David J. Ullman

Sr-Nd-Pb Isotope Evidence for Ice-Sheet Presence on Southern Greenland During the Last Interglacial

Melting of the Antarctic Ice Sheet contributed substantially to the excess sea-level rise of the last interglacial period. To ascertain the response of the southern Greenland Ice Sheet (GIS) to a boreal summer climate warmer than at present, we explored whether southern Greenland was deglaciated during the Last Interglacial (LIG), using the Sr-Nd-Pb isotope ratios of silt-sized sediment discharged from southern Greenland. Our isotope data indicate that no single southern Greenland geologic terrane was completely deglaciated during the LIG, similar to the Holocene. Differences in sediment sources during the LIG relative to the early Holocene denote, however, greater southern GIS retreat during the LIG. These results allow the evaluation of a suite of GIS models and are consistent with a GIS contribution of 1.6 to 2.2 meters to the ≥4-meter LIG sea-level highstand, requiring a significant sea-level contribution from the Antarctic Ice Sheet.

South Greenland ice-sheet collapse during Marine Isotope Stage[thinsp]11

Varying levels of boreal summer insolation and associated Earth system feedbacks led to differing climate and ice-sheet states during late-Quaternary interglaciations. In particular, Marine Isotope Stage (MIS) 11 was an exceptionally long interglaciation and potentially had a global mean sea level 6 to 13 metres above the present level around 410,000 to 400,000 years ago, implying substantial mass loss from the Greenland ice sheet (GIS). There are, however, no model simulations and only limited proxy data to constrain the magnitude of the GIS response to climate change during this ‘super interglacial’, thus confounding efforts to assess climate/ice-sheet threshold behaviour and associated sea-level rise. Here we show that the south GIS was drastically smaller during MIS 11 than it is now, with only a small residual ice dome over southernmost Greenland. We use the strontium–neodymium–lead isotopic composition of proglacial sediment discharged from south Greenland to constrain the provenance of terrigenous silt deposited on the Eirik Drift, a sedimentary deposit off the south Greenland margin. We identify a major reduction in sediment input derived from south Greenland’s Precambrian bedrock terranes, probably reflecting the cessation of subglacial erosion and sediment transport as a result of near-complete deglaciation of south Greenland. Comparison with ice-sheet configurations from numerical models suggests that the GIS lost about 4.5 to 6 metres of sea-level-equivalent volume during MIS 11. This is evidence for late-Quaternary GIS collapse after it crossed a climate/ice-sheet stability threshold that may have been no more than several degrees above pre-industrial temperatures.

Surface‐melt driven Laurentide Ice Sheet retreat during the early Holocene

Received 21 September 2009; revised 3 November 2009; accepted 30 November 2009; published 30 December 2009. [1] To better understand mechanisms of ice-sheet decay, we investigate the surface mass balance of the Laurentide Ice Sheet (LIS) during the early Holocene, a period of known rapid LIS retreat. We use a surface energy-mass balance model (EMBM) driven with conditions derived from an equilibrium atmosphere-ocean general circulation model 9 kilo-years ago simulation. Our EMBM indicates a net LIS surface mass balance of 0.67 ± 0.13 m yr 1 , with losses primarily due to enhanced boreal summer insolation and warmer summers. This rate of loss compared to LIS volume reconstructions suggests that surface ablation accounted for 74 ± 22% of the LIS mass loss with the remaining loss likely driven by dynamics resulting in basal sliding and calving. Thus surface melting likely played a governing role in the retreat and disappearance of this ice sheet. Citation: Carlson, A. E., F. S. Anslow, E. A. Obbink, A. N. LeGrande, D. J. Ullman, and J. M. Licciardi (2009), Surface-melt driven Laurentide Ice Sheet retreat during the early Holocene, Geophys. Res. Lett., 36, L24502, doi:10.1029/ 2009GL040948.

Southern Laurentide ice-sheet retreat synchronous with rising boreal summer insolation

Establishing the precise timing for the onset of ice-sheet retreat at the end of the Last Glacial Maximum (LGM) is critical for delineating mechanisms that drive deglaciations. Uncertainties in the timing of ice-margin retreat and global ice-volume change allow a variety of plausible deglaciation triggers. Using boulder 10 Be surface exposure ages, we date initial southern Laurentide ice-sheet (LIS) retreat from LGM moraines in Wisconsin (USA) to 23.0 ± 0.6 ka, coincident with retreat elsewhere along the southern LIS and synchronous with the initial rise in boreal summer insolation 24–23 ka. We show with climate-surface mass balance simulations that this small increase in boreal summer insolation alone is potentially sufficient to drive enhanced southern LIS surface ablation. We also date increased southern LIS retreat after ca. 20.5 ka likely driven by an acceleration in rising isolation. This near-instantaneous southern LIS response to boreal summer insolation before any rise in atmospheric CO 2 supports the Milankovic hypothesis of orbital forcing of deglaciations.