Walczak Maureen H.

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Last Name
Walczak
First Name
Maureen H.
ORCID
0000-0002-4123-6998

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  • Article
    The role of northeast pacific meltwater events in deglacial climate change
    (American Association for the Advancement of Science, 2020-02-26) Praetorius, Summer K. ; Condron, Alan ; Mix, Alan C. ; Walczak, Maureen H. ; McKay, Jennifer L. ; Du, Jianghui
    Columbia River megafloods occurred repeatedly during the last deglaciation, but the impacts of this fresh water on Pacific hydrography are largely unknown. To reconstruct changes in ocean circulation during this period, we used a numerical model to simulate the flow trajectory of Columbia River megafloods and compiled records of sea surface temperature, paleo-salinity, and deep-water radiocarbon from marine sediment cores in the Northeast Pacific. The North Pacific sea surface cooled and freshened during the early deglacial (19.0-16.5 ka) and Younger Dryas (12.9-11.7 ka) intervals, coincident with the appearance of subsurface water masses depleted in radiocarbon relative to the sea surface. We infer that Pacific meltwater fluxes contributed to net Northern Hemisphere cooling prior to North Atlantic Heinrich Events, and again during the Younger Dryas stadial. Abrupt warming in the Northeast Pacific similarly contributed to hemispheric warming during the Bølling and Holocene transitions. These findings underscore the importance of changes in North Pacific freshwater fluxes and circulation in deglacial climate events.
  • Article
    Global reorganization of deep-sea circulation and carbon storage after the last ice age
    (American Association for the Advancement of Science, 2022-11-16) Rafter, Patrick A. ; Gray, William R. ; Hines, Sophia K. V. ; Burke, Andrea ; Costa, Kassandra M. ; Gottschalk, Julia ; Hain, Mathis P. ; Rae, James W. B. ; Southon, John R. ; Walczak, Maureen H. ; Yu, Jimin ; Adkins, Jess F. ; DeVries, Timothy
    Using new and published marine fossil radiocarbon (C/C) measurements, a tracer uniquely sensitive to circulation and air-sea gas exchange, we establish several benchmarks for Atlantic, Southern, and Pacific deep-sea circulation and ventilation since the last ice age. We find the most C-depleted water in glacial Pacific bottom depths, rather than the mid-depths as they are today, which is best explained by a slowdown in glacial deep-sea overturning in addition to a "flipped" glacial Pacific overturning configuration. These observations cannot be produced by changes in air-sea gas exchange alone, and they underscore the major role for changes in the overturning circulation for glacial deep-sea carbon storage in the vast Pacific abyss and the concomitant drawdown of atmospheric CO.
  • Article
    Ice and ocean constraints on early human migrations into North America along the Pacific coast
    (National Academy of Sciences, 2023-02-06) Praetorius, Summer K. ; Alder, Jay R. ; Condron, Alan ; Mix, Alan C. ; Walczak, Maureen H. ; Caissie, Beth E. ; Erlandson, Jon M.
    Founding populations of the first Americans likely occupied parts of Beringia during the Last Glacial Maximum (LGM). The timing, pathways, and modes of their southward transit remain unknown, but blockage of the interior route by North American ice sheets between ~26 and 14 cal kyr BP (ka) favors a coastal route during this period. Using models and paleoceanographic data from the North Pacific, we identify climatically favorable intervals when humans could have plausibly traversed the Cordilleran coastal corridor during the terminal Pleistocene. Model simulations suggest that northward coastal currents strengthened during the LGM and at times of enhanced freshwater input, making southward transit by boat more difficult. Repeated Cordilleran glacial-calving events would have further challenged coastal transit on land and at sea. Following these events, ice-free coastal areas opened and seasonal sea ice was present along the Alaskan margin until at least 15 ka. Given evidence for humans south of the ice sheets by 16 ka and possibly earlier, we posit that early people may have taken advantage of winter sea ice that connected islands and coastal refugia. Marine ice-edge habitats offer a rich food supply and traversing coastal sea ice could have mitigated the difficulty of traveling southward in watercraft or on land over glaciers. We identify 24.5 to 22 ka and 16.4 to 14.8 ka as environmentally favorable time periods for coastal migration, when climate conditions provided both winter sea ice and ice-free summer conditions that facilitated year-round marine resource diversity and multiple modes of mobility along the North Pacific coast.