Outi Hyttinen

The undatables: Quantifying uncertainty in a highly expanded Late Glacial-Holocene sediment sequence recovered from the deepest Baltic Sea basin-IODP Site M0063

Laminated, organic-rich silts and clays with high dissolved gas content characterize sediments at IODP Site M0063 in the Landsort Deep, which at 459 m is the deepest basin in the Baltic Sea. Cores ...

First direct age determination for the Baltic Ice Lake/Yoldia Sea transition in Finland

The drainage of the Baltic Ice Lake (BIL) to the Yoldia Sea level occurred at around 11 600 years ago and can be considered as one of the most important events in the Late-Glacial Baltic Basin history. Distinctive sandy deposits were formed as a result of the water-level fall. These deposits form a marker horizon at the Pleistocene–Holocene boundary, which has been used as the 0-varve in Finnish clay varve chronology. These sandy deposits have not been dated, and in general, there is lack of direct age determinations from the Pleistocene–Holocene transition. Sediments from three localities: backshore, beachface and upper shoreface environments, interpreted to be connected with BIL drainage, were sampled and dated with an optically stimulated luminescence (OSL) method. Only the beachface deposit appeared to be the suitable material, giving the expected dates of 11 400 ± 1100 and 11 200 ± 1300 years BP for the water-level fall. The other samples were interpreted as only partially bleached, or they represented a mixture of different age generations. This is the first direct dating of a Yoldia shore deposit in Finland and provides a reference for shore displacement studies in Finland. The results also verify the potential of the OSL method in dating ancient shorelines or land uplift history.

Depositional evidence of water-level changes of the Baltic Ice Lake in southern Finland during the Younger Dryas/Holocene transition

Two natural clay sections were described from southern Finland. The sites, Jokela and Koria, are located 5-10 km south of the 1st Salpausselkä end moraine. Six sedimentary units were identified and interpreted: (1) Baltic Ice Lake (BIL) varved sediments – interbedded, laminated clay and silt with load structures and sand layers in places; (2) homogenous, partly deformed clay; (3) BIL varved sediments, deposited in slightly shallower water – interbedded, laminated clay and silt; (4) BIL drainage from Billingen and the 25-m base-level fall 11 600-11 700 BP – homogenous clay with sand pods and discontinuous sand layers; (5) freshwater varves – clay and silt couplets with lenticular and flaser bedding and load structures and (6) brackish water varves – thick clay and thin silt couplets. These units can be associated with previously described units of the Finnish clay-varve chronology. The results confirm the importance of the drainage unit as a basin-wide key horizon and illustrate the differences in sedimentation in different parts of the Baltic Sea basin. This is evidenced by the diachronous nature of the incursion of brackish water and associated facies change in sediments.

Holocene stratigraphy of the Ångermanälven River estuary, Bothnian Sea

This study explores the Holocene depositional succession at the IODP Expedition 347 sites M0061 and M0062 in the vicinity of the Ångermanälven River estuary in the Bothnian Sea sector of the Baltic Sea in northern Scandinavia. Site M0061 is located in a coastal offshore setting (87.9 m water depth), whereas site M0062 is fully estuarine (69.3 m water depth). The dataset comprises acoustic profiles and sediment cores collected in 2007 and late 2013 respectively. Three acoustic units (AUs) were recognized. Lowermost AU1 is interpreted as a poorly to discontinuous stratified glaciofluvial deposit, AU2 as a stratified conformable drape of glaciolacustrine origin, and AU3 as a poorly stratified to stratified mud drift. A strong truncating reflector separates AU2 and AU3. Three lithological units (LUs) were defined in the sediment cores. LU1 consists of glaciofluvial sand and silt gradating into LU2, which consists of glaciolacustrine varves. A sharp contact interpreted as a major unconformity separates LU2 from the overlying LU3 (brackish-water mud). In the basal part of LU3, one debrite (site M0061) or two debrites (site M0062) were recognized. Information yielded from sediment physical properties (magnetic susceptibility, natural gamma ray, dry bulk density), geochemistry (total carbon, total organic carbon, total inorganic carbon and nitrogen), and grain size support the LU division. The depositional succession was formally subdivided into two alloformations: the Utansjö Alloformation and overlying Hemsön Alloformation; the Utansjö Alloformation was further subdivided into two lithostratigraphic formations: the Storfjärden and Åbordsön formations. The Storfjärden (sandy outwash) and Åbordsön (glaciolacustrine rhythmite) formations represent a glacial retreat systems tract, which started at ca. 10.6 kyr BP. Their deposition was mainly controlled by meltwater from the retreating ice margin, glacio-isostatic land uplift and the regressive (glacial) lake level. The Hemsön Alloformation (organic-rich brackish-water mud) represents a period of forced regression, starting possibly at ca. 9.5 kyr BP. At about 7 kyr BP, brackish water reached the study area as a result of the mid-Holocene marine flooding of the Baltic Sea Basin, but the rapid land uplift soon surpassed the associated (Littorina) transgression. Changed near-bottom current patterns, caused by the establishment of a permanent halocline, and the reduced sediment consistency caused by increased organic deposition resulted in a sharp and erosional base of the brackish-water mud. Estuarine processes and salinity stratification at site M0062 started to play a more important role. This study applies a combined allostratigraphic and lithostratigraphic approach over the conventional Baltic Sea stages. This approach makes it more straightforward to study this Baltic Sea deglaciation–postglacial sequence and compare it to other formerly glaciated shallow sea estuaries.

Late Weichselian varve archives re-explored to assess proglacial sedimentary chronologies using the principles of tree-ring analysis

Glaciogenic varves are formed annually in proglacial basins and provide detailed geochronological archives about the dynamics of deglaciation. Varve chronologies are constructed from multiple varve series measured at individual localities. Temporal connection between the series is traditionally achieved by visual comparison of varve-thickness variations. This connection method resembles that of dendrochronological cross-dating. By contrast, dendrochronological correlation proceeds through the routine of statistical (along with visual) comparisons between the tree-ring series, providing quantification of the dating results, their veracity and robustness. Here we demonstrate the advantages and convenience of applying tree-ring statistical methods to varve records, using an extensive varve archive originating from southern Finland. The data series of individual varve-thickness diagrams were metamorphosed by detrending, prewhitening and averaging, that is, by the methods adopted from tree-ring science. Subsequently, it was seen that the series having 80 or more varves have a higher potential to be unambiguously cross-dated than shorter series. The varve-thickness diagrams could be categorized into two types, those having regional chronological importance and those showing only subregional (distance < 20 km) correlativity. The varves of former type ought to be regarded as primary constituents of the geochronology. The varves of latter type have low geochronological validity although they may bear local sedimentological importance. The findings have the potential to modernize the science of varve-based geochronologies.