Giant grains in the Akiyoshi Limestone: A record of atmospheric circulation over the Panthalassic Ocean

Abstract

Models indicate that atmospheric transport distances for large windblown particles are limited to tens to perhaps hundreds of km owing to rapid gravitational settling, yet a number of studies of modern–recent sediment have documented the enigmatic transport of so-called “giant” (generally >63 µm) grains over thousands of km. Additionally, stratospheric injection of tephra by volcanic eruptions has the potential to increase the atmospheric transport distance of volcanically sourced material in particular. Here we report the first finding of giant grains in Earth’s pre-Pleistocene record, within upper Carboniferous–Permian shallow-marine carbonates of the Akiyoshi (paleo)atoll, which formed in the vast equatorial Panthalassic ocean. Grains with diameters commonly ~300 µm and up to ~2 cm occur, transported at least 4,500 km and up to 14,000 km away from the nearest viable source regions. Textural and compositional (both mineralogical and geochemical) data indicate the presence of both continentally sourced eolian grains and volcanically sourced grains, with the relative proportions of volcanic to continental grains peaking in the Moscovian (middle Late Carboniferous). The timing of volcanic versus continental grain deposition indicates that large-magnitude volcanic eruptions affected this part of equatorial Panthalassa in the Late Carboniferous, and that Pangaean continental aridity significantly increased into the Permian. Compositional data indicate likely source regions west of the Akiyoshi atoll, consistent with the common occurrence of westerlies over this equatorial region, thus indicating the operation of monsoonal circulation over the Paleo-Tethys sea.