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- van Mourik, Caroline A., 1969-
(author)
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The Greenhouse - Icehouse Transition : a dinoflagellate perspective
- 2006
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Doctoral thesis (other academic/artistic)abstract
- Through the analysis of the stratigraphic and spatial distribution of organic walled dinoflagellate cysts (dinocysts) from climatologically and oceanographically key sites, this project aims to contribute to a better understanding of the Eocene-Oligocene (E/O) environmental changes and their timing. A central issue is to identify the global environmental changes which are responsible for the Eocene cooling and its underlying mechanisms with the focus on the Oligocene isotope-1 (Oi-1) event, thought to mark the onset of major Antarctic glaciation.Two low-latitude sites were selected, Blake Nose (western North Atlantic) and Massignano (central Italy). For the first time a coherent taxonomy and biostratigraphy of dinocysts was established for the late Eocene at these latitudes. A high resolution correlation was established between the Massignano E/O Stratotype Section and the stratigraphically more extended ‘Massicore’. The composite section was used to analyse sea surface temperature (SST) change across the greenhouse-icehouse transition by means of dinocyst distribution.At Massignano, the Oi-1 event was recognised both qualitatively and quantitatively. In the power spectrum of the SSTdino the ~100 and ~400 kyr eccentricity cycles may be distinguished and correlated with La04. When orbitally tuned, the E/O GSSP dates ~100 kyr older than the Oi-1 event. The boundary’s age could either be ~33.75 or ~34.1 Ma, both differ significantly from the ~33.9 Ma age in the GTS 2004.Furthermore, when the data from the low-latitude sites were combined with extensive datasets from the Proto North Atlantic and adjacent regions, a suite of species sensitive to changes in SST was recognised. Their first and last occurrences reflect seven distinct phases of decreasing SSTs during the Middle Eocene to earliest Oligocene.These results clearly indicate that atmospheric cooling together with higher frequency orbital forcing played a key role in the transition from the Greenhouse to the Icehouse world.
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| 2. |
- Muschitiello, Francesco, 1985-
(author)
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Deglacial impact of the Scandinavian Ice Sheet on the North Atlantic climate system
- 2016
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Doctoral thesis (other academic/artistic)abstract
- The long warming transition from the Last Ice Age into the present Interglacial period, the last deglaciation, holds the key to our understanding of future abrupt climate change. In the last decades, a great effort has been put into deciphering the linkage between freshwater fluxes from melting ice sheets and rapid shifts in global ocean-atmospheric circulation that characterized this puzzling climate period. In particular, the regional expressions of climate change in response to freshwater forcing are still largely unresolved.This projects aims at evaluating the environmental, hydro-climatic and oceanographic response in the Eastern North Atlantic domain to freshwater fluxes from the Scandinavian Ice Sheet during the last deglaciation (~19,000-11,000 years ago). The results presented in this thesis involve an overview of the regional representations of climate change across rapid climatic transitions and provide the groundwork to better understand spatial and temporal propagations of past atmospheric and ocean perturbations.Specifically, this thesis comprises i) a comparison of pollenstratigraphic records from densely 14C dated lake sediment sequences, which provides insight into the regional sensitivity of North European vegetation to freshwater forcing in the Nordic Seas around the onset of the Younger Dryas stadial (~12,900 years ago); ii) a reconstruction of North European hydro-climate, which, together with transient climate simulations, shed light on the mechanisms and regionality of climate shortly prior to the transition into the Younger Dryas stadial; iii) studies of a ~1250-year long glacial varve chronology, which provides an accurate timing for the sudden drainage of proglacial freshwater stored in the former ice-dammed Baltic Ice Lake into the North Atlantic Ocean; iv) a 5000-year long terrestrial-marine reconstruction of Eastern North Atlantic hydro-climate and oceanographic changes that clarifies the hitherto elusive relationship between freshwater forcing and the transient behaviour of the North Atlantic overturning circulation system. The results presented in this thesis provide new important temporal constraints on the events that punctuated the last deglaciation in Northern Europe, and give a clearer understanding of the ocean – atmosphere – ice-sheet feedbacks that were at work in the North Atlantic. This increases our understanding of how the Earth climate system functions in more extreme situations.
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| 4. |
- Aigars, Juris
(author)
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The role of sediments in the biogeochemical cycles of nutrients in the Gulf of Riga
- 2001
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Doctoral thesis (other academic/artistic)abstract
- The sediment biogeochemistry of C, N, P and biologic silica (BSi) plays an important role in cycling of these elements in aquatic ecosystems. In shallow coastal and estuarine systems, water-sediment nutrient dynamics can influence biological processes (e.g., primary production) in overlying waters.The objectives of this study are:characterize C, N, P and BSi geochemistry of the surface sediments in the Gulf of Riga,estimate the spatial distribution, including net flux and sink, for organic C, N and P in the Gulf of Riga,examine if there are any appropriate bulk, regional and/or vertical relationships between the nutrients,examine if nutrient biogeochemistry in surface sediments vary as a response to naturally occurring processes i.e., intensity of sedimentation and bioturbation, oxygen deficiency, variation of temperatureSpatial and vertical distribution reveals that organic C and N ratio is almost independent of sediment characteristics and location, and exhibit a stable value down the sediment core. Over 90 % of carbon and nitrogen in sediments is organic. In contrast, organic P constitutes less than 50 % of the total phosphorus pool. The organic C:N:P ratio in sediments indicate that N and P are decomposed preferentially to C, whereas P is decomposed preferentially to N. The stable C:N ratio in sediments indicates that preferential N decomposition occurs in water column and/or immediately upon settling at sediment surface. Distribution pattern of BSi suggests that accumulation rate of sediments controls BSi concentration. Although nutrient loading from drainage area increased drastically over the past 100 years, vertical profiles of C, N and BSi show no variation in element concentrations except in the top few centimeters. A corresponding increase of benthic biomass most likely results in low elemental accumulation in these sediments.Although Gulf of Riga is one of the most productive areas in the Baltic Sea, seasonal alterations of C and N levels were limited. However, during spring and autumn algae blooms total C and N concentrations increased in the study area. Limited data suggests that burrowing amphipods directly impact the C and N concentrations in the top 2-3 cm of sediments. The vertical distribution of P is more dependent on oxygen concentration, which is largely controlled by bioturbation and sedimentation rates. Moreover, the results suggest that bioturbation is largely responsible for temporal accumulation of inorganic P in surface sediments.The low unidirectional fluxes of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) in early spring is consistent with low water temperature and poor nutritional quality of experimentally added material. The high water temperature and better nutritional quality of material added in summer, comparatively to winter, resulted in large DIN and DIP fluxes. Moreover, flux experiments under low oxygen conditions and pulse input of large quantities of settling seston suggest that sediment surface might experience lack of oxygen despite availability of oxygen in the overlying water column.
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| 6. |
- Alm, Elisabet, 1948-
(author)
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Sveconorwegian metallogenesis in Sweden
- 2000
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Doctoral thesis (other academic/artistic)abstract
- Two main ore types are found in the Sveconorwegian Orogen in southwestern Sweden (Southwest Scandinavian Domain). One of them comprises stratabound Cu mineralizations in the Dal group, located west of lake Vänern. The other comprises quartz veins with varying precious and base metal contents, distributed over 250 km between lake Mjøsa (southeastern Norway) and lake Vänern. In this thesis, both ore types are discussed, although the main emphasis is on Au-bearing quartz veins, particularly those in the Harnäs area near lake Vänern.The Dal group is a 2000 m thick sequence of clastic sediments and intercalated mafic volcanic rocks, metamorphosed under greenschist facies conditions. It records deposition mainly in a shallow marine basin, formed during a rift stage preceding the Sveconorwegian orogeny (c. 1.15-0.9 Ga). The volcanic rocks have been subject to various degrees of sodic and/or potassic alteration. Geochemical and Sm-Nd isotopic evidence indicate a continental setting of volcanism. Cu mineralizations (chalcopyrite and bornite) occur at two stratigraphic levels. An ore-genetic model involving synsedimentary (or syndiagenetic) deposition of sulphides from metal-bearing fluids is favoured.Among Au-bearing quartz veins in the Mjøsa-Vänern ore district, four paragenetic types have been distinguished: Cu-dominated veins with chalcopyrite and/or bornite; Pb-Cu-bearing veins with pyrite, galena and chalcopyrite; Zn-Pb-dominated veins with sphalerite, galena, pyrite and chalcopyrite; Mn-bearing vein(s) with galena, chalcopyrite and hausmannite. In addition, e.g. native gold, argyrodite, hessite, tellurobismuthite and altaite are recognized. The ore lead isotopic composition is complex and metals appear to be derived from a variety of source rocks.The orthogneisses, which constitute the host rocks to the Harnäs veins and the Brustad Au quartz veins (Eidsvoll, near lake Mjøsa), have been investigated with respect to geochemistry, U-Pb zircon age and feldspar lead isotopic composition. The obtained intrusion age of the Brustad augen gneiss is 1674 ± 10 Ma and this rock is considered to belong to the Transscandinavian Igneous Belt. The Harnäs gneiss yielded a protolith age of 1595 +24/-17 Ma and is considered to be a member of the Åmål granitoid suite. Both orthogneisses have undergone ductile deformation during the Sveconorwegian orogeny. A complete isotopic resetting of the feldspar lead through dynamic recrystallization in conjunction with this deformation, at c. 1.0 Ga, has been demonstrated.The steeply dipping Harnäs veins are hosted in a local left-lateral shear zone, which transects the fabric in the surrounding orthogneisses. The moderate wall rock alteration was mainly sericitic. Fluid inclusions show that the ore-bearing vein system at Harnäs developed essentially in three stages: a quartz stage, a pyrite-gold stage and finally a galena stage. The early ore fluid was CO2-bearing, of low salinity and with a temperature of c. 200 oC, while in the galena stage it was purely aqueous, with a slightly higher salinity and a slightly lower temperature. Oxygen and sulphur isotope results imply a predominantly metamorphic origin for the ore fluid and suggest that the fluid constituents were derived from the regional orthogneisses. Ore lead isotopic compositions indicate metal derivation from these orthogneisses shortly after the Sveconorwegian deformation and resetting of feldspar lead. Subordinate Au-anomalous quartz veins in the Harnäs area as well as the Brustad Au quartz veins show characteristics similar to the Harnäs veins. Despite recognized variations, e.g. in mineralogy, a common origin is envisaged for most veins in the Mjøsa-Vänern ore district. They are characterized as late Proterozoic orogenic type Au deposits, with modern analogues e.g. in the western Alps.
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| 9. |
- Andersson, Eva, 1967-
(author)
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Hydrothermal alteration of organic matter at spreading centers
- 1998
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Doctoral thesis (other academic/artistic)abstract
- Alteration of organic matter, mainly amino acids, at sub-seafloor hydrothermal conditions has been investigated by performing laboratory experiments and by studying deep-sea sediments from hydrothermally active areas on the northern Juan de Fuca Ridge, northeast Pacific Ocean.Quaternary sediments from Middle Valley and the eastern flank of the Juan de Fuca Ridge recovered during the Ocean Drilling Program Legs 139 and 168 have been analyzed for total hydrolyzable amino acid concentrations, individual amino acid abundances and stereochemistry in order to evaluate the effects of hydrothermal stress on the decomposition of sedimentary amino acids. In near surface sediments, amino acids account for up to 3.3% of the total organic carbon content and up to 12% of the total nitrogen content. The non-protein amino acid b-alanine and g-aminobutyric acid become increasingly abundant with depth in low temperature holes (Leg 168) as a result of enzymatic decarboxylation of aspartic acid and glutamic acid, respectively. The decomposition of amino acid in high temperature holes (Leg 139) is enhanced with depth and the amino acid patterns indicates that most amino acids are incorporated into geopolymers and that condensation results in increased stability of some amino acids.The effects of low temperature hydrothermal activity on microbially mediated organic matter diagenesis were studied by comparing depth concentration profiles of interstitial sulfate and methane of Holes 1023A, 1024B, 1025B and 1028A, ODP Leg 168. Diffusional exchange between sulfate-rich basement fluids and pore-waters increases the interstitial sulfate concentrations with depth below local sulfate minima caused by bacterial sulfate reduction. The effects of diffusional processes on pore-water chemistry in the sediment column is reflected by the inhibition of methane production and is largely dependent on sediment thickness.The decomposition of alanine, leucine, aspartic acid and serine in aqueous solutions was studied at 200oC and 50 bar with the purpose of evaluating the effect dissolved oxygen on decomposition rates. The redox buffering mineral assemblage pyrite-pyrrhotite-magnetite was used to constrain the oxygen fugacity to geologically realistic values. Comparisons between results obtained from buffered and nonbuffered runs show that the decomposition is faster for most amino acids but serine in nonbuffered experiments.
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