ALBERT

Description: Sea ice is a complex and heterogeneous medium that hosts a rich community of microbial organisms and small invertebrates. This ecosystem is shaped by a network of inhabitable spaces where the upward and downward fluxes of solutes and light support primary production, and ultimately the whole sea-ice trophic network. Describing the optical, physical, biological and biogeochemical processes that drive the functioning of the sea-ice ecosystem at the appropriate, i.e. small scale (micro- to centimeter), is very challenging. This medium is solid, fragile and highly heterogeneous. Traditional sea-ice sampling methods based on coring are most often coarse and destructive. Not only do they not allow the small scale to be explored, they generally alter the material to be analyzed. Here, we present a new approach for measuring relevant variables of the sea-ice ecosystem at small scale and, as much as possible, non-destructively. Inspired by medical endoscopes, the custom-built platform is intended to carry various types of miniaturized optical sensors for radiometry, chemistry and high-resolution imaging of the sea-ice interior. In this presentation, we will describe the concept and present the progress made to date.

Description: The quantity and quality of sunlight transmitted into and through sea ice is a crucial key necessary to understand the thermodynamic development of the ice cover, upper ocean heat and freshwater budget, as well as the associated primary production. Due to its solid impenetrable nature, most optical measurements so far have been conducted above and underneath the sea ice covering our polar oceans. Only very limited measurements have been carried out inside the ice cover itself. This strongly limits our current knowledge of the vertically varying inherent optical properties (IOP) of sea ice, as well as the geometric shape of the in-ice light field. Both factors currently limit our abilities to reliably model radiative transfer in sea ice. Here we present multiple new tools that can fill this observational gap and provide comprehensive optical measurements within the ice: This includes a chain of multispectral light sensors for seasonal long-term monitoring. It is derived from the proven design of the newest generation of ice-mass-balance buoys with digital thermistor strings and enables a non-destructive measurement with flexible geometry. We present data from a first prototype deployed together with an array of drifting ice observatories at the North Pole in September 2018. These vertically resolved in-ice light profiles are compared to in-ice measurements with a newly designed in-ice optical profiler system based on the well-proven TriOS Ramses hyperspectral radiometers. Combining expertise from photonics, medical and sea-ice science enables the ongoing development of a set of endoscopic probes allowing optical studies in sea ice with minimum disturbance of the ice. This includes in-ice microscopy for in-situ ice algal investigations, a UV-spectrometer to observe brine nitrate concentration in situ, a reflectance probe for high-resolution direct determination of inherent optical properties, as well as a radiance camera for quantification of the angular radiance distribution. Here we present data from the first field tests during the Arctic field season 2018. First ruggedized prototypes could be available to the scientific community soon.

Description: Detailed characterization of the spatially and temporally varying inherent optical properties (IOPs) of sea ice is necessary to better predict energy and mass balances, as well as ice-associated primary production. Here we present the development of an active optical probe to measure IOPs of a small volume of sea ice (dm3) in situ and non-destructively. The probe is derived from the diffuse reflectance method used to measure the IOPs of human tissues. The instrument emits light into the ice by the use of an optical fibre. Backscattered light is measured at multiple distances away from the source using several receiving fibres. Comparison to a Monte Carlo simulated lookup table allows, in theory, retrieval of the absorption coefficient, the reduced scattering coefficient and a phase function similarity parameter γ, introduced by Bevilacqua and Depeursinge (1999). γ depends on the two first moments of the Legendre polynomials, allowing the analysis of the backscattered light not satisfying the diffusion regime. The depth reached into the medium by detected photons was estimated using Monte Carlo simulations: the maximum depth reached by 95 % of the detected photons was between 40±2 and 270±20 mm depending on the source–detector distance and on the ice scattering properties. The magnitude of the instrument validation error on the reduced scattering coefficient ranged from 0.07 % for the most scattering medium to 35 % for the less scattering medium over the 2 orders of magnitude we validated. Fixing the absorption coefficient and γ, which proved difficult to measure, vertical profiles of the reduced scattering coefficient were obtained with decimetre resolution on first-year Arctic interior sea ice on Baffin Island in early spring 2019. We measured values of up to 7.1 m−1 for the uppermost layer of interior ice and down to 0.15±0.05 m−1 for the bottommost layer. These values are in the range of polar interior sea ice measurements published by other authors. The inversion of the reduced scattering coefficient at this scale was strongly dependent on the value of γ, highlighting the need to define the higher moments of the phase function. This newly developed probe provides a fast and reliable means for measurement of scattering in sea ice.