GULF OF ALASKA COLLISIONAL MOUNTAIN BUILDING--A FACTOR IN THE PROBABLE PLIOCENE DEMISE OF THERMOHALINE CIRCULATION IN THE BERING SEA BASIN AND THE CONSEQUENT CHILLING OF THE HIGH NORTH PACIFIC

INTRODUCTION: The latest Miocene collision of the Yakutat block with the easternmost rim of the Gulf of Alaska elevated and glaciated coastal mountains belts in southern and southeastern Alaska. Drainage from these highlands entered the west-flowing Alaska Stream, the northern arm of the north Pacific's CCW rotating Subarctic gyre. The voluminous (~9-10 Sv) Alaska Stream flows through the Bering Sea Basin by pouring northward across the Aleutian Ridge via between-island passes. THERMOHALINE CIRCULATION AND THE MEIJI DRIFT: Surface water entering the Bering Sea is presently too fresh by several parts per thousand to support winter-season thermohaline circulation (THC). However, the existence of a large, NW-SE trending sediment drift, the Meiji drift, in the northwestern corner of the Pacific Basin has been cited as evidence that a major cell of THC formerly operated in the Bering Sea Basin. Drift deposition began in the earliest Oligocene and evidently continues today at a reduced but unknown rate. When functioning, the Bering Sea cell of THC is conjectured to have exported water at depths of 3500-4000 m around the western end of the Aleutian Ridge to flow southward into the north Pacific along the path of Meiji drift. PLIOCENE CLIMATIC IMPACT: We explore the notion that surface water circulating in the Subarctic gyre until early or mid Pliocene time was made sufficiently salty with infused subtropical water to support THC in the Bering Sea Basin. By the mid Pliocene, possibly later, freshening of surface water entering the Bering Sea terminated THC. Surface water dilution can be attributed to upwelling-caused chilling and resulting lowering of surface evaporation. But we hypothesize that freshening was also a direct consequence of moisture trapping and increased runoff of water and ice from the rising early Pliocene mountain systems of the Gulf of Alaska. Collapse of THC would have cooled the Subarctic gyre by ending the southward, at depth, export of cold, densified surface water, and by inhibiting the intensified gyre's southward gathering of subtropical water. Yakutat orogenesis is thus posited to have been a factor in the Pliocene climatic deterioration of the high north Pacific and the contribution of this cooling to the onset of northern Hemisphere continental glaciation.