High index reflective glass beads in modern river sediments as a potential marker for the Anthropocene

Defining the terminus of the Holocene and the onset of the Anthropocene as a geological unit of time is currently a key debate in stratigraphy, with multiple markers and potential sites for a Global Boundary Stratotype Section and Point (GSSP) under discussion. Presumably it will be placed in the mid -20th century when planet -wide human impact on conditions and processes on Earth's surface accelerated. Technological materials in environmental samples include retro-reflective glassy microspheres, which are released into the environment during application and degradation of road markings. Here, we report the discovery of high index reflective Ba-Ti-rich glassy microspheres (similar to 150-200 mu m in diameter) in modern river sediment from the Eifel, Germany. Because of their density (similar to 4.1 g/cm(3)), these microspheres are concentrated alongside heavy minerals and are easily identified under the microscope based on their perfectly spherical morphology and high index of refraction (similar to 1.9). Dissolution experiments reveal that barium-titanate glass is practically insoluble in alkaline environments at ambient temperature. Even under acidic conditions (pH = 4), solubility rates are relatively low, although considerably enhanced in the presence of citric acid that forms a soluble Ti4+ complex. Calculated mean lifetimes of a barium-titanate glass bead (100 mu m radius) range between 802 years (in the presence of citric acid at pH = 4) and 1.34 Ma (inorganic acids at pH = 4). These lifetimes are conservative estimates because they assume constant exposure to undersaturated, highly acidic waters, which would be atypical for most sedimentary environments. Based on these results, we argue for high -index reflective glass beads as a persistent, widely dispersed, readily concentrated, optically detectable, and unambiguously anthropogenic stratigraphic marker that allows chronostratigraphic dating of sediments to a terminus postquem between c. 1970 and 2000.