Distribution of iron in the Western Indian Ocean and the Eastern tropical South pacific: An inter-basin comparison

Abstract

The Western Indian Ocean (WIO) and Eastern Tropical South Pacific (ETSP) are distinctly different regimes, yet they share several important features. These include a strong upwelling system, a large oxygen minimum zone (OMZ) with active denitrification, a spreading center with extensive hydrothermal activity, and a vast oligotrophic upper water column. Here, we show that the distribution and geochemistry of iron shows remarkable similarities as well. In particular, both basins exhibit large sub-surface plumes of iron derived from sediments underlying the oxygen minimum zones and from hydrothermal sources. Moreover, the behavior of Fe, especially its redox cycling, is remarkably similar in the Arabian Sea and Peruvian oxygen minimum zone, reflecting similar processes associated with the nitrogen cycle in each region. The large Fe plumes under each OMZ are well below the oxygen-free depth, yet they are probably dependent on internal redox cycling and scavenging of Fe within the overlying OMZs. Globally, both the Arabian Sea and Peru margin sediments are hot spots of carbon oxidation, which probably contributes to their importance as Fe sources.The fate of hydrothermally derived Fe in the WIO and ETSP is particularly interesting to compare because their source terms are very different even though their oxidation rates, once they have entered the water column, should be directly comparable. In the ETSP, hydrothermal inputs from along a section of the ultra-fast spreading Southern East Pacific Rise (SEPR) are integrated into a single coherent plume that is exported west across the ocean interior for >4000km. In contrast, hydrothermal inputs to the WIO derive from multiple sources including outflow from the Red Sea via the Gulf of Aden, the Indonesian Throughflow, and from multiple vent-sources along the Carlsbad Ridge, Central Indian Ridge and SW Indian Ridge which are slow, medium and ultra-slow spreading, respectively. As a result, hydrothermally sourced Fe distributions are more complex in the WIO, with several overlapping plumes and distinctly different distributions of two other hydrothermally-derived metals, aluminum and manganese. Here we argue that comparative studies of these basins, with a future emphasis on benthic-water column interactions, will advance knowledge about Fe biogeochemistry and how distributions in the WIO, in particular, are likely to change in the future.