USF Marine Metal Isotope and Trace Element Laboratory

A number of transition metals (e.g. Fe, Zn, Cd, Ni, Co, Cu, Mn), although present at vanishingly-small quantities in the open ocean, act as key nutrients or toxins for phytoplankton in marine systems. Accordingly, alongside the classical nutrients, these 'trace metals' play a vital role in influencing local, regional and global patterns of primary productivity, ecosystem structure, carbon cycling and climate. Understanding the distribution, cycling and biological role of these metals in the modern oceans is thus of key interest to chemical oceanographers, and also to those studying ocean and climate in the past.

At the broad scale, MarMITE uses high resolution ICPMS to make measurements of the concentration and isotopic composition of these metals in various materials (rocks, sediments, seawater, rain, dust, particles and biological materials) in order to better understand the cycling and role of these trace metals in the earth-ocean-climate system.

 

At present, we are focused on samples collected as part of the International GEOTRACES program, with ongoing collaborations with Australian, Dutch, German, Japanese, Swiss, UK and US scientists. Together with these projects, we also employ innovative laboratory and field experiments in order to address topics in the four following themes:

1) Boundary/Interfaces - how do trace metals enter the ocean (margins, sediments, atmosphere or hydrothermal venting)?

2) Removal - how do trace metals leave the oceans (e.g. biological uptake, particle-scavenging, sediments)?

3) Large-scale - how do physical and biological processes affect metal distributions and isotopic composition?

4) Past processes & large-scale budgets - using trace metal isotope ratios as proxies for past oceanic processes.

The focus of our research is both global (Pacific, Atlantic, Arctic, Southern Oceans) and local (Gulf of Mexico, Florida, Gulf Stream) - find out where.

1) Linking Fe and nitrogen sources in an oligotrophic coastal margin: N₂ fixation and the role of boundary fluxes - in collaboration with FSU, ODU, OSU, UMN and USGS we are funded to investigate the role of submarine groundwater discharge and rivers in influencing DON and Fe cycling on the West Florida Shelf on two cruises in 2023 and with land based sampling. OCE2148836: $238,621 03/22-02/25. Lab Member: Hannah Hunt.

2) GP17-ANT US GEOTRACES Dissolved concentrations, isotopes and colloids of the bioactive trace metals - in collaboration with TAMU and USC we are funded by NSF to measure Fe, Zn, Cd, Cu, Ni, Mn and Pb concentrations, and Fe, Zn, Cd, Cu and Ni isotopes in size-fractionated and filtered seawater, particles and aerosols as part of the GP17-ANT cruise & process study in the Amundsen Sea, which sail November 2023. OCE2123354: $397,537 10/21-9/24. Lab Member: Zach Bunnell.

3) GP17-OCE US GEOTRACES Bioactive Trace Metal Concentrations and Isotopes - in collaboration with Texas A&M and the University of Southern California we are funded by NSF to measure Fe, Zn, Cd, Cu, Ni, Mn and Pb concentrations and Fe, Zn, Cd, Cu and Ni isotopes in size-fractionated and filtered seawater, particles and aerosols on the South Pacific GP17-OCE section from Tahiti to Chile, which sailed November 2022. OCE2049214: $351,982 09/21-08/24. Lab Member: Dylan Halbeisen.

4) Metalgate, Trace metals and the Arctic-Atlantic gateway in a changing world, local processes and global connections - a Dutch Project with lead PI Rob Middag at NIOZ looking at trace metal cycling in the Arctic Ocean around Iceland and Greenland (10/01/19-10/01/24). We are focusing on Fe and Ni isotope cycling. Lab Member: Dylan Halbeisen.

5) GP02 Japanese GEOTRACES and Line P North Pacific metal cycling - we are investigating dissolved metal isotopes along Line P and the Japan-Vancouver GP02 Japanese GEOTRACES cruise which sailed June 2017. This project will allow us to investigate the role of the atmosphere, physical circulation, biological cycling and margin sources for the cycling of these metals in the North Pacific Ocean, in collaboration with a number of Japanese scientists at the University of Kyoto and Jay Cullen at UVic. A paper on GP02 aerosols is in review (Kurisu et al.) and a paper on Fe isotope distributions along the transect is in preparation. Lab Member: Hannah Hunt.

> Iron limitation and viral lysis, phytoplankton caught between a rock and a hard place - a Dutch Project with lead PI Rob Middag at NIOZ looking at trace metal cycling in the Amundsen and Weddell Seas on the Antarctic Margin (04/09/17-03/09/21). Papers have been published on Fe, Zn and Cd concentration distributions (and the role of circulation, biology, sediments, and ice interactions in controlling these) in the Amundsen Sea (van Manen et al., 2022; Tian et al., 2023). Hung-an Tian completed an excellent PhD thesis on this project, and two papers on how Fe isotopes cycle through these regions (the role of sediments, circulation, ice-interactions and biology) are published or in prep (Tian et al., 2024; Tian et al., in prep.). Lastly, a paper looking at the isotope systematics of Cd and Zn in these two regions is in prep (Tian et al., in prep).

> Saandox Project - we are investigating the cycling of metal isotopes during the Saandox time series project in the Saanich Inlet, in collaboration with scientists at University of Victoria, BC, Canada, and elsewhere. A paper is in preparation (Onak et al., in prep.).

> Determining the isotopic signature of iron released via ligand-mediated dissolution of atmospheric dust in the surface ocean - an NSF funded project in collaboration with Dr Rene Boiteau at Oregon State to examine how natural atmospheric dust releases metals into seawater in the presence of different leach media and natural and model organic matter and ligands. This involves collection of dust on Bermuda, experimental work and several cruises and incubation studies with natural phytoplankton communities. OCE1829643: $374,080; 09/18-09/23. Papers looking at the molecular character and persistence of organic matter at BATS (Boiteau et al., 2024) and modeling the surface impact of aerosols on Fe and Fe isotopes (König et al., 2022) are published, and several papers on aerosol or water column Fe isotopes are in review (Kurisu et al.) or in preparation.

> GP15 US GEOTRACES Trace Metal Concentrations and Isotopes - an NSF funded project in collaboration with the University of Southern California to measure Fe, Zn, Cd, Cu and Ni concentrations and isotopes in size-fractionated and filtered seawater, particles and aerosols on the North Pacific GP15 section from Tahiti to Alaska, which sailed Sept. - Nov. 2018. OCE1737136: $412,703; 08/17-01/23. Several papers have been published on Cd and Zn isotopes (Sieber et al., 2023a; Sieber et al., 2024b), an article on global Ni cycling was published in Nature Geoscience (John et al., 2022), a paper on re-evaluating the sulfide sink for Cd and Zn (Buckley et al., 2024), and Zach Bunnell completed a MS thesis (paper is in preparation) on GP15 aerosol samples. A number of collaborative papers on Fe and Fe isotopes in the North Pacific are in preparation.

> GA08 German GEOTRACES Fe isotope cycling (USF) - we are investigating the cycling of Fe isotopes from margin sediments and the Congo river along two oceanic transects from the Angola/Namibian margin into the South Atlantic Ocean. This project is in collaboration with scientists at GEOMAR, Germany. View our findings here - Hunt et al. (2022).

> Gulf Stream Metal Interactions (USF) - this was a USF funded 'New Researcher' project to investigate how the Gulf Stream affects metal cycling in the North Atlantic. We cruised from Florida to the Bahamas onboard R/V Angari in March 2019, investigating the distribution of metals, their speciation and isotopes across the southern jet of the Gulf Stream (in conjunction with the Buck lab at USF). This project was in partnership with the non-profit ANGARI Foundation. A paper is in preparation.

> Antarctic Circumnavigation Expedition (USF) - In collaboration with Swiss (ETH Zürich, U. Geneva, U. Bern) and Australian (ANU) scientists, we were involved in a funded ACE project to investigate how biological activity, local sources and islands affect the distribution and cycling of metals in the Antarctic ocean. The cruise sailed Dec. 2016 - Mar. 2017, and provided a complete circumnavigation of Antarctica, sampling near most of the major island chains and the peninsula, and across the different fronts. Find out more. Graduate student Matthias Sieber carried out a detailed examination of Fe, Zn and Cd isotope systematics in the Southern Ocean, showing how processes in this region act to set the global distribution of these elements and their isotopes (Sieber et al., 2019; Sieber et al., 2020; Sieber et al., 2021). We also were involved with publishing trace metal distributions and a Science Advances paper on Fe speciation and Fe isotopes along the cruise (Janssen et al., 2020; Fourquez et al., 2023).

> GP19 Japanese GEOTRACES South West Pacific metal cycling (ETH Zürich) - we investigated dissolved Fe and Cd isotopes on the Japanese GEOTRACES GP19 section which sailed early 2015. Dissolved Cd and Cd isotopes along this section provide new insight into the role of biological processes, the Southern Ocean and large scale mixing on the distribution of Cd in the oceans (Sieber et al., 2019). Several publications on iron isotopes and inter-comparison with other groups are in preparation.

> GA10 South Atlantic Fe Cycling (USC and USF) - As part of two UK GEOTRACES Section Cruises along 40S in the South Atlantic, we made dissolved Fe isotope measurements in ~300 water-column samples from a number of stations. We were also involved with collaborators at the University of Southampton and Oxford to make pore-water Fe isotope measurements in the sediments underlying the section. We hope the combination of different datasets will generate insights on how different sources of Fe contribute to the dissolved Fe budget, and how water-mass mixing can influence the dissolved Fe cycle.

Published findings include:

Porewater Fe 

- Pore water and sediments provide direct sedimentary evidence for a lithogenic colloidal sedimentary source of Fe to the ocean interior (Homoky et al., 2013; Homoky et al., 2021).

Water column dissolved Fe and inter-comparison 

- The first water column profile of dissolved Fe and dissolved Fe isotopes from within the Cape Basin of the GA10 section implies regional differences in Fe cycling within water masses. Inter-comparison of an Fe isotope  profile (collected 2010) with one collected at the same location in 2008 by the French GIPY4 cruise shows no differences in the intermediate-deep ocean over a two year timescale (Conway, John and Lacan, 2016).

- Water-column section and western margin sediment addition papers are in preparation.

> US GEOTRACES GA03 (USC) - The GA03 US GEOTRACES Section was sampled on two cruises in the winter of 2010 and 2011, and travelled from Lisbon to Mauritania (2010, USGT10) and from Woods Hole to Mauritania (2011, USGT11). We measured dissolved Fe, Zn and Cd isotope ratios in water samples from a range of different marine environments - including 1) the low-oxygen OMZ close to West Africa, 2) the Mid-Atlantic Ridge (where a hydrothermal plume was sampled directly at the TAG site), 3) surface waters close to Africa where dust fluxes are high, 4) margin sediments, and 5) the influence of water masses such as the Mediterranean Outflow, North Atlantic Deep Water, and Antarctic Water masses such as AAIW and AABW. We also measured Fe isotope ratios in particles and aerosol dust from the section.

Published findings include:

Fe (Conway and John, 2014; Revels et al., 2015; Fitzsimmons et al., 2015; Conway et al., 2018; Conway et al., 2019):

- Different Fe sources (oxic sediments, reducing sediments, hydrothermal and dust) can be characterised by isotopically distinct endmembers in the North Atlantic, allowing direct quantification of sources.

- Basin section of particulate Fe isotopes shows insights into particle chemistry.

- Partitioning of Fe isotopes between colloidal and soluble Fe fractions suggests these two phases exchange at depth but cycle independently in the surface oean.

- Gulf stream rings are an important source of Fe to the North Atlantic Gyre.

- Aerosol Fe isotopes can trace anthropogenic sources of Fe to the gyre, and show that these are underestimated in dust deposition models.

Cd (Janssen et al., 2014; Conway and John, 2015):

- Cd and Cd isotopes are dominantly influenced in the surface Atlantic by in situ biological uptake of light Cd and water-mass mixing, leading to a strong vertical gradient in both Cd concentration and Cd isotopes.

- Low Cd/P dissolved ratios (negative Cd*), high Cd/P particulate ratios (positive Cd*) and changes in both dissolved and particulate Cd isotopes point to a removal process for CdS within oxygen deficient, but not anoxic, waters associated with the Mauritanian Oxygen Minimum Zone

- Oxygen Minimum Zones and Mid-Ocean Ridge vents may be removal processes for (isotopically-light) Cd.

Zn (John and Conway, 2014; Conway and John, 2014):

- Scavenging (and regeneration) of isotopically heavy Zn to organic material may be important for the marine distribution of Zn.

- Zn isotope ratios, in contrast to Cd, show much more variability in surface waters, pointing to a balance of uptake, regeneration and scavenging controlling the distribution of both Zn and Zn isotpes.

- Margin sediments and the Mid-Atlantic Ridge are sources of isotopically light Zn to the North Atlantic, whilst the Med Outflow carries high concentrations of Zn.

- Sediments may be a biogenic light sink for Zinc from the oceans. 

Our work at USF is facilitated by the new acquisition and installation in 2017 of a Thermo Neptune Plus Multi-collector ICPMS for high-resolution high-precision isotopic analysis at low-concentrations of the full range of elements (from Li, B through Fe, Zn to Cd, Pb, Mo and U), together with a high-resolution Thermo Element XR ICPMS for elemental concentration analysis over the full periodic table, as part of the Tampa Bay Plasma Lab at CMS.

Thermo Element XR Sector Field High Resolution ICPMS with Jet Interface and SC4DX autosampler.

Thermo Neptune Plus Multicollector ICPMS with Microfast-SC2DX autosampler, Apex-Q and Apex-Omega.

Seawater Processing Clean Laboratory

An ISO 6 Class ~10,000 MarMITE clean laboratory at CMS with filtered laminar flow benches, extraction, acid-distillation and ultrapure water supply was completed in 2017, and provides us with the ideal environment for processing seawater and other samples with ultra-low levels of metal contamination, allowing us to make measurement of isotopic-composition in very low concentration samples, complementing our new MC-ICPMS facility.