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Fe‐rich relict olivine grains are found in a small percentage of cosmic spherules, which are studied here to determine the nature of their precursors. We examined 128 Fe‐rich relict olivine grains with Fa >10 mol% from 53 cosmic spherules of different types collected from Antarctica (Antarctica micrometeorites [AMM]) and deep‐sea sediments (DSS) of the Indian Ocean. Fe‐rich olivines identified in cosmic spherules are close analogs of type II chondrule olivines formed in the early solar system. The olivine analysis shows well‐defined trends in molar Fe/Mn versus Fe/Mg with an affinity for ordinary and carbonaceous chondrites. The minor oxides in olivine are in ranges such as MnO ~0.1–0.8 wt%, Cr2O3 ~0–0.7 wt%, CaO ~0–0.6 wt%, and Al2O3 ~0–0.2 wt%, respectively. The chemical composition suggests that the precursors for these Fe‐rich olivine‐bearing cosmic spherules consist of ordinary chondrites (~21%–23%, AMM‐DSS), carbonaceous chondrites (~17%–36%, AMM‐DSS), and a large fraction overlapping both carbonaceous and ordinary chondrites (~41%–62% AMM‐DSS). The elemental ratios Fe/Si/CI and Mg/Si/CI for the Fe‐rich relict olivines ranging between the values 0.5–1.0 and 1.1–1.7 are compatible with IDPs, Comet 81P/Wild 2 as well as the Asteroid Itokawa and Ryugu, which are indistinguishable from carbonaceous and ordinary chondrites. In addition, pyroxene and olivine assemblages in their Fa versus Fs mol% show strong similarities to EOC chondrites. Our results on Fe‐rich relict olivines show that these grains in cosmic spherules are less common than Mg‐rich olivines, which show a narrow range of chemical compositions identical to those from ordinary chondrites and carbonaceous chondrites, indicating a supplementary contribution of an ordinary chondritic component to the micrometeorite source of dust. [ABSTRACT FROM AUTHOR]

Bioavailable Fe is an essential nutrient for phytoplankton that enables the organisms to flourish and draw down atmospheric CO2 thus affecting global climatic conditions. In marine locales, remote from the continents, extraterrestrial dust provides an important source of Fe and thus moderates primary productivity. Here, we provide constraints on partitioning of extraterrestrial Fe between seawater and sediments from the observations of dissolution and the alteration of cosmic spherules recovered from deep‐sea sediments and Antarctica. Of the ∼3000–14,000 t a−1 extraterrestrial dust that reaches Earth's surface, ∼2–5% material falling in the oceans survives in marine sediments while the remainder is liberated into seawater. Both processes contribute ∼(3–10) × 10−8 mol Fe m−2 yr−1. The Fe contribution of surviving particles due to etching is estimated to be ∼10% of Fe contribution of meteoric smoke. Changes in extraterrestrial dust flux over geological time scales not only vary Fe delivery to the oceans by up to three orders of magnitude but also the partitioning of Fe between surface and abyssal waters depending on entry velocity and evaporation. [ABSTRACT FROM AUTHOR]

Micrometeorites (MMs) are extraterrestrial dust particles, in the size range of tens of µm to mm, recovered from the Earth's surface primarily from deep‐sea sediments, Antarctica, and also from space. The present collection of MMs (>50 µm) obtained by melting ~50 t of ice near the Maitri station, Antarctica, has allowed us to investigate the abundance and properties of the particles by an unbiased collection technique. The collection reveals a large quantity of extraterrestrial material in the ~80−140 µm size range. Previous collections have shown an abundance of particles at diameter ~200 µm, which is in contrast to our findings. This can either be explained by movement of material within the ice or a recent influx of smaller particles. The smaller particles (<80 µm) typically undergo atmospheric entry heating, contrary to earlier observations, which have suggested that they reach the Earth's surface unmelted. Chondrules and refractory inclusions are rare in the collected MMs indicating that their contribution is only a small percentage. The Maitri station collection does not have a well‐constrained ice accumulation rate and terrestrial age. Nevertheless, based on matching the previous well‐documented flux calculation of Antarctica, we suggest a slow ice accumulation rate of <1.0 g cm−2 yr−1 near Maitri station. [ABSTRACT FROM AUTHOR]