Dating early Holocene palaeoseismic event(s) in the Gulf of Bothnia, Baltic Sea

Hutri, K.-L., Heinsalu, A., Kotilainen, A. T. & Ojala, A. E. K. 2007 (January): Dating early Holocene palaeoseismic event(s) in the Gulf of Bothnia, Baltic Sea. Boreas , Vol. 36, pp. 56–64. Oslo. ISSN 0030–9483.
Deformation structures in submarine Holocene sediments caused by palaeoseismicity have recently been found in the Olkiluoto area, Gulf of Bothnia, Baltic Sea, within old fracture zones of bedrock. In this study, the palaeoseismic event(s) was dated and the palaeoenvironment was characterized using palaeomagnetic, biostratigraphical and lithostratigraphical methods, thereby enhancing the reliability of the chronology. The variations in the inclination and declination of the Olkiluoto sediment core showed very good correlation with the palaeosecular variations recorded in the annually laminated long lake sediment record from Lake Nautajarvi in central Finland. Combined litho-, bio- and palaeomagnetic stratigraphy revealed an age estimation of 10 650 to 10 200 cal. yr BP for the palaeoseismic event(s), which coincides with postglacial bedrock faulting in northern Fennoscandia.     

Reconstruction of the White Sea Basin during the late Younger Dryas

Pasanen, A., Lunkka, J. P. & Putkinen, N. 2009: Reconstruction of the White Sea Basin during the late Younger Dryas. Boreas, 10.1111/j.1502‐3885.2009.00128.x. ISSN 0300‐9483 The Weichselian Scandinavian Ice Sheet (SIS) in the White Sea Basin retreated from its maximum position to the Kalevala end moraine between 17 000 and 11 500 years ago. Even though the deglaciation history is relatively well known, the palaeoenvironments in front of the ice sheet are still poorly understood and partly controversial. In the present paper, we use geomorphological, sedimentological and ground‐penetrating radar survey methods to study glaciofluvial plains and shorelines at the Kalevala end moraine. These data are used to define the shoreline gradient for the area and to numerically reconstruct the palaeotopography and the area and volume of the water body in the White Sea Basin during the late Younger Dryas 11 500 years ago. The results indicate that at three sites glaciofluvial plains represent Gilbert deltas deposited to the same water level next to the ice margin. Using the shoreline gradient of 0.42 m/km, it is shown that the water body in the White Sea Basin was extensive and relatively deep, inundating large, currently onshore, areas on the western side of the White Sea and the Arkhangelsk area to the east. The ice margin terminated in the White Sea, which was connected to the Barents Sea via the Gorlo Strait and separated from the Baltic drainage basin to the south.     

A 3000-year palaeoenvironmental record from annually laminated sediment of Lake Korttajarvi, central Finland

High-resolution physical analyses (varve thickness and relative X-ray density) were conducted on a 3000-year varved sediment sequence in Lake Korttajarvi, central Finland. Climate and the local environment strongly influence the properties of the lake sediments, and, through a combination of physical proxies, severe and favourable climate periods and anthropogenic effects on sedimentation with an annual to decadal resolution could be detected. We observed previously identified historical climate periods in the Lake Korttajarvi varve record. The Medieval Climate Anomaly (often termed the Medieval Warm Period) of AD 980–1250, which is characterized by highly organic sediment and a minor minerogenic flux during mild winters, started and terminated abruptly, but also included a short (30-year) colder period lasting between AD 1115 and AD 1145. The Little Ice Age, however, was not clear in our record, although there were two minor cooling periods in AD 1580–1630 and AD 1650–1710. Natural variability in the sediment record was disrupted by increased human impact in the catchment area at AD 1720. There is a distinct positive anomaly in mineral matter accumulation between 907 and 875 BC, which indicates more severe climate conditions. This period exists contemporary with a cold event, recorded worldwide, c. 2800 years ago.     

Late-glacial and post-glacial deposition in a large, low relief, epicontinental basin: the northern Baltic Sea

This paper presents a model of late‐glacial and post‐glacial deposition for the late‐Neogene sedimentary succession of the Archipelago Sea in the northern Baltic Sea. Four genetically related facies associations are described: (i) an ice‐proximal, acoustically stratified draped unit of glaciolacustrine rhythmites; (ii) an onlapping basin‐fill unit of rotated rhythmite clasts in an acoustically transparent to chaotic matrix interpreted as debris‐flow deposits; (iii) an ice‐distal, acoustically stratified to transparent, draped unit of post‐glacial lacustrine, weakly laminated to homogeneous deposits; and (iv) an acoustically stratified to transparent unit of brackish‐water, organic‐rich sediment drifts. The debris‐flow deposits of the unit 2 pass laterally into slide scars that truncate the unit 1; they are interpreted to result from a time interval of intense seismic activity due to bedrock stress release shortly after deglaciation of the area. Ice‐berg scouring and gravitational failure of oversteepened depositional slopes may also have contributed to the debris‐flow deposition. Comparisons to other late‐Neogene glaciated basins, such as the Hudson Bay or glacial lakes formed along the Laurentide ice sheet, suggest that the Archipelago Sea succession may record development typical for the deglaciation phase of large, low relief, epicontinental basins. The Carboniferous–Permian glacigenic Dwyka Formation in South Africa may provide an ancient analogue for the studied succession. Chronological control for the studied sediments is provided by the independent palaeomagnetic and AMS‐¹⁴C dating methods. In order to facilitate dating of the organic‐poor early post‐glacial deposits of the northern Baltic Sea, the 10 000 year long Lake Nautajärvi palaeomagnetic reference chronology ( Ojala & Saarinen, 2002 ) is extended by 1200 years.     

Annual and interannual variation in boreal forest aerosol particle number and volume concentration and their connection to particle formation

We investigated size-resolved submicrometre aerosol particle number and volume concentration time series as well as aerosol dynamic parameters derived from Differential Mobility Particle Sizer (DMPS) measurements at five background stations in the Nordic boreal forest area. The stations in question were Aspvreten, Hyytiälä and Utö in southern Finland and Sweden, and Värriö and Pallas in the Finnish Lapland. The objective of our investigation was to identify and quantify annual and interannual variation observable in the time series. We found that the total number and mass concentrations were much lower at the Lapland stations than at the southern stations and that the total particle number was strongly correlated to particle formation event frequency. The annual total number concentration followed the annual distribution of particle formation events at the southern stations but much less clearly at the Lapland stations. The volume concentration was highest during summer, in line with higher condensation growth rates; this is in line with the assumption that a large part of the particle volume is produced by oxidized plant emissions. The decrease of sulphate emissions in Europe was not visible in our data set. Aerosol dynamic parameters such as condensation sink, condensation sink diameter and the power law exponent linking coagulation losses and condensation sink are presented to characterize the submicron Nordic background aerosol.     

Comparing X-ray densitometry and BSE-image analysis of thin section in varved sediments

A 10-cm long section from varved sequence of Lake Nautajärvi, Finland, has been analysed using both X-ray densitometry and image analysis of Backscattered Scanning Electron microscope Images (BSEI) of thin sections. Lake Nautajärvi's clastic-organi c varves appear in X-ray images as a succession of dark and pale stripes, pale ones being the spring detrital minerogenic layers and dark ones organic matter laid down during the time of summer, autumn and winter ice cover. Owing to the sharp contrast of the minerogenic spring layer, a semiautomatic tree-ring width and density measurement DendroScan system was applied to obtain seasonal/annual data of relative X-ray density. Mean density of BSEI was also acquired. Then BSEI were processed to produce black and white images, where white pixels represent the clay-rich sedimentary matrix and black pixels the particles in the matrix: silt- or sand-sized, terrigenous grains, authigenic particles and diatoms. Measurements of the size of the detrital particles forming the varves were obtained. For each varve, mean grey-level values in BSEI correlated well with the mean grey-level values of X-ray images, but grain size obtained on BSEI did not correlate with any parameters computed from X-ray radiography.