GEOLOGY 445 - GLACIAL GEOLOGY

Lab 9 - Glacioisostasy

Introduction:

Issues relating to isostatic rebound (uplift in response to removal of a load) are among the most complex, and the most interesting, in glacial geology because they involve the interaction among ice sheets, climate, land, and oceans across time and space.   You will use the eustatic sea level curve from last week  (R. G. Fairbanks, 1989, Nature, 342, 637).

Problem:

Use field data on ¹⁴C-dated postglacial uplift at points surrounding Home Bay, Baffin Island, Nunavut, Canada, to interpret the spatial and temporal pattern of postglacial isostatic deformation since deglaciation.

Data:

NOTES:  All dates are on marine shells except that in green, thus relative sea level during deposition was higher than shell elevation.  Marine limits in red are those which are believed to be directly dated by the associated radiocarbon date.  Both the analytical uncertainty in the ¹⁴C dates and the field uncertainty in the geological setting of the samples are given, and must be retained in the analysis.

Method:

1. Apply a eustatic correction to each of your relative sea levels to transform "emergence" to "uplift".
2. Where several data points are available, plot uplift against time to generate local uplift curves.
3. From those curves, estimate the isostatic half-recovery time characteristic of this region.
4. Use the regional half-recovery time to create uplift curves for the remaining points.
5. Select at least two times between 8000 yr BP and 2000 yr BP.  Since each of those times, interpret isostatic uplift from your curves at every point on your map and contour those points to produce maps showing isobases on uplift.
6. Draw a graph of uplift as a function of distance for each of your times, projected to a plane perpendicular to the isobases.
7. On the same or a similar graph, plot the elevation of the marine limits for each site.
8. Finally, annotate the marine limit graph with the known and inferred ages of the marine limits.

Discussion:

Discuss the data, the process of analysis, and each of your graphs.  Summarize the significance of this exercise in the context of eustasy, isostasy, and ice sheet retreat.