What you need to know about Antarctic Bottom Water

The science team onboard the RV Investigator in Fremantle. Image: Aero Leplastrier.

BY DR ALIX POST, GEOSCIENCE AUSTRALIA, ASSOCIATE PROFESSOR HELEN BOSTOCK, THE UNIVERSITY OF QUEENSLAND AND MATT MARRISON, CSIRO

A team of scientists is plumbing the depths in East Antarctica to increase our understanding of Antarctic Bottom Water.

Research vessel (RV) Investigator is once again sailing south to conduct important research in Antarctica. Called CANYONS, scientists on this 47-day voyage will investigate Antarctic Bottom Water in the Cape Darnley region of East Antarctica.

This is what you need to know about Antarctic Bottom Water.

What is Antarctic Bottom Water?

You probably haven’t heard about Antarctic Bottom Water before but it’s very important for our oceans and climate. Put simply, Antarctic Bottom Water is dense, cold, oxygen-rich water that forms in just a few places around the Antarctic continent.

This water forms as cold winds blowing off Antarctica cool the ocean surface and form sea ice. As fresh sea ice forms, the salt in the seawater is ‘rejected’ (released). As a consequence, very salty and cold water is left behind. The same winds blowing off Antarctica then blow the sea ice away, exposing the ocean and forming new sea ice. This process further increases the saltiness of the water. This water then sinks through the water column forming Antarctic Bottom Water in the deepest parts of the ocean.

These bodies of open water, which are called polynya, can be thought of as sea ice factories.

The most important thing to know about Antarctic Bottom Water is that it’s the densest water on the planet. As the densest water mass, Antarctic Bottom Water flows down the Antarctic continental margin and north across the seafloor. In fact, it’s been found to occupy depths below 4000 metres in all ocean basins that have a connection to the Southern Ocean.

For this reason, it has a significant influence on the circulation of the world’s oceans.

Why is it so important?

The flow of Antarctic Bottom Water drives ocean circulation, assists in carbon capture and storage, and also carries oxygen to the deep ocean. As such, Antarctic Bottom Water ventilates the deep ocean.

However, climate change and the melting of the Antarctic ice sheet has led to increased fresh water flowing into the oceans around Antarctica. This has reduced the formation of Antarctic Bottom Water as it impedes the process to make cold, salty water. This reduction is likely to continue as the climate continues to warm.

Potentially, a complete shutdown of Antarctic Bottom Water formation is possible in the future. If this happens, it will likely have dramatic effects on ocean circulation. This will have consequences for weather patterns and the global climate. Moreover, a shutdown would likely create additional warming of the climate, including from reduced carbon capture and storage.

Where are we going and why?

The Cape Darnley region of East Antarctica is one of only four regions where the cold, salty and dense Antarctic Bottom Water forms. Scientists on this voyage aim to determine the flow pathways of this dense water mass down the rugged submarine canyons of the seafloor in this region. At the same time, they will also investigate its impact on seafloor life and ecosystems.

Importantly, they are also seeking insights into Antarctic Bottom Water sensitivity to changes in climate. This will help us predict how a warming climate will influence its future formation and impact on ocean circulation. Changes in the water mass have been detected over recent decades.

However, changes in this region have been little studied.

To address this, a multidisciplinary team of scientists from Australian research institutions and universities has been assembled on board RV Investigator. This team will be led by Dr Alix Post from Geoscience Australia and A/Prof Helen Bostock from The University of Queensland.

Putting together pieces of an icy puzzle

Scientists want to better understand the tipping points that influence the production of Antarctic Bottom Water by investigating different climate states in the past climate record. To achieve this, the team on RV Investigator will undertake detailed seafloor mapping of this area for the first time. Complete seafloor maps will reveal where Antarctic Bottom Water flows through the rugged submarine canyons. This will enable realistic ocean, climate and ecosystem models to be developed.

Multibeam sonar systems on RV Investigator will be used to map the seafloor to study how features in the region, such as deep canyons, influence the flow of Antarctic Bottom Water.

In addition, they will also collect long sediment cores, analyse seawater samples and use deep sea cameras to image seafloor life.

Long sediment cores will reveal past changes in sea ice, ice-sheets and ocean circulation. These records will unlock the history of Antarctic dense water formation during periods of Earth’s history that were warmer than today. As a result, we will gain an important insight into how our global climate is likely to respond to changes in the future.

Furthermore, the team will also collect large volumes of Antarctic seawater. Importantly, this will give us valuable insights into the processes controlling the distribution of trace metals in Antarctic waters. It will also contribute to developing new geochemical tracers for past ocean and ice sheet change.

Protecting Antarctica’s ecosystems

The area is one of 3 regions proposed as Antarctic Marine Protected Areas on the East Antarctic margin. Scientists will use deep sea cameras to take the first images of the seafloor life in this remote part of Antarctica. Altogether, the information they collect will help ensure that this region can be protected into the future.

Join us in the south

The team will be bringing their research to life through photography, video, blogs and podcasts. These will be released through the Australian Centre for Excellence in Antarctic Science. You can also follow the voyage on Facebook, Instagram and Twitter via @CANYONS_voyage and #RVInvestigator.

This voyage is supported by the ARC Australian Centre for Excellence in Antarctic Science and by a grant of sea time on RV Investigator from the CSIRO Marine National Facility.