Dissolved nickel sources: transformation and sinks in the Arabian Sea

17 September 2025

Malla and co-authors (2025, see reference below) present an extensive study of the distribution of dissolved nickel (dNi) in the Arabian Sea. Concentrations range from ∼2 nM at the surface to ∼12 nM in deeper waters, showing a typical nutrient-like profile. Based on their estimates of three major external sources, authors show that atmospheric dust accounts for ~36% of the dNi supply, while riverine input and submarine groundwater discharge (SGD) contribute ~62% and ~2%, respectively.

The vertical distribution of dNi reflects the combined influence of these external inputs, water-mass mixing, biological uptake and remineralization, as well as reversible scavenging. Above 1000 m, scavenging dominates; below 1000 m, regeneration seems to be the main process (see figure).

Last but not least, there are Oxygen Minimum Zones (OMZs) in the Arabian Sea. Interestingly, a significant depletion of dNi over Phosphate in the intense oxygen minimum is observed, implying a complex interplay of factors such as nickel loss as sulphides or iron oxides or particulate organic carbon (POC) and due to varying ecosystem composition.

**Figure:** Section plots of dNi along the cruise track of (a) GI10 and (b) GI05 in the Arabian Sea. The cruise transects are given on the right side of the respective section plots. Please note the colour bar for the dNi concentration (nM). (c) Vertical plots showing the fraction of Ni contributed from the mixing of water masses (blue), inferred remineralised Ni and the observed dNi in the different regions of the Arabian Sea (9°N transect- GI10/10, Eastern Arabian Sea- GI10/17, Western Arabian Sea- GI05/11). The difference in the observed Ni and (remineralised+ water mass) Ni suggests various processes like scavenging in the upper water column (1200-1500m) and regeneration (remineralisation and reversible scavenging) and/or benthic flux in the deeper waters (>1500m).

Reference:

Malla, N., Singh, S. K., Singh, N. D., Shukla, A., Chinni, V., Goswami, V., & John, R. (2025). The Role of Water Masses, Biological Processes, Remineralization and Reversible Scavenging in Controlling the Distribution of Dissolved Nickel in the Arabian Sea. Global Biogeochemical Cycles, 39. Access the paper:10.1029/2024gb008441

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