| 1. |
- Andersson, Sofia, 1975-
(author)
-
Late Holocene humidity variability in central Sweden
- 2010
-
Doctoral thesis (other academic/artistic)abstract
- The aim of this thesis was to reconstruct humidity variability in central Sweden during the late Holocene. A multi-proxy approach was used to infer humidity changes as recorded in a lake and a mire. Age-models were constructed based on radiocarbon dating and the Askja-1875 tephra. Stable isotopes (δ18O and δ13C) on Chara spp encrustations and Pisidium spp mollusc shells and carbon content were analysed in the lake record, whereas peat stratigraphy, humification, testate amoebae assemblages, C/N ratio and stable isotopes (δ13C and δ15N) were analysed in the mire record. Stable isotopes (δ2H and δ18O) on lake water showed that Lake Blektjärnen responded to changes in the balance between evaporation and input water (E/I ratio). A high E/I ratio results from a dry and probably warmer climate during which evaporation and atmospheric equilibration likely enrich lake water in 18O and 13C, respectively, and vice versa for a low E/I ratio. The relatively high Chara δ18O and δ13C values between ca 4400 and 4000 cal yr BP thus suggest relatively dry and likely warm conditions, whereas depleted values suggest wetter and probably cooler conditions between ca 4000 and 3000 cal yr BP. Again, drier and probably warmer conditions were inferred from the relatively enriched δ18O values between ca 2500 and 1000 cal yr BP, and depleted δ18O values were recorded between ca 1000 and 50 cal yr BP indicating wetter and likely cooler conditions. The results from the mire mainly indicated vegetation succession, however, the changes inferred at ca 2600 and 1000 cal yr BP could have been triggered by climate change. This study shows that the proxies responded sensitively to humidity changes in the investigated archives allowing for reconstruction of climate change in central Sweden during late Holocene.
|
|
| 2. |
- Duan, Haochen
(author)
-
Magma generation and formation of continental crust in the Izu arc
- 2022
-
Doctoral thesis (other academic/artistic)abstract
- The Izu oceanic arc is produced by the subduction of the Pacific Sea Plate beneath the Philippine Sea Plate. The proto-Izu oceanic arc was initially formed at ca. 50 Ma, then it was separated into the Kyushu-Palau and Izu arc by the extension of the Shikoku basin from ca. 25 Ma. After the formation of the Shikoku basin, volcanism initiated in the Izu rear-arc at ca. 17 Ma. Nowadays, arc volcanism has built up a thick crust observed by seismic velocities that extends from the Izu fore-arc to rear-arc and the upper crust has felsic compositions. The bimodal mafic-felsic volcanism in the Izu arc-front displays depleted light rare earth elements whereas in the Izu rear-arc the volcanic rocks display enriched light rare earth elements. International Ocean Discovery Program (IODP) Site U1437 is the first scientific drill site in the Izu rear-arc, which drilled 1.8 km into the volcaniclastic successions between the Manzi and Enpo seamount chains. Site U1437 is divided into seven stratigraphic units that range in age from 0 to ca. 16 Ma. This thesis aims to investigate the magma generation and the formation of continental crust in the Izu rear-arc using boron (δ11B) and oxygen (δ18O) isotopes. The felsic melt inclusions from Unit II and Unit IV show that felsic volcanic rocks are formed by partial melting of the pre-existing altered and fresh oceanic crust. Meanwhile, fractional crystallization of mafic magmas affects the diverse compositions of the oceanic crust protolith. Therefore, partial melting of oceanic crust and fractional crystallization of mantle-derived mafic magmas both influence the formation of felsic continental crust in the Izu rear-arc. These findings indicate that extensional settings, like the Izu rear-arc, should be considered as potential locations for the formation of continental crust in the Phanerozoic, Archean and even in the Hadean.The basaltic melt inclusions in Unit VII show that the mafic volcanic rocks in the Izu rear-arc are not derived from the mantle wedge that is metasomatized by slab-derived fluids. Instead, primary magmas in the Izu rear-arc are formed by mélange diapirs ascending into the mantle wedge. This could explain the across arc mantle heterogeneity observed in the Izu arc, with the arc front volcanic rocks are associated with slab-derived fluids and the Izu rear-arc compositions are influenced by mélange. Mélange compositions generated in the rear-arc may be recycled into the upper mantle forming the upper mantle heterogeneity observed in MORB globally.
|
|
| 3. |
- Kaislahti Tillman, Päivi, 1958-
(author)
-
Holocene climate and environmental change in high latitudes as recorded by stable isotopes in peat deposits
- 2012
-
Doctoral thesis (other academic/artistic)abstract
- In this thesis, stable carbon and oxygen isotopes in α-cellulose isolated from Sphagnum fuscum moss remains were used as climate proxies. The main focus was to implement the methods in records from high latitude peatlands in the Northern Hemisphere (west-central Canada and north-eastern European Russia), reconstruct palaeoclimate of the studied regions during the Holocene, and evaluate the compatibility of results with other proxy records, especially tree-ring isotope time-series. The variation of stable carbon and oxygen isotope ratios (δ13C, δ18O) in different moss plant components was investigated and differences between untreated plants and α-cellulose extracts were evaluated. The impact of peat decay on the stable isotope proxies was studied by colorimetric and chemical (C/N) methods. Temperature reconstructions were developed based on the statistically significant relationship between δ13C and modern summer temperature records. Wet/dry periods were derived from a combination of δ18O records, macrofossil analysis, and a peat humification record in west-central Canada. A tentative reconstruction of snow depth in north-eastern European Russian tundra and northern taiga was based on δ18O records. The most promising result of the thesis is that stable carbon isotope variability in α-cellulose isolated from Sphagnum fuscum stems can be used to reconstruct and quantify palaeotemperatures several millennia back in time and to reveal both long-term and rapid climate shifts from peat archives.
|
|
| 4. |
- Larsson, Simon A., 1990-
(author)
-
Ashes to ashes : Applications of tephrochronology in Scandinavia
- 2022
-
Doctoral thesis (other academic/artistic)abstract
- The project presented in this thesis provides two examples of applications of tephrochronology in Scandinavia. The usage of tephra deposits preserved in sediments—i.e. horizons of volcanic ashes, detected either as visible layers or as low-concentration “cryptotephras”—is demonstrated to be a strong and versatile tool for chronological control of individual sediment sequences and for correlation between study sites on both local and wider-regional scales.Fieldwork was done at a number of study sites in southernmost Sweden and mid-Norway to sample sediments formed in lakes and peatlands during the Last Glacial–Interglacial Transition, i.e. c. 16,000–8,000 years ago. Labwork was then performed to analyse the sediments by methods to estimate organic matter, organic and carbonate carbon, and elemental contents as well as to find datable macrofossils and to detect tephra occurrences. Among the detected tephras between the different study sites, identifications were made of the Fosen Tephra, the Hässeldalen Tephra, the Vedde Ash, and the Laacher See Tephra. These were used for the purposes of either application in the project; (1) a palaeoclimate reconstruction at Körslättamossen, with new proxy analyses and correlations to a previous study of the same site as well as to other studies in Europe via linkages established by the tephras, and (2) a palaeoglaciological reconstruction on the Fosen peninsula, based on correlations between study sites using the detected tephras to assess the timing of glacial retreat in the area.The palaeoclimate reconstruction at Körslättamossen and correlation to previous studies provides new information about the climatic development in Europe after the end of the last ice age, specifically regarding the timing of the cold stage known as the Younger Dryas. Detailed knowledge of such events, spatially and temporally, is necessary to understand the behaviour of the climate system in the past and, thereby, its behaviour in the present and the future. This is one of several studies demonstrating the usefulness of tephrochronology for such research questions.The palaeoglaciological reconstruction on the Fosen peninsula, also relating to the Younger Dryas, is one of a few recent studies which demonstrate the strengths of tephra investigations in providing evidence for the timing of events on more local scales than the wider spatial perspective more often utilised in tephrochronological applications. Other than resulting in a significantly revised chronology for the retreat of the Scandinavian Ice Sheet in the study area, this study should also encourage new, similar tephra studies in future Quaternary research.In addition to the direct results of either study (mainly presented in the related scientific papers) and the addition of new reference data for later tephra studies, the thesis also discusses further implications of the results and observations made with emphasis on considerations and study design issues in Quaternary research at large and for the tephrochronologist in particular.
|
|
| 5. |
- Lind, Ewa, 1968-
(author)
-
Tephra horizons in the North Atlantic region during the Early Holocene
- 2014
-
Doctoral thesis (other academic/artistic)abstract
- Uncertainties about the rate and magnitude of future climate change have drawn attention to the climate variability following the Late Glacial-Early Holocene transition, c. 12-8 ka BP. Tephrochronology is a method that uses volcanic ash as isochrons in order to link, date and synchronise geological and palaeoclimatic records which could provide a better understanding of short rapid climate shifts but also the timing of these.The main purpose of this thesis was to improve and refine the Late Younger Dryas-Early Holocene (c. 12-8 ka BP) tephrochronological framework with focus on dating and identification of new and previously known tephra horizons in the North Atlantic region. A special emphasis was to identify tephra horizons contemporary to short rapid climate events that occurred during the period; the Preboreal Oscillation (c. 11.3 ka BP), the Erdalen events (c. 10.0-9.8 ka BP), the 9.3 ka BP and the 8.2 ka BP events. Additional purposes have been to evaluate the potential of XRF ITRAX core-scanning to identify tephra in lake sediments but also as a tool in palaeoclimate studies.The potential of tephrochronology for linking climate archives across the North Atlantic region is a main outcome in this thesis. Specific results include five new tephra horizons; four identified on the Faroe Islands and one in Central Norway. Two previously known tephras, the Hässeldalen and Askja-S tephras, were identified on the Faroe Islands for the first time and at new locations in Sweden which confirmed their widespread distribution in NW Europe. Several of these tephras have the potential to become marker horizons for some of the short rapid climate events which occurred during the Late Younger Dryas-Early Holocene but their importance is yet to be determined. Further results show that the XRF core scanner can be used successfully to identify high concentrations of basaltic tephra but also to identify palaeoclimatic events in lake sediments.
|
|
| 6. |
- Lind, Ewa
(author)
-
Towards an Early Holocene Tephrochronology for the Faroe Islands: Methodology and first results
- 2011
-
Licentiate thesis (other academic/artistic)abstract
- The uncertainties regarding future climate change have put considerable notice to the climate variability following the Late Glacial-Holocene transition (ca. 13-9 ka BP) in the North Atlantic region as well as the forcing mechanisms behind climate changes. Much attention has focused on short climate events in order to understand the mechanisms that drove these changes but also to identify the leads and lags in the climate system. Chronological uncertainties for these events remain but an accurate chronological framework for the North Atlantic region would enhance possibilities to solve some of the chronological questions. Tephrochronology uses volcanic ash from a volcanic eruption which creates a marker horizon in marine and lake sediments, peat bogs and glacier ice as the ash is spread over large areas. These time-parallel markers allow precise correlations between archives. The purpose of this thesis is to improve and refine the Early Holocene tephrochronological framework with focus on dating and identification of new and previously known tephra horizons on the Faroe Islands, especially around the climatic events of the Preboreal Oscillation (11,300-11,100 cal. yr BP), the Erdalen events (10,100-10,050 and 10,000-9800 cal. yr BP) and the 9.3 ka BP event. A second goal is to develop the methodology of tephrochronology for finding cryptotephra (not visible by the eye) horizons in lacustrine sediments. The findings of eight tephra horizons spanning ca. 11,350 to 9700 cal yr BP where three are known from other locations in Europe show the potential of tephrochronology for linking records across the North Atlantic region. Refined ages for the Askja-S and Hässeldalen tephra were obtained from an age model built on eight AMS radiocarbon ages with the Saksunarvatn ash as a constrained age. The results from using XRF ITRAX core scanner to locate tephra in lacustrine sediments illustrate that high concentrations of basaltic tephra could be captured but not lower concentrations of rhyolitic shards.
|
|
| 7. |
- Johansson, Hans, 1956-
(author)
-
Towards a Holocene tephrochronology for the Azores
- 2015
-
Licentiate thesis (other academic/artistic)abstract
- The Azores is situated in the North Atlantic Ocean and is one of the most active volcanicregions in the Northern Hemisphere. The volcanic history of the islands is fairly wellknown and several explosive trachytic eruptions have been reported but the geochemicalcompositions of the glass component of the tephra as well as the dispersal oftephras to distal areas are less well known. The purpose of this study is twofold: (1) topresent major element geochemistry of the glass component from several historic aswell as prehistoric tephras, including the trachybasaltic Capelinhos AD 1957‐58eruption on the island of Faial, and the trachytic explosive eruptions of Sete Cidades(c. AD 1440), Fogo A (c. 5000 BP), Fogo AD 1563 and Furnas AD 1630 on the island ofSão Miguel; (2) to present a refined tephrostratigraphy for the island of Pico. Analyses ofmajor element geochemistry suggest that tephras from the three active stratovolcanoeson São Miguel can be separated in biplots showing e.g. FeOtot vs.TiO2 and FeOtot vs. CaO.The tephrostratigraphy of Caveiro bog on the island of Pico is based on a radiocarbondated core with eight tephra layers extending back to c. 7000 BP. All tephras are oftrachybasaltic/basaltic trachyandesitic composition except the oldest layer, which is ofbasanitic composition. An attempt was made to correlate the tephra record of Caveirobog with the previously investigated Lake Caveiro. A tephra‐based correlation betweenthe Caveiro bog and Lake Caveiro is not straightforward and only three tephras inCaveiro bog can possibly be correlated with tephras found in the sediments of LakeCaveiro.Proximal glass data from the Furnas volcano on São Miguel suggest that distal cryptotephrasfound in Ireland may have an origin in the Azores and not on Jan Mayen aspreviously has been suggested. The similarity of the proximal tephras on São Miguel anddistal tephras in Ireland is demonstrated by high similarity coefficients (>0.95) andbiplots showing major element composition also support a correlation between Azoreaneruptions and distal tephras in Ireland. Thus, trachytic tephras erupted from explosiveeruptions on the island of São Miguel may have a potential to contribute to theconstruction of a European‐wide tephrochronology framework.Trachytic tephras erupted from explosive eruptions on the Azores may be more widelydispersed than previously thought and may provide useful isochrones for correlation ofpaleoclimate archives in the north‐central Atlantic, North Africa and the Iberian regions.The trachybasaltic/basaltic trachyandesitic tephras erupted from cinder cones on PicoIsland are probably only useful for a local tephrochronology in the Azores region, andnot for a wider Atlantic or European framework.
|
|
| 8. |
- Katrantsiotis, Christos, 1982-
(author)
-
Holocene environmental changes and climate variability in the Eastern Mediterranean : Multiproxy sediment records from the Peloponnese peninsula, SW Greece
- 2019
-
Doctoral thesis (other academic/artistic)abstract
- This thesis presents multiproxy reconstructions of the mid to late Holocene climate and environmental changes in the Peloponnese peninsula, SW Greece. The combined dataset consists of diatom, biomarker and X-ray fluorescence spectrometry (XRF) elemental data in radiocarbon-dated sediment cores taken from the Agios Floros fen and the Gialova Lagoon in SW Peloponnese and the Ancient Lake Lerna in NE Peloponnese. Overall, the results highlight the complex interaction between climate, tectonics and human activities in the landscape development and further reveal changes in the W-E precipitation/temperature gradient over the peninsula connected to shifts in the large-scale atmospheric circulation patterns.The Agios Floros study provides a 6000-year hydrological record based on diatoms and hydrogen isotopic (δD) analysis of aquatic plant-derived n-C23 alkanes. The records indicate two decadal-long periods of deep water conditions at ca 5700 and 5300 cal BP, largely attributed to local tectonic processes and the hydrological anomalies of the nearby karst springs. A period of intermediate water level at ca 4600 cal BP is dominated by the new fossil species Cyclotella paradistinguenda described in this thesis. The gradual development of a fen at ca 4500 cal BP is attributed to a combination of human activities and drier conditions, the latter culminating in SW Peloponnese mainly after ca 4100 cal BP. From ca 2800 cal BP and onwards, there is evidence for flooding events probably related to marked rainfall seasonality.The n-alkane δD profiles and XRF data analyzed in the Gialova core co-vary with each other indicating a common climate signal during the last 3600 years, which resembles the Agios Floros record. The n-alkane δ13C values show high contribution of aquatic vegetation to sedimentary organic matter during wet/cold periods. The n-alkane δD signals from the Lake Lerna also exhibit a similar pattern to each other providing further evidence for precipitation/temperature changes over the last 5000 years.Comparison of the δD records reveals sometimes similar and sometimes opposing signals between NE and SW Peloponnese, which can be attributed to the relative dominance of high latitude and low latitude atmospheric patterns over the peninsula. The records show wet conditions at ca 5000-4600 cal BP likely associated with the weakening of the Hadley circulation. High humidity is also evident at ca 4500-4100, ca 3000-2600 (more unstable in SW) and after ca 700 cal BP with drier conditions at ca 4100-3900 and ca 1000-700 cal BP. These periods correspond to regional climate changes, when the North Atlantic Oscillation (NAO) likely exerted the main control with NAO (+) creating conditions of reduced moisture. A NE-SW climate see-saw with drier conditions in NE Peloponnese is evident at ca 4600-4500, ca 3200, ca 2600-1800 and ca 1200-1000 cal BP and a reversal at ca 3900-3300 ca 3200-3000 and ca 1800-1300 cal BP. The dipole pattern is likely driven by shifts in the North Sea–Caspian Atmospheric pattern (NCP), with NCP (+) leading to wetter and colder conditions in NE Peloponnese. The opposing signal can also be explained by changes in summer temperatures driven by the Asian monsoon intensity. Strong monsoonal periods coincide with cool summers in Lerna, due to the northerly winds (Etesians), in contrast to SW Peloponnese, located on the lee side of the mountain and most affected by the large-scale air subsidence.
|
|
