Challenges in the Use of Cosmogenic Exposure Dating of Moraine Boulders to Trace the Geographic Extents of Abrupt Climate Changes: The Younger Dryas Example - Abrupt Climate Change: Mechanisms, Patterns, and Impacts - page 116

resistant mineralogies such as quartz [e.g., Barrows et al.,
2007]. Unless the high points retain polish, boulder surface
relief is a minimum estimate of the thickness of material
removed from the surfaces of the boulders, again assuming
that the boulders were transported subglacially.
These criteria are intended to minimize the chance that the
samples have been shielded from cosmic rays during part of
their postdepositional history. Tall boulders are less likely to
have been covered by sediment or snow; boulders with
polished or striated surfaces have not lost their nuclide-rich
outer surfaces to erosion.
However, tall and fresh boulders sometimes yield expo-
sure dates that are much younger than shorter and more
weathered boulders from the same moraine [e.g., Briner,
2009]. There is no correlation between boulder height and
apparent exposure time for the samples from the inner Tit-
comb Lakes moraine (Figure 4) (boulder heights were not
reported for the Waiho Loop data set). Briner [2009] took
both pebble collections and boulder samples from moraines
in Colorado and found no relationship between clast size and
the apparent exposure time of the sample.
We are unaware of any study that reports on the relation-
ship between surface freshness and apparent exposure time.
However, we believe that surface freshness is a poor predic-
tor of the apparent exposure times yielded by individual
boulders; that is, fresh boulders are not more likely than
weathered boulders to yield exposure dates that are represen-
tative of the moraine

Moraine degradation can explain the failure of these sam-
pling criteria to indicate which boulders to sample on mo-
raines (other scenarios are also possible; see Discussion
section, below). If moraines lose meters of material from
their crests over time, and boulders are distributed through-
out the removed soil column, then there will be no correla-
tion between boulder height and apparent exposure time. If
boulders do not erode while buried, but do erode at a con-
stant rate after exhumation, then the least eroded boulders are
also those that have spent the least amount of time at the
surface.
Our model field observations support these as-

sumptions. The scatter among the beryllium-10 exposure
dates from the Waiho Loop moraine is best explained by the
progressive loss of ~4.2 m of material from the moraine

ts and

' s

crest; similarly, the model fit suggests that the inner Titcomb

Lakes moraine has lost ~2.6 m of material from its crest.
Although these moraine height changes seem extreme, the
average rates of crest lowering that they imply are reason-
able; 4.2 m of removal over 11.6 kyr (Waiho Loop moraine,
Table 2) is ~0.4 mm yr 1 , a geomorphically possible rate for
a steep, unconsolidated deposit.
It is more difficult to assess the correctness of our assump-
tions about boulder erosion from the model ts because the

shape of the modeled distributions is insensitive to the boul-
der erosion rate. However, some support for our statements
about boulder erosion comes from

field observations made
by one of us (P.J.A.) on the Huancané II moraines near the
Quelccaya Ice Cap [Mercer and Palacios, 1977]. On these
moraines, clasts buried less than a meter below the surface
are fresh, but surface boulders adjacent to the soil pits are
weathered. The weathered boulders have pits and pedestals
on their upper surfaces that indicate several centimeters of
lost material. Taken together, these observations suggest that
boulders in this environment do not erode unless they are
exposed at the surface.
Given our model

s actual age. We make this statement on
the basis of our modeling results, which we describe below.

'

fits (Figure 3, top), we can evaluate the
effects of different sampling strategies on the distributions of
cosmogenic exposure dates (Figure 3, bottom). The model
tracks the heights, surface relief values, and exposure dates
for each boulder. Thus, we can identify the modeled boulders
that are taller than a certain height or have less than a certain
amount of surface relief. By comparing the exposure dates
within each subsample to the “ true ” age of the moraine, we
can determine which sampling strategies produce exposure
dates that are closest to this true value.
Very tall boulders are most likely to yield exposure dates
that are close to the true age of the moraine (Figure 3,
bottom). The median exposure date moves closer to the true
moraine age as the minimum boulder height becomes greater.
However, even fairly large boulders, standing at least 1.5 m

Figure 4. Apparent exposure time as a function of boulder height
for the inner Titcomb Lakes moraine [Gosse et al. [1995b] (Table 1).
For this data set, there appears to be no relationship between boulder
height and apparent exposure time. The exposure dates have not
been corrected for snow cover or boulder erosion.