Foraminiferal Evidence of Younger Dryas Age Cooling on the British Columbia Shelf
R. Timothy Patterson
Jean-Pierre Guilbault
Richard E. Thomson
John L. Luternauer
ABSTRACT
Cluster analysis of foraminifera from a ~12,000-9000 radiocarbon year old piston core from Goose Island Trough, Queen Charlotte Sound, indicates that a cold interval correlative with the Younger Dryas stadial occurred during a shallow water phase. The reduction in depth was caused by the passage across the area, between 11,500 and 10,000 years BP, of a glacial forebulge associated with the retreat of the Late Wisconsinian ice sheets. Published sedimentological evidence indicate that water depths decreased to ~75-90 m, placing the site above the permanent North Pacific pycnocline (100 m). Low salinity-near glacial conditions, at these depths, between -11,100 and 10,000 years BP were recognized by abundant populations of Cassidulina reniforme and lslandiella helenae. This cold interval has also been recognized in cores from elsewhere in Queen Charlotte Sound. The depressed salinity and temperature may have resulted from a modification of regional weather patterns. Decreased mean continental summer temperatures could have reduced the seasonal influence of the North Pacific High and lengthened that of the Aleutian Low. This would have resulted in a near continuous onshore surface Ekman transport and enhanced coastal runoff, effectively blocking the movement onto the shelf of deep, saline, warm water of the California Undercurrent. The resultant isolated inshore basin comprised of present-day Hecate Strait and Queen Charlotte Sound is tentatively named the "Hecate Sea". By ~10,000 years BP, weather and ocean circulation had returned to near modern patterns as indicated by the disappearance of lslandiella helenae and by the development of an Epistominella vitrea - dominated biofacies.
RÉSUMÉ
lndications foraminiférologiques d’un refroidissement du Dryas récent sur la plate-forme de la Colombie-Britannique
L'analyse des groupements effectuée sur des populations de foraminifères d'une carotte à piston couvrant la période de 12 000 à 9000 ans BP, provenant du Goose Island Trough, dans le bassin de la Reine-Charlotte, démontre l'existence d'un intervalle froid contemporain du Dryas récent. Il coïncide avec une phase à bathymétrie réduite qui résulte du déplacement vers l'est, entre 11 500 et 10 000 ans BP, du bombement isostatique associé à l'Inlandsis de la Cordillière à la suite de son retrait. Des données sédimentologiques publiées montrent que la profondeur a diminué jusqu'à ~75-90 m, amenant le site étudié au-dessus de la pycnocline permanente du Pacifique Nord (100 m). L'existence de conditions hyposalines de type glaciaire, à ces profondeurs, entre 11 100 et 10 000 ans BP ont été reconnues grâce à la présence d'assemblages à Cassidulina reniforme et lslandiella helenae. Cet intervalle froid a également été reconnu ailleurs dans le bassin de la Reine-Charlotte. Les salinités et les températures réduites sont peut-être le résultat d'un changement dans les configurations météorologiques régionales. Des températures estivales plus basses sur le continent auraient réduit l'influence saisonnière de l'anticyclone du Pacifique Nord et rallongé celle de la dépression des Aloutiennes. Ceci aurait engendré un transport d'Ekman de surface quasi permanent vers la côte et provoqué une augmentation des précipitations, ayant eu pour effet de bloquer la remontée des eaux salées et tièdes du sous-courant de Californie. Nous proposons d'appeler « Mer d'Hécate » le bassin côtier ainsi créé comprenant l'actuel détroit d'Hécate et le bassin de la Reine-Charlotte. Dès avant ~10 000 ans BP, les circulations atmosphérique et océanique étaient revenues à une configuration presque moderne.
ZUSAMMENFASSUNG
Foraminiferen-Belege über die Abktühlung in der jüngeren Dryaszeit auf der Terrasse von British Columbia
Die Analyse der Foraminiferen-Anhàufungen von einem -12,000-9000 Radiokarbonjahre alten Bohrkern von Goose Island Trough im Queen Charlotte Sund zeigt, daB ein kaltes Intervall gleichzeitig mit dem jùngeren Dryas-Stadium wàhrend einer Phase seichten Wassers eintrat. Die Verringerung der Tiefe wurde durch die Passage einer glazialen Emporwôlbung zwischen 11,500 und 10,000 Jahren v.u.Z. quer durch das Gebiet bewirkt, zusammen mit dem Rùckzug der Eisdecken vom spàten Wisconsinium. Die verôfent-lichten sedimentologischen Belege zeigen, daf3 die Wassertiefen auf -75-90 m abnahmen, und so den Platz ùber die stàndige Dichte-Gradient des Nordpazifik (100 m) stellten. Fast glaziale Bedingungen mit niedrigem Salzgehalt wurden in diesen Tiefen zwischen -11,100 und 10,000 Jahren v.u.Z. auf Grund von reichlichen Populationen von Cassidulina reniforme und lslandiella helenae erkannt. Der verringerte Salzgehalt und die niedrigere Temperatur kônnten auch durch eine Ànderung der regionalen Wetterkonstellationen bewirkt worden sein. Gesunkene durchschnittliche kontinentale Sommertemperaturen kônnten den jahreszeitlichen EinfluB des nordpazifischen Hochs verringert und den des atlantischen Tiefs verlàngert haben. Das kônnte einen fast kontinuierlichen Ekman-Transport an der Oberflâche zur Kùste hin bewirkt und die Kùsten-Niederschlàge verstàrkt haben, und so ware die Bewegung von tiefem, salzhaltigem warmem Wasser der kalifornischen Unterstromung zur Terrasse hin blockiert worden. Das daraus resultierende isolierte Binnen-Becken, bestehend aus der heutigen Hecate-Meerenge und dem Queen Charlotte Sund, wird probeweise "Hecate Sea" genannt. Um -10,000 Jahre v.u.Z. sind Wetter und Ozeanstromung zu nahezu modernen Mustern zuruckgekehrt.
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| Auteurs : | R. Timothy Patterson, Jean-Pierre Guilbault, Richard E. Thomson et John L. Luternauer |
|---|---|
| Titre : | Foraminiferal Evidence of Younger Dryas Age Cooling on the British Columbia Shelf |
| Revue : | Géographie physique et Quaternaire, Volume 49, numéro 3, 1995, p. 409-427 |
| URI : | http://id.erudit.org/iderudit/033063ar |
| DOI : | 10.7202/033063ar |
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