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 Th-234: U-238 disequilibria

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 Th-234 method to achieve the highest possible spatial sampling resolution in austral late autumn-early winter, May-June, 2008. Inventories of chlorophyll-a, particulate Th-234 and POC observed in the upper 100 m were all elevated in the mid-salinity water type (34.5 34.8) salinity waters which were of STW origin with low macronutrients. However, Steady-State Th-234 fluxes were similar across the salinity gradient being, 25 +/- 0.78 ((1.5 +/- 0.047) x 10(3)) in the mid-salinity, and 29 +/- 0.53 ((1.8 +/- 0.032) x 10(3)) and 22 +/- 1.1 Bq m(-2) d(-1) ((1.3 +/- 0.066) x 10(3) 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 Th-234 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

Influência da variabilidade ambiental e ecotipos emiliania huxleyi em reconstruções de temperatura derivadas de alkenona no oceano sul subantártico

Long-chain unsaturated alkenones produced by haptophyte algae are widely used as paleotemperature indicators. The unsaturation relationship to temperature is linear at mid-latitudes, however, non-linear responses detected in subpolar regions of both hemispheres have suggested complicating factors in these environments. To assess the influence of biotic and abiotic factors in alkenone production and preservation in the Subantarctic Zone, alkenone fluxes were quantified in three vertically-moored sediment traps deployed at the SOTS observatory (140°E, 47°S) during a year. Alkenone fluxes were compared with coccolithophore assemblages, satellite measurements and surface-water properties obtained by sensors at SOTS. Alkenone-based temperature reconstructions generally mirrored the seasonal variations of SSTs, except for late winter when significant deviations were observed (3–10 °C). Annual flux-weighted averages in the 3800 m trap returned alkenone-derived temperatures ~1.5 °C warmer than those derived from the 1000 m trap, a distortion attributed to surface production and signal preservation during its transit through the water column. Notably, changes in the relative abundance of E. huxleyi var. huxleyi were positively correlated with temperature deviations between the alkenone-derived temperatures and in situ SSTs (r = 0.6 and 0.7 at 1000 and 2000 m, respectively), while E. huxleyi var. aurorae, displayed an opposite trend. Our results suggest that E. huxleyi var. aurorae produces a higher proportion of C37:3 relative to C37:2 compared to its counterparts. Therefore, the dom inance of var. aurorae south of the Subtropical Front could be at least partially responsible for the less accurate alkenone-based SST reconstructions in the Southern Ocean using global calibrations. However, the observed correla tions were largely influenced by the samples collected during winter, a period characterized by low particle fluxes and slow sinking rates. Thus, it is likely that other factors such as selective degradation of the most unsaturated alkenones could also account for the deviations of the alkenone paleothermometer.info:eu-repo/semantics/publishedVersio