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Reed SCHERER Northern Illinois Univ. DeKalb, IL, USA

WAIS Meeting, Isabel Alley, September 20, 2003. “ What would it take?” requires “ How would we know?” Meditations on identifying past WAIS “collapse” events. Reed SCHERER Northern Illinois Univ. DeKalb, IL, USA. What’s the latest?.

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Reed SCHERER Northern Illinois Univ. DeKalb, IL, USA

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  1. WAIS Meeting, Isabel Alley, September 20, 2003 “What would it take?”requires“How would we know?”Meditations on identifying past WAIS “collapse” events Reed SCHERER Northern Illinois Univ. DeKalb, IL, USA

  2. What’s the latest? Is “rapid” collapse of the WAIS still a generally accepted model? And what the heck does “rapid” mean, anyway? Is climate forcing the key?Are basal processes key?What about the role of ice shelves?

  3. After 30 years of study: More known knowns More known unknowns Hopefully fewer unknown unknowns • We now know that some significant changes are faster than even Terry Hughes would have predicted! • Tidal modulation of ice stream flow • Ice shelf collapse

  4. Have we come full circle?What are the lessons from Larsen Ice Shelf collapse, March 5, 2002? Collapse due to warming Feeder glaciers simultaneously accelerate (DeAngelis & Skvarca, 2003) Buttressing hypothesis revisited?

  5. So: “What would it take?” • Addressing this question calls for: • Has it happened before? • If so, • When? • Under what conditions? • Was it catastrophic or progressive?

  6. Geologic records can address: • “If” • “When” • “Under what conditions” • but “how rapid” is tough

  7. To identify past collapse events (if & when) in geologic records requires asking“How would we know?”

  8. Geologic records may be • proximal to the ice sheet • ice cores • glacial-geologic records • sub-glacial sediments • distal • hemispheric or global effects in deep- • sea cores

  9. 18O & 13C records IRD recordsSST estimates Deep-sea (proxy) data include:

  10. Deep-sea cores have the advantage of good chronostratigraphy, but WAIS collapse signal is equivocal Fundamentally, we really don’t know what the oceanographic effects of WAIS collapse would be!

  11. Distal: Marine Isotope Stage 11 (400ka) the most likely candidate ODP Site 1090, Becquey & Gersonde, 2002

  12. bathymetry Proximal evidence Siple Coast Cape Roberts

  13. Hot water drilling and coring at Upstream B camp (Whillans Ice Stream)

  14. Proximal evidence for late Pleistocene WAIS “collapse” Late Quaternary age diatoms and cosmogenic isotopes (10Be) (Scherer et al., 1998) Remains the only “direct” evidence

  15. The Cape Roberts Project Antarctic near shore stratigraphic drilling from a multi-year fast-ice platform

  16. 31.89 IRD interval, including a large granite boulder CRP-1 Unit 3.1 2 m carbonate-rich interval Up to 80% biogenic carbonate Deposited during a single, unusual interglacial period 33.82

  17. Surface water paleoenvironment • Diatoms: dominantly pelagic (open ocean) • Abundant subpolar forms (up to 38%) • Subpolar calcareous plankton • High primary productivity – Shallow ML • Abundant Chaetoceros • Sea-ice related diatoms are rare • = significantly warmer surface waters

  18. Does this mean WAIS collapsed during MIS-31? Don’t know…Seems likely at least no big Ross Ice Shelf

  19. Precisely Dated as MIS-31 by the base of the Jaramillo (1.072 Ma), picked by diatoms, Sr/Sr and Ar/Ar dating

  20. What is the global significance of MIS-31?Very precise global correlation is possible!

  21. High latitude Southern Ocean Site 1094 reflectance (Gersonde et al., 1999)

  22. High latitude North Atlantic – Site 983 Kleiven et al., 2003

  23. North Atlantic planktic record Koç, Hodell, Kleiven, and Labeyrie, 1998

  24. Why MIS-11 and MIS-31? Unknown unknowns among the known knowns

  25. “How would we know?” Targeted modeling can help interpret proxy records Plug for Slawek’s talk (Though his model only runs for 40 years)

  26. WAIS collapsed at least once during the late Pleistocene – almost certainly during MIS-11Nearshore warmth during MIS-31likely incompatible with WAISNo idea about other possible Quaternary eventsDifficult to say whether complete collapse was catastrophic, but seems likely Knowns/unknowns

  27. WAIS -apollo Conclusions To identify past WAIS collapse: We need both proximal and distal geologic records We need better models of what triggers WAIS collapse and the oceanographic response The future?

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