ATMOSPHERIC DUST STIMULATED MARINE PRIMARY PRODUCTIVITY DURING EARTH’S PENULTIMATE ICEHOUSE

Iron is a highly limiting nutrient for terrestrial and marine primary production. While the biogeochemical effects of dust deposition for the modern surface are well studied, they remain virtually unexplored for deep time. Dust deposits are well recognized from the late Paleozoic, and thus this interval might archive evidence of dust-fertilization effects on marine ecosystems. Here, we apply organic and inorganic geochemistry combined with sedimentology to assess dust impacts on marine primary productivity in Moscovian (Middle Pennsylvanian, ~310 Ma) and Asselian (Lower Permian, ~295 Ma) carbonate strata from peri-Gondwanan terranes of Iran.

Sample from the Moscovian and Asselian exhibit facies evidence for fluctuating depositional conditions. Heterozoan-bearing carbonates formed near wave base, whereas photozoan-bearing carbonates formed within the photic zone, and are punctuated with evidence for lowstand (near-exposure). Facies evidence for algal and microbial activity occur in the form of variably rare to common presence of algal fossils (e.g. phylloid algae) and clotted-micrite fabrics. The amount of the finest dust particles (<15 µm) through these intervals similarly varies and tends to positively covary with evidence for algal or microbial activity. Carbonates with facies evidence for microbial activity contain 2-methylhopanoids lipids-- key biomarkers for cyanobacterial oxygenic photosynthesis. These samples are also enriched in highly reactive iron (Fe_HR), whereas facies with low or minimal evidence for algal or microbial activity exhibit lower values of Fe_HR. Moreover, samples dominated by fine-grained dust (<15 µm) are enriched in reactive Fe without a corresponding increase in total Fe. These observations are consistent with the hypothesis that dust stimulated marine primary productivity with evidence for enhanced microbial and cyanobacterial growth, thus fixing nitrogen while also promoting carbonate precipitation. Consequently, dust likely stimulated both organic and inorganic carbon cycling, helping to maintain low pCO₂ even as terrestrial repositories for organic carbon (peat/coal) diminished from Carboniferous to Permian time.