A total of 2.8 kg of micrometeorite-rich sediment from Antarctica (see fig. S6 and Supplementary Text for collection sites) was processed by washing in Milli-Q H2O and sieving at the Vrije Universiteit Brussel (Brussels, Belgium) to separate size fractions of <125 μm, 125 to 200 μm, 200 to 400 μm, 400 to 800 μm, 800 to 2000 μm, and >2000 μm, while the remaining half is kept for reference and other research purposes. All size fractions were subjected to magnetic separation using hand magnets. Using optical microscopy and micro x-ray fluorescence spectrometry, 2039 cosmic spherules and 190 partially melted (scoriaceous) and unmelted micrometeorites were handpicked from the four magnetic fractions between 125 and 2000 μm. On the basis of the inspection of multiple small subsamples, no significant number of residual micrometeorites remained in the nonmagnetic fractions. Surficial textural characteristics and diameters were determined for these particles using a FEI ESEM Quanta 200 environmental scanning electron microscope at the Royal Belgian Institute of Natural Sciences (Fig. 5 and figs. S7 and S8). Additional but smaller subsamples of sediment (<150 g) were processed at the Astrogeobiology Laboratory at Lund University following a protocol similar to the one described above. Here, the studied size fractions of 80 to 200 μm, 200 to 300 μm, 300 to 500 μm, and 500 to 700 μm led to the recovery of an additional 753 cosmic spherules. The total of 2982 micrometeorites was dissolved in HF acid at Lund University. All residual mineral grains were collected on filter paper and analyzed by SEM-EDS for elemental composition (data file S4).

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