CGRG Bibliography of Canadian Geomorphology
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Author : Gosse, J.; Klein, J.; Dyke, A.; and Caffee, M.
Date : 2000.
Title : Cosmogenic nuclide strategies for relative sea level histories.
Publication : 30th International Arctic Workshop, Program and Abstracts, 2000. Institute of Arctic and Alpine Research, University of Colorado at Boulder
Issue :
Page(s) : 72-73.
Abstract
Crustal emergence records are useful but hard to obtain. In high-latitude regions, emergence histories reveal styles and rates of tectonic and isostatic (glacial and non-glacial) processes. Relative sea-level data help constrain glaciological models used to predict ice thicknesses and glacial retreat histories. Emergence rates reflect viscosities, layering, and other rheological properties of the lithosphere and mantle. Unrecognized fault blocks may be detected with emergence shoreline data over a large area. Only after global and local sea-level changes are reconciled can the history of Quaternary sea transgression and regression be understood. In high-latitude regions in the absence of living corals, relative sea-level records have mainly been compiled from radiocarbon dates on marine and terrestrial fauna (mostly bowhead-whale ear bone and shells) and driftwood on emerged shorelines. In particular, the dating of raised deltas with shells in the bottomsets and minimum limiting radiocarbon ages from bog-bottom samples have bracketed the timing of topset-foreset contacts used to infer relative sea level at a geological instance. The chronology of deltas and shoreline records from radiocarbon and other methods has been very successful in areas where datable media exist in stratigraphically meaningful positions. Unfortunately, datable material is often not available where we need it, and even if available, uncertain marine reservoir corrections, unknown thermal histories, and other factors limit the reliability of the dates. Cosmogenic nuclides produced in rocks would seem to be a logical dating method at high latitudes. An array of six nuclides makes it possible to date virtually any rock type. Cosmogenic-nuclide production rates are well understood at high latitudes making uncertainties in the exposure-age calculations comparable to the uncertainties in many calibrated radiocarbon dates. Relative sea-level curves can be constructed from a choice of direct ages on (1) raised beach sediment (cobbles and boulders); (2) emerged bedrock cliff faces; and (3) topsets or other sediment (sand through cobbles) of a delta. Cobbles and boulders on raised beaches and glacially plucked bedrock surfaces on a sea cliff face were collected along the northern coastline of Prescott Island, off the east coast of Prince of Wales Island, Central Arctic, Canada. The concentrations of cosmogenic 10Be produced in quartz phases of Precambrian gneiss and quartz veins were used to calculate the exposure ages of the emerged beach ridges and cliff surfaces. Samples were prepared at the University of Pennsylvania and analyzed at Lawrence Livermore National Laboratory. Our sampling strategy effectively avoided geological factors such as post-depositional beach sediment movement due to sea-ice push or gelifluction, cryoturbation, differential partial shielding due to snow or ice cover, and anthropogenic displacement of cobble-size quartz clasts. The concentrations were corrected for small (<2%) increases in production rate due to a decrease in atmospheric shielding during emergence. A small correction was also made to account for subaqueous production in the rocks before emergence above sea level. Sensitivity tests for reasonable effects of surface erosion and partial shielding due to snow cover indicate the exposure ages would be at most 3% too low. Despite the apparent appropriateness of these strategies, our results are discouraging. Multiple cobble samples collected along single paleoshorelines show large variations about the mean beach age (n=12). Ages on plucked bedrock cliff surfaces were also variable (n=5). Boulder ages were the least variable (n=5) but did not define a simple exponential curve similar to the driftwood curve of Dyke et al. (1991). Because all of the exposure ages are older than the calibrated radiocarbon date for a given elevation, inheritance from pre-exposure is probably the most important source of the scatter. Support for this explanation is afforded by dates from samples collected within the present tidal zone. Collected as geological blanks, they reveal high and variable concentrations of inherited 10Be (as large as 21 kyr!). In the only other published beach sediment study, Trull et al., (1993), also obtained highly variable ages with 3He that were older than expected on beach sediments in Death Valley. However, Zreda et al. (personal communication) has indicated recently that beach sediments dated with 36Cl in the eastern Canadian arctic appears to conform with independent estimates of Holocene emergence. For future studies, a simple pre-test to measure the inherited concentration on modern beach sediments can help determine if the method will be reliable in a particular area. We have recently initiated a project to determine if cosmogenic nuclide dating of raised delta topset/foreset beds is feasible. Subsurface samples of sand, granules, and pebbles were collected in undisturbed fluvial crossbeds in the upper 1m of the topsets of a 150 m high kame delta at Ushuaia, Argentina. The approach is similar to dating fluvial terraces where inheritance is expected or where surface samples have been disturbed. The cosmic ray flux is predictably attenuated with depth into the delta, so concentrations will decrease exponentially with depth. The concentration and variation in the concentration of the 10Be can be used to calculate the age of the topsets and if necessary compensate for any inheritance in the sediment. Additional samples will be collected in Maine and Atlantic Canada this summer.
Bibliography of Canadian Geomorphology