AEOLIAN INFLUENCE ON THE ORGANIC ENRICHMENT OF CRETACEOUS BLACK SHALE

We studied the highest TOC intervals of black shale deposits using Nanomin, a novel sub-micron scale electron imaging and micro X-Ray mineral mapping system specifically designed to identify the grain-scale composition and textural relationships of shale. The sediments are comprised of cyclical 5µm varve like couplets of organic matter interlaminated with mineral layers with a diamictic texture comprised of an unsorted bimodal mixture of clay minerals (~ 1-2 µm) and outsized (10- 50µm) angular grains dominated by feldspar, mica, quartz, framboidal pyrite. These sediments are laminated and lacked evidence of reworking or winnowing by currents indicating deposition by suspension. The association of organic productivity, anoxia, sediment immaturity and lack of sorting in the centre of basins away from obvious upwelling or nutrient sources in these strongly cyclical sediments supports the hypothesis that aeolian deposition triggered nutrient blooms driving anoxic conditions and ballasting. Combining modern values of iron content in dust, the fraction of that iron that is bioavailable to plankton, and the range of Fe:C ratios needed by phytoplankton, we calculate that the amount of organic matter associated with dust mineral particles could easily have been stimulated by the bioavailable iron in that dust. Dust flux is tightly coupled with changes in continental climate and provenance with episodic events accounting for the micro-laminated cyclicity and proportionality of mineral and organic matter rich layers that are concentrated when terrigenous sediments are least abundant during maximum flooding of the basin. The strong cyclicly evident in these samples from the South Atlantic can be readily related to precessional changes in solar insolation facilitating movement of the ITCZ that influenced continental climate and wind delivery of sediment.