Australian Antarctic Magazine — Issue 33: December 2017

Antarctic submarine canyons revealed during sea floor sediment survey

Sediment being removed from a long, rectangular shaped corer.
The box-shaped kasten corer can recover large volumes of sediment. L-R: Jason Fazey (MNF) and Mark Lewis (MNF), Bradley Opdyke (Australian National University). (Photo: Doug Thost/MNF)
A bathymetric map showing the location and structure of sea floor canyons.The RV Investigator at the ice edge.Deploying the piston corer during Sabrina Sea Floor Survey.

Sediment samples collected on the Antarctic ocean floor are set to provide clues to past climate in East Antarctica and how the Earth’s climate system works. At the same time, scientists have mapped a striking submarine landscape of canyons and sediment ridges along the Sabrina Coast of East Antarctica.

The Sabrina Sea Floor Survey voyage was conducted from the CSIRO’s Marine National Facility RV Investigator between January and March this year. It covered 48 000km2 of the continental slope off the Sabrina Coast, close to the Totten Glacier, Moscow University Ice Shelf and Dalton Iceberg Tongue.

The project aimed to identify the former ice extent of the Totten Glacier, which is one of the most rapidly thinning glaciers in East Antarctica, and build a picture of climate evolution in the region. To do this, we decided to study sediment samples from the continental slope, because these sediments are less vulnerable to erosion by advancing ice.

The RV Investigator’s multibeam echo sounders and sub-bottom profiler were used to find the best sediment deposits for climate records. These instruments use the returning echoes from pings of sound directed at the sea floor, to visualise and map sediment deposits. We also used a high resolution seismic reflection system to image beneath the sea floor.

While other surveys have visited the area, they only collected widely spaced soundings. In contrast, our multibeam echo sounders provided us with grid soundings of 50 to 100m resolution over 48 000km2, while the sub-bottom profiler provided 4000 km of data.

Once target areas were identified, we sampled sediments using piston and kasten corers. The piston corer drives 10cm wide tubes, up to 24m long, into the sea floor. The kasten corer is a box-shaped tube, 15cm across, with a one tonne weight on the top, which can recover larger volumes of surface sediment.

We retrieved 11 kasten cores from the top few meters of ridges and canyons, and six piston cores, each containing up to 16m of sediment from ridges. In the kasten cores from the canyons we were surprised by an accumulation of diatoms. These microscopic plants live in the upper ocean and produce skeletons of silica. Sometimes sudden die-offs produce thick masses of diatoms on the ocean floor, but we’ve never seen it in submarine canyons.

The deep piston cores retrieved sediment comprised almost entirely of mud, with physical properties and fossil layers that suggest the cores will provide information on climate cycles going back 300 000 years. What it means for Antarctic climate history is not clear yet. We have a team of 62 researchers from five countries conducting a detailed analysis of the cores, but it will take some time to work through the hundreds of samples.

As well as suitable sediment deposits, our multibeam echo sounders revealed a series of branching canyons near the Moscow University Ice Shelf, originating on the upper continental slope and converging into several major canyons on the lower slope (see map). At the other (western) end of the study area, we found the topography dominated by ridges, separated by a single, relatively straight canyon.

Canyons are typical features of continental slopes around the world, but the sediment ridges are not common and these ones have great potential for recording the climate history of the Antarctic.

The western ridges are particularly unusual. They could be made up of mud dumped from the continent by the Totten Glacier, or they could be the result of a long-lived eddy in the ocean that has been acting as a sediment trap for mud moving along the margin.

The Sabrina Sea Floor Survey has provided a rich source of new scientific data that will take years to exploit. This shows that the RV Investigator has a lot to offer Antarctic marine research and it also shows the benefits that will flow from the new Australian Antarctic icebreaker, RSV Nuyina, which will have some of the same instruments.

Phil O'Brien and Leanne Armand*
Voyage Co-Chief Scientists, Macquarie University

*Australian Antarctic Science Project 4333.

The Sabrina Sea Floor Survey was led by Leanne Armand and Phil O’Brien of Macquarie University, with participants from Istituto Nazionale di Oceanografia e Geofisica Sperimaentale (OGS) Italy, Australian National University, University of Granada, Spain, Colgate University, USA, University of Tasmania and Geoscience Australia. Australian participants were supported by an Australian Antarctic Science Grant and additional financial support was provided by the Australian Research Council, Italian Programa Nazionale Di Richerche in Antartide (PNRA), Spanish Ministry of Economy and Competitivity (MINECO) and the US National Science Foundation.