234Th-based export fluxes during a natural iron fertilization experiment in the Southern Ocean (KEOPS)

Abstract

Abstract Five iron-fertilization experiments in the Southern Ocean have clearly demonstrated that adding iron increases primary production, but the implications for carbon export to the ocean interior have been less clear. This reflects both observational limitations of short-term experiments and their uncertain relevance to quantifying ecosystem level processes that are likely to be structured differently under conditions of punctual versus persistent stimulation. To avoid these biases, KEOPS (KErguelen Ocean and Plateau compared Study) investigated the naturally iron-fertilized Kerguelen Plateau region in the Indian Sector of the Southern Ocean that exhibits an annual phytoplankton bloom. Here, we report particulate organic carbon (POC) and nitrogen export from this system based on the 234 Th approach. Results indicate that the export fluxes were variable both on and off the Kerguelen Plateau (9.0–38.4 mmol C m −2 d −1 and 1.6–4.8 mmol N m −2 d −1 ) and were in the range of values reported for natural Southern Ocean ecosystems. Export fluxes were compared at two reference stations, one above and one outside the Plateau. The station above the plateau was characterized by higher iron supply and export fluxes compared to the station outside the plateau. The difference in the export flux between these two reference stations defines the export excess induced by iron fertilization. It was 10.8±4.9 mmol C m −2 d −1 and 0.9±0.7 mmol N m −2 d −1 at 100 m, and 14.2±7.7 mmol C m −2 d −1 and 2.0±1.3 mmol N m −2 d −1 at 200 m. This POC export excess was similar to those found during other studies of artificial (SOFeX) and natural (CROZEX) iron fertilization in the Southern Ocean. The examination of the export efficiency (defined as the ratio of export to primary production) revealed significant variability over the plateau related to the temporal decoupling of production and export during the demise of the bloom. On average, the export efficiency was lower over the plateau than in surrounding waters, suggesting that increased iron supply may increase total export but lower export efficiency. Our findings are very important for evaluating present and past carbon cycling in the Southern and global oceans and for assessing predictive scenarios of carbon cycling and budget.