Dominance of Wind Blown Minerals in Black Shales, Connecting Continental Climate, Fe Fertilization and Mineral Ballasting

The origin of organic carbon-rich (black) shales is commonly attributed to changes in oceanographic conditions such as anoxia or heightened biological productivity. Here we use sub-micron in situ X-Ray mineral mapping at the grain scale to identify sedimentological and diagenetic processes controlling deposition that are not evident in traditional bulk approaches. Lack of sorting, abundance of angular feldspar and other less stable mineral phases, and absence of evidence of winnowing all imply pelagic deposition of detrital minerals of aeolian origin. Using modern values of iron content in dust, the fraction of bioavailable iron, and the range of Fe:C ratios needed by phytoplankton, we calculate that the amount of organic matter associated with dust mineral particles could have been stimulated by the bioavailable iron in that dust. The anoxic bottom waters would efficiently preserve this high ratio of organic matter to minerals. We propose that aeolian deposition is a fundamental and overlooked component of black shale formation, accounting for minerals that are associated with nutrient influx and assist with ballasting of resultant blooms, and stimulate anoxia in the centre of basins. Such processes would be tightly coupled with changes in continental climate and provenance.