Spatial variability in dissolved iron concentrations in the marginal and open waters of the Indian Ocean

We analyzed 28 full vertical profiles of dissolved iron (DFe) in the Indian Ocean (i.e., between 26°S – 20°N and 72°E – 105°E) during two research expeditions (SK-304: March–May 2013, SK-311: March–April 2014) as part of the GEOTRACES-India programme. Relatively high DFe concentrations in surface waters (≤10 m) were observed in the Arabian Sea (AS: 0.68 ± 0.03 nM, n = 2) and the Bay of Bengal (BoB: 0.44 ± 0.11 nM, n = 21) compared to the stations located south of the equator (0.27 ± 0.16 nM, n = 17). Higher DFe in the coastal stations is presumably sourced from the continental shelves. Depth profiles of DFe in the AS, BoB and Andaman Sea showed sub-surface maxima, coinciding with low dissolved oxygen (DO) concentrations. High DFe concentrations in sub-surface waters of the BoB and the Andaman Sea are attributed to desorption of soluble iron from resuspended sediments of the rivers and the remineralization of sinking particulate organic matter. We also observed a significant contribution of DFe from hydrothermal vents situated at the Central Indian ridge and subduction processes along the Java trench to pelagic waters of the Indian Ocean. Fe:C remineralization ratios of 3.92 ± 0.83, 5.88 ± 2.65, 8.96 ± 3.85 and 5.74 ± 3.06 μmol/ mol were observed in the Indian Ocean, BoB, AS and Andaman Sea, respectively. To assess the surface areas and deep water masses where DFe concentrations potentially limit phytoplankton growth (when reaching the surface) we subtracted the contribution of particulate organic matter remineralization to the DFe pool using the tracer Fe* (≈DFe–RFe:P(Phosphate)). Positive Fe* values in the intermediate waters suggest an external source for DFe, whereas negative Fe* values suggest the absence of such external inputs and/or removal by scavenging processes. Our results contribute to improved understanding of DFe distributions and their coupling with the ocean biogeochemical cycling of carbon and nitrogen in this region.