Radiocarbon, Vol 63, Nr 1, 2021, p 321–342 DOI:10.1017/RDC.2020.118
© 2020 by the Arizona Board of Regents on behalf of the University of Arizona. This is an Open Access
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ERRONEOUSLY OLD RADIOCARBON AGES FROM TERRESTRIAL POLLEN
CONCENTRATES IN YELLOWSTONE LAKE, WYOMING, USA
Christopher M Schiller1* • Cathy Whitlock1,2 • Kathryn L Elder3 • Nels A Iverson4 •
Mark B Abbott5
1Montana State University, Department of Earth Sciences, Bozeman, MT 59717 USA
2Montana State University, Montana Institute on Ecosystems, Bozeman, MT 59717 USA
3Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Woods Hole, MA 02543 USA
4New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro,
NM 87801 USA
5University of Pittsburgh, Department of Geology and Environmental Science, Pittsburgh, PA 15260 USA
ABSTRACT. Accelerator mass spectrometry (AMS) dating of pollen concentrates is often used in lake sediment
records where large, terrestrial plant remains are unavailable. Ages produced from chemically concentrated pollen
as well as manually picked Pinaceae grains in Yellowstone Lake (Wyoming) sediments were consistently 1700–
4300 cal years older than ages established by terrestrial plant remains, tephrochronology, and the age of the
sediment-water interface. Previous studies have successfully utilized the same laboratory space and methods,
suggesting the source of old-carbon contamination is specific to these samples. Manually picking pollen grains
precludes admixture of non-pollen materials. Furthermore, no clear source of old pollen grains occurs on the
deglaciated landscape, making reworking of old pollen grains unlikely. High volumes of CO2 are degassed in the
Yellowstone Caldera, potentially introducing old carbon to pollen. While uptake of old CO2 through
photosynthesis is minor (F14C approximately 0.99), old-carbon contamination may still take place in the water
column or in surficial lake sediments. It remains unclear, however, what mechanism allows for the erroneous ages
of highly refractory pollen grains while terrestrial plant remains were unaffected. In the absence of a satisfactory
explanation for erroneously old radiocarbon ages from pollen concentrates, we propose steps for further study.
KEYWORDS: AMS dating, chronology, contamination, paleoecology, pine.
INTRODUCTION
Lake sediments are the primary archive for reconstructing late-Quaternary terrestrial climate,
vegetation, fire, and limnobiotic processes. Correlation between sites and comparison of
independent proxies and paleoclimate model simulations requires sedimentary records with
high-resolution age-depth control, most commonly accomplished through radiocarbon dating.
The utility of radiocarbon dating pollen concentrates with accelerator mass spectrometry
(AMS) has been well established in lacustrine settings where terrestrial plant remains are
unavailable and where total organic carbon of (bulk) sediments are potentially contaminated by
refractory carbon and reservoir effects. Pollen concentration through mechanical separation
(Brown et al. 1989; Long et al. 1992; Mensing and Southon 1999), chemical maceration (Doher
1980), heavy liquid separation (Vandergoes and Prior 2003), and flow cytometry (Tennant et al.
2013) selectively removes anachronous sources of carbon (e.g. carbonates, algal remains, humic
acids) from the sample. Although some studies suggest dating errors as a result of incomplete
separation of non-pollen material (Regnéll 1992; Richardson and Hall 1994) or incorporation
of reworked pollen grains (Mensing and Southon 1999; Zimmerman et al. 2019), pollen-
concentrate ages typically produce more reliable chronologies than bulk-sediment ages where
reservoir effects are a concern (e.g. Brown et al. 1989; Vandergoes and Prior 2003; Newnham
et al. 2007; Fletcher et al. 2017).
Yellowstone Lake, Wyoming (44.5°N, 110.3°W, elev. 2360 m.a.s.l.), a large (352 km2),
temperate, subalpine lake (Figure 1), is a good candidate for AMS dating of pollen
*Corresponding author. Email: christopher.schiller@student.montana.edu.
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322 C M Schiller et al.
Figure 1 Map of the Yellowstone Lake basin, showing location of the collection of sediment cores and modern
pollen samples. Subaerial hydrothermal areas based on Vaughan et al. (2014) and sublacustrine hydrothermal vents
based onMorgan et al. (2007). All sites are located within or near the eastern half of the Yellowstone Caldera, where
the majority of CO2 degassing occurs from acid-sulfate hydrothermal vents (Werner and Brantley 2003).
concentrates, as few sufficiently large, terrestrial plant remains were recovered from recent
coring efforts (2016, Table 1). The geology of the watershed is complex, resulting from an
interplay of volcanic, glacial, and hydrothermal processes. The northern two-thirds of the
lake basin lies within the Yellowstone Caldera that was formed ~630 ka (Christiansen 2001;
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Terrestrial Pollen Concentrates in Yellowstone Lake 323
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Table 1 Yellowstone Lake and vicinity age controls.
Accession
no.1 Core depth (cm)2/ C δ13C 2-σ cal BP age range
(receipt no.) location Material dated3 (μg) ‰ VPDB Age 14C F14C4 (probability)5 Source6
FD-2
C-5753 24.0 t. plant remains 430 ± 70 0.9479 ± 0.0082 307–552 (0.998) Tiller 1995
614–616 (0.002)
C-5755 123.5 t. plant remains 1450 ± 100 0.8348 ± 0.0103 1179–1558 (1.000) Tiller 1995
C-10034 225.5 humic acid 6060 ± 60 0.4703 ± 0.0035 6750–6767 (0.016) This work
6774–7030 (0.879)
7043–7070 (0.022)
7077–7086 (0.008)
7097–7156 (0.074)
C-10035 225.5 humin 6030 ± 60 0.4721 ± 0.0035 6698–6700 (0.001) This work
6719–7022 (0.977)
7120–7152 (0.022)
C-10048 295.5 t. plant remains 7950 ± 180 0.3717 ± 0.0082 8408–9289 (1.000) Tiller 1995
C-10037 295.5 humic acid 7740 ± 60 0.3815 ± 0.0028 8410–8609 (0.991) This work
8618–8626 (0.009)
C-10036 295.5 humin 7680 ± 60 0.3844 ± 0.0029 8389–8581 (1.000) This work
C-5754 455.3 t. plant remains 9080 ± 100 0.3230 ± 0.0040 9915–10098 (0.149) Tiller 1995
10110–10511 (0.851)
C-10039 475.5 humic acid 10460 ± 80 0.2720 ± 0.0027 12064–12596 (1.000) This work
C-10040 475.5 humin 10280 ± 60 0.2781 ± 0.0021 11815–12244 (0.885) This work
12266–12383 (0.115)
C-4977 476.6 t. plant remains 6290 ± 80 0.4570 ± 0.0045 7615–7879 (0.924) Tiller 1995
7890–7931 (0.076)
C-10038 505.5 t. plant remains 7100 ± 210 0.4132 ± 0.0107 7580–8340 (1.000) Tiller 1995
C-10047 505.5 t. plant remains 7060 ± 100 0.4152 ± 0.0051 7673–8051 (0.996) Tiller 1995
8098–8102 (0.001)
8144–8149 (0.003)
C-9165 505.5 humic acid 11380 ± 80 0.2425 ± 0.0024 13079–13386 (1.000) This work
C-9164 505.5 humin 10280 ± 90 0.2781 ± 0.0031 11651–11662 (0.003) This work
11705–12418 (0.997)
506.5 Mazama ash 7584–7682 Egan et al.
2015
C-4976 582.7 t. plant remains 8290 ± 120 0.3563 ± 0.0053 9010–9503 (0.997) Tiller 1995
9511–9516 (0.003)
C-10042 582.5 humic acid 12350 ± 80 0.2149 ± 0.0021 14067–14830 (1.000) This work
(Continued)
324 C M Schiller et al.
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Table 1 (Continued )
Accession
no.1 Core depth (cm)2/ C δ13C 2-σ cal BP age range
(receipt no.) location Material dated3 (μg) ‰ VPDB Age 14C F14C4 (probability)5 Source6
C-10041 582.5 humin 12050 ± 70 0.2231 ± 0.0019 13748–14088 (1.000) This work
C-4974 657.0 t. plant remains 10200 ± 90 0.2809 ± 0.0031 11404–11568 (0.067) Tiller 1995
11573–11576 (0.001)
11591–12189 (0.901)
12199–12237 (0.015)
12283–12301 (0.006)
12343–12376 (0.012)
C-4973 735.1 t. plant remains 10280 ± 80 0.2781 ± 0.0028 11757–12401 (1.000) Tiller 1995
C-10044 766.5 humic acid 16850 ± 60 0.1228 ± 0.0009 20105–20523 (1.000) This work
C-10043 766.5 humin 17090 ± 150 0.1191 ± 0.0022 20202–21013 (1.000) This work
C-10046 778.3 humic acid 18240 ± 100 0.1032 ± 0.0013 21846–22366 (1.000) This work
C-10045 778.3 humin 17950 ± 100 0.1070 ± 0.0013 21432–22027 (1.000) This work
C-9162 854.0 humic acid 16420 ± 180 0.1295 ± 0.0029 19359–20279 (1.000) This work
C-9163 854.0 humin 18900 ± 250 0.0951 ± 0.0029 22309–23445 (1.000) This work
855.0 Glacier Peak 13410–13710 (1.000) Kuehn et al.
(B or G) ash 2009
13A
0 core top –67
OS-142012 1.0 bulk sediment 1559 5250 ± 25 0.5203 ± 0.0017 5928–6026 (0.729) This work
(153942) 6047–6065 (0.029)
6077–6116 (0.160)
6152–6175 (0.082)
OS-142223 1.0 picked pollen 22 2310 ± 180 0.7505 ± 0.0164 1904–1907 (0.001) This work
(153938) 1924–2755 (0.999)
12.0 1700 CE fire 240–260 Romme &
Despain,
1989
2C
OS-138757 164.0 c.c. pollen 476 3860 ± 20 0.6185 ± 0.0016 4163–4166 (0.006) This work
(149875) 4181–4198 (0.047)
4230–4407 (0.948)
OS-135957 328.0 t. plant remains 670 -26.3 2590 ± 20 0.7242 ± 0.0019 2723–2754 (1.000) This work
(147265)
(Continued)
Terrestrial Pollen Concentrates in Yellowstone Lake 325
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Table 1 (Continued )
Accession
no.1 Core depth (cm)2/ C δ13C 2-σ cal BP age range
(receipt no.) location Material dated3 (μg) ‰ VPDB Age 14C F14C4 (probability)5 Source6
OS-135958 402.0 t. plant remains 1130 -28.0 3150 ± 25 0.6760 ± 0.0022 3272–3285 (0.038) This work
(147266) 3339–3445 (0.962)
OS-138692 524.5 c.c. pollen 440 6280 ± 30 0.4574 ± 0.0017 7164–7263 (1.000) This work
(149876)
OS-136956 624.0 t. plant remains 643 4510 ± 20 0.5707 ± 0.0016 5053–5190 (0.688) This work
(147971) 5213–5296 (0.312)
OS-142084 755.0 t. plant remains 148 -10.9 9980 ± 45 0.2887 ± 0.0017 11259–11623 (0.985) This work
(153943) 11675–11694 (0.015)
OS-138693 792.0 c.c. pollen 307 7220 ± 35 0.4068 ± 0.0018 7963–8072 (0.764) This work
(149877) 8085–8158 (0.236)
OS-142082 934.5 bulk sediment 461 11150 ± 55 0.2497 ± 0.0016 12841–13112 (1.000) This work
(153939)
OS-142018 934.5 picked pollen 58 10250 ± 170 0.2793 ± 0.0059 11348–11380 (0.011) This work
(153935) 11386–12540 (0.989)
940.0 Mazama ash 7584–7682 Egan et al.,
2015
OS-138622 965.5 t. plant remains 190 5740 ± 30 0.4895 ± 0.0019 6453–6459 (0.012) This work
(149869) 6462–6634 (0.988)
OS-138700 978.0 c.c. pollen 569 9870 ± 55 0.2928 ± 0.0019 11188–11405 (0.982) This work
(149878) 11457–11465 (0.004)
11563–11593 (0.014)
OS-138701 986.0 c.c. pollen 248 9390 ± 50 0.3106 ± 0.0019 10500–10741 (1.000) This work
(149879)
OS-138866 1160.5 c.c. pollen 545 12100 ± 110 0.2215 ± 0.0031 13639–13663 (0.004) This work
(149880) 13707–14303 (0.996)
9A
0 core top –67
OS-142011 1.0 bulk sediment 2051 4080 ± 25 0.6017 ± 0.0019 4447–4470 (0.060) This work
(153941) 4516–4644 (0.770)
4677–4692 (0.017)
4761–4800 (0.153)
(Continued)
326 C M Schiller et al.
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Table 1 (Continued )
Accession
no.1 Core depth (cm)2/ C δ13C 2-σ cal BP age range
(receipt no.) location Material dated3 (μg) ‰ VPDB Age 14C F14C4 (probability)5 Source6
OS-142222 1.0 picked pollen 34 1910 ± 110 0.7885 ± 0.0109 1571–1583 (0.006) This work
(153937) 1598–2120 (0.994)
5A
OS-135959 364.5 t. plant remains 730 -27.1 3760 ± 30 0.6261 ± 0.0022 3993–4039 (0.127) This work
(147267) 4074–4184 (0.727)
4188–4192 (0.007)
4195–4235 (0.139)
OS-138998 533.5 c.c. pollen 202 -0.4 6140 ± 95 0.4654 ± 0.0054 6792–7254 (1.000) This work
(149881)
OS-138868 540.5 c.c. pollen 411 6600 ± 45 0.4395 ± 0.0024 7433–7566 (1.000) This work
(149882)
OS-142083 782.0 bulk sediment 405 11050 ± 50 0.2524 ± 0.0016 12779–13057 (1.000) This work
(153940)
OS-142221 782.0 picked pollen 43 5530 ± 130 0.5021 ± 0.0083 5999–6568 (0.977) This work
(153936) 6586–6627 (0.023)
783.0 Mazama ash 7584–7682 Egan et al.,
2015
OS-139001 814.0 c.c. pollen 243 -25.5 9060 ± 50 0.3236 ± 0.0021 10157–10299 (0.975) This work
(149883) 10320–10343 (0.012)
10352–10374 (0.013)
OS-139002 823.5 c.c. pollen 303 -25.2 9100 ± 55 0.3222 ± 0.0021 10184–10409 (1.000) This work
(149884)
OS-138869 837.0 c.c. pollen 413 9340 ± 70 0.3126 ± 0.0027 10297–10357 (0.047) This work
(149885) 10369–10723 (0.953)
OS-139116 941.5 c.c. pollen 390 -25.7 9020 ± 50 0.3254 ± 0.0021 9936–9992 (0.070) This work
(149886) 10008–10030 (0.017)
10035–10062 (0.027)
10128–10255 (0.886)
(Continued)
Terrestrial Pollen Concentrates in Yellowstone Lake 327
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Table 1 (Continued )
Accession
no.1 Core depth (cm)2/ C δ13C 2-σ cal BP age range
(receipt no.) location Material dated3 (μg) ‰ VPDB Age 14C F14C4 (probability)5 Source6
Modern
Pollen
OS-149977 Riddle Lk. TrlHd. modern pollen 247 -30.8 80 ± 15 0.9903 ± 0.0019 –5 (0.003) This work
(160235) 33–74 (0.556)
79–81 (0.004)
98–107 (0.023)
113–136 (0.170)
224–254 (0.243)
OS-149978 Cub Creek Pond modern pollen 343 -31.4 50 ± 20 0.9940 ± 0.0022 –5 (0.024) This work
(160236) 35–70 (0.835)
117–131 (0.056)
231–244 (0.081)
248–250 (0.004)
OS-149979 Mud Volcano modern pollen 268 -30.8 170 ± 20 0.9790 ± 0.0025 –5–31 (0.214) This work
(160237) 138–158 (0.111)
164–222 (0.492)
258–285 (0.182)
1C-## (Chicago), OS-## (National Ocean Sciences Acceleratory Mass Spectrometry).
2Depths reported are tops of 1-cm intervals from which samples were collected.
3c.c. pollen (chemically concentrated pollen), t. plant remains (terrestrial plant remains).
4FD-2 F14C back-calculated from measured 14C age according to method of Reimer et al. (2004).
5Calibrated ranges calculated by CALIB and CALIBomb according to method described in text, parenthetical values relative area under probability distribution.
6Source listed for lithological context (FD-2) or calendar age of event.
328 C M Schiller et al.
Matthews et al. 2015; Wotzlaw et al. 2015), and the entire lake basin was covered by the
Yellowstone glacial complex, 25,000–14,000 BP (Licciardi and Pierce 2018). The lake floor
today has more than 660 active or recently active hydrothermal vents (Figure 1; Morgan
et al. 2007).
Previous studies of sediment cores from Yellowstone Lake provide a postglacial limnogeologic
(Tiller 1995) and paleoenvironmental (Theriot et al. 2006) record, making high-resolution age
control desirable. In addition, Yellowstone Lake sediments preserve deposits from past
hydrothermal explosions (Tiller 1995; Morgan et al. 2009) and volcanic ashes from
eruptions in the Cascade Range to the west (Tiller 1995; Theriot et al. 2006). Sediment
cores collected in 1992 (FD-2; Figure 1) were dated with radiocarbon ages from terrestrial
plant remains (Tiller 1995; Table 1). However, previously unpublished radiocarbon ages
collected from humic acid and humin during that study were up to thousands of years
older than associated terrestrial plant remains (Table 1) and demonstrate the presence of an
old-carbon pool (Figure 2). Radiocarbon ages of bulk sediments and pollen concentrates
from Cub Creek Pond (44.505°N, 110.246°W, elev. 2510 m.a.s.l.) ~4 km east of
Yellowstone Lake also yielded calibrated ages thousands of years older than established by
terrestrial plant remains or tephrochronology (Whitlock 1993; Lu et al. 2017).
Our objective in this study was to answer the following questions: (1) How do radiocarbon ages
from different terrestrial materials in Yellowstone Lake sediments compare? (2) What is the
carbon source for pollen concentrates from Yellowstone Lake sediments that might lead to
erroneously old ages? To answer these questions, we explore a suite of AMS radiocarbon
ages from several sediment cores from the northern basin of Yellowstone Lake and
compare them with independent age controls and the radiocarbon ages of living plant material.
METHODS
Sediment cores from the northwest area (core 2C, 44.53927°N, 110.38922°W, water depth 61
m) and southeast area (core 5A, 44.50782°N, 110.32685°W, water depth 102 m) of the northern
basin of Yellowstone Lake were collected in September 2016 with a Kullenberg sampler
(Figure 1; Kelts et al. 1986). Additional cores were retrieved near the 2C (13A) and 5A
(9A) sites with a gravity sampler in July 2017 to recover the most recent sediments. Cores
were transported to the National Lacustrine Core Facility (LacCore, University of
Minnesota) where they were longitudinally split, photographed, and stored under
refrigeration except when sampled or during core-scanning analyses. Kullenberg and
gravity cores were correlated on the basis of core imagery and charcoal stratigraphy,
producing two complete records for analysis, core 2C/13A and core 5A/9A.
Initial stratigraphic AMS age determinations for core 2C/13A were derived from terrestrial
plant remains (needles, charcoal, or unidentified wood fragments) and pollen concentrates.
Bulk-sediment ages were avoided based on erroneously old ages previously obtained from
humin in the FD-2 core (Figure 2; Table 1). Plant remains were collected wherever present,
identifiably terrestrial, and large enough for analysis. Five terrestrial plant remains were
identified, extracted from core sediments, washed in distilled water, and placed under a
dissection microscope where extraneous sediment was removed with a teasing needle. Initial
pollen samples were collected from 1-cm intervals immediately above and below a thick
hydrothermal explosion deposit and wherever substantial stratigraphic gaps occurred in the
spacing of terrestrial plant remains. Pollen was concentrated by traditional pollen
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Terrestrial Pollen Concentrates in Yellowstone Lake 329
Figure 2 Bacon age-depth models for Yellowstone Lake (FD-2 from Tiller 1995) derived from bulk sediment
components, humin and humic acid (brown), terrestrial plant remains (green; Tiller 1995), and ashes (red).
Younger ash is Mazama (7682–7584 cal BP, 2-σ range, Egan et al. 2015) and the older is Glacier Peak B or G
(13,410–13,710 cal BP, 2-σ range, Kuehn et al. 2009) as determined by petrographic analysis of Tiller (1995).
maceration procedures (Bennett and Willis 2001). Sediments were disaggregated and humic
compounds removed with 0.9N KOH treatment (near-boiling hot bath, 2.5 min) and rinsed
with distilled water, centrifuged, and decanted until the supernatant was clear. Large
particles were wet-sieved with distilled water through a 180 μm mesh. Carbonates were
removed with 2.7N HCl treatment (near-boiling hot bath, until no effervescence was
observed) and silicates were removed with 28.9N HF (near-boiling hot bath, 60 min).
Acetolysis was foregone in favor of oxidation in 15.9N HNO3 (near-boiling hotbath,
3 min) to remove cellulose while avoiding carbon-bearing acetate solutions. Samples were
rinsed with distilled water, centrifuged, and decanted until the slurry was neutral. These
samples are hereafter referred to as chemically concentrated pollen.
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330 C M Schiller et al.
Paired bulk-sediment and picked-pollen concentrate samples also were collected from
stratigraphic locations (in 1-cm intervals) where ages were known independently. Bulk-
sediment and picked-pollen samples were collected at the sediment-water interface (core
top) in the gravity cores (13A, 9A) where the true age was approximately the year of
coring (~–67 cal BP). Further bulk-sediment and picked-pollen samples were collected from
Kullenberg cores (2C, 5A) immediately above what is likely the Mazama ash (Tiller 1995;
Theriot et al. 2006). These tephra (2C, 5A), along with a new analysis of the younger Tiller
(1995) tephra from core FD-2, were sampled and analyzed at the New Mexico Bureau of
Geology and Mineral Resources on a Cameca SX-100 electron microprobe to confirm the
geochemical correlation between the cores, and to the Mazama ash. Picked-pollen samples
were obtained utilizing the maceration procedures described above and a modified pipetting
procedure from Mensing and Southon (1999). Sieving with 90 and 38 μm mesh screens
targeted Pinus pollen and further concentration was managed by picking with a
micropipette under a stereomicroscope (63× magnification). Pipette suction was gently
produced by a syringe in place of the mouth. Identification under the stereomicroscope was
generally only possible at the level of Pinaceae, so picked-pollen samples likely contained
some mixture of Pinus, Abies, and Picea. It was not practical to count grains during the
picking procedure, but resultant mass of C, 22–58 μg, was comparable to the yields of
Mensing and Southon (1999).
Finally, to confirm that the carbon isotope composition of the pollen, like wood, reflects the
atmospheric radiocarbon activity, modern pollen was collected from Pinus contorta trees in the
Yellowstone Lake vicinity (Figure 1). Pollen (male) cones were collected from trees at three
locations near Yellowstone Lake in June 2019: (1) upwind of Yellowstone Lake near the
Riddle Lake Trailhead, (2) downwind of Yellowstone Lake near Cub Creek Pond, and (3)
near Mud Volcano, where uptake of radiometrically old carbon has been previously
documented in Pinus contorta tree rings (F14C up to 0.35 lower than NHZ1 post-bomb
curve; Evans et al. 2010). Due to a cold, wet spring, pollen production was late, and
samples collected in June 2019 were probably from the 2018 crop. Pollen was rinsed off of
cones and sieved through 90 and 38 μm mesh screens to remove extraneous material and
treated with Schulze solution (1:10 wt (g)/vol (mL) mixture KClO3 in 15.9N HNO3, near-
boiling hot bath, 3 min) to remove extraneous organics (Doher 1980). Samples were rinsed
in distilled water, centrifuged, and decanted until the slurry was neutral.
AMS age determinations were made by the National Ocean Sciences AMS (NOSAMS) facility
at the Woods Hole Oceanographic Institute. Traditional pretreatment was conducted only for
terrestrial plant remains using a modified acid-base-acid method to remove non-structural
carbon (de Vries and Barendsen 1954) and bulk sediments were treated with 2.7N HCl
(McNichol et al. 1994). Pollen-concentrate samples were combusted without any further
treatment at NOSAMS since previous concentration procedures made further pretreatment
redundant. Pollen-concentrate samples were generally very small, <0.1 mg total sample
mass. To preserve as much of the sample as possible, concentrates were shipped in dilute
HCl in distilled water with subsequent filtering onto pre-baked quartz fiber filters, rinsed to
neutral, dried, and combusted at 850°C in sealed quartz tubes. Carbon yields from pollen
concentrates ranged from 22 to 569 μg C. Radiocarbon results were blank corrected for the
possible addition of contaminants during sample processing and AMS measurement. For
samples that yielded <100 μg carbon, a mass-balance correction was made according to
Roberts et al. (2019). For larger sample masses, the mass-dependent correction is relatively
small, and an average large mass blank value from acetanilide samples (used as a home
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Terrestrial Pollen Concentrates in Yellowstone Lake 331
blank due to the unavailability of 14C-free pollen material) measured concurrently with the
unknown sample was used to correct the result. When samples were large enough, a split of
CO2 was sent to a stable mass spectrometer for δ13C analysis to aid in the verification of
results and identification of anachronous carbon sources including aquatic plants and
carbonate rock.
Radiocarbon ages were calibrated with CALIB (version 7.1, Stuiver et al. 2019) utilizing the
IntCal13 calibration curve (Reimer et al. 2013). Calibrated ages were further refined and
interpreted in the context of stratigraphic depth through development of two age-depth
models using Bacon software (rbacon version 2.3.9.1, Blaauw and Christen 2011) based on
chemically concentrated pollen and terrestrial plant remains. A priori assignment of mean
sediment accumulation rate was set to 10 yr cm–1, as suggested by Bacon and closely
matching our data, but is somewhat faster than Holocene sediment accumulation rates
estimated for other, deeper parts of Yellowstone Lake (~15 yr cm–1, Tiller 1995; 17–10 yr
cm–1, Johnson et al. 2003). Thickness for spline calculation was set to 20 cm, above which
the model diverged greatly from provided age controls. A priori parameters were held
constant for age-depth models based on chemically concentrated pollen and terrestrial plant
remains, and independent age controls were included in both models. Instantaneous
deposits resulting from volcanic ashfall or hydrothermal explosions were relatively thin in
this core (<5 cm) and not excised from the total core depth.
RESULTS
Radiocarbon Results
Calibrated radiocarbon ages for core 2C/13A are presented in Table 1, along with non-
radiocarbon ages: the core top, a prominent charcoal peak from a large fire that occurred
ca. 1700 CE (based on tree-ring analysis in the watershed; error assigned to ±10 years on
the basis of two prominent decades of stand establishment; Romme and Despain 1989;
Higuera et al. 2010), and the geochemically fingerprinted Mazama ash (Table 2). All three
lake-core tephra are geochemically identical to the Mazama ash (Jensen and Beaudoin
2016) allowing for time to be correlated across Yellowstone lake. The age of the climactic
Mt. Mazama eruption is well established in ice-core records (7777–7477 cal BP, 2-σ range;
Zdanowicz et al. 1999) and by thorough radiocarbon dating efforts (7682–7584 cal BP, 2-σ
range; Egan et al. 2015). One radiocarbon result from unidentified plant remains
(OS-142084) was excluded from the terrestrial plant age-depth model, given its erroneously
old age and a δ13C value of –10.9‰, both of which are consistent with an aquatic plant
fragment.
The age-depth models from chemically concentrated pollen and terrestrial plant remains
diverge, except at the top of the core where closely spaced calendar ages enforce unity
(Figure 3). Modelled median calibrated ages from pollen concentrates ranged from 1700 to
4300 years older than the modelled median calibrated ages from terrestrial plants. Ages
from chemically concentrated pollen were internally consistent, with no ages falling outside
the 2-σ range of the age-depth model, but they too did not agree with the age of the
Mazama ash. The Mazama ash age was rejected by Bacon as an outlier in the chemically
concentrated pollen model. Terrestrial plant remain ages were also internally consistent,
with only one age outside the 2-σ range, and the age model aligned well with the age of the
Mazama ash (Figure 3).
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332 C M Schiller et al.
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Table 2 Summary of normalized1 geochemical data from Yellowstone Lake Mazama ash samples compared with UA2832 of Jensen and
Beaudoin (2016).
Sample n2 P2O5 SiO2 SO2 TiO2 Al2O3 MgO CaO MnO FeO3 Na2O K2O F Cl Total Analytical total
2C 9 0.07 72.77 0.01 0.45 14.22 0.43 1.64 0.05 2.00 5.25 2.84 0.10 0.17 100 99.09
σ 0.03 0.08 0.01 0.03 0.06 0.03 0.04 0.02 0.04 0.08 0.04 0.01 0.01 1.35
5A 12 0.06 72.78 0.02 0.42 14.20 0.43 1.64 0.06 2.03 5.28 2.79 0.11 0.19 100 97.33
σ 0.03 0.15 0.01 0.04 0.10 0.03 0.06 0.02 0.06 0.18 0.07 0.01 0.04 1.32
FD-2 9 0.08 72.87 0.01 0.41 14.25 0.42 1.62 0.05 1.99 5.24 2.77 0.11 0.19 97.56
σ 0.03 0.21 0.01 0.04 0.08 0.03 0.05 0.01 0.06 0.12 0.05 0.03 0.07 2.63
UA2832 21 72.84 0.44 14.73 0.47 1.64 0.05 2.01 4.88 2.77 0.18 100 96.97
σ 0.93 0.07 0.34 0.11 0.25 0.02 0.28 0.16 0.16 0.03
1 Data has been normalized to 100% and the analytical total are presented for each sample. Oxidized reported as wt. %. Glass shards were analyzed on a Cameca SX100 with four
WDS spectrometers an accelerating voltage of 15 kV and a probe current of 10 nA and a beam diameter of 20 microns were used. Count times= 20s except S= 30s, Cl= 40 and
F= 60. Analytical precision from microprobe analyses is calculated by the standard deviation of replicate analyses of secondary standard VG568, are as follows (in wt.%): P2O5
± 0.01, SiO2 ± 0.23, SO2 ± 0.01, TiO2 ± 0.02, Al2O3 ± 0.08, MgO ± 0.01, CaO ± 0.01, MnO ± 0.02, FeO ± 0.06, Na2O ± 0.23, K2O ± 0.06, F ± 0.08, Cl ± 0.01.
2 Number of analyses.
3 FeO total.
Terrestrial Pollen Concentrates in Yellowstone Lake 333
Figure 3 Bacon age-depth models for Yellowstone Lake (core 2C) derived from chemically concentrated pollen
(yellow), terrestrial plant remains (green), the Mazama ash (red), and calendar ages of the coring date and a fire at
1700 CE.
Paired bulk-sediment and picked-pollen sample ages were calibrated utilizing Intcal13 (Reimer
et al. 2013) and plotted with CALIB (version 7.1, Stuiver et al. 2019, Figure 4). Bulk-sediment
ages were 4600 and 6000 cal years too old (F14C 0.6017 ± 0.0019 and 0.5203 ± 0.0017) at the
sediment-water interface and 5300 and 5400 cal years too old directly aboveMazama ash (F14C
0.2524 ± 0.0016 and 0.2497 ± 0.0016). Picked-pollen samples offered an improvement, with
ages that were 1900 and 2400 cal years too old at the sediment-water interface (F14C
0.7885 ± 0.0109 and 0.7505 ± 0.0164). Directly above the Mazama ash, the picked pollen
age from 2C/13A (OS-142018) was 4300 cal years too old directly above the Mazama ash
(F14C 0.2793 ± 0.0059), while the age from 5A/9A (OS-142221) was 1300 cal years too
young (F14C 0.5021 ± 0.0083). The lower precision and resultant posterior distributions of
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334 C M Schiller et al.
Figure 4 Probability density plots of dates from multiple materials at depths with known ages. Core top
ages should be the approximate year of coring or pollen cone collection, –67 and –68 cal BP, respectively.
TheMazama samples were collected immediately above theMazama ash, which was deposited at 7682–7584
cal BP (2-σ range, Egan et al. 2015). All ages from Yellowstone Lake cores, except OS-142221, are
erroneously old by thousands of years. Picked-pollen ages have wide age distributions owing to small
sample sizes.
picked-pollen ages were much wider than bulk-sediment ages, and understandably so, due to
the larger uncertainties involved with small mass sample analysis.
Radiocarbon results from modern pollen from the Riddle Lake Trailhead, Cub Creek Pond,
and Mud Volcano demonstrate virtually modern F14C values of 0.9903 ± 0.0019, 0.9940 ±
0.0022, and 0.9790 ± 0.0025, respectively (Figure 4). These F14C values were calibrated
utilizing CALIBomb (Reimer and Reimer 2004, Figure 4) with the NHZ1 post-bomb
calibration (Hua et al. 2013) and IntCal13 pre-bomb calibration (Reimer et al. 2013).
δ13C Results
Where samples were large enough, a split of CO2 from combusted samples was analyzed for
δ13C as well as for radiocarbon, and the results were compared with existing δ13C values
obtained in earlier studies (Figure 5). Bulk sediments had less depleted δ13C values than
terrestrial plant remains, indicative of a non-terrestrial plant carbon source in the sediment.
With the exception of one outlier, δ13C values from terrestrial plant remains fell within the
accepted range for terrestrial C3 plants (Smith and Epstein 1971). Chemically concentrated
pollen δ13C values were widely variable, however, with one outlier nearly at 0 ‰ VPDB.
All δ13C values from sediment core samples were less depleted than those from modern
pollen cones, which were –31.0‰ VPDB on average. δ13C values did also vary somewhat
from site to site, but with no apparent spatial pattern (Figure 5). Cub Creek Pond samples
had the least depleted values (average –20.8 ‰ VPDB), with slightly more depleted values
from 5A/9A (average –20.8‰ VPDB), Hedrick Pond (located 60 km south of Yellowstone
Lake, 43.754°N, 110.590°W, elev. 2055 m.a.s.l.; average –25.8‰ VPDB), and 2C/13A
(average –27.2‰ VPDB).
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Terrestrial Pollen Concentrates in Yellowstone Lake 335
Figure 5 δ13C values of radiocarbon materials are plotted using beanplot (Kampstra 2008) by material (A) and
by site (B). C3 plant and charcoal ranges are given by Smith and Epstein (1971). n is the number of samples included
in each distribution. Yellowstone National Park (YNP) fumarole gas range taken from Craig (1963) and Werner
and Brantley (2003) with MississippianMadison Limestone range taken from Friedman (1970). Data fromHedrick
Pond and Cub Creek Pond from Lu et al. (2017). Chemically concentrated pollen has a wide range of δ13C values
and the majority of least negative values come from areas near hydrothermal areas (Cub Creek Pond and
Yellowstone Lake).
DISCUSSION
Comparison of Radiocarbon Ages from Yellowstone Lake
In Yellowstone Lake, ages from chemically concentrated pollen were systematically and
erroneously old, compared with ages established from terrestrial plant remains,
tephrochronology, or the date of the sediment-surface interface (Figure 4). Chemically
concentrated pollen ages from core 2C/13A were systematically older than terrestrial plant
remains ages by up to 4300 cal years and disagree with the established age of the Mazama
ash (7682–7584 cal BP, 2-σ range; Egan et al. 2015) by 2300 cal years. Ages from strata of
known age corroborate this finding, as picked Pinaceae pollen ages offered an improvement
on bulk-sediment ages, but resultant ages remained too old by 1900 to 4300 cal years.
These results are consistent with previous efforts at dating bulk sediments from
Yellowstone Lake (Figure 2; Table 1) and bulk sediment and pollen concentrates from Cub
Creek Pond (Whitlock 1993; Lu et al. 2017). Ages from living Pinus contorta pollen are
virtually modern by comparison (in equilibrium with atmospheric CO2).
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336 C M Schiller et al.
Carbon Source of Yellowstone Terrestrial Pollen
Previous studies (referenced below) have reported erroneously old pollen ages in sediment cores
and attributed the introduction of old carbon to (1) laboratory processes; (2) incomplete
removal of non-pollen materials during concentration procedures; (3) reworked pollen; and/
or (4) enrichment of old CO2 in source material. The likelihood of these explanations is
explored below.
First, accidental contamination is a major concern in laboratories that prepare samples for
AMS radiocarbon dating. Most laboratories contain potential sources of old carbon
including sodium bicarbonate (NaHCO3) and petrochemicals. NaHCO3 is found in most
pollen preparation fume hoods to neutralize surfaces exposed to caustic acids; however, it
was not actively used in the fume hood during these analyses and would have been
removed through subsequent acidification. Petrochemicals were not used as reagents in the
pollen preparation laboratory. Definitively, additional studies that used the same pollen
preparation environment and procedure have not reported difficulty with old ages from
chemically concentrated pollen (Krause and Whitlock 2013; Krause et al. 2015; Nanavati
et al. 2019), making systematic laboratory contamination from old carbon or fractionation
unlikely sources of error.
Second, old-carbon contamination from incomplete removal of non-pollen materials is
commonly cited as an issue in procedures for concentrating pollen (Regnéll 1992;
Richardson and Hall 1994), and sedimentological sources of old carbon exist in the
vicinity. The Mississippian Madison Limestone, which is exposed in northern Yellowstone
National Park outside the caldera, produces near-zero δ13C values (Figure 5; Friedman
1970), but does not crop out near or upstream of Yellowstone Lake nor does any other
significant carbonate formation (U.S. Geological Survey 1972). Moreover, lake waters are
relatively depleted in dissolved carbonate (<0.8 mM ΣCO2, Balistrieri et al. 2007). Lake
sediments themselves are not reported to contain carbonate mud (e.g. Tiller 1995),
contain< 1 wt% CaO (Shanks et al. 2007), and do not effervesce when submerged in hot
2.7N HCl. Also, acidification treatments would remove most carbonates. Although our
samples had no bituminous odor, we acknowledge that thermogenic hydrocarbons are
found in several subaerial thermal areas of Yellowstone National Park (Love and Good
1970; Clifton et al. 1990) and probably occur in hydrothermally altered sediments of
Yellowstone Lake (Shanks et al. 2007). Core 2C/13A is not located near known
hydrothermal vents (Figure 1); however, hydrocarbon contamination remains possible in
core 5A/9A. Importantly, pollen ages remained erroneously old (with the exception of
OS-142221) regardless of increasingly thorough concentration procedures through Schulze
treatment and manual picking. Although chemically concentrated pollen ages were
unilaterally more accurate than bulk-sediment ages, suggesting concentration procedures
removed some old carbon, the calibrated ages of chemically concentrated and picked
pollen were similar. The manually picked pollen probably produces the purest pollen
sample currently possible, so erroneous ages in these samples probably reflect the age of
the pollen grains with minimal sediment contaminants. Examination of picked pollen
concentrates under a transmitted light microscope at 400× magnification confirmed the
general absence of sediment contaminants. Remaining contaminant particles would have
been too small to be detected under the stereomicroscope and insoluble in our chemical
treatments.
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Terrestrial Pollen Concentrates in Yellowstone Lake 337
Third, reworked pollen is suggested as a major source of old-carbon contamination in
lacustrine settings where extensive old lacustrine deposits occur at the surface (Mensing and
Southon 1999; Zimmerman et al. 2019) or redeposition occurs from glacial melt (Neulieb
et al. 2013; Zhang et al. 2017). There is no evidence of a Holocene glacial meltwater source
in the Yellowstone Lake watershed. Given the size of the Yellowstone depositional basin
and the preponderance of pollen types that are produced in great abundance and
distributed widely (e.g. Pinus, Artemisia), a hypothetical ancient pollen source would need
to be very large. Most unconsolidated surficial sediments were removed from the basin
during repeated glacial episodes (Licciardi and Pierce 2018). Remaining exposures of
ancient lake sediments occur upstream (Baker 1986) and downstream (Love et al. 2007) but
are patchy and very small. Ancient shoreline facies also occur north of Yellowstone Lake
(Pierce et al. 2007), but are situated upstream of the lake, probably too small to be the
source area, and the coarse gravels and shallow-water depositional environments are not a
good pollen preservation environment. Furthermore, pollen grains subjected to reworking
should be observably broken and degraded under the light microscope (Tweddle and
Edwards 2010; Howarth et al. 2013), but pollen preservation was generally excellent in
Yellowstone Lake, as we observed that <1% of the pollen grains were unidentifiable and
>50% of Pinus pollen had intact, distal membranes that allowed assignment to the
subgeneric level under 400× magnification. Finally, dated terrestrial plant remains were
clearly not out-of-sequence, and Holocene pollen spectra, dominated throughout core 2C
by Pinus (>55%, <79%), are consistent with data from nearby sites within Pinus contorta
forest (e.g., Baker 1976; Whitlock 1993; Whitlock et al. 1995; Theriot et al. 2006), further
evidence against a significant component of reworked pollen.
Fourth, CO2 vented from the Yellowstone Caldera is a somewhat unique source of old carbon
in this region. Yellowstone Lake vents comprise a very active thermal area and account for an
estimated 10% of the thermal water flux in Yellowstone National Park (Balistrieri et al. 2007);
in addition, a small, subaerial thermal area exists upslope of Cub Creek Pond (Vaughan et al.
2014). These areas potentially constitute a large source of old-carbon flux, although no
estimates exist for the hydrothermal carbon budget of Yellowstone Lake, and the Cub
Creek Thermal Area is of unknown chemistry and considered potentially inactive (Vaughan
et al. 2014). δ13C values of pollen from Yellowstone Lake and nearby Cub Creek Pond (Lu
et al. 2017) suggest an admixture of sources with less depleted δ13C values than are typical
for C3 plants (Figure 5). Locally, fumarole gases produce the opposite end member of this
admixture (Craig 1963; Werner and Brantley 2003). Large volumes of CO2 are degassed
from magmatic and pre-Tertiary basement sources at present, with daily, caldera-wide
degassing estimated at 45 (± 16) kt primarily from acid-sulfate hydrothermal features
(Werner and Brantley 2003). Uptake of volcanically derived old CO2 has been inferred
elsewhere. For example, lake-sediment cores from volcanic lakes in Anatolia with bulk-
sediment ages document reservoir effects of up to 14,000 years (Roberts et al., 2001; Jones
2004). In other settings, uptake of old CO2 through photosynthesis has been inferred in
living plant tissue at sites near hydrothermal vents (Pasquier-Cardin et al. 1999; Cook et al.
2001; Evans et al. 2010; Lewicki and Hilley 2014) and industrial areas (Baydoun et al.,
2015), but generally do not occur more than 1 km away from the old-carbon source
(Pasquier-Cardin et al. 1999; Evans et al. 2010; Baydoun et al. 2015). This observation is
consistent with dates from modern pollen near Yellowstone Lake (Figure 4), which suggest
minimal uptake of volcanic CO2 through photosynthesis.
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338 C M Schiller et al.
Potential sources for old-carbon contamination are narrowed by the fact that cone pollen
collected in 2018 has the same F14C as atmosphere and picked pollen from core tops is
erroneously old. This limits the potential place of contamination to the water column or
surficial lake sediments. However, it is unclear how hydrothermal carbon could be
incorporated into pollen grains in either context, given that sporopollenins, which compose
the chemically resistant coating of the pollen exine (outer wall), are “ : : : probably the most
resistant organic materials of direct biological origin found in nature : : : ” (Brooks and
Shaw 1978). This hypothesis is also unsatisfactory given that old-carbon contamination of
organic compounds by hydrothermal carbon apparently did not impact other terrestrial
plant remains in the sediments.
CONCLUSIONS
Pollen concentrate ages are systematically older than terrestrial plant remains ages in
Yellowstone Lake by 1700 to 4300 cal years and we can offer no clear explanation for this
puzzlement. The most likely sources of old carbon are (1) CO2 degassing from magmatic
and pre-Tertiary basement sources, which may be incorporated into pollen grains in the
water column or in surficial sediments; or (2) reworked pollen from older sediments.
Neither explanation is fully adequate. In our consideration, further study of the
radiocarbon signature of modern pollen from its production to its release, transport,
deposition, and final burial in lake sediments is required to locate the source of old-carbon
contamination. Such an investigation should include radiocarbon dating of modern pollen
from the cone, pollen intercepted from the local atmosphere, pollen in the water column
and carbon isotope analysis of the DOC in lake water. Experiments may be also be
employed in the laboratory to explore the possibility of incorporation of old carbon from
artificially sourced CO2 into pollen grains in an aqueous mixture. Future experiments may
also examine the impact of individual steps in pollen concentration procedures for carbon
contamination, obtain 14C-free pollen concentrates as a more representative blank material,
and utilize SEM microscopy to detect remaining contaminants associated with pollen grains.
With the current problem of old radiocarbon ages from pollen concentrates and the lack of
other suitable materials for radiocarbon dating, we continue to be limited in our ability to
produce quality age control for paleoenvironmental and geological reconstructions from
Yellowstone Lake, Cub Creek Pond, and potentially other sites in the Yellowstone Caldera
(e.g. Lewis Lake, glacial Yellowstone Lake). Current results underscore the necessity of
careful consideration of carbon source—even in terrestrial plant remains and pollen
concentrates—where reworked sediments or old-carbon degassing are possible.
ACKOWLEDGMENTS
This research was supported by NSF Grant No. 1515353 to C. Whitlock and sampling in
Yellowstone National Park was conducted under permits YELL-SCI-0009 and YELL-SCI-
5054. Coring efforts at Yellowstone Lake were aided by M. Baker, S.R. Brown, D. Conley,
S.C. Fritz, C. Linder, L.A. Morgan, R. O’Grady, W.C. Shanks III, M.D. Shapley, R.A. Sohn,
and three anonymous Yellowstone National Park rangers. This project was greatly improved
through conversations with J. Southon and S.R.H. Zimmerman and thoughtful reviews of the
manuscript came from D. McWethy, L.A. Morgan, D.A. Orme, and W.C. Shanks III.
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Terrestrial Pollen Concentrates in Yellowstone Lake 339
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