234Th-based carbon export during an ice-edge bloom: Sea-ice algae as a likely bias in data interpretation

Total 234Th and 238U vertical profiles in the water column were determined along a three-station transect perpendicular to the Eastern Weddell Sea ice shelf. The transect was sampled during pre-bloom (6–7 December 2003) and bloom (19–20 December 2003) conditions to follow changes occurring in the particulate organic carbon (POC) flux during an Antarctic phytoplankton ice-edge bloom. During the pre-bloom period, 234Th:238U ratios > 1 (up to 1.3) were measured just above the thermocline at all stations, and throughout the entire mixed layer at the ice-edge. These water column data appear to be the first which support the hypothesis that sea-ice algae could represent an input of 234Th-rich material in polar waters due to cellular uptake of the radionuclide and/or its adsorption onto the extracellular polymeric substances (EPS) produced by these organisms. A simple conceptual model is proposed to link observed 234Th:238U ratios to the input of 234Th-enriched sea-ice algae caused by ice-melting. The extensive, deep-water 234Th deficit observed during the second transect likely results from the combination of vertical particle export and water-seafloor exchange processes. POC steady-state (SS) fluxes out of the photic layer were negative or ∼ nil (− 11 to 2 mmol m− 2 d− 1) under pre-bloom conditions, probably due to ice-algae input at the ice-edge and remineralization elsewhere, whereas POC non-steady-state (NSS) fluxes were generally higher (7 to 33 mmol m− 2 d− 1) when the bloom was developing. We have estimated that these flux values could change by as much as 75% if a sea-ice algae 234Th input term were included in the simple one-box model used. This suggests that failure to consider the sea-ice algae compartment, when using 234Th as a proxy to assess carbon export in seasonally ice-covered regions, could at times lead to significant biases in the interpretation of 234Th-based particle flux estimates.