“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

18O & 13C 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 (10Be) (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

Antarctic near shore stratigraphic drilling from

a multi-year fast-ice platform

CRP-1 Unit 3.12 m carbonate-rich interval

Up to 80% biogenic carbonate

Deposited during a single, unusual interglacial period

33.82

31.89

IRD interval, including alarge granite boulder

• Diatoms: dominantly pelagic (open ocean)Abundant subpolar forms (up to 38%)

• Subpolar calcareous plankton

• High primary productivity – Shallow MLAbundant 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

Insolation 65N

-1200-1100-1000-900-800-700-600-500-400-300-200-1000

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/unknownsWAIS collapsed at least once during the late Pleistocene – almost certainly during MIS-11

Nearshore warmth during MIS-31likely incompatible with WAIS

No idea about other possible Quaternary events

Difficult to say whether complete collapse was catastrophic, but seems likely

WAIS -apolloConclusions

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?The future?