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The 8.2 ka event in Europe: timing, structure and climate impacts from speleothem palaeoclimate records

Kilhavn, Hege ( 2023-04)

The 8.2 ka event is a short-lived climate anomaly that occurred in the end of the early Holocene. The event was first recognized in the Greenland ice cores but has also been identified in marine and terrestrial records over wide parts of the globe. It is widely accepted that the event was triggered by input of freshwater from Lake Agassiz-Ojibway to the North Atlantic Ocean. Although the exact number of freshwater pulses, routing and volume are still debated, it is commonly thought that the freshwater that entered the North Atlantic Ocean was sufficient to disrupt the Atlantic Meridional Overturning Circulation. However, there are still uncertainties regarding the precise timing of the event and details about its climate impacts, even across Europe. The overall aim of this thesis if to advance our understanding of the precise timing and regional climate impacts of the 8.2 ka event in Europe through the use of speleothem palaeoclimate records. Speleothems can provide high-resolution records of the 8.2 ka event that are well-constrained by precise and accurate chronologies. This thesis is guided by two key aims. The first aim seeks to improve the spatial coverage of regional high-resolution records to reconstruct climate changes triggered by the 8.2 ka event across Europe and to achieve a more precise timing and duration for the event through the generation of new high-resolution palaeoclimate speleothem records. Two speleothem records were selected from western and eastern Europe: one from El Soplao Cave in northern Spain, and another from Ascunsa Cave in Romania. The Soplao speleothem record represents early Holocene palaeoclimate in northern Spain, an important region to study the impact of AMOC perturbations on south-western Europe. Throughout the entire record the oxygen isotope variability is related to changes in effective recharge. This is supported by the pattern of changes in the carbon isotopes, Mg/Ca and growth rate. The 8.2 ka event is marked as a negative excursion in the Soplao oxygen isotopes, starting at 8.19 +/- 0.06 ka BP and lasting until 8.05 +/- 0.05 ka BP, suggesting increased recharge at the time. Although this is supported by the other proxies, the amplitude of the changes in the other proxies is minor and largely within the realm of variability of the preceding 1000 years. Further, the shift to lower oxygen isotopes leads the other proxies, which we interpret as the imprint of change in the isotopic composition of the moisture source, associated with the meltwater flux to the North Atlantic. The Ascunsa speleothem record represents Holocene palaeoclimate in Romania, a region where the climate impacts of AMOC perturbations remain uncertain. The oxygen isotope variability is mainly driven by winter temperature changes throughout the entire record. This is supported by cave monitoring and other local speleothem records. The variation in carbon isotopes, trace elements (Mg/Ca and Sr/Ca) and growth rate is related to changes in effective recharge. Our speleothem record displays no significant and convincing evidence for an environmental change through the 8.2 ka event, which could potentially imply that this region was not significantly impacted by the event. The second aim investigates the consistency in the timing of the 8.2 ka event and the spatial reconstruction of its climate impacts across the North Atlantic region and Europe based on an evaluation of published speleothem and other palaeoclimate records. Twenty precisely dated and high-resolution, published speleothem oxygen isotope records were compiled to investigate the timing of the 8.2 ka event. We found that the event was synchronous across Europe, starting at 8.214 +/- 0.049 ka BP and lasting until 8.088 +/- 0.038 ka BP, consistent with the Greenland ice-core records, and with previously published reviews. Furthermore, a total of eighty published palaeorecords (including ice cores, speleothems, marine and lake cores) were compiled to improve our understanding of the climate impacts of the 8.2 ka event. The selected palaeorecords suggest cold conditions prevailed during the event, with the most pronounced change occurring over Greenland, the North Atlantic Ocean, western Europe and the Alps. The precipitation patterns appear to be more complex, with wetter conditions along the North Atlantic seaboard, and a tendency of drier conditions in central and southern Europe. Additionally, several records suggest that no significant environmental changes occurred, especially in the Mediterranean region and eastern-central Europe. Furthermore, we argue that the isotopically depleted values associated with the freshwater pulse have played a major role in influencing the oxygen isotope values during the event, especially in regions that are closely connected to the North Atlantic Ocean. Overall, this thesis provides key insights into the precise timing and climate impacts triggered by the massive meltwater flux at the origin of the 8.2 ka event. An improved understanding of the dynamic links between meltwater in the North Atlantic Ocean and climate response across Europe is important for improving climate model projections of future meltwater episodes from the Greenland ice sheet related to the current global warming, and the projected impacts on European climate.
The Role of Lithospheric-Deep Mantle Interactions in the Evolution and Subduction Dynamics of Arc-Continent Collision settings

RODRIGUEZ CORCHO, ANDRES FELIPE ( 2023-03)

Arc-continent collision is the fundamental process that controls the growth of the continental crust and allows the preservation of critical minerals and energy resources. Conceptual models of arc-continent collision show that collision causes orogenic growth and basin formation, which often occur simultaneously. However, these conceptual models account for the processes occurring on the crust-lithosphere, and not the interactions between the lithosphere and the mantle, hereafter called lithospheric-deep mantle interactions. This thesis aims to understand the effect of lithospheric-deep mantle interactions in the evolution of arc-continent collision by using numerical modelling. The mechanical models presented here explore the effect of material properties such as density/rheology, and geometric boundary conditions, such as angle of collision, in the stress evolution, the mantle flow, and the topography evolution of arc-continent collisional orogens and their associated sedimentary basins. To gain this theoretical insight, I use numerical models of subduction in both two and three dimensions, which couple the dynamics of the lithosphere and the mantle with the landscape evolution at the surface. Modelling results show that a buoyancy contrast of 3% between the arc and the margin controls the style of collision, and that arc buoyancy causes gravitational spreading. This gravitational flow applies a body (buoyancy) force to the adjacent subducting and overriding plates, which decreases the stress transmission (mechanical coupling) from the subducting to the overriding plate, causing lithospheric extension in the continental plate and basin formation/infilling and triggers topography decay in the orogen. Slab-folding and slab-steepening within the mantle, cause episodes of increased compression/shortening in the basin above, poloidal mantle flow, and mechanical coupling in the trench. Increased compression opposes arc body forces and triggers topographic growth in the orogen (0.008-0.310 mm/yr), which increases the sediment supply to the basin in the continental plate. However, the results show that this effect is modulated by the erosion (up to 0.2 mm/yr) and sedimentation (0.03-0.3 mm/yr) rates in the orogen and the basin. The angle of collision triggers a toroidal component in the mantle flow by increasing shortening in the leading edge, which increases the magnitude of body forces and counteracts the poloidal flow. This toroidal flow causes rotation and arcuate geometries in the subducting plate, and is larger in simulations with less acute angles of collision (65-85 degrees). The results of our numerical modelling highlight the competition between arc buoyancy forces and the increased tectonic forces caused by slab-folding in the evolution of arc-continent collision; and how this competition is modulated by erosion in the orogen and sedimentation in the basin. These findings have implications for the formation and preservation of critical energy and mineral resources essential for the transition to a green economy.
Entrepreneurial Javanese migration to Eastern Indonesia: Making a living in Kupang, East Nusa Tenggara

Akhmad, Fandi ( 2023-03)

This thesis focuses on the relationship between trans-local migration, livelihood change, ethnic informal entrepreneurship, and regional development in Eastern Indonesia. Despite the growing literature on entrepreneurial migration in Indonesia, there is limited research on entrepreneurial migration to regional urban centres, where the economic competition is relatively low compared to metropolitan areas. This thesis addresses this research gap by examining the case of Javanese entrepreneurial migration to the town of Kupang. This thesis aims to understand the motivations, experiences, and consequences of Javanese entrepreneurial migration to Kupang. The Javanese are a majority ethnic group in Indonesia, originating from the most populated island in the country. In the last three decades, there has been an increasing flow of spontaneous out-migration—of mainly landless farmworkers—from Java to set up and/or work in informal enterprises in less populated urban centres in Indonesia's outer islands. To address this phenomenon, this thesis asks and addresses three key research questions: 1) Why do Javanese migrate from their hometowns in Java to Kupang? 2) How do the entrepreneurial Javanese migrants navigate the challenges and opportunities in Kupang's informal sector? 3) How has entrepreneurial migration shaped the lives of individual migrants, their families, and their broader communities in Java and Kupang? To answer these questions, this thesis employs a mixed-methods approach by combining secondary data sources and primary data gathered through fieldwork. Firstly, this thesis used secondary data from the 2010 Population Census obtained from BPS-Statistics Indonesia to generate a sampling frame for a representative sample of Javanese migrant entrepreneur households in Kupang. To follow, a household survey (n=344) and in-depth interviews with a sub-sample of the survey respondents (n=28) were conducted in Kupang between June and December 2020, supplemented by research diaries and participant observations. The key findings of the thesis suggest that Javanese migration to Kupang was motivated by factors such as low education levels, the prospect of better economic opportunities and hospitable locals, the desire to escape agricultural work in Java, and the need to provide for their children's education. The majority of Javanese migrants in the study engaged in "step migration," having previous migration experience before moving to Kupang, while others made direct journeys or moved to Kupang after losing their jobs during the financial crisis of 1998 or leaving East Timor after its independence in 1999. The research further provides a rich overview of Javanese informal business practices, including the different modes and segments of businesses based on district of origin, as well as the constraints, opportunities, and accomplishments experienced in running a business in Kupang's traditional markets and surrounding neighbourhoods. Fieldwork findings show how Javanese migrants capitalise on Kupang's informal business potential by utilising what I refer to as migrant infrastructure: a complex web of supporting actors and factors, including Javanese migrant communities and associations, Kupang's traditional markets, internet communication, and local opportunities. Finally, this thesis demonstrates that a set of complex migratory consequences across localities and generations have facilitated a growing flow of Javanese entrepreneurial migration to Kupang. Building on these findings, the central argument in this thesis is that the Javanese engaged in this risky entrepreneurial migration as a livelihood strategy, moving away from mostly agricultural labours in rural Java to seek opportunities in the less competitive informal sector in growing urban centres across Eastern Indonesia like Kupang. Although they encountered a range of hardships that resulted in significant material losses, those who persisted in engaging in Kupang informal businesses were rewarded with better income and broader socio-economic benefits to their communities both back in Java and in Kupang. These largely beneficial consequences associated with ethnic-based entrepreneurial migration drive the consolidation of migrant infrastructure that facilitates growth in the flow and socio-economic importance of this migration corridor. Given that many theoretical frameworks on entrepreneurial migration are based on the socio-demographic and economic contexts of migrant-receiving countries in the West, this thesis contributes to the expanding body of knowledge on internal migration, ethnic informal entrepreneurship, and regional development in the Global South. While refraining from policy analysis, the empirical insights of the thesis can help inform policies aiming to promote entrepreneurial and skill support, employment generation, social protection, and local development in Indonesia and other (lower) middle-income countries in the region.
Retelling Hidden Narratives: An Ethnographic Study of Youth Navigating Disruptive Events in Indian Informal Settlements

De Geest, Febe Phoebe Jorga Maria ( 2023-02)

In the context of ongoing global crises, including climate change and growing urban inequality, the urban poor in India increasingly face disruptive events in their everyday lives. Therefore, it is crucial to understand how those disproportionately impacted by disruptions, such as marginalised youth, navigate spatial and temporal events in their lives. This thesis examines the social and political actions of youth living in informal settlements during social, political, and environmental disruptions, including flooding, eviction, Covid-19 lockdown, and heatwaves. It draws on 14 months of ethnographic fieldwork in three informal settlements in two Indian cities to understand how marginalised people retell and give meaning to these events. In this thesis, I illustrate how social, environmental, and political disruptive events might offer marginalised youth opportunities to perform ‘out of the ordinary’ subjectivities and collective action, challenging local power relations, social hierarchies, and norms. I show how young men and women articulate narratives and hold onto secrets about these events to socially reposition themselves, highlight possibilities, build solidarities, give meaning to their lives, and challenge stereotypes about them. While these narratives and secrets might enhance spaces for agency, I also discuss how they could also generate tension and hardship, and reinforce and strengthen existing social inequalities, norms, and power structures. Moreover, I highlight how these (sometimes hidden) narratives are embedded in already existing forms of hardship that marginalised youth navigate and are shaped by social relations and local notions of religion, class, caste, and gender. As part of bringing event theory into conversation with ethnographic youth studies, I argue how event theory needs to be supplemented with other perspectives, including those that emphasise social inequality, as a basis for reflecting on youth’s actions during disruptions. In doing so, I discuss that although event theory offers a way to understand how disruptions might open up possibilities for new subjectivities and collective action to emerge, it offers limited scope to understand how people navigate the memories of these moments in their aftermath.
Eating in Style: Halal food and Greek food in Melbourne

Liu, Fanqi ( 2022-11)

This thesis aims to develop a working theory of everyday style through producing an ethnography of everyday culinary styles of Halal food and Greek food in Melbourne, Australia. By incorporating relational and more-than-representational thinking in food geography research, this thesis provides a rich understanding of ‘lay’ geographic food knowledge in the context of living together-in-difference. Through pulling bodies back into the discussions of Halal food and Greek food, this thesis extends our understanding of multicultural knowledges as felt geographical assemblages. By investigating the affects and effects of Halal food and Greek food in local places of multicultural Melbourne, this thesis takes up a new direction in food geography that goes beyond consumer-consumed relations and responds to calls in social and cultural geography to better understand the dynamics between a politics of experiences and legacies of ‘Culture’ and representations. It advances our understanding of food cultural politics in contemporary Western multicultural society and adds depth to the well-known trope of a ‘container’ in social sciences. This thesis challenges dichotomies between felt ‘culture’ and representational ‘Culture’ through advancing the concept of style. It develops a playful methodology to document practices of making, recognising, and depicting food cultures through their culinary communities in Melbourne. Using a multi-method approach of interviews, ethnographic fieldwork, and autoethnographic writing, this thesis demonstrates how forms of and practices for Halal and Greek food can shift and multiply while remaining recognisable. By approaching Halal and Greek food as referential effects, this thesis sketches out four key empirical themes: impressions, standards, making dishes and their recipes, and the style of making room for food experiences. It unravels that the ways we make food are embodied and affective, in the force and flows of materials and representations. The empirical findings challenge conventional views on similarities as a fixed reference point and consider how similarities are made in style practices. This thesis demonstrates how everyday culinary styles, such as Halal food and Greek food, play a longitudinal role in shaping public and domestic spaces and contributing to create local publics in multicultural Melbourne. It demonstrates that research through style can produce new understandings of style. This thesis develops a theorisation of style towards referential effects of multiple living mediation. Knowledge generated from this thesis contributes to sensitively evaluating our responsibilities and capacities in capturing the diversity of cultural experiences and offers a fresh approach to thinking about everyday cultures.
New methods for estimating baseline creatinine levels in patients with acute kidney injury

Larsen, Thomas ( 2023-01)

BACKGROUND: Acute kidney injury (AKI) is a syndrome characterised by deterioration in kidney function over a period of 2–7 days. The Kidney Disease: Improving Global Outcomes (KDIGO) guidelines recommend diagnosing and staging AKI by comparing a patient’s current and baseline levels of creatinine, a biomarker of kidney function. However, this presents a challenge for two groups of patients at risk of AKI: those without recorded creatinine values, and those with chronic kidney disease (CKD) whose creatinine values may be outdated. This thesis aims to develop and validate new equations for estimating baseline creatinine levels in these patient groups. METHODS: We conducted two separate observational studies at an academic hospital in Melbourne, Australia, focusing on adult patients who developed AKI during their hospital stay. The first study considered patients without CKD, while the second study considered those with underlying CKD. In both studies, we developed equations to estimate patients’ baseline creatinine levels. We evaluated our new equations’ accuracy and AKI reclassification performance against other methods, including conventional back-estimation approaches that adapt equations originally designed for estimating glomerular filtration rate. RESULTS: Our new equations outperformed other methods in estimating baseline creatinine levels for patients at risk of AKI. In our study of AKI patients without CKD, we found that conventional back-estimation methods were biased for patients younger than 60 or older than 75. To address this bias, we developed equations that considered age-related changes in creatinine levels. In our study of AKI patients with underlying CKD, we noticed inaccuracies in conventional approaches and unadjusted historical creatinine data when creatinine was measured over six months before AKI. We developed equations to adjust for drift in creatinine levels caused by CKD progression. In both studies, our equations significantly improved accuracy and AKI reclassification performance compared to other methods. CONCLUSION: We developed equations to estimate baseline creatinine levels for two groups of patients at risk of AKI: those lacking creatinine data, and those with outdated data due to CKD. These new equations enhance accuracy and AKI reclassification performance to address a significant clinical need. In addition, we developed statistical and computational tools, including a rolling-window technique for establishing reference baseline creatinine levels, and a pioneering application of symbolic regression to find accurate, interpretable models using medical data. These tools will be valuable for future clinical research.
Vegetation controls on incipient dune dynamics

McGuirk, Marita Therese ( 2022-12)

Vegetation is the principal boundary condition on the coast for foredune development both trapping sand and protecting it from erosion. Plants that colonize beaches and dunes grow in a variety of forms and have different life cycles. Plant species survival and growth on the backshore is due to their adaptation to the extreme nature of the environment. As a result, different species display differences in growth attributes such as leaf length, width, and density, which will influence the plants ability to capture and accumulate aeolian sand. This study compared sand accumulation and growth attributes of two species of grass which grow on the backshore, incipient dunes and foredune at Summerlands Beach, Phillip, Australia, indigenous Spinifex sericeus and exotic Thinopyrum junceiforme. The influence of vegetation on dune formation and morphology is studied through the development of Inverloch Spit, in a full dune evolutionary cycle, within a single decade. Leaves of S. sericeus, and T. junceiforme from the backshore, lower stoss slope of the incipient dune, upper stoss slope of the incipient dune and incipient dune crest were analysed. Samples were collected in Autumn, Winter, Spring, and Summer over two seasons. Analysis of leaf area showed that in ninety three percent of samples S. sericeus had a larger leaf area than T. junceiforme. Leaf width was a determining factor in greater leaf area. S. sericeus, consistently produced significantly larger leaves (maximum leaf area of 16.9 mm2 ), than T. junceiforme (maximum leaf area 11.5 mm2). When climatic conditions of higher rainfall and cooler summer temperatures occurred during the study period, T. junceiforme produced larger leaf areas. This indicates that under the predicted hotter and dryer climate scenario T. junceiforme may be less invasive and play less a role in dune building than S. sericeus. A novel method was designed to concurrently quantify changes in sand surface elevation in relation to plant growth character. Sand accumulation was measured, over a single season in 2018/2019, along with the life cycle of an embryo dune from its initial formation, driven by vegetation, to eventual erosion during a storm. Over this time frame S. sericeus accumulated significantly more sand than T. junceiforme. S. sericeus also produced a greater number of longer, wider leaves than T. junceiforme. When erosion occurred the amount of sand lost was related to growth of the grass species as S. sericeus lost less sand than T. junceiforme. A temperate example of dune development lifecycle in Victoria, Australia is examined, through the growth of a spit formation at Inverloch from 2010 to 2020. Remnant mounds of senesced Cakile s found on the spit in 2016 are regarded as the initiation of the hummock shape dunes that dominated the spit formation from 2012 to 2019. The model of dune growth on the spit suggests that T. junceiforme fragments began to grow in the sand mounds created by Cakile The spacing of these annual plants determined the initial dune spacing. The growth characteristics of T. junceiforme in combination with sand supply and aeolian sand transport created the hummock shaped dunes which were the dominant dune formation on the spit. Over time the dunes on the seaward edge of the spit coalesced to form an incipient dune ridge. The spit increased in total area from 2010 (55447 m2) up to a maximum area in 2015 (93349 m2), then steadily decreased in area up to 2020 (23788 m2) due to wave driven erosion. Over this time the ratio of sand to vegetation changed from 26:1 (2012) to 8:1 (2015), then 3:1 in 2020. Although erosion had been decreasing the size of the spit vegetation had managed to persist.
New insights into southern mid-latitude climate change from the terrestrial Roaring Forties

MacGregor, Claire Louise Verden ( 2023-01)

Most of our understanding of Quaternary climate change stems from records derived from the polar regions and the Northern Hemisphere. The forcing mechanisms and terrestrial response to Quaternary climate change in the Southern Hemisphere remain ambiguous owing to a paucity of precisely dated palaeoclimate data. This limits our ability to assess the Southern Hemisphere’s response to large-scale changes in climate beyond the Antarctic region and south of the tropics. The Southern Hemisphere holds significant potential for understanding global climate as a coupled ocean-atmosphere system due to the presence of the mid-latitude south westerly trade winds (SWW) and the Southern Ocean. These key features are fundamental in the transfer of climate signals through global atmospheric and oceanic teleconnections. Tasmania, Australia, is situated between 40-44 degrees south and is unique in that it is one of just three southern mid-latitude landmasses that directly intercept the SWW. It provides the opportunity to track the meridional shifts and climatic significance of the position of the SWW. This thesis explores fundamental aspects of Tasmania's hydroclimate variability over different timescales to understand the driving mechanisms and the regional climatic response. It presents new, precisely dated speleothem records that contribute towards building an integrated understanding of climate forcing mechanisms in a global context. This is crucial in resolving broader palaeoclimate enigmas such as the influence of Antarctica on terrestrial systems to the north and defining the boundary between the Antarctic and Greenland-like responses to millennial-scale events. The new Tasmanian speleothem data reveals a strong coupling with hydroclimate, contributing valuable insights into the meridional movement in the SWW. The speleothem d13C displays a strong, orbitally paced mode of rainfall variability throughout the Last Glacial Period that closely tracks changes in summer insolation at 65N and the intensity of the Asian monsoon. Higher frequency millennial-scale changes are less obvious throughout the Last Glacial Period but appear to resemble the Greenland-like nature of Dansgaard-Oeschger events more closely than the subdued Antarctic Isotopic Maxima events. In contrast, proxy data covering the Last Glacial Termination imply an alternative southern driving mechanism, controlled through feedback effects in response to iceberg discharge from the Antarctic ice sheet. These findings suggest that the southern mid-latitude region responds differently to climate change over orbital, millennial and deglacial timescales, which has important implications for deciphering driving mechanisms of large-scale climate change events.
Aspects of Opal-AG

Herrmann, Jurgen Robert ( 2022-12)

The formation of low-temperature, sediment-hosted opal-AG in eastern Australia is not fully understood. Open questions remain, for example over the timing of opal deposition, the role of intense weathering, the process of silica sphere formation and accumulation, and the origin of precious opal. This thesis on nodular opal-AG at Lightning Ridge (NSW, eastern Australia) attempts to address some of these questions by examining (i) the source of pigment in black, amber and grey opal; (ii) the roles of syn-sedimentary pyrite, changing redox and pH conditions during intense weathering; (iii) the nodule-filling process which produces the nodular opal mined at Lightning Ridge; (iv) the geological age of the opal nodules. New insights gained from these sub-projects are used to refine our understanding of opal formation within the context of the widely accepted ‘deep weathering ’model’. In addition, the thesis also reports on long-term experiments performed to examine the causes of opal ‘crazing’, a form of opal instability which produces cracking in some cut and polished opal. The dark colour of black opal, previously attributed to carbon or manganese impurities, was found here to be produced by disseminated microscopic sulfide and other minerals, and several types of carbon. These trace minerals were identified by X-ray diffraction of residues prepared by dissolving black opal nodules in hydrofluoric acid. Electron microscope (SEM, TEM), ATR-FTIR spectroscopy and laser-ablation ICP-MS were used to further examine this mineral assemblage. Sulfides are mostly pyrite and chalcopyrite, with minor galena and sphalerite. Titanic acid group compounds may also contribute to pigmentation. Carbon compounds are mostly C=O and C-H groups, suggesting an organic origin. Pigment minerals are mechanically (e.g., on the surface of the opaline silica spheres) or chemically bound. Sulfides may have been produced by microbial activity in an anoxic groundwater environment associated with weathering of older syn-sedimentary pyrite; this is consistent with low delta13C values reported for the black opals. Abundant textural evidence from oriented opal nodules indicates that heavy sulfide pigment settled out at the base of the nodules, implying that nodule fills at this stage were of low density/low viscosity. The presence of disseminated pigment in the opal above many of these ‘jet-black’ basal pigment layers suggests that pigment settling became more difficult as silica sols matured to higher density/viscosity gels over time. In some cases, ongoing anoxic conditions may have produced sulfidic pigment until after the cavity was filled with silica sol. However, lightly pigmented, amber-coloured opal also signals a change to more oxidising conditions related to weathering advancing downwards, and thus an end of microbial sulfide production. The inferred interplay between pigment production, particle settling, redox and silica sol/gel density changes imply a short timescale (sol-gel hardening periods on the cm scale) for nodule filling. This is a major outcome as it is inconsistent with the time-consuming incremental nodule-filling model favoured by others. Formation of sedimentary opal-AG in eastern Australia was attributed to deep weathering of the Cretaceous volcanogenic host rocks, possibly during the mid-Tertiary. While this ‘deep weathering‘ model is widely accepted, important aspects remain controversial. Here we propose a revised ‘deep weathering’ model which links the weathering history as recorded in weathering profiles to textural observations in 1000s of black and precious opal nodules. Opal deposits formed at shallow depth where Cainozoic river channels intersected parts of the Cretaceous stratigraphy favourable for high-grade opalization. Groundwater recharge from the river channels drove intense kaolinization of feldspar and glass in the volcanogenic sediments, aided by acidification of the groundwater through oxidation of sedimentary pyrite. Kaolinization released abundant silica, producing pedogenic silicification, including silcrete, at the top of the profile, and ‘steelband’ silicification and opal at the base. Alternating wet and dry cycles indicated by minor smectite and illite and changing redox and pH related to local variations in pyrite and feldspar consumption, were important in the formation of opaline silica spheres and subsequent opal mineralization. Observations from oriented opal nodules provide a record of the cavity filling process and its chemical environment. Textures and model calculations confirm the conclusion based on the pigment distribution (see above) that cavities were filled rapidly under changing Eh-pH. We argue that opal nodules formed when opaline silica spheres, nucleated and grown within perched groundwater bodies, accumulated, and drained into cracks and cavities in underlying claystone, rapidly filling available solution cavities. Drier conditions promoted the sol-gel ripening process that produced the solid opal. Patches and bars of precious opal-AG within common opal nodules suggest it forms through oxidation, diffusion and leaching in common opaline Si gel, during periods of low water flux and limited by the solidification of the Si gel. Ostwald ripening leads to increased silica sphere size which produces the characteristic POC display. Opal formation largely ceases once acidification potential is lost or all feldspar has been converted to kaolinite. The rich textural record preserved in opal nodules provides important insights on the links between cavity filling, weathering history, and ultimately climate. Radiometric dating (Rb-Sr, U-Th-Pb) yielded a preferred age of 33.3 +/- 7.8 Ma for black, amber and POC opal, the first direct date for Lightning Ridge opal. This mid-Tertiary age is consistent with imprecise dating of silcrete, a major regional weathering episode and sedimentary opal at sites elsewhere in the Great Australian Basin, as well as the oldest Ar-Ar date for hollandite associated with Lightning Ridge opal. In principle, the imprecision of this opal age leaves room for models involving multiple or prolonged opal formation, although a single, short-lived event is more likely based on other evidence. If confirmed by further dating, this early Oligocene age constrains not only formation of opal but also the exhumation of the mid-Cretaceous sediments, the age of a regional river system with riparian flora resembling that in modern rainforest gullies, intense deep weathering associated with the (perhaps seasonal) availability of abundant water, and regional silcrete formation. Finally, the discovery in this work of microscopic mineral blooms associated with shallow fractures in post-mining ‘controlled’ opal surfaces suggests that water loss, previously thought to be the major cause of opal ‘crazing’, may not the major trigger for this phenomenon. Experiments mimicking the ambient temperature and air humidity stresses that often lead to cracking in cut/polished opal show that magnitude and duration of such stresses are the major controls on both cracking and pitting. Transport of molecular water from interstitial spaces in opal to the surface is thought to occur via a process akin to ion hopping rather than diffusion through connected porosity. Surficial cracks develop as ions held in solution precipitate to form often sheeted but sometimes euhedral-subhedral mineral parageneses in near-surface porosity and on crack surfaces. A method for pre-treating commercial high-value opal using a vacuum method is suggested.
Mercury methylation in low-oxygen environments and the evolution of mercury-methylating genes

Lin, Heyu ( 2023-02)

The Ocean is a huge mercury (Hg) reservoir with a large amount of Hg deposited. Low-oxygen water columns are widespread in the ocean and play an important role in the global biogeochemical cycle. Many studies have focused on the nitrogen and sulfur cycles in the marine low-oxygen zones, whereas Hg cycling in these zones remains unclear and potentially underestimated. Hg methylation is conducted by microorganisms and produces a potent neurotoxin methylmercury (MeHg) accumulating in terrestrial and aquatic food webs. With the discovery of the Hg-methylating gene pair hgcAB, the investigation of Hg methylation has entered an era of multi-omics. The Hg-methylating community has been expanded massively by the advantage of culture-independent technologies, providing opportunities for in-depth studies of the community structure, diversity, phylogenetics, and metabolic potentials of Hg methylators. In this thesis, I investigate the potential for Hg methylation in marine low-oxygen environments, and explore the origin and evolution of Hg methylation from both phylogenetic and ecological aspects. First, I investigate a seasonally anoxic fjord, Saanich Inlet (British Columbia, Canada), to search for potential Hg methylators and study Hg cycling in this oxygen gradient. Combining chemical oceanography, large-scale multi-omics and computational modelling, I discover a novel putative MeHg producer Marinimicrobia, and identify it as a group of substrate-versatile and oxygen-tolerant marine bacteria. By searching for the Marinimicrobia-hgcA gene in a large-scale public database, the role that Marinimicrobia plays in global Hg cycling is highlighted. Next, I investigate the genetic potential for Hg methylation in the world’s deepest blue hole, Yongle blue hole (Sansha, China). Samples collected along the depth are divided into free-living and particle-associated to study Hg methylation by microbes with different lifestyles. Diverse Deltaproteobacteria contribute to Hg methylation in the anoxic bottom water, while microaerophilic Nitrospina and Myxococcota dominate the Hg-methylating community in the suboxic intermediate layer. Also, these methylators with different lifestyles occupy different ecological niches. Finally, I collected Hgc homologs from public databases and the novel Hgc sequences found in this thesis, to study the evolution of Hg methylation. The study suggests that the hgc gene has undergone generally vertical evolution with extensive loss. With consideration of the phylogeny of the alkylmercury lyase encoding gene merB, we propose a novel hypothesis that Hg methylation is an ancient metabolism that evolved as an antimicrobial production mechanism to compete with bacteria that could not demethylate mercury. As a response, other microbes may have evolved alkylmercury lyase encoded by merB as a defense against Hg methylators. Overall, the findings in this thesis highlight import role that low-oxygen environments play in global Hg cycling, provide new evidence that microaerophilic microbes take part in MeHg production, and propose a new hypothesis explaining the evolution of Hg methylation. While many fundamental questions about Hg methylation remain to be addressed, I hope this thesis put forward our understanding of Hg methylation and shed light on future Hg methylation studies.

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