No abstract

Multibeam sonar surveys in the past decade, augmented by single-beam data from the OLEX charting system, reveal landsystems on Atlantic Canadian shelves that are diagnostic of Late Wisconsinan ice-sheet dynamics. Four landsystems are described. (1) The Bay of Fundy landsystem comprises two contrasting sets of bedforms, and is interpreted as evidence of topographically controlled fast-flowing ice adjacent to slower-moving ice.(2) The German Bank landsystem off southwest Nova Scotia is comprised of glacially fluted terrain overprinted by De Geer moraines and arcuate recessional moraines. We infer that a flow of grounded glacial ice out of the Bay of Fundy was followed by steady retreat, punctuated by at least one major re-advance. (3) The Placentia Bay landsystem consists of a convergent field of streamlined landforms with superimposed De Geer moraines, overprinted in one area by flutings. We infer that this landsystem was formed in the onset zone of fast-flowing ice, and that overprinting was due to a re-advance of ice from offshore. (4) The south coast of Newfoundland landsystem, which includes arcuate, fjord-mouth moraines and a coast-parallel, fluted moraine, indicates strong topographic control on a retreating marine ice margin as it reached a fjord coastline. These submarine glacial landsystems are not inconsistent with a conceptual model showing Late Wisconsinan ice advance to shelf edges, rapid calving retreat along deepwater channels and slower retreat of ice margins grounded in shallow water. The re-advances documented two of the study areas have parallels in the Last British Ice Sheet, confirming that the reorganization of marine-based ice sheets, caused by calving in embayments, led to internally forced re-advances.

2000 (December): Submarine and onshore end moraines in southern Newfoundland: implications for the history of late Wisconsinan ice retreat. Boreas, Vol. 29, Recessional positions of the Newfoundland ice sheet 14-9 ka BP are represented by fjord-mouth submarine moraines, fjord-head emerged ice-contact marine deltas, and inland moraine belts. The arcuate submarine moraines have steep frontal ramparts and comprise up to 80 m of acoustically incoherent ice-contact sediment (or till) interfingered distally with glaciomarine sediment that began to be deposited c. 14.2 ka BP. The moraines formed by stabilization of ice that calved rapidly back along troughs on the continental shelf. The ice front retreated to fjord-heads and stabilized to form ice-contact delta terraces declining in elevation westward from 26 m to just below present sea level. Stratified glaciomarine sediments accumulated in fjords, while currents outside fjords eroded the upper part of the glaciomarine deposits, forming an unconformity bracketed by dates of 12.8 and 8.5 ka BP. The delta terraces are broadly correlated with the 12.7 ka BP Robinson's Head readvance west of the area. The ice front retreated inland, pausing three or four times to form lines of small bouldery stillstand moraines, heads of outwash, sidehill meltwater channels, and beaded eskers. Lake-sediment cores across this belt yield dated pollen evidence of three climatic reversals to which the moraines are equated: the Killarney Oscillation c. 11.2 ka BP, the Younger Dryas chronozone 11.0-10.4 ka BP, and an unnamed cold event c. 9.7 ka BP. Relative sea level fell in the early Holocene because of crustal rebound, so that outwash and other alluvium accumulated in deltas now submerged due to relative sea-level rise.

Tidal models for the Bay of Fundy, Canada -site of the highest recorded modern tide -show that tidal amplification began in the early Holocene and by ca. 5000 BP the range was almost 80% of the present range. Empirical data consisting of 146 sea-level index points and other observations appear to contradict model results. Aggregated relative sea-level data for Chignecto Bay and Minas Basin show that rapid tidal expansion began ca. 3400 BP. However, if we separate these two geographically separate data sets, evidence for this rapid late-Holocene tidal expansion is confined to Minas Basin. We explain this singularity by positing a barrier at the mouth of Minas Basin, at the Minas Passage, that delayed tidal expansion. With the rapid breakdown of this barrier and near-instantaneous tidal expansion, water temperature dropped, tidal currents and turbidity increased, and the form of the inner estuary was changed from lagoonal-mesotidal to macrotidal. We argue that the catastrophic breakdown of the barrier is related in the aboriginal legend of Glooscap, showing that aboriginal peoples observed the rapid environmental changes and preserved an oral record for 3400 years.Résumé : Les modèles pour les marées dans la baie de Fundy, Canada -lieu des plus fortes marées enregistrées dans les temps modernes -montrent que l'amplification des marées a débuté à l'Holocène précoce et que vers 5000 ans avant le présent, l'étendue des marées était d'environ 80 % de l'étendue actuelle. Des données empiriques comportant 146 points indiquant le niveau de la mer et d'autres observations semblent contredire les résultats du modèle. Les données agrégées pour le niveau relatif de la mer pour la baie Chignecto et le bassin Minas montrent qu'une rapide extension des marées a débuté vers 3400 ans avant le présent. Toutefois, si nous séparons ces deux ensembles de données géographiquement distinctes, les preuves pour cette expansion rapide des marées à l'Holocène tardif se limitent au bassin Minas. Nous expliquons cette singularité en positionnant une barrière à l'embouchure du bassin Minas, au passage Minas, retardant ainsi l'expansion des marées. Avec la destruction rapide de cette barrière et l'expansion presque instantanée des marées, la température de l'eau a chuté, les courants de marée et la turbidité ont augmenté et la forme de l'estuaire interne a changé de lagunaire-mésotidal à macrotidal. Nous postulons que la destruction catastrophique de la barrière soit reliée à la légende autochtone de Glooscap, montrant que les peuples autochtones ont observé les changements environnementaux rapides et en ont conservé une tradition orale depuis 3400 ans.[Traduit par la Rédaction]