Deglaciation of Nova Scotia: Stratigraphy and chronology of lake sediment cores and buried organic sections
Rudolph R. Stea
Robert J. Mott
Abstract
The deglaciation of Nova Scotia is reconstructed using the AMS-dated chronology of lake sediments and buried organic sections exposed in the basins of former glacial lakes. Ice cleared out of the Bay of Fundy around 13.5 ka, punctuated by a brief read- vance ca. 13-12.5 ka (Ice Flow Phase 4). Glacial Lake Shubenacadie (1) formed in central Nova Scotia, impounded by a lobe of ice covering the northern Bay of Fundy outlet. Drainage was re-routed to the Atlantic Ocean until the Fundy outlet became ice free after 12 ka. When this lake drained, bogs and fens formed on the lake plain during climatic warming. Organic sediment (gyttja) began to accumulate in lake basins throughout Nova Scotia. Glacierization during the Younger Dryas period (ca. 10.8 ka) resulted in the inundation of lakes and lake plains with mineral sediment. The nature and intensity of this mineral sediment flux or "oscillation" varies from south to northern regions. Southern lakes simply record changes in total organic content whereas northern lakes, where most buried peat sections are found, feature a thick inorganic sediment layer. Glacial ice or permanent snow cover and seasonal melting are essential in the formation of this mineral sediment layer; both to provide the water source for erosion, and to prevent plant re- colonization and landscape stabilization. Some northern lakes do not appear to record the Younger Dryas event, with organic accumulation starting around 10 ka. During the Younger Dryas, fine and coarse-grained deposits were deposited in Glacial Lake Shubenacadie (2) and other lowland areas at elevations similar to former (12 ka) lake levels, impounded by re-invigorated residual ice caps and permanent snow/aufeis.
Résumé
La déglaciation en Nouvelle-Écosse : stratigraphie et chronologie à partir de carottes sédimentaires lacustres et de coupes de matériaux organiques enfouies.
Il s'agit d'une reconstitution à partir de la chronologie par spectométrie de masse (SMA) de sédiments lacustres et de certaines coupes de matériaux organiques enfouis dans les bassins d'anciens lacs glaciaires. La baie de Fundy, libre de glace vers 13,5 ka, a connu une récurrence vers 13-12,5 ka. Le Lac glaciaire Shubenacadie (1), qui s'est formé au centre de la Nouvelle-Écosse, a été endigué par un lobe glaciaire qui recouvrait la sortie de la baie de Fundy au nord. L'écoulement a alors été détourné vers l'océan Atlantique jusqu'au retrait de la glace vers 12 ka. Une fois le lac asséché, tourbières et marécages se formèrent sur l'ancien fond lacustre au cours du réchauffement climatique. Ensuite, la gyttja s'accumula dans les bassins lacustres à travers la Nouvelle-Écosse. L'englacement au cours du Dryas récent (vers 10,8 ka) a entraîné le comblement des lacs et des plaines lacustres par des sédiments minéraux.Lanatureetl'intensitédecefluxdesédiments ou « oscillation » varient du sud au nord. Dans les lacs du sud, la teneur organique totale a simplement varié, tandis que dans les lacs du nord, une importante couche de sédiment inorganique a été déposée. Certains lacs du nord ne semblent pas avoir enregistré l'épisode du Dryas récent, l'accumulation de matière organique ayant commencé vers 10 ka. Au Dryas récent, des dépôts à grains fins et grossiers ont été mis en place dans le Lac Shubenacadie (2) et autres basses terres à des altitudes comparables aux anciens niveaux lacustres (12 ka), endigués par des calottes glaciaires résiduelles ravivées et de neige permanente ou de lentilles de glace.
zusammenfassung
Die Enteisung von Nova Scotia: Stratigraphie und Chronologie von See-Sedimentkernen und Torf-Profilen.
Man hat die Enteisung von Nova Scotia rekonstruiert, indem man mittels Massenspektroskopie die Chronologie von See-Sedimenten und vergrabenen organischen Profilen, die in den Becken ehemaliger glazialer Seen ausgesetzt sind, bestimmt hat. Etwa um 13,5 ka verschwand das Eis aus der Fundy-Bai, unterbrochen durch einen kurzen Rückvorstoß um etwa 13-12,5 ka (Eisflussphase 4). Der glaziale Shubenacadie-See (1), der sich im Zentrum von Nova Scotia bildete, wurde durch eine Eislobe eingedämmt, welche den Nordausläufer der Fundy-Bai bedeckte. Die Dränierung wurde zum Atlantischen Ozean umgeleitet bis der Fundy-Ausläufer nach 12 ka eisfrei wurde. Als dieser See austrocknete, bildeten sich Torfmoore und Fehne auf der See- Ebene während einer Klimaerwärmung. Organisches Sediment (Gyttja) begann sich in den See-Becken durch ganz Nova Scotia anzusammeln. Die Vereisung in der jüngeren Dryas-Zeit (etwa 10,8 ka) führte zur Anschwemmung der Seen und See-Ebenen mit Mineral-Sedimenten. Die Art und Intensität dieses Mineral-Sedimentflusses oder seiner Oszillation ist von Süden nach Norden unterschiedlich. Die südlichen Seen belegen einfach Veränderungen im gesamten organischen Gehalt, wohingegen die nördli- chen Seen, in denen die meisten vergrabenen Torfprofile gefunden werden, eine dicke anorganische Sedimentschicht aufweisen. Glaziales Eis oder eine permanente Schneedecke und jahreszeitbedingtes Schmelzen sind bei der Bildung dieser Mineral-Sedimentschicht entscheidend, sowohl, um die Wasserquelle für die Erosion bereitzustellen, als auch um die Wiederansiedlung von Pflanzen und die Stabilisierung der Landschaft zu verhindern. Einige nördliche Seen scheinen das jüngere Dryas- Ereignis nicht zu belegen, denn ihre organische Ablagerung begann um etwa 10 ka. Während des jüngeren Dryas wurden feine und grobkörnige Ablagerungen im glazialen Shubenacadie-See (2) und anderen Gebieten des Tieflands in Höhen, die den früheren See- Ebenen (12 ka) entsprechen, abgelagert, eingedämmt durch wiederbelebte Alluvialeiskappen und permanenten Schnee/Aufeis.
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| Auteurs : | Rudolph R. Stea et Robert J. Mott |
|---|---|
| Titre : | Deglaciation of Nova Scotia: Stratigraphy and chronology of lake sediment cores and buried organic sections |
| Revue : | Géographie physique et Quaternaire, Volume 52, numéro 1, 1998, p. 3-21 |
| URI : | http://id.erudit.org/iderudit/004871ar |
Tous droits réservés © Les Presses de l'Université de Montréal ,1998