Burial of calcium carbonate in deep sea sediments: Does it all add up?

Supervisory Team:

Primary supervisor: Zanna Chase

Co-supervisor: Lennart Bach

Brief project description:

The calcium carbonate cycle in the ocean plays an important role in the global carbon cycle on timescales of decades to millions of years. Production of calcium carbonate in surface waters lowers alkalinity and increases pCO₂, thus decreasing the ocean’s uptake of carbon from the atmosphere. The dissolution of CaCO₃ on the seafloor will neutralise anthropogenic CO₂ additions to the ocean in the coming centuries (Archer et al., 2009; Ridgwell & Hargreaves, 2007). Indeed, a recent study suggests such dissolution has already started in some parts of the ocean (Sulpis et al., 2018). However, an accurate prediction of this effect is hampered by our incomplete understanding of what controls carbonate burial in the deep sea (Sulpis 2019).

The CaCO₃ cycle in the ocean is relatively simple (see figure), and one can expect a simple mass balance at the seafloor, where B = P – W – S. That is carbonate burial (B) is equal to the production of carbonate in the upper ocean minus dissolution in the water colum (W) and dissolution in the sediment (S). However, major uncertainties persist in our understanding of CaCO₃ cycling in the ocean.

In particular, the following key components of the carbonate cycle are poorly constrained:

• The rate at which carbonate is produced in surface waters (Krumhardt et al., 2019)
• The amount of dissolution that occurs during transit through the water column (Berelson et al., 2007; Sulpis et al. 2021)
• The dissolution rate at the seafloor and whether this is controlled primarily by sediment-side dynamics or water-side dynamics (Boudreau et al., 2020)

This project will take advantage of a number of recently published datasets and model results to test the carbonate mass balance in the ocean. Specifically, the project will use recent estimates of carbonate burial in sediments, carbonate production in surface waters, and dissolution at the seafloor, to test the extent to which carbonate burial can be predicted based on estimates of carbonate production and dissolution.

If a mass balance can be demonstrated, this will be a strong confirmation of the accuracy of current theory and observations. Discrepancies in the mass balance can be used to diagnose any gaps in understanding.