Insights into Southern Ocean carbon export from the δ¹³c of particles and dissolved inorganic carbon during the SOIREE iron release experiment

¹³C contents of total dissolved inorganic carbon (δ¹³C-DIC) and particulate organic matter (δ¹³C_org) were determined to examine the factors influencing phytoplankton ¹³C contents and carbon export from the SOIREE iron-induced algal bloom. Suspended particles sieved into 200, 70, 20, 5, and 1 micrometer (μm) size classes displayed an extremely large range in δ¹³C_org of 8‰. δ¹³C_org values increased from - 28‰ for the 1-5 μm class to a maximum of - 20‰ for the 20-70 μm class, which was dominated by the large pennate diatom Fragilariopsis kerguelensis. Larger particles (70-200 and > 200 μm) had similar δ¹³C_org to the smaller (1-5 and 5-20 μm) particles, reflecting both the presence of long narrow Thalassiothrix antarctica diatoms and zooplankton that grazed on small phytoplankton. Comparison of results inside and outside the bloom identified cell surface/volume ratio (mainly reflecting cell size) as the dominant control of phytoplankton δ₁₃C_org, with subsidiary roles for growth rate and seawater [CO₂]_aq. The SOIREE iron fertilization provoked an increase in the proportion of large (> 20 μm) diatoms. This increased the δ¹³C_org of the bulk suspended particles within the mixed layer, but there was minimal increase in the δ¹³C_org of sub-surface suspended particles and negligible change in the δ¹³C_org of particles obtained with sediment traps suspended below the bloom. This suggests that there was no increase in carbon export over the ∼ 13 day observation period. However, comparison to δ¹³C_org results from previous voyages, and to vertical changes in δ¹³3C-DIC, suggests that large diatoms control carbon export from the Antarctic Zone over the summer growth season. This result must be viewed with great caution as it is based on very sparse data and involves several assumptions.