Marine Metal Isotope and Trace Element Laboratory

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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 and local (Gulf of Mexico, Florida, Gulf Stream) - find out where.

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1) Determining the isotopic signature of iron released via ligand-mediated dissolution of atmospheric dust in the surface ocean (2018-present) - an NSF funded project in collaboration with 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; 09/18-09/21.

2) US GEOTRACES GP15 Trace Metal Concentrations and Isotopes (2017-present) - 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: $400,703; 08/17-08/20.

3) Gulf Stream Metal Interactions (2017-present) - a USF 'New Researcher' project to investigate how the Gulf Stream can affect metal cycling in the North Atlantic. We will cruise from Florida to the Bahamas onboard R/V Angari in 2018, 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 is in partnership with the non-profit ANGARI Foundation. $9,975; 05/17-05/18.

4) GA02 Dutch GEOTRACES Atlantic metal cycling (2017-present) - As part of a NWO supported project in collaboration with Rob Middag at NIOZ in the Netherlands, we are measuring dissolved metal isotopes (δ56Fe and δ66Zn) along the Dutch GA02 cruise which sailed in 2010. This project will allow us to assess margin sources and long-distance transport of these elements in the Atlantic.

5) GP02 Japanese GEOTRACES North Pacific metal cycling (2017-present) - We are measuring dissolved metal isotopes (δ114Cd and δ56Fe) along 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.

6) The role of circulation, biology and islands on the distribution of metals and their isotopes in the Antarctic (2016 - present) - Through collaboration with Swiss (ETH Zürich, U. Geneva, U. Bern) and Australian (ANU) scientists, we are 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.


1) GA03 North Atlantic Metal Cycling at the University of South Carolina with Seth John (2010-2014) - 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, 2014Revels et al., 2015Fitzsimmons et al., 2015):

- 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 and the signature of labile and total particulate phases.

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

Cd (Janssen et al., 2014Conway 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 δ114Cd.

- 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 maye be removal processes for (isotopically-light) Cd.

Zn (John and Conway, 2014Conway 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 δ66Zn.

- 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. 

2) GA10 South Atlantic Fe Cycling at USF and the University of South Carolina with Seth John (2010-2014) - As part of two UK GEOTRACES Section Cruises along 40S in the South Atlantic, we made dissolved δ56Fe measurements in ~200 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 δ56Fe 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 non-reductive sedimentary Fe source that was previously proposed by Radic et al., 2011 (Homoky et al., 2013).

Water column dissolved Fe and intercomparison 

- The first water column profile of dissolved Fe and dissolved δ56Fe from within the Cape Basin of the GA10 section implies regional differences in Fe cycling within water masses. Inter-comparison of a δ56Fe 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).

3) North Pacific metal cycling at SAFe and San Pedro, at the University of South Carolina with Seth John (2014) - Measurements of trace metal isotopes at 2 stations in the North East Pacific provide insights into the supply of Fe and Zn from continental margins, and how low-oxgyen waters may affect transition metal cycling, with the dominance of isotopically light Fe emphasizing the importance of reductive sediment sources of Fe in the North Pacific, compared to the North Atlantic (Conway and John, 2015).

4) GP19 Japanese GEOTRACES South West Pacific metal cycling, at ETH Zürich with Matthias Sieber and Derek Vance (2015 - 2018) - 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., accepted).

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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 new Tampa Bay Plasma Core Facility based at CMS.

Thermo Element XR Sector Field High Resolution ICPMS with Jet Interface and SC4DX autosampler (April 2017).

Thermo Neptune Plus Multicollector ICPMS with Microfast-SC2DX autosampler, Apex-Q and Apex-Omega (May 2017).

Clean Laboratory (2017)

Construction of a brand-new 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.