Lithogenic inputs to the Southern Ocean assessed with thorium and rare earth elements

Abstract

<p> </p><p>The Southern Ocean is a key regulator of global climate, and it’s responsible for about one-quarter of the global carbon export. Part of this export is fuelled by phytoplankton, however, in some areas of the ocean, phytoplankton activity remains to a minimum despite the abundance of nutrients. These areas, known as high nutrient low chlorophyll (HNLC), from which the Southern Ocean is the largest one, exist due to the absence of the micro-nutrient iron (Fe). The Kerguelen Plateau located in the Indian sector of the SO hosts one of the largest phytoplankton blooms in the SO during spring and summer. This study focused on the role of Heard and McDonald Islands, located in the southern part of the Kerguelen Plateau, on the Fe fertilization of the whole region, and their role in overcoming HNLC conditions. For this, we combined thorium, rare earth element concentrations and Nd isotopic composition to trace the lithogenic supply of Fe. Our results highlight the importance of Heard and Mc Donald Islands, surrounding shallow areas and probably the melting of glaciers from Heard Island on the supply of Fe that fuels the bloom in the region.  We used thorium isotopes (<sup>230</sup>Th and <sup>232</sup>Th) to estimate the flux of Fe produced by the dissolution of lithogenic particles. These results suggest that thorium isotopes can be used to calculate the fluxes of other elements not only in the remote open ocean but also near continental margins. Cerium and europium anomalies, together with the neodymium isotopic composition data seem to indicate that Heard and McDonald Islands are the main contributors to the natural Fe fertilization of the region. This study highlights the need for more systematic studies to obtain a better understanding of the functioning of the Southern Ocean carbon pump and its relationship with global climate.</p>