SOFeX: Southern Ocean Iron Experiments. Overview and Experimental Design

Current numerical models suggest that, over time scales of 102 - 105 years, atmospheric CO2 is controlled largely by conditions in the polar ocean. These polar regions act as the source for abyssal water in all the oceans, and under present conditions, are the main route by which the huge reservoir of dissolved carbon in the deep sea communicates with the atmosphere. As a consequence, the Southern Ocean in particular has a disproportionately large influence on atmospheric carbon dioxide. Although it represent only 10-20% of the ocean surface area, processes controlling production and export of carbon from these HNLC waters will relate directly to carbon cycling and climate change. To date several factors conspire to limit the uptake of carbon dioxide by phytoplankton in these waters. These factors include the availability of major nutrients (particularly silicate), light and iron. Iron enrichment experiments in the Southern Ocean are of particular utility in understanding carbon cycling at high latitudes. In a landmark experiment, three US ships (R/V Revelle, R/V Melville and RIB Polar Star) conducted progressive iron enrichment experiments in two distinct regions of the Southern Ocean: One north of the Antarctic Polar Fronts Zone in low silicate waters (<3 μM), and another to the south, below the APFZ in high silicate waters (~60 \muM) along 170°W. At both locations iron sulfate, infused with sulfurhexafluoride, was spread over 225 square kilometers and to the depth of the mixed layer. These experiments were reinfused with iron and tracked by multiple vessels over a seven week period to observe the physiological response of plankton, bacteria, changes in plankton community structure, the drawdown of major nutrients, the partitioning of carbon into POC, DOC, TCO2 and pCO2, and the flux of carbon from the surface waters. At this writing, values are being refined. The drawdowns in nutrients in the South Patch (NO3, 2.1 \muM; SiO2, 3.7 \muM and PO4, 0.25 \muM) seem to be concomitant with increases in chlorophyll (6μg/L) and decreases in TCO2. (17\muM). Low Si concentrations in the North Patch did not limit biological production and, in fact, may have eclipsed the South Pat6ch after increased horizontal mixing and patch dilution is factored in. In addition, this is the first Southern Ocean experiment in which carbon flux has been detected from the surface waters. This overview presentation will focus on the driving hypotheses, experimental design, execution, timeline and the progression of the major biogeochemical signals, providing some background for other talks in this session.