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

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?

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 After 30 years of study: More known knowns More known unknowns Hopefully fewer unknown unknowns

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?

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?

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

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

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

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

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!

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

bathymetry Siple Coast Cape Roberts Proximal evidence

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

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

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

CRP-1 Unit m carbonate-rich interval Up to 80% biogenic carbonate Deposited during a single, unusual interglacial period IRD interval, including alarge granite boulder

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 Surface water paleoenvironment

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

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

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

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

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

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

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

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

Knowns/unknowns WAIS collapsed at least once during the late Pleistocene – almost certainly during MIS-11 Nearshore warmth during MIS-31 likely incompatible with WAIS No idea about other possible Quaternary events Difficult to say whether complete collapse was catastrophic, but seems likely

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?