Processes influencing carbon cycling: observations of the lower limb of the Antarctic overturning (PICCOLO)

Through a 52-day cruise, scientists set out to learn more about the biological, chemical and physical processes that draw atmospheric carbon deep into the Southern Ocean. Taking place on the Royal Research Ship Sir David Attenborough (SDA) – one of the most advanced polar research vessels in the world – PICCOLO marks the ship's first real test of her capabilities in an interdisciplinary programme.

The journey took scientists from Chile, across Drake Passage and ashore to James Ross, Seymour and Vega Islands, and down through ice floes to the Larsen Ice Shelf (southernmost latitude 67.6 °S) – over 5,600 nautical miles.

PICCOLO is a direct response to the NERC strategic programme on the Role of the Southern Ocean in the Earth System (RoSES) to inform international climate policy – particularly quantifying the crucial processes that determine carbon cycling.

Seawater at the surface near Antarctica interacts with the atmosphere, absorbing carbon and losing heat. In the water, the carbon is incorporated into plankton, before particles descend to the ocean depths, taking the carbon with them. Understanding this process in greater depth can improve models that predict future climate change.

Linked closely to the carbon, the Southern Ocean's waters are rich in major nutrients, including nitrate, phosphate and silicate, which all nourish phytoplankton. However, the Southern Ocean lacks iron – something phytoplankton's ecosystem is reliant on. So much so, that previous experiments that added iron to the waters resulted in phytoplankton blooms so large that they can be viewed from space.

The multi-institutional project made use of the latest technologies, including ocean gliders and floats, to observe previously inaccessible and unstudied regions of the Weddell Sea continental shelf. Using local Antarctic microorganisms, experiments were undertaken to determine how their health improved when iron and manganese were added to their seawater. This included how this impacted the rates of photosynthesis of phytoplankton which harvest light and turn it into biomass – crucial for life and global carbon cycles.

This project aims to shed light on the role of micronutrients on the ecophysiology of Antarctic microbiology.

Antarctic krill play a key role in recycling iron and other nutrients back into the sea via their digestion of plankton. Scientists looked at the process of krill egesting or excreting the iron they ingest rather than storing it. As their faecal pellets sink out of the upper ocean within one or two days, the aim was to determine whether these pellets strip out iron from the surface waters or release it to benefit phytoplankton.

Using 'clues' from a series of natural geochemical isotope tracers found in seawater (radium/oxygen/iron isotopes), scientists have previously detected sub-atomic differences in the mass of iron and radium atoms from water samples and oxygen atoms that make up the water molecule. This has helped to provide clues about the origins of these elements in the sea and whether they come from melting sea ice, glaciers on land, or finely crushed rock minerals around Antarctica and beneath the Weddell Sea.

On PICCOLO, research was done in collaboration with the University of Leeds using radium (a natural radioactive element found in seawater) and made use of its predictable rate of radioactive decay. These measurements can provide information on how quickly seawaters bearing radium are mixing and moving, which in turn helps researchers understand how iron and other nutrient trace elements are similarly being transported, moving them away from the seafloor and towards the surface for phytoplankton to use.