Sedimentological and chronological aspects of the Younger Dryas : Holocene transition record in southern Finland and northern Baltic
Year: 2012
Type: dissertation
Abstract: In this study, different types of sediments deposited in the Baltic Sea Basin in Southern Finland and the Gulf of Finland before and after the Baltic Ice Lake (BIL) drainage were examined. The aim was to gain a better understanding of changes in sedimentation in offshore, shallow water and onshore beach environments, to provide an independent age control for the drainage event, and to test the applicability of dendrochronological cross-correlation methods to varve clay data. The study consisted of acoustic sounding data from offshore, one offshore marine sediment core, six outcrops related to the BIL/Yoldia Sea transition sediments, and a digitized version of original varve measurements by Sauramo. In the Baltic basin area, the drainage of the BIL occurred close to end of the Younger Dryas cold event. This sudden 25 28 m fall in water level had originally been chosen as the zero datum for the Finnish varve clay chronology, but the "key horizon" concept was not developed further and its chronostratigraphical connection and importance remain unclear. Since the early 20th century annually laminated, or varved, sediments have been used succesfully in constructing Late Pleistocene - Holocene ice retreat chronologies and in dating ice marginal formations. This also applies to Finland. Correlating varve chronologies across the Salpausselkä zone is difficult, due to slow ice retreat rates and ice front oscillations during the Younger Dryas period. Therefore, the older part of the chronology, which pre-dates the drainage event, is only loosely connected to Holocene varve series. In offshore environment (water depth > 40 m), the falling water level triggered debris flows, which eroded and redeposited older, varved sediments creating a distinct deformation unit. In the northern Baltic proper and Gulf of Finland, up to 4 m thick, discontinuous deposits of homogeneous clay bearing traces of rotational slump and deformation were deposited. In Jokela, where the original zero varve was first described by Sauramo (1923), a homogeneous clay unit containing deformed sandy pods and layers was observed. This unit corresponds to a zero varve which was formed as a consequence of a sudden water level drop in the BIL. In shallow water environment (water depth PIENEMPI 40 m), deposition of varved sediments ceased and, as a result of rapid regression, shore processes started to operate, during which progressive marine terraces were formed. The first signs of saline incursion into southern Finnish area were ca 100 varve years after the BIL drainage. On newly emerged land, exposed sediments were prone to wind erosion. The occurrence of massive cover sands, "Lammi loess", in the Second Salpausselkä area has traditionally been attributed to rapid dust-storm type of deposition. However, there are also indications of non-aeolian origin of these deposits. Morphologically well-defined coastal terraces are related to the oldest Yoldia Sea level (YI), which was developed after the BIL drainage. One of these terraces within the First Salpausselkä zone was dated by optically stimulated luminescence (OSL) method. This is the first direct YI-level date in Finland and it yielded ages of 11 200 11 400 ± 2 700 years. The finding highlights the potential of shore terraces in verifying Lateglacial early Holocene varve chronology in Finland. Another approach to strengthen the deglaciation chronology could be applying to clay varve data the statistical methods used in dendrochronology.This approach separates the local variation within sedimentary basin from the climatic signal, and enables varve correlations over longer distances. The main findings can be summarized as following: -The BIL drainage was a basin-wide sedimentological event, leading to the deposition of a distinct drainage varve facies expressed in the annually laminated glaciolacustrine sediment as a debris-flow unit and in the shallow water sediment as the change into non-annual deposition rhythm. -After the BIL drainage, freshwater conditions prevailed in the early Yoldia sea phase for at least ca 100 200 years. -The first OSL-dating of oldest Yoldia Sea terrace (YI) in Finland gave ages of 11 200 and 11 400 ± 2 700 years. This gives also a minimum age to the BIL drainage. -Although drainage facies makes a good stratigraphic marker horizon, the erosion and redeposition related to drainage event combined with a possible hiatus in sedimentation make drainage varve facies problematic as a chronostratigraphic clay varve key horizon.
Source: Työväentutkimus Vuosikirja
Author: Outi Hyttinen
Language: English
Subfield: Oceanography
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W774146193