Sediment cores retrieved over summer from the Siple Coast region of Antarctica's Ross Ice Shelf were carefully opened and examined at a recent workshop at the Otago Repository for Core Analysis (ORCA). It marked an important step in the hunt for clues about the past behaviour of the West Antarctic Ice Sheet.

The 10 sediment cores, extracted in December 2023 by the Sensitivity of the West Antarctic Ice Sheet to 2°C of Warming (SWAIS2C) project, have had quite a journey. They were hoisted up through 55 m of ocean cavity and 580 m of ice, travelled by air 860 km to Scott Base then 4000 km to Christchurch, and finished with a 350 km car ride to Otago University’s ORCA facility in Dunedin.

This was precious cargo. With only a handful of cores previously collected from under the Ross Ice Shelf, an area the size of France, these new cores are an extremely rare geological record.

“These samples offer a huge increase in the amount of material we have to help us understand the present-day environment beneath the ice shelf, and the history those sediments represent,” says SWAIS2C co-chief scientist Richard Levy, from GNS Science and a professor in the Antarctic Research Centre at Te Herenga Waka—Victoria University of Wellington.

This initial workshop was focused on using largely non-destructive analysis techniques to learn more about the cores, so that the wider research team can plan its research and put in sampling requests.

To begin unlocking the sediment’s secrets, the cores were X-rayed and CT-scanned. They then had to be split in half longitudinally, a painstaking process that initially uses a small circular saw to score the sides and then requires manual splitting with a thin wire.

“Antarctic sediment often has large clasts in it. If you pull the wire through too quickly, or cut through with the saw, there’s a risk of dragging a rock with you all the way through the core and ruining it,” says Linda Balfoort, SWAIS2C sedimentologist and PhD candidate studying at Victoria University and GNS Science.

Following successful splitting, visual description began. The team looked for changes in sediment colour and grain size, and calcareous materials (materials mostly made up of calcium carbonate) and signs of fossilised marine algae, creating a visual core log. They also took smear slides—tiny samples of sediment placed on microscope slides—and some bigger samples to use for dating analysis.

“Antarctic sediments are notoriously hard to date. We’re particularly looking out for fossilised foraminifera (single-cell organisms with a calcium carbonate shell), as they’re very useful for pinpointing dates and revealing information about ocean chemistry,” says Linda.

The split cores were scanned to a high resolution and a range of other measurement techniques were used to identify the chemical elements present.

“Any new record from this area is incredibly important and valuable, so I felt very privileged to be among the first people to see these sediments. But to me the most exciting thing will be to get the data, analyse the smear slides, and find out what’s hiding inside the sediments,” says Linda.

The team chose to split and analyse three of the cores, including the longest core ever retrieved from the Siple Coast—a 1.92 m core obtained through the physically demanding hammer coring technique.

SWAIS2C co-chief scientist Tina van de Flierdt, from Imperial College London, says that without the dating results and further analysis it’s too soon to know conclusively how far back in time the sediment in this core reaches, but there are positive signs that it holds information about how the Ross Ice Shelf retreated in the past.

“We’re confident that what we are seeing is a glacial retreat sequence, when the grounding line—the point where the floating Ross Ice Shelf meets the Western Antarctic Ice Sheet that is grounded on bedrock—moved further inland. We’re not sure yet whether it’s the long retreat following the last glacial maximum, or a much more recent process within the past 200 years or so,” says Tina.

The team of drillers, engineers, and researchers will be heading back to the Siple Coast site for the 2024/25 Antarctic field season. They plan to drill up to 200 m below the seafloor to recover a geological record going even further back than these shorter cores, to provide key insights into West Antarctica’s potential contribution to future sea-level rise.

This article was originally published by GNS Science.