Extraterrestrial dust as a source of bioavailable Fe for the ocean productivity

Bioavailable Fe is an essential nutrient for phytoplankton that allows these organisms to flourish and drawdown atmospheric CO₂ affecting global climatic condition. In marine locales remote from the continents extraterrestrial (ET) dust provides an important source of Fe and thus moderates primary productivity. Here we provide the first constraints on the partitioning of extraterrestrial Fe between seawater and sediments from observations of the dissolution and alteration of 5228 cosmic spherules recovered from the deep sea sediments and Antarctica. Of the ~3,000-6,000 t/a ET dust that reaches the Earth surface, ~2–5% material survives in marine sediments whilst the remainder is liberated into seawater by dissolution and etching. Both processes contributes ~(3–10)×10^-8mol Fe m^-2 yr^-1. Also, the Fe contribution due to evaporation of survived particle is estimated to be ~0.2–0.5 t d^-1 that is ~10% of Fe contribution to meteoric smoke. The contribution of Fe from dissolution is small in comparison to recondensed meteoritic smoke particles. Cosmic spherules, also indicate that Fe is enhanced in smoke particles compared to surviving micrometeorites (MMs) by partial evaporation. Whilst smokes are delivered directly to the photic zone of the ocean, dissolved MMs also deliver their Fe in those areas. Changes in the ET dust flux vary not only the amount of Fe by up to three orders of magnitude, but also the partitioning of Fe between surface and abyssal waters depending on entry velocity and its effect on evaporation.