DOI: 10.5061/dryad.k98sf7mfs
title: Concentrations of the rare earth elements (REE) and Thorium-232 (232Th) in glacial dust from the northern Gulf of Alaska region

description: 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 &lt;0.49 micrometers (um) to &gt;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 &lt;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.
description: <strong>Sampling
 approaches:</strong><br>Alaskan glacial dust samples were
 collected in a variety of ways from two different locations, during
 multiple years. Bulk dust was collected using a Thermo Partisol Plus 2025
 on Middleton Island, Alaska (59.42144 °N, 146.3493 °W) continuously during
 two years from 2011-2012, as described in Schroth et al, 2017. The
 sampling location, at a high point in the southwestern third of the
 ~50-meter (m) high island, was selected to be close to a source of needed
 electricity but far from frequently traveled roads and other local sources
 of aerosol (e.g. diesel generators at the northeastern end).
 Size-fractionated samples were also collected on Middleton Island (same
 location) during a large glacial dust event from 10-12 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 &lt;0.49 micrometers (um) to
 &gt;7.2 um in diameter. This sampler technology is discussed in
 greater detail in Morton et al, 2013. Samples were filtered with Whatman
 41 (W41) cellulose fiber filters. Filters were acid-washed using
 environmental grade 0.5 M HCl and very thoroughly rinsed with milli-Q
 water, then dried, in a HEPA filtered laminar flow hood.
 Finally, one large-volume sample was generated by collecting dust
 on July 29, 2011, from the shelves of a shed in the Copper River delta
 (close to Million Dollar Bridge, at 60.67454 °N, 144.74811 °W) that was in
 the path of the Copper River-derived dust plume and had a hole in its
 roof, which served as the sample collection device. This large dust sample
 probably integrated over a few years prior to collection. The validity of
 this sample, despite a very unconventional sampling approach, is confirmed
 by the striking similarity of its rare earth element (REE) signature to
 the REE signature of the other samples collected from Middleton Island
 (especially to the &gt;7.2 um diameter sample)).
 <strong>Digestion methods:</strong><br>Samples
 of a few milligrams (mg) of dust (typical for the largest size fractions;
 sometimes less) were weighed after equilibration in lab air overnight in a
 laminar flow hood to minimize moisture content fluctuations. Moisture
 content of the dust has the potential to alter mass estimates
 substantially, hence this step was essential. Total particle digestions
 were carried out in Savillex 15-milliliter (mL) Teflon vials on a hotplate
 using a three-step digestion, at 140-150 degrees Celsius (°C), patterned
 after Morton et al (2013) using: 1) Optima concentrated HNO3; 2) a 4:1
 mixture of Optima concentrated HNO3 and Optima concentrated HF; 3) Optima
 concentrated HNO3, followed in each case by evaporation to dryness.
 Samples were then redissolved in 4 M HNO3 at 90°C for two hours.
 Digestions and evaporation steps were carried out within a polycarbonate
 enclosure, with HEPA-filtered air intake, on top of clean polyethylene
 sheet, within an exhausting fume hood.
 <strong>Analyses:</strong><br>Concentrations of
 the REE and 232Th were determined on 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 ~7 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
 &lt;0.49 um size fraction with the smallest amount of dust, and ~3
 orders of magnitude smaller than the signal of the largest samples. Hence,
 the blanks did not impact our REE concentration estimates
 significantly. Solid reference materials were analyzed
 to evaluate the completeness of the particle digestion process and the
 accuracy of the standard solution calibrations. These included PACS-3 and
 a large sample of Columbia River Basalt (BCR-UW) collected, ground, and
 homogenized by Prof. Bruce Nelson of UW Earth and Space Sciences, from the
 same location as BCR-1 and BCR-2. In addition, Arizona test dust and a
 large-volume dust sample from the Copper River delta (see sampling methods
 discussion) were analyzed multiple times as additional constraints on
 reproducibility. Our REE concentration estimates agree within roughly four
 percent of the published concentrations for the reference materials PACS-3
 and BCR-UW, while 232Th concentrations agree within ~5%. We used BCR-UW as
 a reference material because the USGS Geochemical Reference Materials lab
 was not able to provide us with any BCR-2 upon request, and we assume that
 this sample is of the same composition as BCR-2, as is suggested by the
 REE concentrations. <strong>Double normalized REE
 data processing:</strong><br>REE concentrations were double
 normalized as follows. They were normalized to the average Post-Archean
 Australian Shale (PAAS) REE concentration (McLennan, 1989), following
 recent practice (Grenier et al, 2018; Zhang et al, 2008; Friend et al,
 2008). Such normalization generates smooth plots of the REE because it
 helps to reduce the effect whereby even atomic-numbered REE are more
 abundant than odd atomic-numbered REE (see Grenier et al, 2018).
 Concentrations were then normalized again to the mean PAAS-normalized REE
 concentration of each sample, similar to the approach Serno et al (2014)
 used normalizing to average Upper Continental Crust. See Serno et al
 (2014) for more detail on double normalization. This normalization
 approach differs slightly from the Upper Continental Crust normalization
 used by Serno et al (2014), although there is a very minor impact on the
 REE patterns (Garcia-Solsona et al, 2014, Appx. A.). Another reason we
 used this PAAS normalization approach is that it has been used in recent
 publications where the europium anomaly, Eu/Eu*, is estimated (e.g. Friend
 et al, 2008; Zhang et al, 2008; Grenier et al, 2018) and is thus
 consistent with recent practice in the oceanographic community.
description: # Concentrations of the rare earth elements (REE) and Thorium-232 (232Th)
 in glacial dust from the northern Gulf of Alaska region
 [https://doi.org/10.5061/dryad.k98sf7mfs](https://doi.org/10.5061/dryad.k98sf7mfs) These include raw rare earth element (REE) and thorium (Th) data for samples of glacial dust collected from the northern Gulf of Alaska region.  These also include double normalized data (see methods) that help to reveal subtle differences among samples of different provenance and among samples of different grain size.  These data can be useful to discern the sources of lithogenic sediments, and of trace metals, to the Gulf of Alaska.  See the relevant paper to be published in 2024 in Geophysical Research Letters (currently in review).