An unexpected delay in the onset of particle export during an iron fertilisation experiment in the Southern Ocean Book uri icon

abstract

  • While much has been learned about iron-limitation and low phytolankton biomass in high-nutrient, low-chlorophyll (HNLC) region, less is know about the effect of Fe on particle export. Here we present results from the first comprehensive study of particle dynamics during a mesoscale iron fertilisation experiment (Southern Ocean Iron RElease Experiment: SOIREE). We conducted time-series measurements of the natural particle tracer thorium-234 (half-life = 24.1 days) which indicated no change in this parameter between the iron-fertilized patch and the control station. Application of a non-steady-state model for super(234)Th to evaluate particle export from the upper 100 m revealed a super(234)Th flux of -40 plus or minus 760 dpm m super(-2)d super(-1). These results indicate minimal particle export within 14 days after the initial infusion of iron. We attribute this lack of response to colder water temperatures which were responsible for slower cell metabolism in phytoplankton and hence slower secondary responses of grazers and/or particle aggregation. Because of the significant delay in the onset of particle export, the magnitude of export following SOIREE remains unconstrained, yet is key to our understanding of the ocean's role in carbon cycling over the last glacial/interglacial cycle. Most importantly, our results indicate that short pulses of iron do not necessarily lead to a net export of carbon from the surface ocean. Therefore an iron-induced response in chlorophyll (increasing) or pCO super(2) (decreasing) does not necessarily lead to a proportional response from the biological pump. These results raise fundamental questions about the fate of organic carbon in iron-induced phytoplankton blooms in the Southern Ocean.

publication date

  • 2000