The extraterrestrial component in marine sediments: Description and interpretation

Abstract

The influx of extraterrestrial matter to the Earth is dominated by two size‐fractions: sub‐millimeter interplanetary dust and impacting asteroids and comets. Over geologic time the major contribution of extraterrestrial matter is from the largest impactors. Interplanetary dust is largely vaporized by atmospheric entry, but some surviving material is found in sediments as cosmic spherules. Impacting asteroids and comets can produce fallout layers either regionally or globally which may contain a significant fraction of meteoritic material as well as shock‐metamorphosed and shock‐melted terrestrial material. Iridium is a sensitive tracer of extraterrestrial matter. Although high concentrations of Ir in marine sediments probably have an extraterrestrial origin, they do not necessarily indicate the presence of a major impact event. Of the thousands of impact horizons which must be resolvable in the sedimentary record, to date only seven probable impact horizons have been identified in the entire Phanerozoic. The marine geochemistry and occurrence of Ir is still poorly understood, but the thousands of Ir analyses performed in the last several years have demonstrated that the global occurrence high Ir (>10 ng/g) concentrations in Cretaceous‐Tertiary boundary sediments is a truly anomalous phenomenon.