Iron from a submarine source impacts the productive layer of the Western Tropical South Pacific (WTSP)

Cécile Guieu,Valérie Chavagnac,Anne Petrenko,Thierry Moutin,Sophie Bonnet,Karine Desboeufs,Christophe Menkes,Christophe Maes

doi:10.1038/s41598-018-27407-z

Abstract

In the Western Tropical South Pacific, patches of high chlorophyll concentrations linked to the occurrence of N2-fixing organisms are found in the vicinity of volcanic islands. The survival of these organisms relies on a high bioavailable iron supply whose origin and fluxes remain unknown. Here, we measured high dissolved iron (DFe) concentrations (up to 66 nM) in the euphotic layer, extending zonally over 10 degrees longitude (174 E−175 W) at ∼20°S latitude. DFe atmospheric fluxes were at the lower end of reported values of the remote ocean and could not explain the high DFe concentrations measured in the water column in the vicinity of Tonga. We argue that the high DFe concentrations may be sustained by a submarine source, also characterized by freshwater input and recorded as salinity anomalies by Argo float in situ measurements and atlas data. The observed negative salinity anomalies are reproduced by simulations from a general ocean circulation model. Submarine iron sources reaching the euphotic layer may impact nitrogen fixation across the whole region.

Highlights

The Western Tropical South Pacific (WTSP) Ocean has recently been identified as a hotspot of N2 fixation[1], the main external source of new fixed nitrogen (N) to the surface ocean, which controls primary productivity and carbon export[2,3]
Our results reveal that hydrothermal submarine inputs would explain these high dissolved Fe (DFe) concentrations, rather than atmospheric deposition
Results strongly support the remote and crucial role of a shallow, intermittent and strong submarine source located in the Tonga arc, in shaping the spatial and temporal DFe field observed during OUTPACE (Fig. 1B)

Summary

Introduction

The Western Tropical South Pacific (WTSP) Ocean has recently been identified as a hotspot of N2 fixation[1], the main external source of new fixed nitrogen (N) to the surface ocean, which controls primary productivity and carbon export[2,3]. In the WTSP, Fe could be supplied by runoff from islands in the vicinity of Melanesian archipelagos[6] or by atmospheric volcanic inputs as the WTSP includes the Tonga and Vanuatu volcanic arcs. These potential Fe sources have not been quantified to date, even though it has recently been shown that volcanic aerosols facilitate natural Fe ocean fertilization around Iceland and the Mariana back-arc[7,8]. Our results reveal that hydrothermal submarine inputs would explain these high DFe concentrations, rather than atmospheric deposition

Methods

Results

Conclusion