Abstract
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.
Highlights
The Subtropical Front (STF) (Fig. 1a) is a circum-global oceanographic feature, typically between about 35◦ S and 45◦ S, where cold, high macro-nutrient, iron-limited subantarctic waters (SAW) mix with warm, low macro-nutrient, subtropical waters (STW) (Longhurst, 1998; Orsi et al, 1995)
Such enhancement in primary production (PP) in the STF zone has been suggested to be induced by natural iron fertilization processes, with the iron sourced from atmospheric deposition, shelf boundary exchange processes and/or mixing with iron-replete subtropical waters (Boyd et al, 1999, 2004; Pollard et al, 2009)
We show that 234Th-based Particulate Organic Carbon (POC) export is not significantly enhanced in the frontal zone, despite high Chl-a and inferred PP levels, compared to the adjacent SAW and STW
Summary
The Subtropical Front (STF) (Fig. 1a) is a circum-global oceanographic feature, typically between about 35◦ S and 45◦ S, where cold, high macro-nutrient, iron-limited subantarctic waters (SAW) mix with warm, low macro-nutrient, subtropical waters (STW) (Longhurst, 1998; Orsi et al, 1995). Elevated integrated production rates have been observed in the STF around 152◦ E off Australia in summer (Clementson et al, 1998) and in the South African sector at 20◦ E in winter (Froneman et al, 1999). Such enhancement in PP in the STF zone has been suggested to be induced by natural iron fertilization processes, with the iron sourced from atmospheric deposition, shelf boundary exchange processes and/or mixing with iron-replete subtropical waters (Boyd et al, 1999, 2004; Pollard et al, 2009). Like other STF zones, this site is the transition zone from mesotrophic STW in the North, characterized by the relatively high temperatures (summer >18 ◦C; winter >14 ◦C), salinities (>35.1) and dissolved
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
