Relative Sea-Level Change in the Northern Strait of Georgia, British Columbia*

Abstract

Twenty-four new radiocarbon dates from isolation basin cores, excavations and natural exposures, and an archeological site, constrain relative sea-level change since the last glaciation in the northern Strait of Georgia, British Columbia. Relative sea level fell rapidly from about 150 m elevation to 45 m elevation from 11 750 to 11 000 BP (13 750 to 13 000 cal BP), then its rate of fall slowed. The initial rapid emergence began soon after the transition from proximal to distal glaciomarine sedimentation, when the glacial front retreated from the Strait of Georgia and the Earth’s surface was unloaded. A sea-level lowstand a few metres below present-day sea level may have occurred in the early Holocene, but sea level was near its present level by 2000 BP. Sea-level change in the northern Strait of Georgia lagged the mid Strait of Georgia, 80 km to the south, by a few hundred years during initial emergence. The lowstand in the northern strait was later and probably shallower than in the mid strait. Isostatic depression inferred from the sea-level observations can be fit with two decaying exponential terms with characteristic decay times of 500 and 2600 years. The faster decay time corresponds to a shallow mantle viscosity of about 1019 Pa s, consistent with previous glacio-isostatic modelling. The present-day crustal uplift rate from the residual isostatic effects of the Cordilleran Ice Sheet is about 0.25 mm/a. Crustal uplift is not expected to significantly ameliorate projected sea-level rise in the mid and northern Strait of Georgia because present-day vertical crustal movements are inferred to be small.Vingt-quatre nouvelles dates au 14C provenant de carottes sédimentaires de bassins isolés, d’excavations, de coupes naturellement exposées et archéologiques permettent de cerner les changements du niveau marin relatif depuis la dernière glaciation dans la partie nord du détroit de Géorgie, en Colombie-Britannique. Le niveau marin relatif a rapidement passé de 150 m à 45 m d’altitude entre 11 750 et 11 000 ans BP (13 750-13 000 cal. BP), le taux d’abaissement ayant ralenti par la suite. L’émersion initiale rapide correspond à la fin de la sédimentation glaciomarine distale qui accompagnait le retrait du front glaciaire. Un bas niveau de quelques mètres sous le niveau actuel a pu s’instaurer durant l’Holocène inférieur, jusqu’en 2000 ans BP. Au début, le soulèvement isostatique a accusé un retard de quelques siècle sur celui de la zone du détroit à 80 km plus au sud. Le bas niveau de la partie nord du détroit est survenu plus tard et fut moins prononcé. Les données indiquent un enfoncement isostatique de type exponentiel avec des constantes de désintégration de 500 et de 2600 ans 14C. Le taux le plus rapide correspond à une viscosité du manteau terrestre d’environ 1019 Pa s, ce qui concorde avec les résultats de la modélisation glacio-isostatique. Le taux de soulèvement glacio-isostatique actuel résultant de l’inlandsis de la Cordillère s’établit à environ 0,25 mm/a. Ce taux ne peut contrer les effets du rehaussemen...

Post-glacial sea-level change along the Pacific coast of North America

“…Further north on Quadra and Cortes islands in the northern Strait of Georgia, RSL dropped from above þ146 m amsl after 13.8 ka BP to þ46 m amsl by 13.4 to 12.9 ka BP (James et al, 2005). A low stand in the northern Strait of Georgia has not been identified, although James et al (2005James et al ( , 2009a suggested that a low stand a few meters below present was probably reached prior to 10 ka BP. They argue that after 5 ka BP, RSL rose to, or slightly below þ1.5 m amsl by 2 ka BP.…”

“…From 2 ka BP to the present, RSL dropped to its present level (Fig. 6) (James et al, 2005). Further still at Port McNeill on northeast Vancouver Island, marine shells at þ53 m amsl record a high stand around 13.9 to 12.9 ka BP (Howes, 1983) (Fig.…”

Deglaciation landforms and C-14 chronology of the Lac Guillaume-Delisle area, eastern Hudson Bay: A report on field evidence

“…Corner et al, 2001;Long et al, 2003;Bentley et al, 2005;James et al, 2005;Sparrenbom et al, 2006aSparrenbom et al, , 2006b). This approach is even viewed by some authors as the preferred method for obtaining high-resolution information on sea-level changes (Hutchinson et al, 2004a).…”

Holocene Relative Sea-Level Changes from Near-, Intermediate-, and Far-Field Locations

“…Deglacial RSL histories of the Pacific coast [75][76][77][78][79][80][81][82][83][84][85] are supplemented by studies of the earthquake and tsunami history of the Cascadia subduction zone along the coasts of British Columbia, Washington, Oregon, and northern California [86][87][88][89][90][91][92][93].…”