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.