Concentrations of the rare earth elements (REE) and Thorium-232 (232Th) in glacial dust from the northern Gulf of Alaska region

Concentrations of the rare earth elements (REE) and Thorium-232 (232Th)
are presented for filtered air (dust) samples collected from the northern
Gulf of Alaska region, including from Middleton Island (AK)(59.4214 N,
146.3493 W) and the Copper River delta  (60.4324 N, 145.0954 W).
Size-fractionated samples were collected in November 2019, using a Tisch
Volumetric Flow Controlled (VFC) high volume sampler (Tisch Environmental,
TE-5170V- BL) outfitted with a Cascade impactor. The six size fractions
collected ranged from <0.49 micrometers (um) to >7.2 um in
diameter. This sampler technology is discussed in greater detail in Morton
et al, 2013. Samples were filtered with acid-washed Whatman 41 (W41)
cellulose fiber filters. Additional bulk dust samples were collected in
October 2012, using a  Thermo Partisol Plus 2025 using Teflon
filters. Samples were fully digested using concentrated nitric and
hydrofluoric acids, following the approach of Morton et al, 2013. Samples
were analyzed using a Thermofisher iCAP inductively coupled plasma mass
spectrometer (ICP-MS) in KED mode, with He as a collision cell gas,
adapted from the approach of Trommetter et al (2020). Concentrations were
determined from standard curves using a REE ICP-MS standard from
High-Purity Standards (that also contained 232Th). Three internal
standards (Ge, In, and Bi) were added to both samples and standards, to
correct for short-term variability in the instrument response and to
evaluate stability of mass response during the ICP-MS run. Concentration
estimates for the REE and 232Th were blank-corrected using full-process
blanks that included filters deployed during times when there was no known
dust deposition. Most of the full-process blank concentrations were 100
times or more smaller than the concentrations of our lowest standard (with
the exception of Ce, the concentration of which was ~seven times smaller
than our lowest standard. This means that our blank concentrations were
very low but also not quantified extremely accurately. Our best estimates
are that the full-process blanks, including filters, ranged from 0.02
picograms per square centimeter (pg cm-2) for Eu, Tb, and Ho, to 2 pg cm-2
for Ce. These blank concentrations were in all cases 40 times or more
smaller than our lowest REE sample concentration for the <0.49 um
size fraction with the smallest amount of dust, and ~3 orders of magnitude
smaller than the signal of the largest samples. The REE data are also
presented in a double-normalized format that first normalizes to
concentrations of Post Archean Australian Shale and then normalizes to the
mean REE concentration. The normalization approach is slightly modified
from that of Serno et al, 2014.