Doctoral Degrees (Earth Sciences)

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Browsing Doctoral Degrees (Earth Sciences) by Author "Loock, Jean Christian"

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    Austral summer and winter trace metal distributions in the Southern Ocean and Antarctic seasonal sea ice
    (Stellenbosch : Stellenbosch University, 2021-03) Loock, Jean Christian; Roychoudhury, Alakendra N.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: The metabolic efficiency of carbon (CO2) sequestering microorganisms, such as marine phytoplankton, is subject to the availability of trace metal nutrients such as Manganese (Mn), Cobalt (Co) and Cadmium (Cd) among others. However, much of the Southern Ocean is characterised by a paucity of trace metals, resulting in low productivity despite excess availability of the macronutrients Nitrate (NO3), Phosphate (PO4) and Silicate (SiO4). Areas exhibiting this paradoxical condition are classified as High Nutrient Low Chlorophyll (HNLC). The paucity is related to the regional remoteness, which increases vulnerability to weak inputs via traditional sources (e.g. atmospheric dust, riverine discharge, and continental margins). Thus, regional productivity may also rely on alternative supply pathways such as internal recycling, hydrothermal vent inputs, winter resupply, and indirect inputs from the seasonal sea ice inventories. However, our understanding of these processes, and by association their impact on global carbon cycling, remains relatively limited with large gaps in the database, particularly concerning the winter state and trace metals in sea ice. This study endeavoured to constrain these poorly understood facets by reporting observations from a novel trace metal dataset spanning three research cruises (1 summer, 2 winter). This includes the first winter data for Cd (Chapter 2), and Co and Mn (Chapter 3) from within the HNLC Southern Ocean (GEOTRACES GIPY05_e transect), and the first multi-element trace metal (Mn, Fe, Co, Cu, Ni, Cd and Pb) and sympagic (sea ice) phytoplankton community data for pancake sea ice in winter (WOCE IO6 transect). Concentrations are reported for the total dissolvable (T, unfiltered) and dissolved (D, <0.2 μm) fractions. Analogous distributions of TCd and DCd were found in the austral summer across the sector, albeit in the surface (and mixed layer) where TCd > DCd owing to depletion of DCd. This results in partitioning of the Cd fractions (ΔTM = total – dissolved) as DCd is incorporated into the TCd fraction. Larger ΔCd values correlated to productivity, but further dependent on the phytoplankton community structure (diatom links) and metabolically related trace metal concentrations. Winter reoccupation of the Antarctic Circumpolar Current (ACC) revealed that TCd and DCd concentrations had increased within the mixed layer, while ΔCd values decreased (consistent with lower winter productivity). Moreover, despite elevated mixed layer concentrations in winter, and weak uptake, DCd maintained a highly predictable linear correlation to PO4 (summer = 592x – 539, R2 = 0.93; winter = 584x – 515, R2 = 0.87). I propose that the relationship is dominated, or set, by mixing with the upwelling UCDW within the Antarctic Zone (AAZ) but may ultimately be susceptible to uptake in the northernly advecting surface waters. In summer, LCo concentrations ranged from ca. 5.15 pmol/kg in surface waters in the Sub-Tropical Zone (STZ) up to ca. 37.7 pmol/kg proximal to the Sub-Antarctic Front (SAF). High concentrations in the surface near the SAF were linked to the Antarctic Intermediate Water (AAIW). In summer, surface DMn concentrations ranged from ca. 0.034 nmol/kg at 60°S in the Weddell Gyre to ca. 0.96 nmol/kg in the STZ. Extremely low concentrations (<0.125 nmol/kg) also characterised surface waters near the Polar Front (PF) at 50°S and at 65°S in the Weddell Gyre, and may co-limit productivity in the region. Thus, supplementation of Mn even via sporadic hydrothermal inputs, such as at 54°S (DMn = 0.67 nmol/kg at 300 m) may be hugely beneficial. LCo and DMn concentration increased significantly in winter in the surface and mixed layer along with decreased ΔCo and ΔMn. LCo and DMn remained strongly coupled in summer and winter, albeit with linear correlation plots unique to the season. Ice cores drilled from pancake ice samples collected within the Antarctic marginal ice zone (MIZ) at 61°S, 30°E revealed that DFe concentrations, and to an extent DMn, were heavily elevated (up to 40 times) in sea ice relative to their open seawater concentrations. Cores were analysed in segments (3 x ± 10 cm). Two trace metal cores were melted using different methods (direct melting and matrix matched melting), but the results were consistent in all segments (no melting method bias) and compared well to ranges reported in external studies. Chlorophyll-a (Chl-a) concentrations were also significantly elevated (>10 times) in sea ice relative to under ice seawater. Trace metal and Chl-a concentrations followed a high (top), low (middle), high (bottom) distribution within the core segments (c-shaped distribution). The phytoplankton community structure was dominated by diatoms and Phaeocystis.