Modern-ocean ground-truthing of planktic foraminifer nitrogen isotopes : a proxy for surface ocean nutrient conditions
| dc.contributor.advisor | Roychoudhury, Alakendra N. | en_ZA |
| dc.contributor.advisor | Fawcett, Sarah E. | en_ZA |
| dc.contributor.advisor | Sigman, Daniel M. | en_ZA |
| dc.contributor.advisor | Haug, Gerald H. | en_ZA |
| dc.contributor.advisor | Schiebel, Ralf | en_ZA |
| dc.contributor.advisor | Ren, Haojia | en_ZA |
| dc.contributor.advisor | Martínez-García, Alfredo | en_ZA |
| dc.contributor.author | Smart, Sandi M. | en_ZA |
| dc.contributor.other | Stellenbosch University. Faculty of Science. Dept. of Earth Sciences. | en_ZA |
| dc.date.accessioned | 2020-01-10T16:32:14Z | |
| dc.date.accessioned | 2020-04-28T12:01:54Z | |
| dc.date.available | 2020-01-10T16:32:14Z | |
| dc.date.available | 2020-04-28T12:01:54Z | |
| dc.date.issued | 2020-04 | |
| dc.description | Thesis (PhD)--Stellenbosch University, 2020. | en_ZA |
| dc.description.abstract | 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. | en_ZA |
| dc.description.abstract | AFRIKAANSE OPSOMMING: Die stikstof (N) isotoopverhoudings (δ15N) van organiese materiaal wat vasgevang is in die fossielskulpe van planktoniese foraminifere, soöplankton wat die boonste lae van die oseaan bewoon, verskaf ‘n nuwe perspektief waardeur die skakel tussen die afname in biologiese voedingstowwe in die oppervlakwater van die oseaan en die wêreldklimaat van die verlede bestudeer kan word. Hierdie tesis gebruik die moderne oseaan as ‘n proefterrein om die bepalende faktore te karakteriseer wat die δ15N bepaal van lewende en onlangs afgestorwe foraminifere in twee kontrasterende voedselstelsels: die voedingstofarme subtropiese Noord-Atlantiese Oseaan en die Suidelike Oseaan, waar die voedingstowwe in die oppervlakwater nooit heeltemal verbruik word nie. In beide hierdie omgewings is daar geen sistematiese verskil te bespeur tussen die δ15N van die oorgrote meerderheid foraminifere-weefsel of net dié wat in skulpdoppe gehul is nie, wat die gebruik van die δ15N, wat in skulpdoppe gehul is, as aanwyser van lewende foraminifere ondersteun. In die Sargasso-see, waar nitraat uitgeput is, stel foraminifere spesies met dinoflagellate (alge) simbionte, wat in die oppervlakkige waters woon, by benadering die δ15N vas van die nitraat wat aan die oppervlakte verskaf word (2.6‰). Daarteenoor het foraminifere spesies, wat dieper in die see woon sonder dinoflagellate, ‘n hoër δ15N (~3.6‰). Hierdie bevindinge stem ooreen met vorige grondslaggewende pogings in die lae breedteliggings in die oseaan, wat die gasheer-simbionte herwinning van lae δ15N ammonium impliseer. Vergelykings tussen die foraminifere-monsters vanuit die oppervlakkige oseaan (lewend), middel-diepte (dalend) en die seebodem (onlangs neergedaal) onthul ‘n swak (~0.6‰) toename in die δ15N, wat in skulpdoppe gehul is, gedurende die neerdaling deur die boonste 500 m van die waterkolom. Dit kan moontlik toegeskryf word aan die verlies van lae δ15N skulpe of skulpgedeeltes. Daar word egter geen verdere veranderinge in die δ15N bespeur wanneer die foraminifere in die sedimente geïnkorporeer word nie. Hierdie tesis bied die eerste grondslaggewende studie wat uitgevoer is in die hoë breedteliggings van die nitraatryke oseaan. Die data toon ruimtelike tendense in die laatsomer foraminifere δ15N wat ooreenstem met die suid-na-noord afname (en δ15N toename) in nitraat regoor die Suidelike Oseaan. Foraminifere δ15N verskil egter in sy afwyking van die nitraat wat opgeneem word in die Subantarktiese oppervlakswaters. Die δ15N volg eerder die deeltjies wat deur die foraminifere opgeneem word as voedsel. Die beskikbaarheid van hierdie deeltjies is seisoenaal - dit neem toe in die winter as gevolg van ontbinding en dit neem af in die somer as gevolg van laatsomer N herwinning. Dus bevat die foraminifere nie ‘n direkte rekord van die δ15N van nitraat wat in die boonste laag van die oseaan opgeneem word, soos voorheen vermoed is nie, maar weerspieël dit eerder ‘n meer ingewikkelde wisselwerking van die N siklusprosesse. Ten spyte van alles wat uit hierdie en ander grondslaggewende werk geleer is oor die foraminifere-gebonde δ15N aanwyser, bly een van die grootste uitdagings die interpretasie daarvan – dit is nog onduidelik of die omvang van die nitraat assimilasie isotoop-effek (die graad van verdeling van die isotoop wat plaasvind wanneer die nitraat deur die fitoplankton opgeneem word) deur die tye verskil. Om hierdie vraag te beantwoord, is die opname van die isotoop-effek van nitraat assimilasie in die moderne oseaan en om vas te stel of hierdie parameter verskil tussen omgewings ‘n goeie eerste stap. Voorlopige beraminge vanuit ‘n biogeochemiese model van die Suidelike Oseaan, wat seisoenale faktore insluit, dui op ‘n isotoop-effek van ~8 ‰ vir Subantarktiese nitraatverbruik, >2‰ hoër as wat bepaal is vir die naburige Antarktiese waters. Die bevestiging van die bevindinge van ‘n ruimtelik-veranderlike isotoop-effek behoort ‘n prioriteit te wees, aangesien dit ‘n sleutelparameter is wat benodig word om die historiese benutting van voedingstowwe van enige paleo- δ15N-argief te rekonstrueer. Tesame bied die ondersoeke oor die modern oseaan wat in hierdie tesis gedetailleer word, ‘n positiewe, maar meer genuanseerde vooruitsig vir die foraminifere-gebonde δ15N aanwyser, wat die lens verskerp waardeur ons eertydse oseaanproduktiwiteit en -klimaat bestudeer. | af_ZA |
| dc.description.version | Doctoral | en_ZA |
| dc.format.extent | xv, 186 leaves : illustrations (some color), map | |
| dc.identifier.uri | http://hdl.handle.net/10019.1/107746 | |
| dc.language.iso | en | en_ZA |
| dc.publisher | Stellenbosch : Stellenbosch University | en_ZA |
| dc.rights.holder | Stellenbosch University | en_ZA |
| dc.subject | Oceanography | en_ZA |
| dc.subject | Mineral cycle (Biogeochemistry) | en_ZA |
| dc.subject | Biogeochemistry | en_ZA |
| dc.subject | Nitrogen -- Isotopes | en_ZA |
| dc.subject | Planktic foraminifera -- Nitrogen content | en_ZA |
| dc.subject | UCTD | en_ZA |
| dc.title | Modern-ocean ground-truthing of planktic foraminifer nitrogen isotopes : a proxy for surface ocean nutrient conditions | en_ZA |
| dc.type | Thesis | en_ZA |