Timing of Cordilleran Ice Rafting, Freshwater Discharge, and Implications for Subsurface Ventilation in the Northeast Pacific During the Last Deglaciation

Abstract:

An ocean seesaw has been proposed to operate between the North Pacific and North Atlantic, such that enhanced deep or intermediate water is formed in the North Pacific during times when deepwater formation is suppressed in the North Atlantic. Data and modeling studies have suggested a North Pacific deep water mass (NPDW) may have developed during Heinrich Stadial 1 (~18 – 15 ka, HS1), a time when the strength of the Atlantic meridional overturning circulation (AMOC) was reduced. However, the flux of ice rafted debris (IRD) in sediment cores from the northeast Pacific has been observed to peak from ~18 – 16.5 ka, implying that ice-melt reduced the surface salinity during at least part of the time period implicated for NPDW formation. We developed high resolution records of sea surface temperature based on the alkenone Uk’37 unsaturation index, paired with planktonic foraminiferal oxygen isotopes in marine sediment cores from the Gulf of Alaska (GOA), allowing us to reconstruct changes in the δ¹⁸O of seawater (related to surface salinity) throughout the last deglaciation. These records indicate generally higher surface ocean salinities in the GOA during 18 and 16 ka, implying relatively low freshwater inputs to the Alaska Coastal Current at these times. However, superimposed on this trend are brief freshening events and large pulses of IRD occurring between 17.5 - 16.5 ka, which suggests roughly synchronous timing for major ice rafting sourced from both the Cordilleran (Pacific) and Laurentide (Atlantic) ice sheets. Radiocarbon evidence indicates reduced subsurface ventilation of the northeast Pacific during HS1, making a scenario of sustained (millennial-scale) NPDW formation appear unlikely.