Doctoral Degrees (Earth Sciences)

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Browsing Doctoral Degrees (Earth Sciences) by browse.metadata.advisor "Fawcett, Sarah E."

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    Modern-ocean ground-truthing of planktic foraminifer nitrogen isotopes : a proxy for surface ocean nutrient conditions
    (Stellenbosch : Stellenbosch University, 2020-04) Smart, Sandi M.; Roychoudhury, Alakendra N.; Fawcett, Sarah E.; Sigman, Daniel M.; Haug, Gerald H.; Schiebel, Ralf; Ren, Haojia; Martínez-García, Alfredo; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: The nitrogen (N) isotope ratios (δ15N) of organic matter trapped within the fossil shells of planktic foraminifera, upper-ocean dwelling zooplankton, are providing a new lens through which to examine the link between biological nutrient drawdown in oceanic surface waters and past global climate. This thesis uses the modern ocean as a testing ground to characterize the controls on the δ15N of living and recently living foraminifera in two contrasting nutrient regimes: the nutrient-poor subtropical North Atlantic and the Southern Ocean, where surface nutrients are never completely consumed. In both environments, no systematic difference between bulk foraminifer tissue and shell-bound δ15N is observed, supporting the use of shell-bound δ15N as an indicator of living foraminifera. In the nitrate-depleted Sargasso Sea, shallow-dwelling foraminifer species with dinoflagellate (algal) symbionts (average δ15N ~2.3‰) approximate the δ15N of the nitrate supplied to surface waters (2.6‰), while deeper dwellers without dinoflagellates have a higher δ15N (~3.6‰). These findings are consistent with earlier ground-truthing efforts in the low-latitude ocean, implicating host-symbiont recycling of low δ15N ammonium. Comparison between upper-ocean (living), mid-depth (sinking) and seafloor (recently deposited) foraminifer specimens reveals a weak (~0.6‰) increase in shell-bound δ15N during sinking through the upper 500 m of the water column, possibly due to the loss of low-δ15N shells or shell portions, but no further change in δ15N upon incorporation into the sediments. This thesis presents the first ground-truthing study conducted in the nitrate-replete high-latitude ocean. The data show spatial trends in late-summer foraminifer δ15N that are consistent with the south-to-north drawdown (and δ15N rise) of nitrate across the Southern Ocean. However, foraminifer δ15N varies in its offset from nitrate consumed in Subantarctic surface waters, instead tracking the δ15N of the foraminifer’s particulate food source, which rises (due to winter decomposition) and falls (due to late-summer N recycling) with the seasons. Therefore, foraminifera do not directly record the δ15N of nitrate consumed in the upper ocean (as previously thought), but rather reflect a more complex interplay of N cycling processes. Despite all that has been learned about the foraminifer-bound δ15N proxy from this and previous ground-truthing work, a major obstacle remains for its interpretation: It is not yet known whether the magnitude of the nitrate assimilation isotope effect (the degree of isotope partitioning during nitrate consumption by phytoplankton) has varied through time. A first step towards answering this question is mapping the isotope effect of nitrate assimilation in the modern ocean to determine whether this parameter varies among environments. Preliminary estimates from a seasonally resolved biogeochemical model of the Southern Ocean suggest an isotope effect of ~8‰ for Subantarctic nitrate consumption, >2‰ higher than that determined for neighbouring Antarctic waters. As a key parameter required for reconstructing past nutrient utilization from any paleo-δ15N archive, verifying this finding of a spatially variable isotope effect should be a priority. Taken together, the modern-ocean investigations detailed in this thesis present a positive but more nuanced outlook for the foraminifer-bound δ15N proxy, polishing the lens through which we view past ocean productivity and climate.