Supplementary File 2 Exploratory Worksheets

Stage 1 Registered Report Worksheets; not yet peer reviewed. An earlier study of a sediment core from Körslättamossen Fen in southern Sweden found that iron-rich and glassy microspherules were abundant near the onset of the Younger Dryas. In addition, numerous nanoparticles containing platinum-group elements (PGEs) peaked 1–2 cm higher in the core. After extracting a new core, we will ask whether the sediment records a local environmental transition consistent in age with the Younger Dryas onset (question #1, Q1). We will also evaluate eight hypotheses for the origin of the PGE enrichment and other sedimentary anomalies at the corresponding horizon. These include (1) mineral formation or alteration within the sediment, (2) transport of terrestrial material, (3) human activity or modern contamination, (4) lightning, (5) natural combustion, (6) volcanic input, (7) diffuse extraterrestrial influx, and (8) an extraterrestrial airburst or impact. To locate the corresponding horizon, we will compare photographs of the new core with those of two earlier cores, matching distinctive sediment contacts and using dated volcanic ash layers where identifiable. Before sample coding, we will designate six consecutive 1-cm samples as the Target and center a 50-cm study section on that interval. An independent referee will distribute coded samples and withhold their depths until the laboratories finalize their measurements and investigators record their initial grain classifications. In all 50 samples, we will measure grain size, total mercury, total organic carbon, total nitrogen, and the carbon and nitrogen isotope ratios. We will also analyze bulk elemental composition using both acid digestion and peroxide fusion. We will pair adjacent samples into 2-cm composites to screen quartz grains for evidence of shock metamorphism and to measure helium concentrations and isotope ratios in the magnetic fraction. At 20 depths, including all six Target samples, we will analyze nanoparticle abundances and compositions, bulk PGE concentrations, and osmium isotope ratios. At 25 depths, we will measure soot-carbon concentrations and the carbon isotope composition of the soot fraction. We will compare each Target observation with measurements from older and younger sediments and report which explanations the evidence can distinguish and where more than one source hypothesis is plausible.