The Role of the Pacific Ocean and Adjacent Cryosphere in Past Abrupt Climate Oscillations

Séminaire du Geotop / Geotop seminar

Événement hybride / Hybrid event:

En personne/In person:
LOCAL/ROOM 7605
201 ave. du Président-Kennedy, Montréal, QC

Zoom ID: 832 1632 7466
Passcode: isotope

Dre Maureen Walczak, University of Washington

"The Role of the Pacific Ocean and Adjacent Cryosphere in Past Abrupt Climate Oscillations"

Recent years have brought new depth and dimension to our understanding of the dynamic role of the Pacific Ocean in millennial-scale climate variability. Detailed chronologies suggest that changes in North Pacific circulation occur early in the sequence of millennial events over the past 50,000 years, but the mechanisms remain an area of inquiry. Here we present new radiocarbon-dated foraminiferal oxygen isotopic records derived from marine sediment cores (OC1706B-11JC, OC2006A-27JC) on the NE Pacific continental margin adjacent to the major southernmost drainages of the Cordilleran Ice Sheet – the Columbia River and Strait of Juan de Fuca. We find that discharge from these drainages over the past 21,000 years is most active when the adjacent NE Pacific is at its coldest – likely associated with an ice sheet that is advancing towards or has achieved its greatest southward extent. Available ice sheet models (e.g., GLAC-1D, ICE-6G) suggest that during periods when the Cordilleran Ice Sheet advances into the Columbia River drainage basin, >40% of the total discharge of the ice sheet may be routed through these outlets, with the drainage basin potentially crossing the continental divide if and when the ice sheet dome covers the Canadian Rockies. The Strait of Juan de Fuca and Columbia River reach the Pacific Ocean within the range of latitudes at which the northern limb of the North Pacific Subtropical Gyre (NPSG) bifurcates into the northward-flowing Alaskan Current (the eastern limb of the North Pacific Subpolar or Alaskan Gyre) and southward-flowing California Current (the eastern limb of the NPSG). Diversion of the subtropical-subpolar transition zone associated with a large ice sheet during glacial periods would likely increase return of freshwater via northward flowing Alaskan Current to the subpolar ocean, enhancing stratification and reduced overturning. However, when the ice sheet extends to its southernmost position an increased fraction of summer runoff from the ice sheet may more readily enter the southward-flowing California Current; increased net freshwater export from the subpolar North Pacific would export buoyancy southward and favor North Pacific overturning and enhanced northward heat transport. The NE Pacific – Pacific Northwest region may thus operate as a sensitive salinity (buoyancy) flux switch to trigger abrupt changes in the North Pacific overturning system with global consequences.

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