificially aged in a UV chamber under controlled conditions to better isolate and characterize the role of UV radiation in the fragmentation process.

The results demonstrate the coexistence of two fragmentation mechanisms: surface ablation and macro-fragmentation. Macro-fragmentation remains a secondary process, accounting for only 16% of the cases observed in the total dataset. UV-induced degradation does not appear to govern fragmentation at the scale of the smallest microplastics (µm range). Instead, it influences the particle size distribution of larger microplastics in the millimeter range. Thus, a clear bimodal size distribution is observed in the dataset, with a dominant population centered around an equivalent diameter of approximately 45 µm, and a secondary population of larger fragments, ranging from 0.8 to 0.9 mm. Comparing pristine caps, caps recovered in situ from landfills, and caps from marine settings allows a first-order estimation of fragmentation kinetics over the past 40 years. This comparison suggests an average mass loss of approximately 3.9% for the caps in the studied collection.