Seasonal dynamics of dissolved iron in the marginal ice zone of South Atlantic and Southern Oceans
| dc.contributor.advisor | Roychoudhury, Alakendra N. | en_ZA |
| dc.contributor.author | Jansen van Vuuren, Lide | en_ZA |
| dc.contributor.other | Stellenbosch University. Faculty of Science. Dept. of Earth Sciences. | en_ZA |
| dc.date.accessioned | 2025-02-05T10:40:02Z | |
| dc.date.available | 2025-02-05T10:40:02Z | |
| dc.date.issued | 2024-12 | |
| dc.description.abstract | The Southern Ocean (SO) plays a central role in connecting various ocean basins while also sequestering ~40-50% of the global anthropogenic carbon uptake by oceans (Terhaar et al., 2021). The dynamic region of the marginal ice zone (MIZ, situated between sea ice and open ocean), is known for being a primary production hotspot while influencing surrounding biogeochemical processes (Tagliabue et al., 2017). Dissolved Fe (dFe), a critical micronutrient for phytoplankton, therefore directly linked to the efficiency primary productivity and therefore of the Biological Carbon Pump (Boyd et al., 2007). Improving our understanding of dFe distribution and the key physical and biological processes can provide a more comprehensive view of the SO's carbon cycle. Therefore, this study aimed to examine the spatial and temporal variability of dFe in the Southern Ocean's MIZ and its relationship to sea ice dynamics and biogeochemical processes. Samples were collected at 21 stations during two oceanographic expeditions (59.5 - 51.4 °S, Southern Ocean Seasonal Experiments, 2019). Samples were analysed using a SeaFAST preconcentration module and ICP-MS analysis method. The dataset covers 348 duplicate samples, which were combined with physical, chemical and biological data to aid in evaluate the processes governing Fe distribution. Sub-nanomolar dFe concentrations (nmol/kg) increased with depth during both seasons. Vertical profiles were characterised as nutrient distributions at most stations, with depleted dFe concentration in surface waters (0.053 - 0.082 nmol/kg in upper ~150 m depth) and variable concentrations in deeper waters (0.374-1.44 nmol/kg at depths of ~1000 to maximum 3750 m). Remineralisation depths ranged from the Mixed Layer Depth (MLD) base to ~200-500 m. MLDs shoaled from winter (115 ± 18 m, n=7) to spring (87.0 ± 38 m, n=17 2019). Spring MLD’s also shoaled northwards below the pack ice (59.5 – 55.0 °S), due to sea ice melt and SML freshening (shoaling from 96.0 ± 30 m to mean 44.0 ± 18 m, n=3). An overall deepening of the MLD was seen when comparing stations under ice to those in the open ocean. Macro-nutrients (NO3-, PO43- and Si(OH)4) and oxygen proxies were utilised for internal water column remineralisation estimates. Integrated remineralised dFe [dFerem] stock ranged from 6.06 - 156 μmol/m2 (n=296), increasing northwards and comparable with previous studies (Tagliabue et al., 2014; Van Horsten, 2022). When looking at some of the internal processes (e.g., remineralisation and scavenging) influencing dFe distribution, a surprising lack of winter replenishment was noted. Demonstrated by higher dFe concentrations in spring compared to winter (dFeSML,Winter = 0.190 ± 0.12 nmol/kg, n=26; dFeSML,Spring =0.340 ± 0.17 nmol/kg, n=53). Early spring sampling thus still showed an end-of-winter signal of winter replenishment before the onset of depleting, semi-consistent phytoplankton blooms during spring and summer. Some phytoplankton blooms were observed in spring (19-22nd October and 6-9th November), with chlorophyll-a (Chl-a) exceeding 2.67 mg/kg (n=101), while moderate dFe concentrations (0.350 ± 0.12 nmol/kg, n=37) prevailed. In combination with low Apparent Oxygen Utilisation (AOU) values, this suggests an additional source of dFe to the region other than internal cycling. This thesis provided some of the first insights into the remote MIZ of the SO. Remineralisation estimates highlight the prominent role of internal cycling in the distribution of dissolved iron within this remote region. While hydrothermal vents, were noted as potential source for dFe to this region, internal cycling, winter entrainment, and upwelling were concluded as main drivers of Fe dynamics within this region. In addition to adding to the existing global dataset, this study improved understanding of dFe distributions within this remote, yet pivotal HNLC region. These findings give new perspectives on potential Fe sources, challenging existing theories and opening new avenues for underexplored processes and their global implications. | en_ZA |
| dc.description.version | Masters | en_ZA |
| dc.format.extent | 124 pages : ill. | |
| dc.identifier.uri | https://scholar.sun.ac.za/handle/10019.1/131679 | |
| dc.language.iso | en | en_ZA |
| dc.publisher | Stellenbosch University | en_ZA |
| dc.rights.holder | Stellenbosch University | en_ZA |
| dc.title | Seasonal dynamics of dissolved iron in the marginal ice zone of South Atlantic and Southern Oceans | en_ZA |
| dc.type | Thesis | en_ZA |