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Australian boulder stream samples reveal was from the bottom of a boulder accom- [2008]) based on the constant production
a cluster of minimum limiting exposure panied by three underlying clasts (Fig. rate model (Lal, 1991; Stone, 2000) using
histories around 21 ± 0.5 ka (LGM), while 2A). We photographed and recorded the the regional northeastern U.S. production
other samples from the same field have min- dimensions, sub-meter resolution UTM rate (Balco et al., 2009).
imum total near-surface histories of coordinates, sample thickness, and lithol-
60–480 ka (Barrows et al., 2004). Samples ogy of each boulder. Additionally, we used RESULTS
from boulder streams in the Falkland eCognition software to automatically
Islands (n = 16) have 10Be histories of extract boulder outlines from aerial imag- Boulders at Hickory Run have experi-
42–730 ka (Wilson et al., 2008). A Korean ery to test for trends in boulder size and enced widely varying and substantial near-
boulder field has 10Be histories (n = 4) orientation. surface exposure. Hickory Run samples
between 38 and 65 ka (Seong and Kim, have 10Be concentrations ranging from
2003), while samples from Swedish boul- We purified quartz (Kohl and Nishiizumi, 0.44 to 3.26 × 106 atoms g−1 (Fig. 3), the
der fields have histories of 33 and 73 ka 1992) and extracted 10Be and 26Al (Corbett equivalent of between 70 and 600 k.y. of
(n = 2) (Goodfellow et al., 2014). Analysis et al., 2016) at The University of Vermont. surface exposure.
(n = 15) of paired 26Al and 10Be in block We measured 10Be/9Be ratios at Lawrence
streams suggests some boulders have Livermore National Laboratory, normal- There is no significant correlation
histories that include either exposure under izing them relative to ICN standard between 10Be concentration and boulder
cover and/or burial after near-surface 07KNSTD3110 with an assumed value of lithology, size, or proximity to the edge of
exposure (Goodfellow et al., 2014; Seong 2.85 × 10−12 (Nishiizumi et al., 2007). We the field. Boulders downslope are more
and Kim, 2003; Wilson et al., 2008). corrected our data using process blanks rounded, smaller (Fig. DR1 [see footnote
(see GSA Data Repository1 Table DR1) and 1]), and have more developed weathering
METHODS processed four replicates to test reproduc- rinds than those upslope, suggesting that
ibility; the difference between replicates boulder weathering increases downslope.
We sampled in and around the Hickory ranged from <1%–4% (mean 2%). We then We also observe spatial trends in boulder
Run boulder field in eight slope-normal selected the boulder bottom and clast sam- orientation; downslope boulders align
transects, collecting a total of 52 samples ples (n = 4) along with a subset of upslope with the main axis of the field (NE-SW),
by removing the surficial few centimeters (n = 10) and downslope (n = 11) boulder whereas upslope boulders align E-W
of rock. Of these samples, 30 were from samples for 26Al/27Al analysis at PRIME (Fig. DR1 [see footnote 1]).
boulders in the main field, six were from Lab. Minimum near-surface histories were
the southeastern sub-field, seven were calculated using the CRONUS Earth Our 10Be results support the inference
from boulders in the surrounding forest, online calculator (http://hess.ess.washing- of increased weathering and near-surface
five were from bedrock tors cropping out ton.edu/), wrapper script 2.2, main calcula- exposure time downfield. The strongest
on a ridgeline 700 m NE (Fig. 1C), and one tor 2.1, constants 2.2.1 (see Balco et al. correlation we observe is between down-
field distance and 10Be concentration
(r2 = 0.45; Fig. 3); additionally, 10Be

Figure 2. Measurement of boulder HR10 and underlying clasts. (A) Photograph of boulder HR10 on top of clasts; (B) side view of HR10 samples and
underlying clasts; (C) 10Be production decreases exponentially with depth. The black dashed line represents the 10Be concentrations expected in HR10B
and samples 10C1–C3 if they remained in place at depth for their entire histories. (D) Depth profile assuming the boulder flipped 180° at 25 ka—the
concentration in HR10T is too high to have flipped then. (E) Sample HR10T aligns with the depth profile assuming the boulder flipped at 200 ka.

1 GSA Data Repository Item 2017393, a detailed description of methodology, is online at www.geosociety.org/ft2017.htm.

6 GSA Today | March-April 2018