Particle flux, carbon sequestration and the role of mesoscale spatial variability in the Iceland Basin

Abstract

Organic carbon sequestration is driven by the biological carbon pump (BCP), which transfers organic‐rich biomass and detritus to the deep ocean, storing carbon on climatically significant timescales. The BCP exports 5 – 11 Gt C yr‐1 globally into the interior ocean and 0.33 – 0.66 Gt C yr‐1 reaches 2000 m depth. Understanding the functioning of the BCP, and the factors controlling the magnitude and composition of particle flux to the deep ocean in the current climate system, is crucial to detecting and predicting future changes. Records of deep ocean particle flux are usually limited to a single mooring in one location. Uniquely, this study utilises four sediment traps deployed below 2000 m in a mesoscale spatial array in the Iceland Basin (60 °N, 20 °W) from November 2006 to June 2008. In this thesis, the effects of spatial variability and particle flux composition on the magnitude of carbon sequestered in the deep ocean Iceland Basin will be investigated and the observed spatial variability in the context of upper ocean biological and physical processes will be explored. In the first results chapter of this thesis, the flux of organic carbon to the deep ocean Iceland Basin is quantified for the first time. The mean annual particulate organic carbon (POC) flux to 2000 m in the Iceland Basin is 101.7 (± 12.3) mmol m‐2 yr‐1, which is lower than the global average. The data indicate considerable mesoscale spatial variability, evidenced by differences in POC flux captured by the 4 sediment traps. Averaging POC fluxes over increasingly long temporal scales, decreases the magnitude of the observed mesoscale spatial variability, particularly for time scales > 1 month. The influence of localised spatial variability on observed POC fluxes should be considered when investigating particle fluxes at given locations, or using individual traps, for less than annual timescales. However, reassuringly, mesoscale spatial processes likely do not impact deep ocean annual carbon budgets derived from long‐term time‐series, such as at sustained observatories. In the second results chapter of this thesis, I explore what controls spatial and intra‐annual variability in carbon sequestration to the deep ocean Iceland Basin. The particle source regions of each sediment trap were estimated and used to explore how upper ocean biological spatial variability relates to deep ocean particle flux. Increases in POC flux coincide with increased biogenic silica fluxes and upper ocean diatom abundance, in line with previous literature, suggesting diatoms are a major contributor to deep ocean particle flux. The role of eddies on deep ocean particle flux is explored for periods with the greatest observed mesoscale variability. Four of the five case studies were found to be associated with periods of high eddy kinetic energy in the surface ocean above the trap location and increased biogenic silica fluxes suggesting deep ocean carbon fluxes may be influenced by mesoscale eddies. In the third results chapter, I confirm the hypothesis that there is a regular annual sedimentation of Acantharian cysts to the deep ocean Iceland Basin in spring. Extremely high cyst fluxes were sampled, which allowed for improved estimates of cyst sinking rates, and POC and Strontium cyst content. Acantharian cysts can contribute significantly to the pre‐spring bloom POC flux, and dominate the annual particulate Strontium flux in the Iceland Basin, with fluxes up to 2.50 mmol m‐2 yr‐1. The role of celestite, rapid sedimentation and the viability of the Acantharian cysts in the Iceland Basin is explored to further the current understanding of the cyst reproductive strategy, as well as discussing the implications for deep ocean carbon sequestration and the Strontium cycle. The composition of deep ocean particle flux, especially in terms of biomineral content and plankton community, is key to furthering our understanding of the factors controlling carbon sequestration in the deep ocean and can have important implications for other elemental cycles, such as Strontium. Further work is needed to determine the exact role of mesoscale eddies on deep ocean particle flux using data with greater spatial, temporal and vertical resolution.

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ORCID for Chelsey Adrianne Baker: orcid.org/0000-0002-0840-2333

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