Insignificant enhancement of export flux in the highly productive subtropical front, east of New Zealand: a high resolution study of particle export fluxes based on ²³⁴Th: ²³⁸U disequilibria

Abstract. We evaluated the export fluxes of Particulate Organic Carbon (POC) in the Subtropical Frontal zone (STF) of the SW Pacific sector of the Southern Ocean. The site is characterized by enhanced primary productivity, which has been suggested to be stimulated through so-called natural iron fertilization processes where iron-depleted subantarctic water (SAW) mixes with mesotrophic, iron-replete subtropical water (STW). We adopted the small-volume ²³⁴Th method to achieve the highest possible spatial sampling resolution in austral late autumn-early winter, May–June, 2008. Inventories of chlorophyll-a, particulate ²³⁴Th and POC observed in the upper 100 m were all elevated in the mid-salinity water type (34.5 < S < 34.8), compared with low salinity waters (S < 34.5) which were of SAW origin with high macronutrients and high (S > 34.8) salinity waters which were of STW origin with low macronutrients. However, Steady-State ²³⁴Th fluxes were similar across the salinity gradient being, 25 ± 0.78 ((1.5 ± 0.047) × 10³) in the mid-salinity, and 29 ± 0.53 ((1.8 ± 0.032) × 10³) and 22 ± 1.1 Bq m⁻² d⁻¹ ((1.3 ± 0.066) × 10³ dpm m⁻² d^–1) in the high and low salinity waters respectively. Bottle POC/Th ratios at the depth of 100 m were used to convert ²³⁴Th fluxes into POC export fluxes. The derived POC flux did not appear to be enhanced in mid-salinity waters where the primary productivity was inferred to be the highest at the time of sampling, with a flux of 11 ± 0.45 mmol C m⁻² d⁻¹, compared to 14 ± 0.39 mmol C m⁻² d⁻¹ in high salinity waters and 8.5 ± 0.66 mmol C m⁻² d⁻¹ in low salinity waters. This study thus implied that natural iron fertilization does not necessarily lead to an enhancement of POC export in STF regions.