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REmineralisation, OPTIcs and Marine partIcle siZE

Periodic Reporting for period 1 - REOPTIMIZE (REmineralisation, OPTIcs and Marine partIcle siZE)

Reporting period: 2016-06-13 to 2018-06-12

"Microscopic marine particles that sink from the surface to the deep ocean are a crucial component of the biological carbon pump, because they can be remineralised back to CO2 and ultimately influence the Earth’s climate. Little is known, however, about the distribution of these particles in the open ocean and the variability of mesopelagic (200-1000m) particle remineralisation as a consequence of observational difficulties. Only autonomous robotic platforms equipped with bio-optical sensors (e.g. Biogeochemical-Argo floats) can allow us to monitor and understand the dynamics of these particles at the global scale, with the required high spatial and temporal resolutions.
The light scattered by marine particles can be used as a proxy to quantify particle size distributions (PSD), size-resolved carbon biomasses and ultimately, mesopelagic particle remineralisation rates. However, optically-derived particle size distributions suffer from large uncertainties. An in-depth knowledge of the relationships linking marine particles and optical scattering is, therefore, vital to improve our understanding of oceanic particle dynamics and, ultimately reduce uncertainties in current and future estimates of the ocean carbon budget.
The REmineralisation, OPTIcs and Marine partIcle siZE (REOPTIMIZE) project primarily focused on analysing the relationships between optical scattering measurements, particle size distributions and concentrations in open-ocean environments. The project was developed within the Earth Observation Science and Application (EOSA) team at the Plymouth Marine Laboratory (PML), in Plymouth, United Kingdom. REOPTIMIZE analysed extensive and globally-relevant, unique, in-situ datasets of coincident particle size distributions and optical properties, from the surface to the mesopelagic zone, collected during two “Atlantic Meridional Transect” (AMT) cruises (#22, and #26, which encompassed a wide range of oceanic conditions including oligotrophic mid-ocean gyres.
REOPTIMIZE opened the door to a new way to investigating marine particle dynamics and had an impact on the European strategy for global ocean observations.
Overview of REOPTIMIZE final results (2 papers in peer-reviewed journals):
Part 1. The optical scattering as a proxy of marine particle size
No significant relationships between particle size distributions and optical scattering proxies were found across the oligotrophic Atlantic Ocean, from the surface down to 500 m depth. The slopes of the particle size distributions were lower than expected from previous bio-optical models based on the spectral beam attenuation coefficient (i.e. total scattering). Optical proxies based on the backscattering coefficient, could only match the average value of the slope of PSDs as predicted from previously published models, though with high uncertainty. The lack of significant relationships between the slopes of PSDs and backscattering, observed here for the first time in open-ocean oligotrophic waters, confirmed findings of growing literature for other regions of the global ocean. In stark contrast with earlier studies, the spectral backscattering properties of marine particles analysed in our study were not related to co-located particle size distributions. This is because a single slope of the PSDs cannot describe the huge variability of marine particle sizes and concentrations in the open ocean.

Part 2. The marine particles seen by optical scattering measurements
The observations of the particulate optical backscattering coefficient are critically limited by an incomplete mechanistic understanding of what particles generate the backscattering signal. To achieve this understanding, optical models must be employed. The simplest of these models – the homogeneous sphere (i.e. Mie theory) – severely underestimates the particulate backscattering in the open ocean, and the missing signal has been attributed to submicron particles. This long-standing issue is known as the “missing” backscattering enigma.
In REOPTIMIZE, we developed a slightly more complex optical model for particles – the coated sphere – that predicted all the measured backscattering across the Atlantic Ocean using realistic parameters. Thus, for the first time, we could provide an interpretation of in-situ oceanic particulate backscattering measurements that was not based on the homogeneous-sphere model. In contrast with previous interpretations based on the homogeneous-sphere model, our theoretical and experimental results suggested that the majority of the particulate backscattering coefficient in the studied area was due to particles with equivalent diameters between 1 and 10 μm, and that submicron particles generated less than 40% of the measured signal. Additional 1-µm size-fractionation experiments independently confirmed these findings. Our results thus demonstrated that the structural complexity of particles is critical to understand open-ocean backscattering, provided a simple solution to the long-standing backscattering enigma, and opened a new direction towards exploiting the full potential of optical backscattering observations for investigating the biological carbon pump.
REOPTIMIZE undoubtedly improved our knowledge on the relationships between the size of marine particles and optical proxies that were investigated – for the first time – in open-ocean oligotrophic waters, from the surface down to the mesopelagic region. Planned tasks have been accomplished, though with totally unexpected results. The lack of relationships between particle size distributions and optical proxies prevented us from investigating the particle remineralisation rates from spectral backscattering measurements acquired by autonomous Biogeochemical-Argo floats. The important implications of these findings have been presented to, and discussed with, the European Argo community and infrastructures (e.g. Euro-Argo ERIC) during a dedicated meeting, as well as with the international community upon invitation. The findings of REOPTIMIZE have finally promoted European Argo actions for improving the study of particle dynamics and related biogeochemical processes, such as new technological development and the integrations of new sensors on the autonomous platforms.
The analysis of Biogeochemical-Argo data has been carried out thanks to the complementarity of REOPTIMIZE with earlier projects. The quality-controlled dataset for the global ocean has been published on Earth System Science Data (Organelli et al., 2017). Two papers have been published in the Journal of Geophysical Research – Oceans (Organelli et al., 2017; Barbieux et al., 2018) on the variability in the bio-optical behaviours of various oceanic regions. In particular, this last work by Organelli et al. (2017) has been featured as research spotlight in the US Eos: Earth & Space Science News magazine.
Thanks to REOPTIMIZE and its unique database of particle size distributions, we produced breakthrough results by providing a solution to the long-standing open-ocean missing backscattering enigma. REOPTIMIZE helped opening the door to a new way of interpreting marine optical backscattering measurements, and thus to investigating marine particle dynamics and ocean biogeochemistry.
REOPTIMIZE study area