GEOLOGY 445 - GLACIAL GEOLOGY

Lab 9 - Eustasy and the Last Great Ice Sheets

Introduction:

The great ice sheets are the dominant physical and climatic phenomena of the Quaternary.  The effects of the last great ice sheets are evident in global landscapes, climate, sea level, and even land level.  This exercise examines the effects on global "true" sea level - eustatic sea level.   Eustatic sea level is a chimera, in that no coastal area is known to be absolutely stable with respect to long-term (100,000+ years) sea level change.  However, it has been reliably estimated (below) during deglaciation from radiocarbon-dated coral on Barbados (R. G. Fairbanks, 1989, Nature, 342, 637) and for the last million years by correlation with ¹⁸O isotope records (N. J. Shackleton, 1987, Quaternary Science Reviews, 6, 183).

Problem:

Compare the approximate contribution of the decaying Laurentide Ice Sheet to global sea level, and interpret the differences.

Method:

1. Use the isochrones (lines of equal age) on Laurentide deglaciation of Bryson et al., 1969 (Arctic and Alpine Research, 1, 1) and the planimetry techniques you mastered earlier to estimate the area (in km²) of the Laurentide Ice Sheet in 1000-yr increments from about 14,000 to 6000 yr BP. 
2. Once you have calculated those areas, use the relationship defined by W. S. B. Paterson (1972, Reviews of Geophysics and Space Physics, 10, 885): log₁₀V = 1.23 x ((log₁₀S)-1)  to estimate the volume (in km³) of the Laurentide Ice Sheet at each increment.
3. Next, change ice volume to volume of water and divide by the area of the world's oceans (about 3.61 x 10⁸ km²) to estimate the sea level equivalent of the Laurentide Ice Sheet at those times, and the incremental contribution of Laurentide decay to global sea level. Note that the continental shelves, now considered part of the ocean, were largely exposed about 18,000 years ago.  Their area is about 10% of the total modern ocean area.
4. Finally, compare your results to Fairbanks' curve and explain the differences.  HINT:  differences could result from inaccuracies in Fairbanks'  reconstruction, in Bryson et al.'s reconstruction, in Paterson's model, and in your methods; from contributions from other ice masses; and, of course, from a combination of the above!  I will place a copy of each paper in my outer office, as well as those available in Renne - if you read them for inspiration, please do so in place and leave them where you found them for others.

Show your work!