Isotopic constraints on declining oceanic nutrient inventories weakening the marine soft-tissue biological carbon pump during the last deglaciation

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Understanding the mechanisms controlling the nearly 80 ppm increase in atmospheric CO2 concentrations during the last deglaciation (19,000 to 9,000 years before present) remains a major challenge in paleoceanography. It has been suggested that up to half of the deglacial atmospheric CO2 rise may be driven by a reduction the marine biological carbon pump, i.e. the sequestration of CO2 in the deep ocean through primary production and vertical export of organic carbon. Even for the marine biological carbon pump, many mechanisms and nutrient cycles have been proposed to have driven these deglacial changes. This issue is extensively discussed and it remains largely unresolved since most studies focus only on single mechanisms and thus a coherent picture is lacking. In this project, we will use the Model of Ocean Biogeochemistry and Isotopes (MOBI) coupled within an Earth System Climate Model to estimate how declining marine nutrient inventories of nitrogen, iron, and dissolved organic carbon impacted the soft-tissue biological carbon pump in a transient simulation of the last deglaciation. The novel approach of our project will be utilizing the isotope tracers of carbon (13C and 14C) and nitrogen (15N) incorporated in our model. Direct comparisons of simulation results with isotopes measured in sedimentary records provide important quantitative constraints on processes controlling marine nutrient inventories and thus the strength of the soft-tissue biological pump. We will provide a comprehensive analysis on changes to key limiting nutrient inventories, as well as their individual and combined contribution to rising deglacial atmospheric CO2 using a coherent multi-proxy model-data comparison. This project will indicate the most important climate-sensitive marine nutrient cycle processes that need to be incorporated into global climate models to improve predictions of the future ocean and climate.