A case study on productivity, ocean ventilation and carbonate dissolution from the S-SE Brazilian margin: insights for marine carbon storage

Abstract

Greenhouse-gas emissions are rapidly transforming the Earth's system through complex feedback mechanisms affecting biogeochemical cycles. The rise in atmospheric CO2 causes global warming, leading to ocean stratification and modifying ocean circulation patterns and deep-water ventilation, while the CO2 absorbed by the oceans causes ocean acidification and promotes calcium carbonate dissolution. Anthropogenic activities are also resulting in ocean eutrophication and expanded hypoxia. The study of past environmental conditions similar to those projected under anthropogenic effects is essential to elucidate the roles of the different feedback mechanisms involved in the carbon cycle. The ideal setting to study such mechanisms would be one where strong changes in productivity, deep-water ventilation, and carbonate dissolution took place. The late Quaternary S-SE Brazilian continental margin meets all these requirements due to past changes in terrestrial fertilization, upwelling systems, bottom water masses geometry and oscillations of pH levels. Thus, it is a natural laboratory for studying processes linked with the carbon sequestration in the oceans. The research hypotheses are: (1) an enhanced sea surface productivity does not actually increasing the carbon storage, but due to the decomposition and remineralization of the exported organic matter at the seafloor the process is acting opposite; (2) the organic matter decomposition decreases the seafloor pH levels, leading to an enhanced dissolution rate of calcium carbonate, once again decreasing the carbon storage; (3) the higher decomposition rate of organic matter is depleting the oxygen levels at the seafloor, thus profoundly impacting benthic life; (4) there is a synergistic effect of the presence of corrosive southern-sourced Antarctic bottom water masses and enhanced sea surface productivity to dissolution of calcium carbonate; and (5) the selected combination of geochemical and micropaleontological proxies is a robust tool to identify the temporal and spatial variation in the contribution of both effects stated in (4). These hypotheses will be tested through geochemical and micropaleontological proxies obtained from the analysis of four piston cores spanning the last 125 kyrs, retrieved from the mid-continental slope of the S-SE Brazilian Margin. The selected proxies will provide estimates of past sea surface productivity and organic matter flux to the seafloor, changing calcium carbonate dissolution intensities, possible pH fluctuations at the seafloor, changes in water masses geometry, therefore providing a comprehensive summary of the combined effects and feedbacks affecting carbon storage under different oceanographical settings. (AU)