NORTHEAST PACIFIC COOLING AND FRESHENING EVENTS DURING EARLY HEINRICH STADIAL 1 AND THE YOUNGER DRYAS: LINKS TO THE MISSOULA MEGAFLOODS? (Invited Presentation)

During the last deglaciation (19-12 ka), a series of megafloods sourced from glacial Lake Missoula are estimated to have repeatedly funneled 0.5-10 sverdrups (10⁶ m³ s^-1) of freshwater through the Columbia River. The subsequent dispersal pathways and regional impacts of this freshwater influx to the Northeast Pacific are largely unknown. In order to evaluate changes in surface salinity and accompanying impacts on subsurface ventilation, we compiled new and existing records of alkenone-based Uk’37 sea surface temperature (SST) estimates, reconstructions of local surface salinity from foraminiferal δ¹⁸O_seawater, and subsurface records of ventilation inferred from benthic-planktic radiocarbon age differences within marine sediment cores spanning 40-60°N in the Northeast Pacific. Surface salinity reconstructions indicate episodes of surface freshening during the early part of Heinrich Stadial 1 (~18.5-17 ka), a brief freshening event in the Bølling-Allerød (~14.3-14.0 ka), and a relatively sustained interval of surface freshening during the Younger Dryas (12.9-11.7 ka). Sea surface cooling events accompanied intervals of reduced sea surface salinity, with coldest temperatures occurring during early Heinrich Stadial 1 (17.5 ka) and the Younger Dryas (12.5 ka). Benthic-planktic radiocarbon ages indicate aged deep water masses during both Heinrich Stadial 1 and the Younger Dryas, concurrent with sea surface freshening and cooling. Global climate models simulate regional to hemispheric cooling in response to freshwater influx in the Northeast Pacific, whereas they typically simulate warming in the North Pacific in response to North Atlantic freshwater forcing; thus we infer that cooling during the early part of Heinrich Stadial 1 and the Younger Dryas were at least in part driven by North Pacific deglacial freshening events – possibly associated with the Missoula megafloods.