Radium in Changing Environments: A Novel Tracer of Iron Fluxes at Ocean Margins

Phytoplankton are microscopic plants that live in the sunlit surface ocean. Iron (Fe) is an essential nutrient for phytoplankton, which use Fe for photosynthesis, converting carbon dioxide (CO2) to oxygen and organic matter. Organic carbon that sinks to the deep ocean or sediment removes CO2 from the atmosphere. This is called the biological pump, and is an important process regulating CO2 in the atmosphere, consequently affecting global climate.
Iron is present at very low concentrations in the ocean (less that 1 Fe atom per billion water molecules). Large areas of the ocean are Fe-limited, where supply of Fe does not support healthy phytoplankton and a strong biological pump. To understand the strength of the biological pump, we need to know the sources of Fe to the ocean. Fe ultimately comes from lithogenic material, which can be delivered to the ocean as dust, in rivers/groundwater, in melting glaciers, from sediments of the continental shelf, or from hydrothermal vents. However, measuring the rate of Fe supply from each of these sources is challenging and not yet constrained.
My project will use radium (Ra) to determine rates of Fe supply and removal, in order to better understand the cycling of Fe in the ocean. Three key gaps in our knowledge of the Fe cycle are: 1) how much Fe comes from continental shelf sediments, 2) how much Fe is supplied by glacial meltwater, and 3) how rapidly is Fe scavenged from the metal-rich fluids at hydrothermal vents? All of these processes are vital to understanding the Fe cycle.
Radium and Fe have a common source (lithogenic material), but Ra decays over time by natural radioactivity, at a very precise rate. This decay allows us to use Ra as a clock in the ocean. A parcel of seawater will have high Ra near a source, but the amount of Ra will decrease over time as the water parcel moves away. By measuring how much Ra has decayed, we can calculate how long since the parcel of water was in contact with the source. Measuring Fe at the same time, we can calculate how much Fe was supplied along with the Ra, and if any of that Fe has been lost. Fe does not decay, but can be removed by two processes: biological uptake by phytoplankton for photosynthesis; or scavenging, when it sticks to particles in seawater. Scavenged Fe sinks along with the particle and is no longer available for uptake.
I will measure Fe and Ra to determine Fe supply from the continental shelf of the western Antarctic Peninsula to the open ocean, as the Southern Ocean is the largest Fe-limited region in the world. I will test the hypothesis that high-Fe waters from the shelf are transported offshore, and have the potential to mix upwards into the surface water supplying Fe to phytoplankton. I will also monitor Fe and Ra in glacial meltwater in this region. The supply of glacial Fe may be changing, as warming in this region is accelerating rates of glacial melt. By using Ra to calculate how much time has passed since the water contacted sediment, I will assess how quickly the Fe in this meltwater is lost (either to uptake or scavenging).
To compare supply and removal rates in other locations, I will follow the same approach at other glaciers on the western Antarctic Peninsula, and in Greenland, to estimate input of Fe from glacial melt globally.
Finally, I will measure Fe and Ra near hydrothermal vents along the Mid-Atlantic Ridge. Combining these two elements to determine the removal of Fe from vent fluids as they drift away from vent sites will provide vital information for evaluating the contribution of this source to the total amount of Fe in the world's oceans.
My results will address key gaps in our understanding of the marine Fe cycle. Improving our knowledge of this essential nutrient will help us determine how sensitive marine systems are to current Fe supply, as well as predict the impacts of changes in Fe supply on phytoplankton health, the biological pump, and global climate.

Who will benefit from the proposed research?
- Extreme environments such as the Southern Ocean and hydrothermal vents are of great interest to the public. They are an ideal gateway for engagement with students, teachers, educators and the general public on topics of environmental science and climate change. This group will form the main focus of my impact initiatives, as communicating the importance of the iron cycle in the oceans to global climate, marine productivity and regional effects on fisheries or pollution are vital to maintaining public support for environmental research, increasing awareness of marine and climate change issues, and inspiring future generations of oceanographers.
- The proposed work will have immediate benefits for the academic community, in particular those interested in iron biogeochemistry, physical oceanographers who can utilise radium-derived data to provide independent constraints on mixing processes, and scientists interested in the links between iron cycling, marine productivity and carbon drawdown. In particular, the rates of iron transport from the ocean margins will be used by climate modelers who need to accurately parameterise the iron cycle.
- Constraining cycling of iron and its impacts on ocean productivity is a fundamental component of coupled ocean-atmosphere circulation models that underpin climate forecasting, from long-term projections of future change to daily weather prediction. Thus this work will benefit agencies involved in projecting the impacts of future climate change and advancing coupled climate models, such as the IPCC assessment report committees, and UK Met Office.
- A better understanding of the processes supporting primary productivity on the WAP shelf has applications for organisations such as CCAMLR and SOOS through BAS's long-term monitoring and survey. The international support and commitment for this type of monitoring initiative, providing data to monitor climate change and biodiversity, have been highlighted by the recent Tsukuba Communique from the G7 meeting.

How might the potential benefits be realised?
- The results of this work will be communicated directly to the oceanographic community through publications in academic journals and presentations at national and international conferences (e.g. Ocean Sciences, Goldschmidt and Challenger Society). Archiving data with BODC and incorporating the data into GEOTRACES data products released every 3 years will increase the use of radium isotopes for tracing biogeochemical processes. Moreover, the integration of Ra data into the NEMO-PISCES model will act as a springboard for applying my results to other global biogeochemical models.
- The results generated from this Fellowship will influence the modeling efforts and advances through the ORCHESTRA programme, which will inform the next generation of coupled ocean-atmosphere climate models and underpin the UK climate change policies via the links to the IPCC process and Met Office. By working to incorporate my results directly into the NEMO-PISCES model that includes iron biogeochemistry, sensitivity experiments based on my work will inform the outputs and interpretation of the ORCHESTRA programme, including NEMO-CICE.
- I will participate in the outreach programs at SOES Southampton including public lectures, open days and departmental social media pages to communicate the outcomes of my research and how it relates to big picture questions in oceanography, environmental science and climate change.
- Finally, working with the outreach resources at the University of Southampton and SOES, I will design and establish a local Southampton Oceanography Ambassador Project (SOAP) based on my current work at Rutgers University, using real scientific data to communicate the key scientific concepts and outcomes of this research.