The cause for the onset of Northern Hemisphere glaciation (NHG) at ~2.7 Ma remains widely debated. One hypothesis involves the development of a strong, permanent halocline in the North Pacific Ocean, which occurred at the same time as and may have induced NHG through its effects on Northern Hemisphere climate. The cause for the development of the North Pacific halocline, however, remains unknown. I will test the hypothesis that the halocline formed due to rapid uplift of the Alaska Range and Alaskan Coastal Mountains (>1 km/Myr) starting ~5 Ma, creating an orographic rain shadow that reduced export of atmospheric freshwater from the North Pacific. To accomplish this, I will collect ~30 dated volcanic tephras deposited between ~8 Ma and 100 ka from the interior of Alaska. The associated δ¹⁸O meteoric water signal contained in the tephras' weathered smectites will be used to constrain timing of changes in paleoelevation and associated rain-shadow dynamics. This eight million year record of δ¹⁸O values of meteoric water will have implications for uplift history in Alaska, paleoelevation, regional tectonics and the cause of the onset of Northern Hemisphere Glaciation.