Browsing by Author "Marshall, Tanya"

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    Investigating the biogeochemistry of the Mozambique Channel thermocline using an optimum multiparameter analysis approach
    (2022) Harris, Eesaa; Fawcett, Sarah; Marshall, Tanya
    The ocean's thermocline represents the transition zone between the surface and deep ocean. Its formation is linked to vertical and horizontal diffusion, and it constitutes part of the ocean's wind driven circulation, playing a critical role in the lateral advection and vertical supply of nutrients. In the Indian Ocean, the topography, winds, and inter-ocean exchange make the basin's thermocline unique. Particularly complex thermocline circulation occurs in the Mozambique Channel, the highly dynamic region between the coast of south eastern Africa and Madagascar, owing to the confluence of tropical and subtropical regimes in the southwestern Indian Ocean. Our current understanding of the Mozambique Channel thermocline is largely derived from hydrographical analyses (i.e., conservative property relationships such as temperature-salinity), which do not resolve the influence of mesoscale features such eddies on the regional biogeochemistry. Additionally, the Mozambique Channel is one of the three source regions to the Agulhas Current, such that its biogeochemistry may influence the waters of this western boundary current. Here, the thermocline array approach within the optimum multiparameter (OMP) analysis framework was used to determine the contributions of different source waters to the thermocline across four transects sampled in the southwestern Indian Ocean, one at either end of the Mozambique Channel and two across the upper Agulhas Current. Three thermocline source regions proximate to the Mozambique Channel were identified (equatorial, tropical and subtropical) and used to initialise the OMP analysis. This localised approach was validated by the low standard deviations (average <20%) associated with perturbing the model parameters in a Monte Carlo analysis and the low residuals (average < 5%) associated with the model solution, allowing for an evaluation of relative biogeochemical changes across the region. A decline in the upper thermocline nutrient concentrations between the source regions and the transects can be explained by mixing with nutrient-deplete surface waters. By contrast, an increase in the nutrient concentrations of the thermocline evinces in situ remineralization, presumably following primary productivity in Mozambique Channel surface waters. The presence of tropical waters across the two Agulhas transects confirms a supply of tropical nutrients to the current, with the coincidence of significant tropical water contributions (up to 60%) and mesoscale eddies suggesting that these features represent a mechanism by which tropical thermocline water is supplied to the Agulhas Current region. A rise in the thermocline nitrate-to-phosphate ratios across the Agulhas transects but not in the Mozambique Channel transects indicates that nitrogen is added to the Agulhas Current region by local N2 fixation occurring in the subtropical waters south of the Mozambique Channel (25°S). This finding contradicts recent suggestions that the channel itself is a hotspot for N2 fixation. The fact that the OMP analysis applied at the regional scale captures the complexity and heterogeneity of the southwest Indian Ocean indicates that this approach can be used to quantitatively assess thermocline contributions to shallow nutrient cycling in hydrodynamically complex regions.
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    Nitrogen cycling in the South Atlantic and South Indian Oceans investigated using nitrate isotopes: implications for nutrient supply, ocean fertility, carbon export, and climate
    (2023) Marshall, Tanya; Fawcett, Sarah
    Bioavailable nitrogen (N) limits phytoplankton growth across much of the (sub)tropical ocean, thereby modulating ocean fertility and climate. Dinitrogen (N2) fixation is the dominant source of new N to the ocean and is thought to occur mainly in well-lit, warm, oligotrophic waters. The under-sampled South Atlantic and South Indian Ocean basins are predicted by models to host widespread N2 fixation; for the South Atlantic, this predication contradicts the limited available observations and for the South Indian, is yet to be confirmed by measurements. In this thesis, four new nitrate isotope datasets from the South Atlantic and South Indian Oceans are presented alongside coincident nutrient and hydrographic data, and other published nitrate isotope datasets. Combined, these data provide a means of quantifying the rate and distribution of N2 fixation, along with characterizing additional co-occurring N cycle processes, mechanisms of subsurface nutrient supply, and water mass circulation. Measurements of nitrate N isotope ratios (15N) and nutrient stoichiometry (i.e., nitrate to phosphate ratios; N:P) from a zonal transect of the tropical South Atlantic (at ~12S) and a meridional transect along the Angola margin (at ~12E) reveal an N2 fixation hotspot in the eastern tropical Angola Gyre. Here, thermocline nitrate 15N is low and N:P is high relative to the underlying source water and the western tropical basin thermocline. The N2 fixation rate estimated from the Angola Gyre nitrate 15N data of 1.4-5.4 Tg N.a-1 accounts for 28-108% of the rate predicted for the South Atlantic basin. These findings contradict recent model diagnoses of N2 fixation, which predict high rates in the western tropical basin and none to the east. The overlapping biogeography of a basin-wide P excess relative to N and bioavailable iron supplied locally from the Angola margin likely control N2 fixation in the Angola Gyre. Analogous conditions elsewhere in the ocean, such as in other eastern boundary shadow zones and retentive near-coast subtropical systems, should also favour N2 fixation. The western boundary current of the South Indian Ocean, the Agulhas Current, is the strongest boundary current on Earth, yet nutrient cycling in this subtropical system remains largely uncharacterized. Measurements of the dual isotope ratios (N and oxygen) of nitrate from within and upstream of the greater Agulhas region provide insights into regional circulation and N cycle dynamics. The nitrate isotopes reveal both local and remote signals of Indian Ocean N cycling such as denitrification in the Arabian Sea and partial nitrate assimilation in Southern Ocean surface waters, as well as evidence of local N2 fixation and coupled partial nitrate assimilation and nitrification. Using a one-box model to simulate the newly-fixed nitrate flux, the local N2 fixation rate for the greater Agulhas region is estimated to Thesis abstract be 7-25 Tg N.a-1; this value is the first observation-based N2 fixation rate estimate for the South Indian Ocean. Local N cycling imprints an isotopic signal on Indian Ocean nitrate that can be tracked beyond the Indian Ocean because it persists in Agulhas eddies that “leak” into the South Atlantic at the Agulhas Retroflection. If this signal is retained in plankton that sink to the seafloor, it could be used to reconstruct past Agulhas leakage, yielding quantitative insights into the strength of the Atlantic Meridional Overturning Circulation in the past. The Agulhas Current system, like other western boundary current systems, is characterised by high energy and turbulence. A novel application of the dual isotopes of nitrate reveals the occurrence of three (sub)mesoscale mechanisms of upward nitrate supply; entrainment at the edges of a mesoscale anticyclonic eddy, inshore upwelling likely driven by a frontal eddy, and overturning at the offshore edge of the current core likely driven by coupled mesoscalesubmesoscale instabilities. The intensity and (sub)surface expression of these nutrient supply events are not always apparent in the hydrographic data, highlighting the utility of the nitrate isotopes for exploring physical ocean processes. The conditions driving the nitrate supply mechanisms in the Agulhas region are common to western boundary currents, implying that the (sub)mesoscale vertical nitrate supply is quantitatively significant at the global scale. Additionally, these events of upward nitrate supply likely increase regional fertility in all western boundary current systems, with implications for the sustenance of higher trophic levels. Finally, increasing turbulence observed along mid-latitude western boundaries may enhance the upward nutrient supply to subtropical surface waters, and possibly compensate for the diminished productivity predicted as a result of increasing subtropical gyre stratification. Collectively, the work detailed in this thesis reveals the strong regionality of N cycling in the historically under-studied South Atlantic and South Indian Oceans, as well as the importance of interpreting biogeochemical data in the context of ocean dynamics across various scales. Improved predictions of N fluxes at the basin- and global scale, which are critical for estimating the ocean's CO2 sink and fertility, will require careful consideration of these southern basins so as not to mischaracterise their functioning, as has occurred in the past.