Dissertations / Theses on the topic 'Coastal carbon cycling'

Pacific Northwest coastal wetland extent has been significantly reduced due to development. To understand the effects of land use change on carbon cycling in coastal wetlands, we compared soil carbon dynamics in restored, disturbed (by diking or draining), and reference wetlands in both freshwater and saline conditions in Coos Bay, Oregon. We quantified soil carbon pools, measured in situ fluxes of methane (CH4) and carbon dioxide (CO2), and estimated sediment deposition and carbon sequestration rates. We found that land use change influences carbon cycling and storage in coastal wetlands. The disturbed marshes have likely lost all their organic material after draining or diking, except for a shallow A horizon. The restored marsh in situ CH4 and CO2 fluxes were intermediate between the disturbed and reference marshes. Generally, restored marshes showed a partial return of carbon storage functions, or an indication that reference level functions may be achieved over time.

Geochemical analyses of Holocene coastal sediments from eastern England were made to better understand the cycling of organic carbon and nutrients in the coastal zone in the past, present and future. Sediments and peat were deposited in freshwater marshes, saltmarshes and intertidal mud- and sand-flat environments that were much more extensive during the Holocene than they are at present. The reduction in these areas, largely through human activities, has decreased the potential annual accumulation and storage of organic carbon, nitrogen and phosphorus associated with sediments. While the carbon and nitrogen contents of modem intertidal environments are similar to Holocene intertidal areas, phosphorus is enriched in modem sediments by up to a factor of two. Budgets of nitrogen and phosphorus cycling in Fenland, eastern England, suggest that the Holocene estuaries in this area were sinks of nutrients from the North Sea despite nitrogen isotopic evidence suggesting that nitrogen buried in freshwater marshes was predominantly terrestrially derived. The present-day estuaries are sources of nutrients to the North Sea as riverine loads and atmospheric deposition are much higher than during the Holocene and sedimentation is also greatly reduced. The southern North Sea is probably autotrophic, in contrast to the coastal zone global average which is heterotrophic. The major differences between these two areas are: 1) the global coastal zone receives much greater loads of riverine particulate matter than the southern North Sea, and 2) sedimentation in the global coastal zone occurs in large river deltas which are absent from the relatively small European estuaries, thus much of the sediment supplied to the North Sea is exported to the shelf edge. Approximately 4x 109 t C, 0.3 x 109 tN and 0.1 x 109 tP are currently stored in fine-grained Holocene sediments in the southern North Sea coastal zone.

The western Antarctic Peninsula (WAP) is a hotspot of climatic and oceanographic change, with a 6°C rise in winter atmospheric temperatures and >1°C warming of the surface ocean since the 1950s. These trends are having a profound impact on the physical environment at the WAP, with widespread glacial retreat, a 40% decline in sea ice coverage and intensification of deep water upwelling. The main objective of this study is to assess the response of phytoplankton productivity to these changes, and implications for the marine carbon and nitrogen cycles in the WAP coastal zone. An extensive suite of biogeochemical and physical oceanographic data was collected over five austral summer growing seasons in northern Marguerite Bay between 2004 and 2010. Concentrations and isotopic compositions ( 15N, 13C, 14C) of dissolved nitrate, dissolved inorganic carbon species, particulate nitrogen, organic carbon and chlorophyll a are used in the context of a substantial ancillary dataset to investigate nutrient supply, phytoplankton productivity and nutrient uptake, export flux and the fate of organic material, and the factors underpinning pronounced seasonal and interannual variability. High-resolution biogeochemical time-series data for surface and underlying seawater, sea ice brine, sediment trap material and coretop sediments allow detailed examination of carbon and nitrogen cycle processes under contrasting oceanographic conditions and the interaction between these marine processes and air-sea exchange of climate-relevant CO2. This study shows that the WAP marine environment is currently a summertime sink for atmospheric CO2 in most years due to high productivity and biological carbon uptake sufficient to offset the CO2 supply from circumpolar deep waters, which act as a persistent source of heat, nutrients and CO2 across the shelf. For the first time, CO2 sink/source behaviour is parameterised in terms of nitrate utilisation, by exploiting the relationship between CO2 and nitrate concentrations, and deriving the nitrate depletion at which surface ocean CO2 is undersaturated relative to atmosphere and carbon sink behaviour is achieved. This could have vast utility in examining CO2 sink/source dynamics over greater spatial and temporal scales than by direct CO2 measurements, of which availability is more limited. This study documents abrupt changes in phytoplankton productivity, nitrate utilisation and biological CO2 uptake during a period of rapid sea ice decline. In fact, nitrate utilisation, particulate organic matter production and biological CO2 uptake all decrease by at least 50 % between a sea ice-influenced, high productivity season and one of low sea ice and low productivity. The key driver of interannual variability in production and export of organic material is found to be upper ocean stratification and its regulation of light availability to phytoplankton. Productivity, CO2 uptake and export are maximal when stratification is sufficient to provide a stable well-lit surface environment for phytoplankton growth, but with some degree of mixing to promote export of suspended organic matter. Strong stratification causes intense initial production, but retention of suspended organic particles in the surface ocean induces a self-shading effect, and overall productivity, CO2 uptake and export fluxes are low. When stratification is weak, mixing of phytoplankton over a larger depth range exposes cells to a wider range of light levels and reduces photosynthetic efficiency, thus total productivity and CO2 uptake. A conceptual model is developed here, which attempts to describe the mechanism by which sea ice dynamics exert the principal control on stratification and therefore productivity and CO2 uptake at the WAP, with potential application to other regions of the Antarctic continental shelf. Although meteoric waters (glacial melt and precipitation) are more prevalent in surface waters throughout the study, sea ice meltwater variability is driven by large and rapid spring/early summer pulses, which stabilise the upper ocean and initiate phytoplankton growth. The timing and magnitude of these sea ice melt pulses then exert the key control on stratification and seasonal productivity. In a low sea ice year of this study, the sea ice trigger mechanism was absent and productivity was low. This strongly suggests that ongoing sea ice decline at the WAP and greater frequency of such low sea ice years is likely to drive a dramatic reduction in productivity and export, which would substantially reduce the capacity of the summertime CO2 sink in this region. Ongoing warming and ecosystem change are thus likely to have severe impacts on net CO2 sink/source behaviour at the WAP over the annual cycle, and the role of the Southern Ocean in regulating atmospheric CO2 and global climate. Finally, factors influencing the stable isotopic signature of particulate organic carbon ( 13CPOC), a common paleo-proxy, are assessed. 13CPOC is greatly influenced by seasonal shifts in diatom assemblages and isotopically heavy sea ice material, so cannot be used as a robust proxy for ambient CO2 in the coastal Southern Ocean.

Les rivières sont une source importante de constituants biogéochimiques pour les océans. Jusqu’à présent, les modèles océaniques globaux représentaient de manière inadéquate ou ignoraient simplement les apports continentaux de nutriments, de carbone, d’alcalinité provenant des rivières. En particulier, les perturbations anthropiques des apports fluviaux au cours du 20 ème siècle et leurs conséquences sur l’état physique et biogéochimique des océans - notamment la zone côtière - n’ont pas encore été analysées à l’aide d’un modèle global prenant en compte la circulation tridimensionnelle de l’océan. L’objectif principal de cette thèse était donc d’intégrer les apports biogéochimiques provenant des rivières dans un modèle océanique global afin d’améliorer la compréhension du cycle du carbone de l’océan côtier et son évolution au cours du 20 ème siècle. Dans un premier temps, mon travail a visé à l’amélioration des connaissances concernant le rôle des apports biogéochimiques fluviaux sur le cycle du carbone océanique à long-terme, en se focalisant sur la période préindustrielle. Pour cela, j’ai estimé les apports des rivières en utilisant des modèles permettant d’estimer l’érosion chimique et le transfert de matière organique desécosystèmes terrestres à l’océan. Ces apports fluviaux ont ensuite été ajoutés dans le modèle biogéochimique océanique HAMOCC et leurs impacts sur la production primaire océanique et les flux de CO2 entre l’atmosphère et l’océan ont été analysés. Les résultats nous ont permis de quantifier un dégazage de CO 2 préindustriel de 0.23 Pg C yr -1 pour l’océan global, principalement localisé à proximité de l’embouchure des rivières. Le modèle a également démontré l’existence d’un transfert inter-hémisphèrique de carbone, avec un plus grand apport des rivières à l’océan dans l’hémisphère nord, et un transfert de l’hémisphère nord à l’hémisphère sud où un dégazage net se produit. Une augmentation considérable de la production primaire océanique induite par les apports des rivières a également été prédite.La modélisation biogéochimique de l’océan côtier a ensuite été améliorée, en augmentant la vitesse de minéralisation de la matière organique dans les sédiments côtiers et en incluant la dégradation de la matière organique dissoute d’origine terrestre (tDOM) dans l’océan. Par ailleurs, notre analyse suggère un temps de résidence des eaux dans la zone côtière significativement plus courte (14-16 mois en moyenne) que celui estimé jusqu’à présent (>4 ans). Ce temps de courte résidence implique un transfert efficace de matière organiquede l’océan côtier à l’océan ouvert, un état autotrophe net de l’océan côtier, ainsi qu’un puit de CO 2 (0.06-0.08 Pg C yr -1) pour la période préindustrielle, contrairement aux hypothèses précédemment proposées dans la littérature.Dans le dernier chapitre, les perturbations océaniques induites par les changements de la concentration en CO 2 dans l’atmosphère, de la physique de l’océan et des apports biogéochimiques fluviaux au cours du 20 ème siècle ont été analysées. Les résultats indiquent que la réduction de production primaire nette (NPP) observée dans les océans tropicaux et subtropicaux, pourrait être entièrement compensée par une augmentation de la NPP dans l’océan austral et dans les systèmes côtiers de type «EBUS». Les simulations montrent aussi que l’augmentation des apports fluviaux provoque une augmentation de NPP océanique à l’échelle de l’océan côtier (+15 %) et à l’échelle globale (+ 4 %). En conclusion, cette thèse a permis de démontrer l’importance d’inclure la variabilité spatio-temporelle des apports fluviaux et des processus biogéochimiques de l’océan côtier dans la description du cycle du carbone océanique global. Les améliorations apportées au modèle océanique global HAMOCC permettront d’affiner les prédictions du rôle de l’océan dans le cycle du carbone au cours du 21 ème siècle.
River deliver vast amounts of terrestrially derived compounds to the ocean. These fluxes are of particular importance for the coastal ocean, which is recognized as a region of disproportionate contribution to global oceanic biological fluxes. Until now, the riverine carbon, nutrient and alkalinity inputs have been poorly represented or omitted in global ocean biogeochemistry models. In particular, there has yet to be a model that considers the pre-industrial riverine loads of biogeochemical compounds to the ocean, and terrestrial inputs of organic matter are greatly simplified in their composition and reactivities in the ocean. Furthermore, the coastal ocean and its contribution to the globalcarbon cycle have remained enigmatic, with little attention being paid to this area of high biological productivity in global model analysis of carbon fluxes. Lastly, 20 th century perturbations in riverine fluxes as well as of the physical and biogeochemical states of the coastal ocean have remained unexplored in a 3-dimensional model. Thus, the main goals of this thesis are to integrate an improved representation of riverine supplies in a global ocean model, as well as to improve the representation of the coastal ocean in the model, in order to solve open questions with respect its global contributions to carbon cycling.In this thesis, I first aimed to close gaps of knowledge in the long-term implications of pre-industrial riverine loads for the oceanic cycling of carbon in a novel framework. I estimated pre-industrial biogeochemical riverine loads and their spatial distributions derived from Earth System Model variables while using a hierarchy of state-of-the-art weathering and organic matter land-ocean export models. I incorporated these loads into the global ocean biogeochemical model HAMOCC and investigated the induced changes in oceanic biological production and in the air-sea carbon flux, both at the global scale and in a regional shelf analysis. Finally, I summarized the results by assessing the net land sink of atmospheric carbon prescribed by the terrestrial models, and comparing it to the long-term carbon outgassing determined in the ocean model. The study reveals a pre-industrial oceanic outgassing flux of 231 Tg C yr -1 ,which is found to a large degree in proximity to the river mouths. The model also indicates an interhemispheric transfer of carbon from dominant northern hemisphere riverine inputs to outgassing in the southern hemisphere. Furthermore, I observe substantial riverine-induced increases in biological productivity in the tropical West Atlantic (+166 %), the Bay of Bengal (+377 %) and in the East China Sea (+71 %), in comparison to a model simulation which does not consider the riverine inputs.In addition to considering supplies provided by riverine fluxes, the biogeochemical representation of the coastal ocean is improved in HAMOCC, by firstly increasing organic matter remineralization rates in the coastal sediment and by secondly explicitly representing the breakdown process of terrestrial dissolved organic matter (tDOM) in the ocean. In an analysis of the coastal fluxes, the model shows a much shorter residence time of coastal waters (14-16 months) than previously assumed, which leads to an efficient cross-shelf transport of organic matter and a net autotrophic state for both the pre-industrial timeframe and the present day. The coastal ocean is also revealed as a CO2 sink for the pre-industrial time period (0.06-0.08 Pg C yr -1 ) in contrary to to the suggested source in published literature. The sink is however not only caused by the autotrophic state of the coastal ocean, but it is likely also strongly influenced by the effects of biological alkalinity production, as well as both physical and biogeochemical characteristics of open ocean inflows.In the final chapter, 20 th century oceanic perturbations due to changes in atmospheric CO 2 concentrations and in the physical climate, and to increases in riverine nutrient supplies were investigated by using sequential model simulations. The model results show that the decrease in the net primary production (NPP) in the tropical and subtropical oceans due to temperature-induced stratification may be completely compensated by increases in the Southern Ocean and in Eastern Boundary Upwelling Systems (EBUS). The model also reveals that including increases in riverine supplies causes a global ocean NPP increase of +4 %, with the coastal ocean being a particularlystrongly affected region (+15 %).This thesis shows a strong necessity to represent spatio-temporal changes in riverine supplies and of the coastal ocean state in spatially explicit global models in order to assess changes of the global cycling of carbon in the ocean in the past and potentially in the future.
Doctorat en Sciences
info:eu-repo/semantics/nonPublished

Coastal wetlands store immense amounts of carbon (C) in vegetation and sediments, but this store of C is under threat from climate change. Accelerated sea level rise (SLR), which leads to saltwater intrusion, and more frequent periods of droughts will both impact biogeochemical cycling in wetlands. Coastal peat marshes are especially susceptible to saltwater intrusion and changes in water depth, but little is known about how exposure to salinity affects organic matter accumulation and peat stability. I investigated freshwater and brackish marsh responses to elevated salinity, greater inundation, drought, and increased nutrient loading. Elevated salinity pulses in a brackish marsh increased CO2 release from the marsh but only during dry-down. Elevated salinity increased root mortality at both a freshwater and brackish marsh. Under continuously elevated salinity in mesocosms, net ecosystem productivity (NEP) was unaffected by elevated salinity in a freshwater marsh exposed to brackish conditions (0 à 8 ppt), but NEP significantly increased with P enrichment. Elevated salinity led to a higher turnover of live to dead roots, resulting in a ~2-cm loss in soil elevation within 1 year of exposure to elevated salinity. When exposing a brackish marsh to more saline conditions (10 à 20 ppt), NEP, aboveground biomass production, and root growth all significantly decreased with elevated salinity, shifting the marsh from a net C sink to a net C source to the atmosphere. Elevated salinity (10 à 20 ppt) did not increase soil elevation loss, which was already occurring under brackish conditions, but when coupled with a drought event, elevation loss doubled. My findings suggest these hypotheses for the drivers and mechanisms of peat collapse. When freshwater marshes are first exposed to elevated salinity, soil structure and integrity are negatively affected through loss of live roots within the soil profile, leaving the peat vulnerable to collapse even though aboveground productivity and NEP may be unaffected. Subsequent dry-down events where water falls below the soil surface further accelerate peat collapse. Although saltwater intrusion into freshwater wetlands may initially stimulate primary productivity through a P subsidy, the impact of elevated salinity on root and soil structure has a greater deleterious effect and may ultimately be the factors that lead to the collapse of these marshes.

Permafrost-affected ecosystems including peat wetlands are among the most obvious regions in which current microbial controls on organic matter decomposition are likely to change as a result of global warming. Wet tundra ecosystems in particular are ideal sites for increased methane production because of the waterlogged, anoxic conditions that prevail in seasonally increasing thawed layers. The following doctoral research project focused on investigating the abundance and distribution of the methane-cycling microbial communities in four different polygons on Herschel Island and the Yukon Coast. Despite the relevance of the Canadian Western Arctic in the global methane budget, the permafrost microbial communities there have thus far remained insufficiently characterized. Through the study of methanogenic and methanotrophic microbial communities involved in the decomposition of permafrost organic matter and their potential reaction to rising environmental temperatures, the overarching goal of the ensuing thesis is to fill the current gap in understanding the fate of the organic carbon currently stored in Artic environments and its implications regarding the methane cycle in permafrost environments. To attain this goal, a multiproxy approach including community fingerprinting analysis, cloning, quantitative PCR and next generation sequencing was used to describe the bacterial and archaeal community present in the active layer of four polygons and to scrutinize the diversity and distribution of methane-cycling microorganisms at different depths. These methods were combined with soil properties analyses in order to identify the main physico-chemical variables shaping these communities. In addition a climate warming simulation experiment was carried-out on intact active layer cores retrieved from Herschel Island in order to investigate the changes in the methane-cycling communities associated with an increase in soil temperature and to help better predict future methane-fluxes from polygonal wet tundra environments in the context of climate change. Results showed that the microbial community found in the water-saturated and carbon-rich polygons on Herschel Island and the Yukon Coast was diverse and showed a similar distribution with depth in all four polygons sampled. Specifically, the methanogenic community identified resembled the communities found in other similar Arctic study sites and showed comparable potential methane production rates, whereas the methane oxidizing bacterial community differed from what has been found so far, being dominated by type-II rather than type-I methanotrophs. After being subjected to strong increases in soil temperature, the active-layer microbial community demonstrated the ability to quickly adapt and as a result shifts in community composition could be observed. These results contribute to the understanding of carbon dynamics in Arctic permafrost regions and allow an assessment of the potential impact of climate change on methane-cycling microbial communities. This thesis constitutes the first in-depth study of methane-cycling communities in the Canadian Western Arctic, striving to advance our understanding of these communities in degrading permafrost environments by establishing an important new observatory in the Circum-Arctic.
Permafrost beeinflusste Ökosysteme gehören zu den Regionen, in denen als Folge der globalen Erwärmung eine Veränderung des mikrobiell-kontrollierten Abbaus von organischem Material zu erwarten ist. Besonders in den Ökosystemen der feuchten Tundralandschaften kommt es zu einer verstärkten Methanpoduktion unter wassergesättigten und anoxischen Bedingungen, die durch immer tiefere saisonale Auftauschichten begünstigt werden. Die vorliegende Doktorarbeit kontenzentrierte sich auf die Untersuchung der Abundanz und Verteilung der am Methankreislauf beteiligten mikrobiellen Gemeinschaften in vier unterschiedlichen Polygonen auf der Insel Herschel und an der Yukon Küste in Kanada. Trotz des relevanten Beitrags der kanadischen West-Arktis am globalen Methanhaushalt, sind die dortigen mikrobiellen Gemeinschaften im Permafrost bisher nur unzureichend untersucht worden. Die zentrale Zielstellung der vorliegenden Arbeit besteht darin, die derzeitige Lücke im Verständnis der Kohlenstoffdynamik in der Arktis im Zuge von Klimaveränderungen und deren Bedeutung für den Methankreislauf in Permafrost-Ökosystemen zu schließen. Dies erfolgt durch Untersuchungen der am Abbau der organischen Substanz im Permafrost beteiligten methonogenen und methanothrophen mikrobiellen Gemeinschaften und ihrer möglichen Reaktionen auf steigende Umgebungstemperaturen. Um dieses Ziel zu erreichen, wurde ein Multiproxy-Ansatz gewählt, der die Analyse der Gemeinschaften mittels genetischen Fingerprintmethoden, Klonierung, quantitativer PCR und moderner Hochdurchsatzsequenzierung („Next Generation Sequencing“) beinhaltet, um die in der Auftauschicht der vier untersuchten Polygone vorhandenen Bakterien- und Archaeen-Gemeinschaften zu charakterisieren sowie die Diversität und Verteilung der am Methankreislauf beteiligten Mikroorganismen in unterschiedlicher Tiefe eingehend zu analysieren. Diese Studien wurden mit physikalisch-chemischen Habitatuntersuchungen kombiniert, da diese die mikrobiellen Lebensgemeinschaften maßgeblich beeinflussen. Zusätzlich wurde ein Laborexperiment zur Simulation der Klimaerwärmung an intakten Bodenmonolithen von der Insel Herschel durchgeführt, um die Veränderungen der am Methankreislauf beteiligten Gemeinschaften aufgrund steigender Bodentemperaturen zu untersuchen, sowie sicherere Voraussagen bezüglich der Methanfreisetzung in polygonalen Permafrostgebieten im Zusammenhang mit dem Klimawandel treffen zu können.

New Zealand forests have been reduced and degraded by gross removal, logging, and the effects of mammals introduced by Polynesian and European settlers. These changes increase the value of the remaining forests, so information on the effects of these disturbances will be useful to inform the management of forest protection. Integrated measurements of C and N cycling within forests can be obtained using foliar stable isotope ratios, which may detect differences between forests resulting from natural or anthropogenic disturbances. This thesis characterises the stable isotopic composition distribution and likely drivers of isotopic variation of vegetation in several central South Island forests, and provides a baseline for future ecological New Zealand studies of present and pre-human vegetation. The largest detected stable isotope variation in modern leaf material was that of δ15N values between the eastern and western podocarp-broadleaf forests. This variation was probably controlled by the lower soil N availability associated with the high rainfall of western forests causing low δ15N values (-8.5 ± 3.5 ‰) relative to an eastern forest (+1.6 ± 1.3 ‰) and global temperate forests (average -2.8 ± 2.0 ‰ (Martinelli et al. 1999)). The significant but slightly higher mean δ15N (0.6 ‰) of a historically selectively logged forest (Saltwater Forest) in comparison to the mean in an unlogged forest (Okarito Forest), on the West Coast, could be attributed to either alteration to N cycling from logging, site differences in topography, or local soil N differences between the forests. Although δ13C showed no significant geographical variation, the well-described ‘canopy effect’ was observed in all modern forests, manifested as a positive covariation between δ13C and vegetation height. Similarly, large taxon-specific differences were observed between δ15N and δ13C values in both modern and fossil leaves. Well-preserved fossil leaves, from sediments c. 4500 years B.P in Pyramid Valley, North Canterbury, had higher δ13C (4.2 ‰) and δ15N (2.5 ‰) values than modern vegetation from Riccarton Bush, Christchurch. The difference between ecosystems spanning several millennia probably reflects ecosystem-scale changes in C and N cycling within New Zealand forests following human arrival, particularly from the degradation caused by invasive animals.

A ciclagem de matéria orgânica (MO) no ambiente marinho é um processo-chave para o ciclo global de carbono. Os sedimentos costeiros são de suma importância para a ciclagem de carbono pois atuam como receptores de grandes quantidades de MO alóctone (i.e. terrestre) e autóctone (i.e. marinho). A miríade dos componentes orgânicos e suas diferentes características dificultam o entendimento das fontes de MO em ambientes costeiros. Este trabalho visou entender a origem e a composição da MO (através de biomarcadores lipídicos) e a dinâmica da comunidade microbiana (método ATP) em sedimentos superficiais de diferentes ecossistemas marinhos da costa sudeste do Brasil: (i) margem continental de Cabo Frio; (ii) sistema lagunar de Saquarema; (iii) áreas costeiras e a plataforma continental de Ubatuba; (iv) e a plataforma adjacente ao estuário de Santos. Os resultados apontaram uma origem predominantemente autóctone para a MO nestes sistemas, com contribuição terrestre reduzida e limitada à áreas próximas à costa. Processos oceanográficos e forçantes ambientais são cruciais para a composição da MO sedimentar e são discutidas para cada um dos ecossistemas estudados.
The cycling of the organic matter (OM) in the marine environment is a key process in the global carbon cycle. Coastal sediments are important to the global carbon cycle, since they receive large inputs from both marine and terrestrial OM. The myriad of organic compounds and their spectrum of reactivity complicate the understanding of OM sources in coastal environments. In this work, we aimed to access the origin and composition of the OM (through lipid biomarkers) and the microbial dynamics (ATP method) in surface sediments of diverse marine ecosystems from the SE Brazilian coast: (i) the continental margin off Cabo Frio; (ii) the lagoonal system of Saquarema; (iii) coastal and shelf areas from Ubatuba; and (iv) the continental shelf adjacent to Santos estuary. The results showed a dominance of autochthonous OM, with a minor fraction of the OM derived from terrestrial sources and restricted to areas close to the coast. Oceanographic processes and environmental forces are crucial to the composition of sedimentary OM and are discussed for each of those ecosystems.

Much research has been devoted to understanding the ocean carbon cycle because of its prominent role in controlling global climate. Coastal oceans remain a source of uncertainty in global ocean carbon budgets due to their individual characteristics and their high spatial and temporal variability. Recent attempts to establish general patterns suggest that temperate and high-latitude coastal oceans act as sinks for atmospheric carbon dioxide (CO2). In this thesis, carbon cycling in two Canadian coastal ocean regions is investigated, and the uptake of atmospheric CO2 is quantified. A combination of ship-board measurements and highly temporally resolved data from an autonomous mooring was used to quantify the seasonal to multi-annual variability in the inorganic carbon system in the Scotian Shelf region of the northwestern Atlantic for the first time. The Scotian Shelf, unlike other shelf seas at similar latitude, acts as a source of CO2 to the atmosphere, with fluxes varying over two orders of magnitude in space and time between 1999 and 2008. The first observations of the inorganic carbon system in the Amundsen Gulf region of the southern Beaufort Sea, covering the full annual cycle, are also presented. Air-sea CO2 fluxes are computed and a carbon budget is balanced. The Amundsen Gulf system acts as a moderate sink for atmospheric CO2; seasonal ice-cover limits winter CO2 uptake despite the continued undersaturation of the surface waters. Biological production precedes the ice break-up, and the growth of under-ice algae constitutes nearly 40% of the annual net community production. The Scotian Shelf may be described as an estuarine system with an outflow of surface water, and intrusion of carbon-rich subsurface water by a combination of wind-driven mixing, upwelling and convection, which fuels the CO2 release to the atmosphere. In contrast, Amundsen Gulf may be described as an anti-estuarine, or downwelling, system, with an inflow of surface waters and an outflow of subsurface waters. Wind-driven and convective mixing are inhibited by ice-cover and restrict the intrusion of carbon- and nutrient-rich waters from below, maintaining the CO2 uptake by the surface waters.
PhD Thesis

Coastal systems in semi-arid areas are characterised by complex physico-chemical processes involving mixing of marine and continental water sources as well as precipitation of evaporitic and carbonate minerals. The latter processes involving carbonate cycling also represent an important but currently poorly constrained component of the coastal carbon budget. This thesis fills important knowledge gaps in our understanding of water source mixing and local carbonate cycling in a semi-arid coastal system in South Australia – the Coorong, Lower Lakes and Murray Mouth (CLLMM) estuary, using selected alkaline earth metals (Ca and Sr) and their isotopes with the following research components: 1. Application of radiogenic Sr isotopes ((87)Sr/(86)Sr), stable Ca isotopes (δ(44/40)Ca) and elemental ratios, complemented by mineralogical analysis of top-sediment samples and geochemical (PHREEQC) modelling of carbonate saturations in the CLLMM waters to constrain the water source apportionment and local carbonate output in the Coorong lagoon. 2. Development and validation of high-precision stable Sr isotope analysis (δ(88/86)Sr) using thermal ionisation mass spectrometry (TIMS) and follow up calibration of δ(88/86)Sr in the CLLMM waters with respect to changing salinity and carbonate saturation states. 3. Application of (87)Sr/(86)Sr and δ(88/86)Sr tracers, along with elemental concentration data, to monitor seasonal variations (i.e., every 3 months) in water source mixing and carbonate dynamics (i.e., dissolution vs precipitation) in the CLLMM. 4. Reconstruction of palaeo-hydrology and salinity in the Coorong South Lagoon throughout the past ~2400 years, based on (87)Sr/(86)Sr, δ(88/86)Sr and Mg/Sr analysed in fossil bivalve shell species (Arthritica helmsi) collected from sediment cores. The above data were complemented by radiocarbon (14C) and pollen-based geochronological models. Overall, the results from the thesis showed that the modern North Lagoon waters are mainly sourced from the Southern Ocean, with transient freshwater inputs sourced from the River Murray and Lower Lakes and/or local groundwater discharge. In contrast, the hypersaline South Lagoon waters are basically highly evaporated ‘brackish waters’ with significant contribution of Sr from continental water sources. Importantly, stable Ca and Sr isotope tracers and water chemistry data indicate that the South Lagoon acts as a net sink for dissolved inorganic carbon (DIC) in the form of precipitated carbonate minerals (mostly aragonite). Both δ(44/40)Ca and δ(88/86)Sr in the CLLMM waters seem to be controlled by mass-dependent isotope fractionation, most likely related to carbonate dissolution and precipitation. Despite the current uncertainty regarding the role of local groundwater discharge on the chemistry of Coorong waters, the results indicate that an increased alkalinity supply (mainly from the Salt Creek) may locally promote CaCO3 precipitation and increase in δ(88/86)Sr of waters in the South Lagoon. Finally, the multi-proxy analysis ((87)Sr/(86)Sr, δ(88/86)Sr and Mg/Sr) of fossil shells revealed that over the past two millennia the South Lagoon waters were never purely marine but originally rather comprised brackish waters (estimated minimum salinities of ~6-23 PSU) with at least 60% contribution from continental water. Overall, the findings of this thesis improved our understanding of modern and past water source mixing and carbonate cycling in the CLLMM system and can hopefully benefit future water management strategies and plans.
Thesis (Ph.D.) -- University of Adelaide, School of Physical Sciences, 2021