Hydrothermal plumes and processes in Indian Ocean

Abstract

The predicted cycling of the whole ocean through hydrothermal plumes is comparable to the mixing time of the oceans (few thousand years). Hence, understanding hydrothermal plume processes is crucial if their impact on the global geochemical cycles of elements is to be assessed. One of the most important processes that has been demonstrated to modify the gross chemical flux from venting to the oceans is the oxidative precipitation of dissolved Fe (II). It has been hypothesised that this might vary significantly from one ocean basin to another along the path of thermohaline circulation. To test that hypothesis, hydrothermal plume samples were collected from the first confirmed hydrothermal vent fields in the Indian Ocean, at Kairei and Edmond, close to the Rodriguez Triple Junction, during the RRS Charles Darwin cruise CD128 in 2001. The samples were analysed to determine the concentrations of dissolved iron and manganese and particulate Fe, Mn, Al, Ca, Mg, Cu, Zn, P, V, As, Y and the rare earth elements. For a subset of the samples, the concentrations of Fe, Mn, Cu and P in different size fractions of the particulate phase were also measured. Dissolved Fe and Mn concentrations are high in the Kairei and Edmond hydrothermal plumes compared to Atlantic and Pacific hydrothermal plumes previously studied.
Particulate Fe concentrations are also high while particulate Mn concentrations remain low throughout the plume. Of the total (i.e. particulate plus dissolved) Fe which emerges from the vents, approximately 20-30% is lost from the plume via the removal of Fe sulfide phases formed early in the buoyant plume. Further loss of Fe due to the oxidation and formation of particulate Fe-oxide phases results in a total Fe loss of 50-70%. For the very young non-buoyant plume samples, there is very little in situ particulate Fe present.
The behaviour of the chalcophile elements (Cu, Zn, Cd and Pb), elements which exist as oxyanions in seawater (P, V and As), as well as the rare earth elements and Y are consistent with previous studies of elemental behaviours in hydrothermal plumes in the Atlantic and Pacific Oceans. The observed behaviours of these elements with respect to particulate Fe suggests that the differing Fe (II) oxidation rates between ocean basins do not impact the processes taking place within hydrothermal plumes. In addition, fractionation of Fe, Mn, Cu and P in the Edmond hydrothermal plume between the dissolved, colloidal and fine and coarse particulate phases shows consistency with previous conclusions based on only one of these phases. The P/Fe and V/Fe ratios of the hydrothermal particulate samples are intermediate to those of particulate samples from the Atlantic and Pacific Oceans suggesting that as previously hypothesised, these ratios are dependent on dissolved ambient phosphate concentrations. Hence, there remains the potential to use these ratios from sediment cores as paleo-proxies for dissolved phosphate concentrations.

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