Use of Ra isotopes to deduce rapid transfer of sediment-derived inputs off Kerguelen

Abstract

Abstract. The Southern Ocean is known to be the largest high-nutrient, low-chlorophyll (HNLC) region of the global ocean due to iron limitation. However, a large phytoplankton bloom develops annually downstream of the Kerguelen Islands, a bloom which is sustained partly by iron released from the sediments deposited onto the shelves. In the framework of the KEOPS-2 project, we used radium isotopes ( 224 Ra, T 1/2 = 3.66 d; 223 Ra, T 1/2 = 11.4 d; 228 Ra, T 1/2 = 5.75 yr) to provide information on the origin of iron fertilization and on the timescales of the transfer of sedimentderived inputs (including iron and other micronutrients) towards offshore waters. Significant 224 Ra and 223 Ra activities were found in the near vicinity of the Kerguelen Islands, in agreement with the short half-lives of these isotopes. Significant 224 Ra and 223 Ra activities were also detected up to 200 km downstream of the islands and more unexpectedly in offshore waters south of the polar front. These observations thus clearly indicate (i) that the sediment-derived inputs are rapidly transferred towards offshore waters (on timescales on the order of several days up to several weeks) and (ii) that the polar front is not a physical barrier for the chemical elements released from the sediments of the Kerguelen Plateau. The Ra data set suggests that iron and other micronutrients released by the shelves of the Kerguelen Islands may contribute to fueling the phytoplankton bloom downstream of the islands, despite the presence of the polar front. However, the heterogeneous distribution of the 224 Ra and 223 Ra activities in surface waters suggests that this supply across the front is not a continuous process but rather a process that is highly variable in space and time.

High variability in dissolved iron concentrations in the vicinity of the Kerguelen Islands (Southern Ocean)

“…Moreover, Bowie et al (2014) showed that atmospheric inputs were of the order of 50 nmol m −2 d −1 , which is insignificant compared to the lateral supply of dFe in the same area (180-2400 nmol m −2 d −1 ). Significant 224 Ra and 223 Ra activities were detected in offshore waters south of the PF (Sanial et al, 2015). These observations clearly indicated that dissolved sediment-derived inputs of Ra can be rapidly transferred towards offshore waters.…”

Production regime and associated N cycling in the vicinity of Kerguelen Island, Southern Ocean

“…By this process, subsurface water is becoming enriched in iron, which subsequently can be efficiently supplied to the mixed layer via vertical mixing Park et al, 2008). At the polar front, iron fertilization results from the previously described enrichment over the plateau and the spreading of this enrichment eastward through the additional interaction between the polar front jet and local bathymetry (Mongin et al, 2008;Sanial et al, 2015). This mode of supply can be compared with the polar-front-associated bloom in the Atlantic sector, where iron is supplied by lateral advection from the Patagonian shelves and islands in the Scotia Ridge area (de Baar et al, 1995;Löscher et al, 1997) or by direct lithogenic aerosol deposition (Quéguiner et al, 1997).…”

Delineating environmental control of phytoplankton biomass and phenology in the Southern Ocean

“…In order to estimate iron delivery due to horizontal stirring, we use an advection scheme based on altimetry, extending to the Southern Ocean a Lagrangian model used for predicting the development of the Kerguelen phytoplanktonic plume . The model has been extensively calibrated and validated in the Crozet and Kerguelen regions by integrating satellite data (altimetry and ocean color), lithogenic isotopes, iron measurements, and drifters Sanial et al, 2014Sanial et al, , 2015. model seeds each open ocean location with a particle and finds the particle's most recent contact with a shallow bathymetry by a back trajectory issued from altimetry data.…”