Zinc and nickel isotopes in seawater from the Indian Sector of the Southern Ocean: The impact of natural iron fertilization versus Southern Ocean hydrography and biogeochemistry

Abstract

The Southern Ocean is the site of 20% of global ocean carbon export, and the origin of water masses that significantly impact the chemistry of the global ocean. Much of the Southern Ocean is a high nutrient low chlorophyll (HNLC) region. Iron is the primary limiting nutrient for phytoplankton growth, and may also be an important control on the biogeochemistry of the water masses emanating from the Southern Ocean. Here we investigate the response of Zn and Ni and their isotopes to natural Fe fertilization from Southern Ocean islands, using samples collected during the KEOPS-2 (Oct-Nov 2011) program in the early austral spring near the Kerguelen plateau, Indian Sector of the Southern Ocean.Zinc and Ni concentrations show depth distributions and relationships to major nutrients that are very similar to published data from the Atlantic sector of the Southern Ocean, away from island fertilization. As in the Atlantic sector, and other regions outside the Southern Ocean, there is a slight minimum in d 66 Zn at about 100-150m depth and a slightly heavier value at the surface. At the KEOPS-2 sites, the light sub-surface Zn isotopes occur at the depth level where measured chl-a has declined to near zero. The heaviest Zn isotope values in either the Atlantic or Indian sectors occur in bottom waters, are associated with samples that show high dissolved Si relative to Zn. These latter represent possible evidence for a heavy input directly to deep water or from sediment, and the Zn-Si-d 66 Zn systematics are consistent with a source in diatom opal. Nickel concentrations show much less variation than those of Zn, with the most depleted surface value only 15-20% lower than those in upwelled deep water in the Southern Ocean. These small variations in Ni concentration are associated with no significant variability in Ni isotopes, as also previously observed in the Atlantic sector.Overall, the key feature of the dataset is the apparent absence of any significant impact of natural Fe fertilization on the distribution of Zn and Ni, with the dominant control being the station location relative to the Polar Front in both the Atlantic and Indian sectors. Zinc and Ni isotopes are invariant in surface waters, whether inside or outside the fertilised zone and whether south or north of the Polar Front.

Some observations on the biogeochemical cycling of zinc in the Australian sector of the Southern Ocean: a dedication to Keith Hunter

“…These deep-water values are consistent with other Southern Ocean deep-water ($700 m) measurements (e.g. Wang et al 2018), where mean (AE2s.d.) d 66 Zn dissolved was 0.40 AE 0.05% (n ¼ 13).…”

Zn Isotope Composition in the Water Column of the Northwestern Pacific Ocean: The Importance of External Sources

“…In the surface ocean, previous studies observed relatively low dissolved δ 66 Zn in almost all of the major oceanic regions with the exception of the SO. In the surface water of the SO, dissolved δ 66 Zn is comparable to or slightly isotopically heavier than the values observed in the deep water (Sieber et al, ; Wang et al, ; Zhao et al, ). A recent study in the subarctic Northeastern Pacific Ocean also observed some isotopically heavy signal in the surface water (Vance et al, ).…”

“…Spatial distribution patterns of Zn isotope composition (δ 66 Zn) would provide further insights in understanding its sources, sinks, and internal cycling processes in marine water column (Moynier et al, ). Recent studies on dissolved δ 66 Zn have provided further insight for Zn cycling in different oceanic regions, including the North Atlantic Ocean (Conway & John, ), the Southern Ocean (SO; Zhao et al, ; Sieber et al, ; Wang et al, ), the Southeastern Pacific Ocean (SEPO; John et al, ), and the Pacific Ocean (Bermin et al, ; Conway & John, ; John & Conway, ; Samanta et al, ; Vance et al, ). Several mechanisms have been proposed to explain the distribution of dissolved Zn elemental and isotopic composition in marine water column globally.…”

The Distribution of Nickel in the West-Atlantic Ocean, Its Relationship With Phosphate and a Comparison to Cadmium and Zinc

“…Combining the Ni data reported by Lai et al (2008) with the associated PO 4 for the upper water column (upper 300 m) [provided by Prof. Y. Sohrin (Kyoto University) and Prof. A. Bowie (University of Tasmania)], indeed the steepest slope (∼4.94 nmol µmol −1 ) is found for a station in summer in the PFZ, whereas less steep slopes are observed to the north and south, implying relatively high Ni uptake in the PFZ. However, this is based on a single station and, in contrast, a very gentle slope is found in the PFZ in spring over the Kerguelen plateau (Wang et al, 2019). Cloete et al (2019) also report an elevated Ni:PO 4 uptake ratio of >2 nmol µmol −1 at a single station, which they attributed to increased metal uptake by diatoms, but lower uptake ratios at other stations.…”

“…The northern origin (NADW) and Antarctic origin (AAIW, uCDW, and AABW) deep water masses have different slopes for the Ni-PO 4 relationship (1.43 and 2.62 nmol µmol −1 , respectively), implying different uptake and remineralization ratios in the various source regions, as previously suggested for Zn and Cd. There is little data available for dissolved Ni in the high latitude oceans, but reported observations do suggest higher surface concentrations in the Southern Ocean [∼5 nmol kg −1 or higher (Lai et al, 2008;Butler et al, 2013;Cloete et al, 2019;Wang et al, 2019)] than in the open Arctic Ocean outside the Transpolar Drift [∼ 4 nmol kg −1 ; (Danielsson and Westerlund, 1983;Cid et al, 2012;Gerringa et al, unpublished)] or at the northern end of the current transect (Figure 2). The higher concentrations in the Southern Ocean are most likely due to upwelling of older deep water in this region whereas in contrast, the Arctic is largely supplied by nutrient poor surface water transported north with the Gulf stream [and only a modest amount of old (Pacific) water] (Van Aken, 2007).…”