Controls on moraine exposure-age clustering and implications for sampling strategy

Past glacier fluctuations can be reconstructed successfully via cosmogenic nuclide exposure dating of boulders protruding from the moraine surface. However, post-depositional processes like denudation, slope failure and weathering, together with nuclide inheritance, potentially affect nuclide concentrations and diminish the accuracy of moraine age estimates. Post-depositional exhumation of boulders leads to incomplete cosmic-ray exposure and thus underestimated ages. Conversely, some boulders contain nuclides produced prior to their deposition (nuclide inheritance) due to insufficient depth of glacier erosion, incorporation of older glacigenic sediments or material from surrounding non-glaciated areas. Nuclide inheritance yields age estimates older than the true age of moraine formation.

With an aim to evaluate the controls on moraine denudation and to gain insights to the reliability of exposure dating and sampling strategy, we compiled a global dataset of 10,083 ¹⁰Be-based exposure dates from the Expage database. Clustering of exposure ages from each moraine was used as an indicator of dating quality, assuming that boulders without prior or incomplete exposure should yield a well-clustered (MSWD<2) age. Moraine age-clustering was analysed with respect to climate, topography, location and type of ice mass. 

We find that just 23% of moraine ridges with at least three exposure dates show well-clustered exposure ages, increasing to 69% after iterative removal of outliers with the highest deviation. Exposure-age clustering is mainly a function of moraine age: clustering is best among moraines of 15–10 ka age and decreases notably for moraines that are either younger or older. Climate also matters: well-clustered moraine ages are more frequent in regions with milder climates experiencing higher mean annual temperature and precipitation and lower annual temperature range. Conversely, poorly-clustered ages (e.g. Antarctic Ice Sheet, Cordilleran Ice Sheet, northeastern Asia, High Mountain Asia and Greenland) appear to reflect aridity, extreme cold, or large annual temperature range, but may also stem from complex glacial histories involving multiple glacier readvances.

A key implication for moraine boulder sampling strategies is the effect of the number of samples per moraine. While 36% of the examined moraines comprise only three samples, the likelihood of obtaining a well-clustered age increases significantly by sampling four. The optimal number of samples varies with moraine age and climate. For moraines dated to 20–10 ka, four samples are generally sufficient, whereas younger or older moraines typically require seven or more samples to achieve a similar level of accuracy. The optimal number of samples increases toward colder climates, from temperate (3–4 samples or more) through boreal (5–6 samples or more) to polar climates (7 or more).