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

[摘要] We evaluated the export fluxes of Particulate Organic Carbon (POC) in theSubtropical Frontal zone (STF) of the SW Pacific sector of the SouthernOcean. The site is characterized by enhanced primary productivity, which hasbeen suggested to be stimulated through so-called natural iron fertilizationprocesses where iron-depleted subantarctic water (SAW) mixes withmesotrophic, iron-replete subtropical water (STW). We adopted thesmall-volume ²³⁴Th method to achieve the highest possible spatialsampling resolution in austral late autumn-early winter, May–June, 2008.Inventories of chlorophyll-a, particulate ²³⁴Th and POC observed in theupper 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 wereof SAW origin with high macronutrients and high (S > 34.8) salinity waterswhich were of STW origin with low macronutrients. However, Steady-State²³⁴Th 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 thehigh and low salinity waters respectively. Bottle POC/Th ratios at the depthof 100 m were used to convert²³⁴Th fluxes into POC export fluxes. The derivedPOC flux did not appear to be enhanced in mid-salinity waters where theprimary 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 to14 ± 0.39 mmol C m⁻² d⁻¹ in high salinity waters and8.5 ± 0.66 mmol C m⁻² d⁻¹ in low salinity waters. This study thus impliedthat natural iron fertilization does not necessarily lead to an enhancementof POC export in STF regions.

[学科分类] 地球化学与岩石