Transition metals and their stable isotopes
Transition metals such as Ni are essential nutrients that serve as metal co-factors in vital enzymes in marine organisms. The stable isotopes of transition metals thus have strong potentials to help us better understand the marine ecosystem functioning and the carbon cycle. However, transition metals also face the problem of budgetary imbalance. Together with collaborators, I have modeled how the composition of the phytoplankton community controls the distribution of Ni isotopes in the ocean and showed that Ni may play a key role in nitrogen fixation. I am currently co-supervising a Ph.D. project to quantify the sedimentary fluxes and processes of Ni, Cu, and Zn in a variety of sedimentary settings.
Related publications
2023
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The nickel output to abyssal pelagic manganese oxides: A balanced elemental and isotope budget for the oceans
Sarah Fleischmann, Jianghui Du, Aditi Chatterjee, James McManus, Sridhar D. Iyer, Ankeeta Amonkar, and Derek Vance
Earth and Planetary Science Letters, 2023
The development of nickel isotopes as a chemical tracer of past ocean environments requires a sound understanding of the modern oceanic budget. Our current understanding of this budget implies a large elemental and isotope imbalance between inputs to and outputs from the dissolved pool of the ocean. This imbalance is mainly caused by the dominant oxic sink of Ni to Mn oxide-rich sediments. Though the Ni isotope composition of Fe-Mn crusts has previously been used as proxy for the Ni isotope composition of these sediments, crusts and nodules represent a very small part of the total Mn oxide output. Instead, Mn oxide microparticle supply to pelagic and hemi-pelagic sediments dominates the removal of Mn to sediments, but there are very few isotope data for such samples. Here we present the first extensive Ni concentration and isotope dataset from fully oxic Mn-rich pelagic sediments, from 6 different sites across the open Pacific and 10 closely-spaced sites in the Indian Ocean. We also present data for one hemi-pelagic site representing a suboxic setting on the California Margin. Abyssal Pacific and Indian Ocean sediments have a Ni/Mn ratio of 0.02 (similar to Fe-Mn crusts) and their authigenic Ni is isotopically lighter (δ60Ni = +0.26 to +1.08‰) than seawater (+1.33‰) and crusts (+1.55±0.38‰). Data presented here for organic carbon-rich suboxic sediments of the Californian margin have lower Ni/Mn ratios (0.004 to 0.014 for the oxic top of the core, where Mn oxide is present in abundance) and even lighter authigenic Ni isotope compositions (δ60Ni = -0.08±0.11‰). We show that the Ni isotopes of nearly all Mn-rich sediments and deposits analysed to date, including the new data presented here, are correlated with Co/Mn ratios, suggesting that both are controlled by accumulation rate, progressive incorporation of Ni into the metal oxide structure and isotopic re-equilibration between the solid and aqueous phase. At sites where sediments are diagenetically processed, such as the California Margin, differential diagenetic remobilisation of Mn, Ni and Co cause deviations from this correlation. We present a new mass balance calculation that recognises the importance of scavenging of oceanic Ni to Mn oxide-rich proximal hydrothermal sediments, with low Ni/Mn and light isotope compositions. The mass balance produces a budget that can be simultaneously balanced for both amounts and isotope compositions of Ni. This result provides a strong basis for the application of Ni isotopes as records of the evolution of the metal sink from the oxic oceans through Earth history.
2022
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The essential bioactive role of nickel in the oceans: Evidence from nickel isotopes
Nolwenn Lemaitre, Jianghui Du, Gregory F. Souza, Corey Archer, and Derek Vance
Earth and Planetary Science Letters, 2022
The role of nickel (Ni) in ocean biogeochemical cycles is both under-studied and controversial. Strong correlations between Ni and organic carbon in modern and ancient marine sediments suggest a prominent biogeochemical role over a substantial portion of Earth history. Addition of Ni to culturing and seawater incubation experiments produces strong responses in terms of cell growth, particularly of nitrogen-fixing organisms. But the implied limiting role for phytoplankton growth is inconsistent with observations in the real ocean, specifically that photic zone Ni concentrations never descend to the very low values that characterise other bioactive, and often bio-limiting, metals like iron. These two observations can be reconciled if a large portion of the total dissolved Ni present in open-ocean surface waters is not bio-available on short timescales. Here we present new Ni concentration and stable isotope data from the GEOVIDE transect in the North Atlantic. We interpret these new data in the light of the growing database for Ni stable isotopes in the modern ocean, with implications for the biogeochemical importance of Ni. In the new North Atlantic dataset, the lowest Ni concentrations (1.8-2.6 nmol/L) and highest delta Ni-60 (up to +1.67 parts per thousand) are associated with low nitrate, south of the subarctic front (SAF). By contrast, stations at latitudes north of the SAF, with higher surface nitrate, show very subdued variation in Ni concentrations throughout the entire depth of the water column (3.6 +/- 0.3 nmol/L, mean and 2SD), and no variation in delta Ni-60 beyond the narrow global deep-ocean range (+1.33 +/- 0.13 parts per thousand). These North Atlantic Ni isotope data also show relationships with nitrogen isotope effects, observed in the same samples, that are suggestive of a link between Ni utilisation, isotope fractionation and nitrogen fixation. The global dataset, including the new data presented here, reveals a biogeochemical divide with Ni isotope fractionation only occurring in low latitude surface waters. A simple observationally constrained three-dimensional model of Ni cycling suggests that the creation of this isotopically heavy, Ni-poor, end-member, together with the physical circulation and remineralisation at depth, can explain the global Ni-delta Ni-60 systematics. Taken together, these findings hint at Ni-N co-limitation in the modern ocean. We advocate for more extensive and detailed culturing/incubation studies of this neglected metal in order to elucidate its potentially crucial biogeochemical role. (C) 2022 The Author(s). Published by Elsevier B.V.
