Research project: Radium in Changing Environments: A Novel Tracer of Iron Fluxes at Ocean Margins (RaCE:TraX)
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RaCE:TraX is a NERC-funded Research Fellowship project to quantify key rates of iron input and removal at ocean margins, targeting specific areas of uncertainty in the biogeochemical iron cycle. Focusing on three key processes (oxic margin sediments, glacial inputs, and hydrothermal vents), this project will apply novel radium isotope measurements to different marine margin inputs to increase our understanding of contemporary fluxes of iron to the ocean.
Project Overview
Type of project: NERC Independent Research Fellowship
At present a key uncertainty in our understanding of the iron (Fe) cycle is the rate of supply from margin sources. The novel approach of combining short-lived radium (Ra) isotopes with Fe measurements to quantify benthic Fe flux overcomes many of the limitations of existing methods. The unifying aim of RaCE:TraX is to use Ra isotopes to provide chronometric information to determine rates of input and removal of Fe at three important ocean margins: oxic shelf-slope sediments, glaciers, and hydrothermal vents.
In targeted locations (the western Antarctic Peninsula, hydrothermal vent fields, Greenland fjords), RaCE:TraX will determine activities of four Ra isotopes, which in combination allow an assessment of short-term input and removal processes and long-term mixing and advection required to evaluate the longevity of Fe supply in Fe-limited regions. These Ra measurements will provide the time information necessary to constrain the iron fluxes from oxic sediments, glacial processes and hydrothermal vents to the global ocean. Ultimately, these Ra-derived Fe input and removal rates will be incorporated into an Fe biogeochemistry model, to reduce model uncertainty and better understand how changes in Fe supply affect contemporary and future ocean biological productivity.
Fieldwork on several of these components has already been completed. In 2017, the ICY-LAB cruise (led by collaborator Dr. Kate Hendry at the University of Bristol) surveyed the southwest coast of Greenland, collecting samples for trace metals and radium isotopes. This has been complemented by two summer expeditions to sample endmember waters in the glacially-influenced fjords near Nuuk, providing new insights on the glacial delivery and cycling of trace metals in the coastal environment. See the “Greenland” tab for a look at our fieldwork!
The hydrothermal component of RaCE:TraX took place aboard the FeRIDGE expedition along the mid-Atlantic Ridge, lead by Profs Alessandro Tagliabue (Liverpool) and Maeve Lohan (Southampton). Working with Sean Selzer at Oxford University, profiles of radium isotopes were collected around two major vent fields, Rainbow and TAG, to better understand the dispersion and fate of trace metals with neutrally buoyant hydrothermal plumes.
In 2018 and 2019 the RaCE:TraX project joined two expeditions to the western Antarctic Peninsula, collaborating with the ICEBERGS project at the British Antarctic Survey and the University of Exeter. ICEBERGS is investigating the Impacts of deglaciation on benthic marine ecosystems in Antarctica, studying the marine in three locations of rapid glacial retreat, with RaCE:TraX providing complementary radiotracer insights into the delivery and dispersion of trace metals in melt water and from glacial sediments. This work synergises with surveys of meltwater and sediment fluxes at Rothera Research Station (Adelaide Island) and an upcoming oceanographic cruise along the shelf of the Antarctic Peninsula. See the “Antarctica” tab for more information about preliminary results and fieldwork opportunities.
RaCE:TraX fieldwork a beautiful day on the western Antarctic Peninsula
Greenland
In September 2019, fieldwork was carried out in the Ameralik fjord and Godhabs fjord in southwest Greenland as part of the RaCE:TraX project in collaboration with the Biogeochemical Cycling in Greenlandic Fjords project headed by Dr. Kate Hendry and colleagues at the University of Bristol. Glacially-derived sediment has been shown to be enriched in nutrients including the micronutrient iron, low concentrations of which limit productivity in large regions of the open ocean. With rising air and ocean temperatures, both fjords are receiving increasing discharge of meltwater, although the ultimate effects of this meltwater upon the fjord and open ocean is relatively unknown. Both fjords sit just outside of Nuuk, the capital of Greenland, and the area is an important environment for fishing and tourism. Of the two fjords, Ameralik hosts a land-terminating glacier, whilst Godhabs glacier is marine-terminating, and so the meltwater and sediment delivery regimes to both fjords are in contrast, despite their proximity.
Samples for radium were taken at 10 sites along the fjords from glacier to fjord mouth, along with surface samples for trace metal and macronutrient analysis, and full depth CTD casts for physical oceanography and chlorophyll data. At sites close to the glacier edge in Ameralik fjord, the waters were visibly murky and thick with resuspended sediment. This was in contrast with Gothabs fjord, which was home to icebergs and much clearer fjord water as we approached the glacier. Radium isotopes are natural daughter products in the U-Th decay series, and further decay or addition of these isotopes is a robust indicator for transport and sedimentary inputs to the water column, acting as a tracer for sediment delivery and transport when taken in concert with hydrographic data such as salinity and d18O. Radium samples were analysed immediately on board to test for short-lived isotope decay, with further counts for longer-lived isotopes done back in the lab. The results of this suite of measurements will help us determine the role of glaciers in nutrient cycling and the biological processes occurring in the fjords. This data will provide comparison with Greenlandic field data from the same fjords in 2018, and link to a broader survey of trace metals and radium isotopes from the 2017 ICY-LAB expedition that probe the longevity and fate of these glacial trace elements, as well as further inputs from continental shelf sediments as surface waters move from coastal regions to iron-limited offshore areas.
Approaching icebergs provide new challenges when sampling
Hong and Tim collect samples for macronutrients from the Niskin bottle
Antarctica
Along the western Antarctic Peninsula, 87% percent of glaciers have retreated in the past ~60 years. The increasing meltwater delivers both dissolved and particulate material, which have different impacts on the surrounding ecosystem. Meltwater supplies dissolved nutrients and minerals that support growth of marine primary producers, but little is known about the delivery, distribution and fate of this dissolved matter. Meltwater also contains fine particulate material, which sinks to the seafloor and can supply essential macro- and micro-nutrients to overlying waters. The relative importance of surface (dissolved) versus benthic (particulate) micronutrient sources in sustaining healthy coastal ecosystems is unknown. This project focuses on the micronutrient iron, which is very scarce in the ocean due to its very low solubility. As a result, the Southern Ocean as a whole is considered iron-limited, primary production there cannot reach its full potential for supporting ecosystems and fisheries, or for drawing carbon dioxide out of the atmosphere.
To understand how the balance of these sources is changing with on-going melting, RaCE:TraX will characterise both meltwater and sediment end-members near rapidly retreating glaciers. Three key locations of rapid deglaciation have been identified, and the southernmost site, Sheldon Glacier, will be complemented by additional studies across a broad range of surrounding sediment types via work out of Rothera Research Station. Our main tool is radium (Ra), a naturally-occurring radioactive element produced in very tiny amounts from decay of thorium, which is present in any lithogenic material such as rock or sediment. There are 4 different isotopes of Ra, each one with its own half-life, so these isotopes are used together to understand how far and how fast glacial inputs are dispersed, and how strong sediment sources are.
In January 2020, the RaCE:TraX project headed south on the RRS James Clark Ross to the Antarctic Peninsula to carry out fieldwork in conjunction with the ICEBERGS project (at the British Antarctic Survey and University of Exeter). ICEBERGS carried out their 3rd and final cruise to the region, investigating the impacts due to ice loss and deglaciation on Antarctic benthic ecosystems. RaCE:TraX complements this work by investigating trace metal cycling, incorporating the roles of both meltwater discharge and shelf-sediment resuspension into trace metal and nutrient budgets of the region through full depth water column and sediment core analysis. Joining the project is Rhiannon Jones, a 1st-year PhD student (INSPIRE cohort 1) and Harry Elderfield scholar. Rhiannon’s project seeks to trace biogeochemical cycling and fluxes of trace metals such as iron in glaciated environments, from both sediment and meltwater sources. Rhiannon will use the U-Th radium decay series along with physical oceanographic data to supplement trace metal sampling.
The rusty-coloured cliffs mean the rocks here are rich in iron
