Microplastics and the Anthropocene: Toward a new class of planetary markers

The mid-twentieth-century initiation of the Great Acceleration left an indelible anthropogenic imprint on the Earth's stratigraphic record, including the global dispersal of novel materials. Microplastics (particles 1 μm - 5 mm) are emblematic diagnostic markers of this interval due to their mostly entirely novel polymeric chemistry and ubiquity across diverse depositional environments. Microplastics represented an important component of the ultimately rejected proposal to formalize an Anthropocene epoch beginning in 1952 CE, but the stratigraphic utility of microplastics as near-synchronous markers of human transformation of the planet remains a critical frontier in Earth System science. This study provides a comprehensive synthesis of the role of microplastics as a stratigraphic marker, reflecting their growing environmental impacts. We evaluate their performance against the Anthropocene's proposed primary marker of plutonium and introduce a novel framework for their geological interpretation. We propose a polymer-based chemostratigraphic model incorporating abundance, lineage, and interval zones to capture the evolution of synthetic materials since the 1950s. Crucially, we address the often-neglected principles of taphonomy in microplastic research. Our analysis reveals that, although methodological inconsistencies and post-depositional mobility pose challenges, specific polymers with documented production histories can provide highly precise horizons for global correlation. By harmonising analytical protocols and integrating taphonomic assessments, microplastics can provide a durable, multidimensional record of the Anthropocene that complements traditional geochemical and biotic signals.