The mobile drilling operation for quantitative coring of the sub-ice platelet layer. Photo: Natalie Robinson
Platelet ice sampling system: the first deployment
The 22/23 Antarctic field season was the first scientific deployment of the new platelet sampling system. This custom-designed and bespoke-engineered system successfully sampled the biota associated with sea ice and platelet ice habitats.
Sub-ice platelet layers (SIPLs) refer to loosely consolidated accumulations of disc-shaped ice crystals that form in supercooled water and buoyantly accrete beneath sea ice. SIPLs are known to support the highest concentrations of primary productivity (e.g., algae) of any type of ice in the ocean (read more about this ecosystem here). But SIPLs and the species that depend on them are susceptible to effects of a warming climate.
Our research aims to understand the environmental conditions that control development and evolution of SIPLs on various timescales, and their role as a critical biological habitat. To do this, we needed a method to quantitatively sample the organisms associated with the interior and under-surface of sea ice, including platelet ice.
In the 2021/22 season, the new platelet sampling system was trialled, prompting a number of modifications prior to the 2022/23 deployment. In 2022/23, the focus was on training personnel and collaboratively developing a standard operating procedure for use in future seasons, whilst also collecting samples to advance the research. The system will next be deployed in the 2024/25 season.
Illustration using colour pencil and pen of the mobile drilling operation for coring the sub-ice platelet layer. Credit: Jacqui Stuart.
Illustration using colour pencil and pen of using the Kovacs drill at an angle to flood the 'well' hole as part of the site preparation. Credit: Jacqui Stuart
Collecting cores from beneath ‘old’ and ‘new’ first year sea ice
In light of the unprecedented sea-ice conditions of the 2022 winter growth season, the focus in 22/23 was to sample ‘old’ and ‘new’ first year sea ice to document the physical and biological differences between ‘mature’ and ‘immature’ sub-ice platelet layers.
The ‘old’ ice had been stable since February (i.e., 7-8 months of continuous growth and thickening) and so had developed a ‘mature’ physical sub-ice platelet layer structure and associated biological community. Conversely, ‘new’ ice had only stabilised in late August / early September (i.e., ~6 weeks prior), permitting only ‘immature’ development of the physical sub-ice platelet structure and associated biological community.
In eight days, the platelet sampling system successfully collected 12 full, segmented cores from 10 separate locations. Unfortunately, some samples were lost….to be expected when using new equipment under very challenging conditions.
Phases of sample collection. Left: The sub-sample of the SIPL is tipped from the sampler into a waiting bucket, passing through a sieve that separates ice from water as it does so. Right: After the final section of SIPL has been extracted, active supercooling and frazil ice formation can be observed in the ocean water drawn up into the sampling chamber. Photo: Craig Stewart
As each 25 cm section of SIPL was extracted, the ice:water ratio was directly measured by weighing the separated components. These observations represent the first direct and quantitative measurements of SIPL composition.
Clear and consistent physical differences were observed between the snow, freeboard (ice above sea level), sea-ice and SIPL thicknesses, with universally thicker components for the ‘old’ sea-ice. This was entirely as expected, given the ice growth history of the preceding winter.
The lowest section of solid sea ice was collected from each site. This was followed by sequential sections of platelet ice – to the full thickness of the platelet layer – which was drawn up through the sampler. Each section of the core, destined for laboratory analysis back in New Zealand, was placed in a separate sterilised bucket, kept in low light conditions, and returned to the field laboratory where they were allowed to melt. The samples were then filtered and preserved ready for the trip back to New Zealand.
Subsequent analysis of these samples has shown substantial differences in both the abundance and make-up of the algae communities associated with the two types of ice. This is likely due to differences in the time of year that the SIPL became established and available for recruitment; and differences in light availability within the SIPL due to the thickness of snow on the surface of the sea ice.
This image represents the first ever quantitative profile of primary productivity within a sub-ice platelet layer. Tubes of filtrate from one of the of the high-resolution sub-ice platelet cores collected from beneath the ‘old’ sea ice. The sequential change (left-to-right) in colour intensity reflects the reduction in concentration of algae present in the top-to-bottom sequential samples of the sub-ice platelet layer. This variability is most-likely due to the diminishing quality and intensity of the sunlight filtering through to support the primary production of the platelet layer. Photo: Jacqui Stuart
The research team lived in a containerised camp, which provided a field base suitably located to facilitate both science and living, while also minimising downtime generated by bad weather. Photo: Vanessa Wells
The K892 field camp from the air. Photo: Anthony Powell