Dissolved iron cycling in the Arabian Sea and sub-tropical gyre region of the Indian Ocean

Abstract

Understanding the sources and biogeochemical processes influencing the dissolved iron (DFe) distribution is crucial because of its coupled role in C- and N-fixation in the oceans. Vertical depth profiles of DFe in the northwestern Indian Ocean along two meridional transects (~65 �E and ~57 �E) during spring and fall seasons are presented here under the GEOTRACES-INDIA initiative. The DFe in the surface waters exhibited both spatial and seasonal variabilities; N-S gradient with higher values in the north; and notably higher concentrations in the fall season. The photic zone-integrated DFe (57.5 � 13.4 �mol m -2) was double in the Arabian Sea compared to the subtropical Indian Ocean (31.81 � 7.63 �mol m -2) with a significant positive correlation with the photic zone integrated primary production. This suggests higher availability of DFe is responsible for high primary production in the Arabian Sea compared to the subtropical Indian Ocean. Lateral and vertical advection along with particle dissolution from the atmospheric dust fluxes and scavenging-adsorption processes could play prominent role in controlling photic DFe concentrations in the Arabian Sea. The continental margin sources, redox chemistry, and remineralization are responsible for the observed high DFe concentration within the intense oxygen minimum zone. The in-situ remineralization ratios of Fe to C in the Arabian Sea are higher than any other region of the Indian Ocean. It could be due to the high uptake of Fe during primary production in the Arabian Sea, despite huge spatio-temporal variations in the abundance of phytoplankton communities. Our DFe data from the undersampled region over the Carlsberg Ridge suggests the presence of chronic/event plumes contributing to the DFe inventory in the western Arabian Sea region. We have approximated the geochemical budget of Fe in the top 100 m for open ocean waters of the Arabian Sea. These preliminary estimates suggest that lateral and vertical fluxes for DFe are prominent in the region compared to soluble atmospheric Fe dust fluxes and these sources are balanced by the biotic and abiotic removal processes in the region. Based on these estimated fluxes, the calculated residence time of DFe varies from ~0.1 to 1 year in photic waters of the Arabian Sea. Our results highlight the intrusion of Fe-deficit Southern Ocean waters in the Arabian Sea and the upwelling of Fe-starved deep waters alongside the northwest of the Murray Ridge are likely responsible for Fe limitation in the region.