The oceanic biogeochemistry of nickel and its isotopes: New data from the South Atlantic and the Southern Ocean biogeochemical divide

• Published in: Earth and Planetary Science Letters
• Main Authors: Archer, Corey, Vance, Derek, Milne, Angela, Lohan, Maeve C.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2020
• Subjects: Southern Ocean; North Atlantic Deep Water; North Atlantic
• Online Access: https://eprints.soton.ac.uk/441295/

Nickel (Ni) is important for a number of enzymes in oceanic phytoplankton. It has received less attention than some other bioactive metals because it is not reduced to extremely low dissolved concentrations in the photic zone. However, there are strong indications in previous studies that this residual pool is not bio-available. Oceanic Ni isotope data are still scarce, but have great potential for understanding this issue, as well as for understanding the Ni mass balance of the oceans now and in the past. Here, we present new concentration and isotope data for the UK GEOTRACES section at 40°S in the Atlantic (GA10). Nickel concentration data show typical nutrient-like profiles, slightly modified by variable pre-formed concentrations in sub-surface water masses, e.g. North Atlantic Deep Water. Nickel isotopes, in common with findings in previous studies, are homogeneous beneath 500 m, at about +1.3‰ in δ 60 Ni, in samples with Ni concentrations above 3-3.5 nM. The surface South Atlantic, however, has concentrations below 3 nM, and shows significantly higher δ 60 Ni, up to +1.74‰, that are closely anti-correlated with Ni concentrations. The data for the deep South Atlantic dissolved pool, with a δ 60 Ni = 1.31 ± 0.12‰ (average and 2SD) confirm the homogeneity of the global deep ocean, which previous data demonstrate extends all the way to the surface in the upwelling zone of the Southern Ocean south of the Polar Front. This Ni isotope composition is significantly heavier than known inputs to the oceanic dissolved pool. This mass balance requires an isotopically light sink that may be represented by sedimentary Mn-oxide associated Ni. The magnitude of the isotope fractionation implied by the upper ocean data is not consistent with plausible potential abiotic removal processes. Rather, these data are best explained by biological uptake. However, consideration of the detailed relationships between Ni concentrations and isotope compositions requires that a substantial portion of the oceanic dissolved Ni pool is not ...