Microspherules, meltglass, and nanoparticles from the Younger Dryas boundary layer at White Pond, USA

Sediment cores from White Pond, South Carolina, preserve a well-constrained Younger Dryas Boundary (YDB) interval dated by Bayesian age modeling to ~12,875–12,775 cal BP and previously shown to contain a platinum (Pt) anomaly, elevated Pt/Pd ratios, a peak in pyrogenic carbon, and a decline in coprophilous spores. Here we report additional high-temperature proxies from this same horizon, including microspherules, vesicular meltglass, and metal-bearing nanoparticles, integrating laser ablation ICP-MS (LA-ICP-MS) and single-particle ICP time-of-flight mass spectrometry (SP-ICP-TOF-MS) to evaluate their origin and carrier phases. Scanning electron microscopy reveals Fe-rich and aluminosilicate microspherules exhibiting dendritic crystallization, quench textures, and vesiculation, together with vesicular meltglass, all confined to the YDB layer. Laser ablation ICP-MS analyses show that individual microspherules contain strongly elevated absolute platinum concentrations (up to ~82–89× upper continental crust) and correspondingly elevated Pt/Fe ratios. However, Fe-normalized Ni/Fe ratios remain within or near terrestrial compositional fields, indicating that the microspherules are dominantly derived from melted local sediment rather than from bulk meteoritic metal. In contrast, SP-ICP-TOF-MS data document synchronous increases in platinum-group-element (PGE)–bearing (Ir, Pt, Pd, Os, Ru) and transition-metal–bearing (Ni, Co, Cr, V, Fe) nanoparticles at the YDB. This interval is marked by coincident peaks in total nanoparticle mass and number, together with a reduction in average mass per particle, indicating enrichment in numerous low-mass nanoparticles. This population structure is consistent with high-temperature processing and rapid formation of fine metal-rich particles during a short-lived depositional event, potentially involving vaporization and condensation processes, rather than accumulation of a small number of large particles. Nanoparticle Pt/Fe, Ir/Fe, and HSE/Ni ratios overlap fields defined by metal-rich extraterrestrial materials, including refractory metal nuggets and cometary matter, and show minimal overlap with magmatic or upper-crustal compositions. Chondrite-normalized multi-element, rare-earth-element (REE), and PGE patterns show that the YDB layer does not reflect bulk meteoritic mixing or large-scale melt differentiation but instead records a subtle enrichment of background terrestrial sediment. The coexistence of terrestrial melt spherules and compositionally distinct metal-rich nanoparticles supports a two-population framework involving sediment melting alongside condensation or delivery of siderophile-enriched nanoparticles during a short-lived event. Absence of cryptotephra argues against a volcanic source. Together, these multiproxy observations are consistent with an abrupt, high-energy depositional pulse at the onset of the Younger Dryas and are compatible with processes proposed under the Younger Dryas Impact Hypothesis, including low-altitude airbursts or fragmented impacts by metal-bearing primitive bodies such as comets or rubble-pile asteroids.