Investigating the Southern Ocean: Part 1…by Carol Arrowsmith

Carol organising her equipment at BAS prior to departure

In a few days I will be embarking on my leg of the major NERC project called ORCHESTRA (Ocean Regulation of Climate through Heat and Carbon Sequestration and Transport) to collect seawater samples for isotope analysis. My leg is called ANDREX II - Antarctic Deep Water Rates of Export (ANDREX), and is the second time this part of the ocean has been sampled. I will be boarding the RRS James Clark Ross in Punta Arenas and following a stop off in the Falklands will start sampling from the tip of the Antarctic Peninsula along the 60°S parallel and across the Southern Ocean to 30°E, before returning to the Falklands in mid April.

Why are we collecting seawater samples from the World’s oceans?

Since the industrial revolution, the global ocean has absorbed around 30% of anthropogenic (human-produced) CO₂ emissions. In addition, 93% of the total extra heat in the Earth system since the onset of global warming has been absorbed by the global ocean. Improving climate prediction requires us to learn more about how the global ocean works, and how it interacts with the atmosphere to control the split of heat and carbon between them, especially given the extra heat and carbon we are currently producing.

The Southern Ocean is key

A key region in this context is the Southern Ocean, the vast sea that encircles Antarctica. The Southern Ocean occupies around 20% of the total ocean area, but absorbs about three-quarters of the heat that is taken into the ocean, and approximately half of the CO₂. This is because of its unique pattern of ocean circulation: it is the main region where deep waters rise to the surface, allowing new water masses to form and sink back into the ocean interior. This exposure of “old” waters to the atmosphere, and the production of new waters at the surface, is fundamental to the exchanges of heat and carbon with the atmosphere.

The track of the ANDREXII cruise

Despite knowing the key role that the Southern Ocean plays in global climate, there are many important unknowns. These include how exactly heat and carbon are taken up by the oceans and how fast this occurs (especially important because of the Anthropocene period we are living in), and how much heat and carbon is currently stored in the oceans. These questions are being addressed using various chemical and physical measurements of the ocean, including the stable isotope composition of the seawater (which we are responsible for at the BGS). Oxygen isotopes will tell us about how much freshwater to seawater there is at particular locations (which will help us understand melting of the Antarctic ice mass and therefore heat) and carbon isotopes will tell us where the carbon is formed and how the ocean uses the carbon.  

The ANDREX leg in particular seeks to assess the role of the Weddell gyre in driving the southern closure of the meridional overturning circulation, in ventilating the deep global ocean, and in sequestering carbon and nutrients in the global ocean abyss.

Progress

ORCHESTRA is in the second year of a five year collection programme around the World’s oceans. I will be collecting samples from the RRS James Clark Ross. I will be tweeting @CarolArrowsmith and @ORCHESTRAPROJ and Facebooking (Orchestra project) along the way, as well as updating the BGS Geoblogy. Carol Arrowsmith is a chief technician in the stable isotope facility at the BGS. 

ORCHESTRA (Ocean Regulation of Climate by Heat and Carbon Sequestration and Transports) is a programme funded by NERC and includes partners at the British Antarctic Survey (lead), the National Oceanography Centre, Plymouth Marine Laboratory, and many more including BGS.