Hi,

Here are a few comments on the Antarctic reconstruction.ice6g for glacial (21 k)

The first look to figure 2 and 4 gives a rather strange impression because of the split between East and West Antarctica. This split has surely geophysical basis (not the same plate) but it is not realistic from the glaciological point of view (ice would flow to smoothe the step

I think most glaciologists will agree on a slighlty thinner (100 m ) Antarctic Plateau at LGM, From the color scale in figure 4(21-0), it seems almost ok

In the Ross sector of West Antarctica, rather small changes (especially compared with older reconstructions). It is consistent with the fact that Siple dome was still a dome. My own modelling agrees as well.

In the Weddel sector. The most striking feature is the huge increase in the Ellsworth ranges and where is presently the Rosnes ice shelf. The reconstruction displays a 4 km high dome in that place and I must say that from the ice sheet modelling point of view I am a bit skeptical.

Constrains from Berkner (Rob please correct is needed)

- Berkner Island was an isolated dome based on Raymond bump, underlying sand. It seems to be the case in the ice6g reconstruction - change in surface elevation at Berkner:

• from isotope 400 m to 2000 m, best estimation 550 m
• from total gas content 200-900 m
• to keep a cold base requires a change smaller than 1500 m

Such a configuration means that this ice dome was likely surrounded by two active ice streams. In the reconstruction it seems a bit thick on the Filchner side to be an ice stream (but one could have resolution effets). At the present Rosnes ice shelf location, it seems to me that there is a depression and it seems too thick for an ice stream.

The thickness at the grounding line is larger than the one inside and is globally too high for all this region (1000 m high at the grounding line means ~ 10 km thick ice !)

Some other constrains : change in Shackleton range < 340 m change in Ellsworth Mountains > 1000 m,

I remember Rob saying that there are doubts on this point because the trimlines could be much older than LGM ? (as in many other places in Antarctica) I do not know whether there are new published data on this point but it is the key for the existence on a “Ellsworth dome ”

In conclusion and keeping in mind that I had only a look at the pictures, not the numerical data.

Compared to previous reconstructions

- East Antarctica more in agreement with glaciological model and ice core data. - West Antarctica : Ross sea, ok - West Antarctica : Weddel sea a bit too thick (especially at the grounding line) and I doubt from the existence of the huge dome first because I do not see which glaciological process could lead to it ; second because, with just a dome nearby the depression between this dome and Berker Island would be quickly filled and Berker Island would not be a local dome as we think it was.

At this point I can only say that geomorphological information at Frontier Mountain (~50 km away from Talos Dome) indicate at maximum an increase of ice sheet elevation by ~200 m during the Last Glacial Maximum compared with today.

Our newest results working with Dominique Raynaud (total gas) and David Fisher (Canadian ice cores) are the following:

1) The Camp Century site (NW Greenland) was some 600m higher than present in the early Holocene. This suggest that the Canadian high arctic and Greenland was connected by an ice ridge and that the Greenland ice sheet (GIS) extended far out into Melville bay.

2) The DYE-3 site (SE Greenland) was 400m higher than present in the early Holocene. This suggest a wide southern GIS.

3) The GRIP and NGRIP sites (central Greenland) were 150m higher than present in the early Holocene. This suggest a a very wide GIS that could have extended out onto the entire continental shelf.

So to sum up, the GIS was both thicker and wider all over in the early Holocene and it was connected to the Innuitian ice sheet that covered the Canadian high arctic.

wrt thickness history of Greenland since 12000 BP all future modelling efforts now must measure themselves against the very recent paper about that topic by Bo Vinther et al. in Nature (Sept 17 2009). Most of the 3D models for Greenland assumed the GRIP/GISP2 O18 records could be used for the temperature history. This has proven to be wrong and the Vinther et al. paper shows what the temperature history should be and indeed what the thickness history was.

While its good to see that the Keewatin Dome of the LIS is now thinner in 6G than 5G, the LIS is still really thick if its maximum height above sea level is in excess of 4 km. Also, I don't think there is good evidence to have the 21 ka global sea level ~12 m higher than the 26 ka sea level as the margin chronologies of all ice sheets do not show any significant retreat until 19 to 20 ka (see Clark et al., 2009, Science), at least not enough to justify 9 m of sea level rise from the Laurentide and 3 m from the Scandinavian/Barents/Kara. In addition, relative sea level records during this interval mainly consist of M. annularis (20 m growth habitat) (Peltier and Fairbanks, 2006) (one New Guinea coral with a 5 m growth habitat is in conflict with these data; Cutler et al., 2003), which doesn't allow determination of a sea level change between 26 and 21 ka. The first discernible rise comes between 20 and 19 ka. So, what is the basis for thinning of the Cordilleran Ice Sheet, the Keewatin Dome and the new dome over Hudson Bay?

In this vein, the LGM flow patterns of the LIS would not place a fourth dome over Hudson Bay, rather they indicate a dome over Keewatin, a dome over Baffin/Foxe Basin and a dome over Labrador/Quebec with a saddle running across southern Hudson Bay (i.e., the Dyke and Prest, 1987 pattern, or Prest et al., 1968).

Another odd feature is the very thick ice of the Barents Ice Sheet margin in the eastern Barents Sea at 26 ka, what is this based on?

The Barents/Kara component of ICE6G with a large ice-cube in the middle of a pancake makes no glaciological sense. Dick and I both face the challenge of trying to find places to stick all the ice apparently indicated by the Barbados record. Dick's geophysical hand-tuned Lego block approach lends itself to sticking ice in places where there are not geophysical observations to constrain the ice load (ie the Barents/Kara ice cube). I don't have this option with a calibrated glaciological model. Furthermore, the max/bestguess/min deglacial ice margin chronology for Eurasia from our Bergen colleagues (Gyllencreutz, Mangerud, Oystein, Svendsen has about 20% less ice area at 26ka than at 21 ka, yet ICE6G has 23% more ice volume at 26ka than at 21ka

Wrt Anders' comments for North Am, contrary to Eurasia, I think some 26ka to 21ka thinning for North Am is plausible with Heinrich event 2. The M. annularis data actually does provide a challenging constraint if one takes (which Dick and I do) the 20 m growth habit of as the error bar. Unless Dick's VM2/5a earth rheologies are way out, it's a major challenge to get enough ice on land in a glaciological model to fit the Barbados record given the ice margin, RSL, and geodetic constraints.

More fundamentally, this community needs to decide on what criteria will be used for choosing the deglaciation models. At this stage, there are significant trade-offs between:

-fit to RSL data (I'll leave Dick to deal with an associated earth rheology issues)

-fit to geodetic data (””)

-glaciological self-consistency

-fit to the best available margin chronologies

-derivable from plausible climatological forcing

-fit to paleo lake level indicators (strandlines)

-inclusion of error bars

-inclusion of self-consistent meltwater drainage pointer field

I like Lev's description of these as “Lego ice sheets” - they are certainly not glaciological phenemona, and their use in climate models with increasingly high resolution leaves something to be desired. As far as I understand, the geomorphic “trimlines” in Antarctica are very ambiguous with respect to constraining LGM ice limits (cosmogenic nuclides suggest that they likely represent transition from warm-based ice below to cold-based ice above). What is the basis for reducing E. Antarctica down to 1.45 m excess ice? Why can't E. Antarctica have substantially more ice? Regarding Lev's comments, I don't know of any evidence for thinning of ice from 26-21 ka (any sea level contribution from H2 is unknown), so what is the basis for thinning in the model? If Barbados, see below. For clarification, I'm not sure what Lev means by the 20 m error on the Barbados M. annularis corals providing a “challenging constraint” - as in it's a challenge to use these data with this error to constrain ice volume? What are the RSL and geodetic constraints on 21-26 ka ice? With respect to modeling sea level with Dick's VM2/5a earth rheologies, I refer the reader to Jerry Mitrovica's model results of LGM sea level and Barbados using a different Earth rheology that are presented in Clark et al. (2009, Science - see more complete discussion and results in the accompanying Supporting Online Material).

I have trouble not seeing a LIS dome over Foxe Basin, and instead a strong dome over Hudson Bay for Ice 6g at both 26 and 21 ka. The large scale geomorphic flow patterns on Baffin Is suggest the dome is over Foxe Basin most of the time.

Refering to the ice_ng_compare_polar.pdf: Kuhle (Prof. Kuhle, Goettingen) provided for the LGM and Last Glacial Period as he calls it since more than a decade quite a lot of hard data (ice scoures, ice-levels etc etc) from the Tibet Plateau (also presented on the IGC in Oslo 2008).

Although it is physical geography (geomorphology, mapping) these are data, comparable to ODP/IODP cores. The evidence as reported and document I regard as normal credible geological data.

Several figures from above show on the relevant maps “fraction of grid-cell covered with ice” no ice on the Tibet Plateau.

If I understood the maps correctly (reading quite fast) I recommend to include above-mentioned data.

I have an updated version of a newer manuscript by Prof. Kuhle here (intended for a special issue). Thus you might either refer directly to him or to me for this.

If, reading quite fast, I misunderstood something, e.g. respective maps with ice on the Tibet Plateau appear below Fig. 33 of above please apologize this comment.

The _data_ I think should be included in the reconstructions.

The interpretations regarding the impact of the ice on the Tibet-Plateau are a “different story” - independent of the data themselves. I referred to the data.

Commenting on the points made by Catherine about the Weddell Sea region. The evidence from a bedrock ice core from Berkner Island seems to imply that the Weddell Sea ice sheet was not as thick as the model suggests.

It looks as though there was no (or little) basal melting during the LGM which a quick re-run of the model requires ice less that 1600m thicker than today (if thicker than that, even with a surface temperature 20 degrees colder and an accumulation rate half that of today, geothermal heat flux leads to basal melting).

Data from analysis of the core are also limiting on the thickness, with the enclosed air suggesting <900m, and the isotope derived temperature profile around 600m (though could be interpreted as up to ~1500m). Sub-ice sand is morphologically consistent with a stable ice sheet, rather than one that is subsumed into the larger WAIS and subject to ice flow near the base.

Mike Bentley at Durham will have the data on the exposure ages and trimlines in the Shackleton and Ellesworth ranges and be better placed to give a definitive response.

Hi

I have some comment about Ross sea area:

Geomorphological evidence points out that the former profile of outlet glaciers along TAM and Ross Sea coast afford surface elevation changes of about 1100 m at Hatherton Gl. (80 S, 157 ° 30’E), 600 m at Mackay Gl. (77° S 162°E), and 360 m Reeves Gl (74°45’ S 162° E).

Whereas the glacier surface at margin between TAM and East Antarctic Ice Sheet (EAIS) remained virtually unchanged in the western Ross Sea area.

While glacier surface at margin between TAM and EAIS showed a modest thickening (100–200 m) of glacier drainnig directly to Southern Ocean (Rennick Glacier 160°E from 70 to 73°S). The extensive presence of meteorites in the Frontier Mountain blue-ice-field (73°S 160°E) area and their terrestrial ages, attributed to the presence of blue-ice meteorite traps during full glacial times, suggest that a moderate change in elevation occurred in agreement with modest thickening (100–200 m) of Rennick Gl.

Isotopic composition of Talos Dome Ice core indicates that Talos Dome was not overrun by East Antarctic ice during the LGM.

The elevation changes at Talos Dome are due to a combination of glacial dynamic effects during LGM and subsequent climate effects (increase of accumulation) during deglaciation. The former is related to an advance of the grounded ice sheet up to the continental margin during the sea level drop occurring at LGM. The latter is related to a doubling of accumulation rate linked to atmospheric warming and sea-ice reduction during deglaciation. Analogously, the thinning of the dome started after the retreat of the grounding line from LGM position in Ross Sea occurring from 14.3 to 12 ky BP.

On the base of C14 date Terra Nova Bay area was free from Ross Ice Sheet up to around 25 kyr BP, this suggest that the main thickening of glaciers along the Ross west Coast (Mackay and Reeves) occur between 25 kyr and 16 kyr and a significant difference should be occur between Ice6G26 and Ice6G21. I think that is also important to see an Ice6G15 before the start of retreat of Ross Ice Sheet.

The drastic change of morphology between present and LGM at the boundary between TAM and EAIS is not compatible with knowledge and appear as an artefact of model constrain.

In agreement with Peter and Giff, I think it is quite important to have the ice domes, particularly for the LIS, at the best estimate of height and location as this clearly impacts atmospheric circulation (see Otto-Bliesner et al., 2006 for ICE-5G vs. 4G and the warming over Alaska from the high Keewatin Dome). The impact will only grow as models become higher resolution and add isotopes to the hydrologic cycle, which requires then the ice ablation to interact with the ocean and its circulation. For example, at the last PMIP meeting, Allegra showed ModelE-R simulations using 5G and a lower LIS from Licciardi et al. (1998). AMOC was reduced in the Licciardi experiment because of the high overturning of the thinner ice sheet, while AMOC was enhanced in the 5G run.

Dear fellow scientists and researchers,

I am working as Associate Professor and have more than 17 years of thematic and research for societal needs. I belong to the Thar Desert of India and it has always fascinated me since my childhood days. I have studied about its origin and other geological upheavals during my masters and doctoral study. Still I am engaged.

I would appreciate receiving support (in the form of un/published research materials; financial and others) from all of you in framing a research proposal to study (and even try to reconstruct) its glacial and holocene age. I am keen to associate (and even undergo training) with any research organization / Institution / university Department to undertake this study. I would feel obliged I am offered training (with financial support) for carrying out this study.

A quick response of assistance will be highly solicited. I am looking forward to hear favorably from world scientific community in my endeavor.

best regards,

Mahesh Gaur

PALSEA meeting WHOI September 21-25, 2009, consensual statement on PMIP ice sheet boundary conditions:

1. Alternative ice sheet boundary conditions must be considered as a means to consider the sensitivity of the PMIP simulations to ice sheet boundary conditions.

2. We recommend as an ice sheet boundary condition the existing Dyke (2004) database of ice sheet extent covering the last 20 kyr of ice-sheet retreat. This is a reasonable alternative reconstruction of the Laurentide for this purpose. A parabolic ice sheet can be fitted to this reconstruction in a similar fashion to Carlson et al (2008).

3. We recommend, as an ice sheet boundary condition, alternative, independent isostatic rebound models such as that of Kurt Lambeck (e.g. Lambeck et al 2002). These boundary conditions will be made available to PMIP.

4. We note that an underlying inconsistency in isostatic models is with the relative sea level databases input to them. These are not the same between models and almost certainly contain relative sea level data which has been dated with inconsistent and inaccurate techniques. To address this problem in the longer term PALSEA will construct an open access, quality controlled, self consistent, state-of the art database of relative sea level for use in isostatic models.

Carlson, A.E., A.N. LeGrande, D.W. Oppo, R.E. Came, G.A. Schmidt, F.S. Anslow, J.M. Licciardi, and E.A. Obbink, (2008), Rapid early Holocene deglaciation of the Laurentide ice sheet. Nature Geosci., 1, 620-624.

Dyke, A. S. in Quaternary Glaciations — Extent and Chronology Part II Vol. 2b (eds Ehlers, J. & Gibbard, P. L.) 373–424 (Elsevier, Amsterdam, 2004).

Lambeck, K., Y. Yokoyama, and A. Purcell (2002), Into and out of the Last Glacial Maximum: sea-level change during Oxygen Isotope Stages 3 and 2, Quat.Sci.Rev., 21, 343-360

signed (workshop participants):

Morten Andersen, University of Bristol Andrew Ashton, Woods Hole Oceanographic Institution Edouard Bard, CEREGE, College de France Hai Cheng, University of Minnesota Dan Condon, British Geological Survey H. Allen Curran, Smith College Pierre Deschamps, CEREGE Andrea Dutton, Australian National University Norbert Frank, LSCE Edward Gasson, University of Bristol Paul Hearty, University of North Carolina Gideon Henderson, Oxford University Mick O'Leary, Manchester Metropolitan University David Richards, University of Bristol Laura Robinson, Woods Hole Oceanographic Institution Peter Rowe, University of East Anglia Denis Scholz, University of Bristol Mark Siddall, University of Bristol Alex Thomas, Oxford University William G. Thompson, Woods Hole Oceanographic Institution

As I was saying when the nasty wiki cut me off! I had already entered text discussing the new data that has been employed to adjust the north american element of the model from ICE-5G to ICE-6G. There has been no recognition in the previous comments of this important new information which is entirely of a space geodetic origin. Please read the Argus and peltier paper submitted to GJI and the Peltier et al paper submitted to nature. Simply put these data require that LGM ice was thinner to the west and to the south of HB but thicker over Labrador and northern Quebec and along the northern border between Alberta and British Columbia.

Some consternation has been expressed over the ridge of LGM ice that is evident in ICE-6G within HB itself. This is clearly a transient feature that is posited to exist in the model prior to the time of Heinrich event 1 which leads to a rapid draw down of ice from the Bay itself. Because i is lost quickly it makes no contribution to the time dependent gravitational field signal which the GRACE satellites are now recording to be present over the north american land mass. Inspection of the prediction of the GRACE signal by ICE-6G documented in the Peltier et al (2009, submitted) publication demonstrates that this model beautifully reconciles this observation whereas the prediction by the previous ICE-5G model, with its overly thick ice to the west of Hudson Bay, provides a very poor fit to this important observation. Since the GRACE data has not been employed to construct ICE-6G this is an important confirmation of the quality of the model.

Concerning the Antarctic component of ICE-6G we have employed the following types of information to constrain this component of the reconstruction (Much of this information was unavailable at the time ICE-5G was produced). First we have now exlicitly employed the ice-core derived constraint provided by the work of Valerie Masson-Delmotte et al This work very strongly suggests that there was very little if any loss of ice from the central dome of the east Antarctic ice sheet. What loss of ice that did occur from east Antarctica must therefor have been associated with a diminution of its lateral extent. At LGM East Antarctic ice extended outwards to the shelf break. During deglaciation it retreated inwards in the same way as the Greenland ice sheet adjusted to the marked change in climatic conditions in the northern hemisphere (more to follow).

In West Antarctica the situation is known to have been much more dynamic. In constraining the geographical regions from which ice was lost from this region during deglaciation use has been made of the original geomorphological constraints provided by the work of George Denton and colleagues. So far as I am aware this work remians unchallenged. Comparing this aspect of the reconstruction with the IJ05 reconstruction of Ivins and James there is very little difference between the models in this respect. In constraining the amount of ice that was lost from these geographical regions, two sources of information have been employed. First all available relative sea level data have used to constrain the mass loss in regions from which such data is available. These data are not voluminous, however, and furthermore are available only from coastal regions! The additional constraint that has been employed to approximate the amount of mass loss is that provided by the most recent glaciological reconstructions by Ritz et al and Philippon et al, analyses which appear to restrict the total mass loss to be equivalent to approximately 11 m of eustatic sea level rise. The timing of this mass loss is also critical, of course. In ICE-6G this aspect of the deglaciation history of Antarctica has been fixed by the work of Eugene Domack and colleagues whose work in the context of the US Antarctic programme has allowed them to identify the timing of the pull-back of grounded ice from the shelf-break at a very large number of sites surrounding both East and West Antarctica to the time of meltwater pulse 1b in the extended Barbados sea level curve of Peltier and Fairbanks (2006). In ICE-6G it is assumed that the fraction of the total Antarctic ice that was lost during the glacial-interglacial transition was such as to explain the observed amplitude of meltwater pulse 1b in the Barbados record.

I thought it might be helpful, prior to the discussion tomorrow morning (?-depending upon where you are)if I were to comment upon the two complementary methodologies that are being employed by Lev and I. Although they are closely linked, these two methodologies each have strengths and weaknesses. I' ll refer to them below as follows: (1) The geophysics based methodology. This is the methodology employed to develop the ICE-NG (VMX) sequence of models.

  The only glaciological inputs to this methodology consist of time dependent margin positions from LGM to present, wherever 
  this information is available. The only exception to this in the past has been for Greenland in the ICE-5G (VM2) 
  reconstruction for which the model developed in the two papers by Tarasov and Peltier (referenced by Lev above) has been
  introduced.

(2) The trained ice sheet methodology employed in the several papers by Tarasov and Peltier now available from our eb sites.

  In this method the same geophysical data employed in method (1) are used to train a fairly standard shallow-ice 
  representation of the dynamics of a continental scale ice sheet for either North America and seperately for Eurasia. The
  same methodology may of course be applied to Antarctica. The data required to apply the method in this region, however, is
  still rather sparse.

The breakthrough that has occurred in the past year or so that has enabled to refinement of the ICE-5G (VM2) model to produce the the ICE-6G (VM5a) model consists of the space geodetic information presented for the first time in the paper by Argus and Peltier (2009, submitted; available as a preprint from the PMIP3 eb site. The new model has been developed so as to satisfy these newly available constraints. The new model is described in some detail n the Peltier et al (2009,preprint) paper also available for download frmo the PMIP3 web site.Since these results have only recently been finalized they have yet to be proprly integrated into the reconstructions that have been performed using method (2) above. Through the application of method (1), however, they have been shown to have an extremely important impact upon the inferred topography of the LIS that is expected to have a significant impact upon the atmospheric circulation of the northern hemisphere at LGM.

Method (1) of course does have an important weakness. This is that no attempt is made to ensure that the ice sheets delivered by the analysis are glaciologically rationale. This is why Lev and I have been working to develop method (2). The weakness of method (2)is that it requires knowledge of the climate forcing that is responsible for driving the accumulation of mass on the continents that evolves so as to produce the LIS and other ice sheets. This is of course unknown and so must be varied so as to produce a “best possible” fit to the available constraints. This is a highly under constrained inverse problem and has to be regularized by the aplpication of a number of assumptions. A crucial constraint, however, is on the total mass of ice that is removed from the continents across the glacial-interglacial transition. We employ far field relative sea level data to provide the necessary constraints, Barbados being the most significant data set available for this purpose.

The circumstance in which we find ourselves at present is that method (2) seems unable to deliver models that both satisfy the constraints on regional ice thickness provided by the rsl and space geodetic data from the ice-covered regions AND the far field constraint on total mass of all ice sheets provided by the record from Barbados. My expectation is that once the new space geodetic constraints are fully integrated into the training procedure employed in method (2) this unfortunate circumstance will be alleviated. Until this has been carefully checked, however, we cannot rule out the possibility that the simple shallow ice approximation to the dynamics we have been employing might in fact be the cause of the problem. Of course the net mass constraint is a zeroth order constraint that any accaeptable model must satisfy.

Assessing the three presented ice topographies for the LGM, it looks like the main difference between the 6G and MOCA reconstructions for the Eurasian Ice Sheet is the thickness of the ice and extent. Based on the cosmogenic dates of Goehring et al. (2008, QSR) and Linge et al. (2006, QSR), ice was at least up to 1400 m asl in SE Norway near the coast and 1800 m at the Norway-Sweden border, which would indicate that 6G and ANU are more in line with the data on ice sheet height than MOCA (Moca is <1000 m near the coast). The ANU model does not, however, have a British ice sheet, which there clearly was.

With regards to the North American Ice Sheet, I would say the ANU and MOCA models look similar, but with MOCA model having a more pronounced dome structure, similar to the flow patterns of Dyke and Prest (1987) (Keewatin, Labrador, Foxe Domes with the trough between the Keewatin and Lab Domes). Nevertheless, I think MOCA does a better job of resolving the Foxe Dome than ANU. It would be nice to know what the ice volume is in MOCA. As I mentioned before, 6G does not produce the dome pattern one would expect from the well mapped flow patterns of the LGM and early deglacial. Similarly, it looks like the Laurentide is flowing over the Canadian Rockies in 6G to the Pacific Ocean, which did not occur. The other two models produce a discrete Cordilleran Ice Sheet. Both 6G and the ANU models produce lower southern LIS lobes than the MOCA model, in agreement with reconstructions (Clark, 1992, GSA Bull; Hooyer and Iverson, 2002, J. Glac.).

There is reasonably good evidence that significant deglaciation of the Antarctic ice sheet began between 15-14 ka.

Constraints from trimlines:

The significance of the vertical zonation of a landscape according to differences in rock-surface weathering (so-called weathering zones) to ice-sheet history in Scandinavia and the eastern Canadian Arctic has been debated for over a century, with the upper limit of any given weathering zone (trimline) interpreted as either a former ice surface (Boyer and Pheasant, 1974; Ives, 1978; Nesje and Dahl, 1990) or an englacial change in basal thermal regime from non-erosive cold-based ice above to erosive warm-based ice below the limit (Sugden and Watts, 1977). Terrestrial cosmogenic nuclide (TCN) ages from upper weathered zones on Baffin Island (Briner et al., 2005; Miller et al., 2002) and Fennoscandia (Goehring et al., 2008) have now conclusively demonstrated that the limit marks an englacial thermal boundary, whereby the upper, more weathered surface was covered by cold-based, largely non-erosive ice at the LGM. Recent work from Antarctica has similarly used TCN ages to demonstrate that highly weathered rock surfaces were preserved beneath cold-based ice during the last glaciation (Di Nicola et al., 2009; Stone et al., 2003; Strasky et al., 2009; Sugden et al., 2005). Trimlines are thus not a reliable constraint on former ice thickness.

Constraints from the shallow-marine record.

Most constraints on deglaciation from marginal shelf areas are based on AMS radiocarbon of the bulk acid insoluble organic (AIO) carbon fraction. AIO 14C ages from surface sediments in the western and central Ross Sea, however, are substantially older than the reservoir age, with a range from core to core of 2000 to >6000 14C yr B.P. (Andrews et al., 1999; Licht et al., 1999). Corresponding age corrections for each core site are conventionally established by subtracting the age of the AIO at the modern sediment-water interface (Andrews et al., 1999; Domack et al., 1999), which assumes that both the reservoir age and the amount of reworked organic matter has remained the same at each core site. This seems highly unlikely. The marine record of deglaciation from marginal shelf areas is established by using a characteristic stratigraphic succession reflecting grounding line retreat as represented by the transition from subglacial facies to sub-ice shelf facies to open-marine facies (Anderson et al., 2002). AIO 14C ages obtained from the diatomaceous mud or ooze that comprises the open-marine facies are the most reliable (Licht et al., 1996; Andrews et al., 1999; Domack et al., 1999), and thus closely constrain the timing of the transition from sub-ice shelf to open marine conditions, but provide only limiting minimum ages for grounding line retreat. In any event, Mackintosh, Domack et al. recent presentation at the ACE-Grenada meeting conclude the following: “Our reconstruction demonstrates that deglaciation of deep-shelf troughs and lowering of the ice sheet surface occurred in two phases, from 14-12 and 12-7 ka before present (BP). Our consideration of possible mechanisms for the observed retreat of the marine ice margin of Mac.Robertson Land favours rapid rates of eustatic sea level rise associated with Meltwater Pulse 1a (MWP-1a) at ~14 ka BP and warming of the marginal oceans and atmosphere to near-modern levels ~2 ka later.” (http://www.apcongress.es/misc/webantartic/docs/resumen/V11-9.40.pdf) It certainly seems reasonable, given dating uncertainties, to argue that initial ice retreat was associated with a contribution of Antarctica to MWP-1A.

Constraints from ice cores:

The isotopic (delD and del18O) records from two ice cores bordering the Ross Sea (Siple and Taylor Domes) show an abrupt warming at ~14.5 ka, which may reflect ice-surface lowering or some regional climate change induced by ice-surface lowering. If attributed solely to a change in ice-surface elevation, the 3-4oC warming at Siple Dome (Steig et al., 2000) would indicate 500-650 m of ice-surface lowering assuming a free atmospheric lapse rate of 6oC/1000 m. This magnitude of lowering is supported by ice-sheet modeling, which suggests thinning of Siple Dome ice by 350 m between 14 and 15 ka (Price et al., 2007).

Constraints from Antarctic relative sea level: Bassett et al. (2007) approached the question of an early and rapid deglaciation of Antarctica associated with MWP-1A by using an earth model to compute the near-field Antarctic sea-level history for two scenarios, one in which MWP-1A was produced entirely from the northern hemisphere, and one in which there was a dominant contribution from Antarctica. The former scenario would most closely correspond to the mid-Holocene deglaciation and retreat of the grounding line in the Ross Sea suggested from the RSL history (Baroni and Hall, 2004; Hall et al., 2004), whereas the latter scenario would test the hypothesis of whether deglaciation associated with MWP-1A can be accommodated by the RSL history. The best RSL records from Antarctica come from the Ross Sea region. For both scenarios, there is a sea-level rise at the Ross Sea sites associated with the northern hemisphere melting until this region deglaciates, after which there is a sea-level fall resulting from the isostatic uplift. The combined effect of these two processes is a sea-level highstand, which occurs earlier in the Antarctic source model and thus results in a more gradual sea-level fall during the mid-to-late Holocene than is the case for the northern hemisphere source model. Bassett et al. (2007) concluded that the more gradual sea-level fall predicted by the Antarctic source model is more consistent with the data than the larger and steeper fall predicted by the northern hemisphere source model.

Constraints from the Ross Sea Glacial Geological Record:

As summarized in Clark et al. (2009, Science), most of our best-constrained information on the chronology of the Antarctic ice sheet comes from the extensive radiocarbon dating from the Ross Sea sector of the WAIS, particularly that from Taylor Valley, by Denton and colleagues.  (1) The youngest 14C age (14.7-15.1 cal ka; AA-20667) constraining the time of the local last glacial maximum (LLGM) ice margin at the Hjorth Hill moraine provides a limiting maximum age for retreat from the moraine. (2) Two 14C ages on algae that grew in a glacial lake that formed following ice-margin retreat from the Hjorth Hill moraine provide limiting minimum ages for this retreat (14.8-15.2 cal ka; AA-13576, and 14.9-15.2 cal ka; AA-17342). (3) The glacial lake in Taylor Valley permanently dropped below ~80 m after 13.9-14.7 cal ka (AA-17314) (14C age on youngest high-elevation delta) and before 13.9-14.1 cal ka (QL-1707) (14C age on oldest low-level delta after permanent drop).  (4) Lacustrine deposits occur in a stream exposure cut into the moraine deposited by the LLGM ice margin on the floor of Taylor Valley, with a 14C age requiring deglaciation from the moraine before 14.3-14.8 cal ka (AA-17333). (5) Glaciolacustrine sediments containing dropstones occur along the distal side of the valley-floor threshold moraine, indicating that the ice margin was at the moraine.  The youngest 14C age from this unit (13.6-14.7 cal ka; QL-1794) provides a limiting maximum age for retreat from the moraine. Given these dating constraints, we suggested an age for the termination of the LLGM (13.9-15.2 cal ka)in this region, and assigned the error based on the youngest limiting maximum 14C age for ice-margin retreat (13.9-14.7 cal ka (AA-17314) and the oldest limiting minimum 14C age for ice-margin retreat (14.9-15.2 cal ka; AA-17342).  This only identifies when ice began to retreat, but does not constrain any subsequent rate of retreat.

Peter's comments above concerning the (re)interpretation of Eugene Domack's data are interesting. I've sent a copy of his text above to Gene in order to have him (Eugene) respond directly.Peter's interpretation above would appear to contradict Gene's own as recently expressed to me.

From a physical perspective, however, I would find it unusual if there were not some reaction around Antarctica to the dramatic warming of the northern hemisphere that was caused by the re-invigoration of the NADW formation that in turn caused the Bolling-Allerod warming and the release of meltwater that was MWP1A. This re-invigoration of northern hemisphere deep water formation occurred as a consequence of the cessation of the freshwater forcing associated with Heinrich event 1 (H1). The evidence of a strong contribution to MWP1A from the northern hemisphere is clearly evident in records from the GOM as is well known. The northern hemisphere warms because the NADW process resumes. This causes the main reaction of the northern hemisphere ice sheets which deliver the majority if not all of MWP1A. I would expect that the grounded ice surrounding Antarctica should have reacted to some degree to this. The question is to what degree. I do not believe that we have any evidence that would allow us to quantitatively measure this. I can say , however, that all of the relative sea level records available from Antarctic sites are well fit by the ICE-6G (VM5a) model, including those of Hall et al from the Scott coast referred to above. These data fits have been sent previously to the PMIP3 group. Clearly this model for Antarctica is based upon the radical assumption that the only mass loss from east Antarctica occurred from the exposed shelf that would have existed at the time of the low stand of the sea. However the discrepency between the eustatic equivalent ice amounts melted from Antarctica in the ANU model are ~29 m rather than the ~13 m in ICE-6G, ie more than double. I believe this model has been constructed on the assumption that essentially ALL of MWP1A was delivered by Antarctica. Since the proponents of this model have not seen fit to provide us with the time series of the meltwater production by their model I'm in no position to know whether this interpretation is correct. So , again, please provide us all with the detailed ice thickness history for all ice sheets for this model so that they may intercompared directly and fully.

What I would find helpful insofar as the ANU model is concerned is to have Kurt forward the complete thickness history of all of the ice sheets in this model to me in Toronto so that I can run them through the complete suite of analyses that will allow me to compare the predictions of this model to the space geodetic constraints that are now available, especially the GRACE data which are crucial in enabling us to clearly see what wee the dominant geographical regions from which unloading of ice occurred. In order for the model we fasten upon to be a truly community model there needs to be full disclosure of the thickness isopacks so that detailed inter-comparisons of the models can be peformed.

In quick response to one of Gene's comments provided by Dick, the evidence for a large MWP-1A contribution from the LIS in the form of benthic and planktonic d18O, particularly in the Gulf of Mexico, is not supported by the existing data as shown in Carlson (2009, QSR). Rather all of these records (Aharon, 2003, Paleoc, 2006, EPSL; Keigwin et al., 2005, Paleoc; Poore et al., 1999, Geology; Hall and Chan, 2004, Paleoc) record just a small 1A LIS contribution. When considering a reasonable ice-melt d18O end member (-25 per mil for conservative, noting that -35 per mil reduces the calculations below), the maximum contribution the LIS provided to MWP-1A through the Mississippi River is 2.7 m of sea level rise. Aharon (2006, EPSL) assumed a -25 per mil end member as well (and -35 per mil), but also assumed a constant P-E-ice melt relationship where only 12% of the water down the Mississippi River was sourced from LIS melt. This led to the use of a total river end member d18O value of -9.8 to -11 per mil (-25 and -35 per mil melt end members, respectively), and thus his high discharge calculations. In reality, he should have then taken the 0.28-0.33 total Sv calculated and adjusted for the 88% contribution from P-E he modeled, which gives a southern LIS sea level rise contribution (P-E does not raise sea level) of only 1.4-1.7 m. Similarly, the northern outlet record indicates a maximum contribution of 0.5 m and the eastern outlet record a maximum contribution of 2.1 m. Thus the total LIS MWP-1A contribution is less than 5.3 m; not a significant contribution.

The records from other Northern Hemisphere ice sheets also do not show a large MWP-1A contribution, contrary to the Gene's assertion. The southern margin of the Scandinavian Ice Sheet retreated less than 100 km during MWP-1A (Rinterknecht et al., 2006, Science). Similarly, planktonic and benthic d18O records from the Norwegian Sea suggest a reduction in freshwater discharge during MWP-1A relative to earlier meltwater discharge from the Scandinavian Ice Sheet (Lehman et al., 1991, Nature; Karpuz and Jansen, 1992, Paleoc; Clark et al., 1996, Paleoc). The much smaller Cordilleran Ice Sheet also retreated only a relatively small distance during MWP-1A when compared to later retreat (Dyke, 2004). The major retreat and melt-back of the Barents-Kara Ice Sheet occurred ~18–17 ka, well before MWP-1A leaving significantly less ice available at ~14.6 ka to contribute to MWP-1A (Jones and Keigwin,1988, Nature; Koç and Jansen,1994, Geology; Svendsen et al., 2004).

This is a joint comment from the Bergen group (wemade the ice margin constraints for MOCA and ICE-6G):

1) ICE-6G: The apparent ice bridge between British-Irish ice sheet (BIIS) and the Scandinvavian ice sheet (SIS) in ICE-6G at 21 k must be a result from coarse resolution, which can be seen in the MOCA model, where there is just a narrow strait separating the two. There is firm evidence that BIIS-SIS were not confluent after 25 k.

2)ANU: BIIS is entirely missing, which must have an important impact on climate modelling based on this model. There is no ice on easternmost Iceland. Western Barents Sea ice sheet has a marked bight in Bjornoyrenna, which shouldn't be there. The ice in Russia between 45-55 deg E, 70 deg N is too large and goes slightly beyond the maximum Weichselian limit from QUEEN.

Richard Gyllencreutz, Jan Mangerud, John Inge Svendsen, Oystein Lohne