An urban collection of modern-day large micrometeorites: Evidence for variations in the extraterrestrial dust flux through the Quaternary

Contrary to expectation we report the discovery of significant numbers (500) of large micrometeorites (MMs; >100 ??m) from roof tops in urban areas. The identification of particles as MMs is achieved on the basis of their compositions, mineralogies and textures. All particles are silicate-dominated (S-type) cosmic spherules with sub-spherical shapes that form by melting during atmospheric entry and consist of quench crystals of magnesian olivine, relict crystals of forsterite and iron-bearing olivine within glass. Four particles also contain Ni-rich, metal-sulphide beads. Bulk compositions are chondritic apart from depletions in the volatile, moderately volatile and siderophile elements as observed in MMs from other sources. The reported particles are likely to have fallen on Earth in the past 6 years and thus represent the youngest large MMs collected to date. The relative abundance ratio of barred olivine to cryptocrystalline spherule types amongst the urban particles of 1.45 is shown to be higher than a Quaternary average of ~0.9 suggesting variations in the extraterrestrial dust flux over the last 800 k.y. Changes in the entry velocities of dust caused by quasi-periodic gravitational perturbation during transport to Earth are suggested to be responsible. Variations in cosmic spherule abundance within the geological column are thus unavoidable and can be a consequence of dust transport as well as major dust production events