Massive deposition of oceanic diatom mat and its impact on the carbon-nitrogen cycle over glacial-interglacial periods

The course of diatom blooming and deposition links the oceanic primary productivity, carbon export, burial of organic carbon, and even the production and storage of dissolved inorganic carbon in the deep ocean. At present, study on the blooming and deposition of mat-forming diatoms is becoming a hotspot of research, and shall have a significant impact on the marine carbon-nitrogen cycle. By reviewing available published studies , we summarized the specific function of mat-forming diatoms at cellular level, the mechanism of their blooming, and the spatiotemporal characteristics of diatom mat deposits, discussed the potential outcomes of diatom mat deposition under the global carbon-nitrogen cycle on the orbital time scale. Mat-forming diatoms are able to grow under low-light conditions, store nutrients, regulate buoyancy via large vacuoles, be symbiotic with nitrogen-fixing cyanobacteria, and features unique urea cycle, etc. These characteristics allow mat-forming diatoms to gain a competitive advantage and bloom in oligotrophic, stratified waters or across the oceanic frontal convergence zone. Based on downcore records, the massive blooming of Ethmodiscus rex occurred in the tropical-subtropical Atlantic as well as in the tropical western Pacific-eastern Indian Ocean during the Quaternary glacial periods, which transported a large amount of organic matter into the ocean interior. The vast majority of the sinking organic carbon was degraded in the water column, and a tiny fraction entered into marine sediments. We infer that the E. rex blooming contributed to an increase in the “respired carbon pool” in the deep oceans and a decrease in the atmospheric CO₂ level in the past glacial periods. Moreover, the E. rex blooming also increased nutrient turnover in the upper ocean, which might have facilitated the expansion of glacial oceanic nitrogen reservoir. In addition, the widespread E. rex blooming during the marine isotope stages 14/12 and 4/2 has been found to be associated with the marine inorganic carbon isotope maxima events, suggesting their causal relationship. Therefore, mat-forming diatoms are an important component of the coupling oceanic carbon and nitrogen cycle. Further studies are required to constrain the extent of mat-forming diatom blooming in the geological past and its role in marine element cycling, which can help to solve the puzzle of the global carbon-nitrogen cycle over glacial-interglacial periods.