The role of particle characteristics in determining transfer efficiency of the biological carbon pump

Abstract

The Biological Carbon Pump (BCP) exerts a major control on atmospheric CO2. Through the surface production, subsequent transfer to depth and remineralisation of organic carbon, the BCP acts to maintain atmospheric CO2 approximately 120 – 200 ppm lower than they would otherwise be. The size of the remineralised carbon pool stored in the deep ocean- out of contact with the atmosphere- is strongly influenced by the efficiency with which sinking organic carbon reaches the deep ocean: the “transfer efficiency” of the BCP. Yet despite the importance of transfer efficiency, the importance of the many factors governing transfer efficiency remain unclear.

One group of factors believed to be highly important in determining transfer efficiency are the characteristics of sinking organic particles. In this thesis I examine the influence of several particle characteristics such as size, biomineral ballast content, and morphology on particle sinking velocities and transfer efficiency, combining a meta-analysis, shipboard Marine Snow Catcher (MSC) deployments, and in situ imaging methods.

Particle size alone is shown to represent a poor predictor for particle sinking velocities and transfer efficiencies for in situ particles, in contrast with a common paradigm that has been largely informed from ex-situ measurements. This result highlights the importance of incorporating additional particle morphological information (such as compactness), with particular implications for the estimation of fluxes for in situ imaging methods which are increasingly used to study the BCP. I also show that particulate organic carbon (POC) is more efficiently transferred to depth than biogenic silica at several sites in the subpolar Southern Ocean, suggesting diatoms may not always efficiently transfer carbon to depth and that added density due to biomineral ballast may be compensated by ecological processes. This result highlights the need to move away from a binary view of biomineral ballasting and to characterise the conditions that determine the balance between biomineral ballasting and sometimes counteracting effects of ecological processes. Lastly, I present measurements from a field trial of a novel Marine Snow Catcher design, a tool to safely and swiftly sample marine particles with minimal alteration- currently essential to the calibration of in situ image-based methods of flux estimation. Collectively, results presented in this thesis serve to highlight the need to incorporate additional ecological information and particle characteristics to improve mechanistic understanding of BCP efficiency, accurately model fluxes, and estimate fluxes from in situ image datasets.

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ORCID for Mark Moore: orcid.org/0000-0002-9541-6046

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