The impact of circulation and dust deposition in controlling the distributions of dissolved Fe and Al in the south Indian subtropical gyre

Abstract

The South Indian Subtropical Gyre (SISG) is one of the least studied gyre systems of the world ocean with respect to trace elements. Here we report dissolved (<0.45μm) Fe and Al measurements collected during two high-resolution US-CLIVAR CO2 Repeat Hydrography sections, which transected the upper 1000m of the gyre zonally along ~32°S (I05) and meridionally along ~30°E (I06S). Particulate Fe and Al concentrations in waters influenced by the Agulhas Current are also presented. The distributions of dissolved Fe and Al in the gyre are primarily impacted by mineral dust deposition at the surface and the large-scale circulation patterns of the gyre at depth. Using mean mixed layer dissolved Al concentrations, we estimate that the deposition and partial dissolution of mineral dust emanating from South Africa and Australia vary from 60 to 685mg (dust) m−2yr−1 across the 32°S transect. This translates into a dust source of dissolved Fe ranging from 1.7 to 20μmolFem−2y−1. The zonal patterns of aeolian deposition and those of N*, an indirect geochemical tracer for nitrogen fixation, show remarkable similarities along the I05 transect, suggesting that aeolian delivery of Fe may regulate nitrogen fixation rates in the SISG. In the western SISG (west of 60°E), which receives some of the highest aeolian Fe fluxes of the 32°S section, the Fe:AOU ratio in Indian Central Water was elevated relative to that observed in the Indian Central Water occupying the eastern Indian Ocean. These elevated Fe:AOU ratios may reflect the remineralization of Fe-rich organic material from nitrogen fixing organisms at the western end of the basin. Below the mixed layer, the distribution of dissolved Al appears to trace the principal features of the large-scale circulation of the SISG. Elevated subsurface concentrations of dissolved Al (>4nM) in the southwest Indian Ocean west of 45–50°E are most likely sustained by leakage of Al-rich waters from the Agulhas Return Current. Along the southeast African margin, the elevated particulate Fe (up to 230nM) and Al (up to 690nM) concentrations reflect the resuspension and transport of shelf sediments by the highly energetic Agulhas Current. However, while the particulate inputs at the margin are massive and appear to supply modest amounts of dissolved Fe, the distribution of dissolved Al is decoupled from the particulate phase. This observation suggests that the elevated subsurface dissolved Al concentrations observed near the African shelf are not the result of sediment resuspension processes occurring in situ along I05 but are more likely an advected signal originating from the upper reaches of the Agulhas Current.