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. Reference Brown, Nelson, Mathewes, Vogel and Southon1989; Long et al. Reference Long, Davis and Lanois1992; Mensing and Southon Reference Mensing and Southon1999), chemical maceration (Doher Reference Doher1980), heavy liquid separation (Vandergoes and Prior Reference Vandergoes and Prior2003), and flow cytometry (Tennant et al. Reference Tennant, Jones, Brock, Cook, Turney, Love and Lee2013) 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 Reference Regnéll1992; Richardson and Hall Reference Richardson and Hall1994) or incorporation of reworked pollen grains (Mensing and Southon Reference Mensing and Southon1999; Zimmerman et al. Reference Zimmerman, Brown, Hassel and Heck2019), pollen-concentrate ages typically produce more reliable chronologies than bulk-sediment ages where reservoir effects are a concern (e.g. Brown et al. Reference Brown, Nelson, Mathewes, Vogel and Southon1989; Vandergoes and Prior Reference Vandergoes and Prior2003; Newnham et al. Reference Newnham, Vandergoes, Garnett, Lowe, Prior and Almond2007; Fletcher et al. Reference Fletcher, Zielhofer, Mischke, Bryant, Xu and Fink2017).
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 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 Reference Christiansen2001; Matthews et al. Reference Matthews, Vazquez and Calvert2015; Wotzlaw et al. Reference Wotzlaw, Bindeman, Stern, D’Abzac and Schaltegger2015), and the entire lake basin was covered by the Yellowstone glacial complex, 25,000–14,000 BP (Licciardi and Pierce Reference Licciardi and Pierce2018). The lake floor today has more than 660 active or recently active hydrothermal vents (Figure 1; Morgan et al. Reference Morgan, Shanks, Pierce, Lovalvo, Lee, Webring, Stephenson, Johnson, Harlan, Schulze and Finn2007).
1 C-## (Chicago), OS-## (National Ocean Sciences Acceleratory Mass Spectrometry).
2 Depths reported are tops of 1-cm intervals from which samples were collected.
3 c.c. pollen (chemically concentrated pollen), t. plant remains (terrestrial plant remains).
4 FD-2 F14C back-calculated from measured 14C age according to method of Reimer et al. (Reference Reimer, Brown and Reimer2004).
5 Calibrated ranges calculated by CALIB and CALIBomb according to method described in text, parenthetical values relative area under probability distribution.
6 Source listed for lithological context (FD-2) or calendar age of event.
Previous studies of sediment cores from Yellowstone Lake provide a postglacial limnogeologic (Tiller Reference Tiller1995) and paleoenvironmental (Theriot et al. Reference Theriot, Fritz, Whitlock and Conley2006) record, making high-resolution age control desirable. In addition, Yellowstone Lake sediments preserve deposits from past hydrothermal explosions (Tiller Reference Tiller1995; Morgan et al. Reference Morgan, Shanks and Pierce2009) and volcanic ashes from eruptions in the Cascade Range to the west (Tiller Reference Tiller1995; Theriot et al. Reference Theriot, Fritz, Whitlock and Conley2006). Sediment cores collected in 1992 (FD-2; Figure 1) were dated with radiocarbon ages from terrestrial plant remains (Tiller Reference Tiller1995; 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 Reference Whitlock1993; Lu et al. Reference Lu, Fritz, Stone, Krause, Whitlock, Brown and Benes2017).
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.
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. Reference Kelts, Briegel, Ghilardi and Hsu1986). 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 maceration procedures (Bennett and Willis Reference Bennett, Willis, Smol, Birks and Last2001). 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.
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 Reference Tiller1995; Theriot et al. Reference Theriot, Fritz, Whitlock and Conley2006). These tephra (2C, 5A), along with a new analysis of the younger Tiller (Reference Tiller1995) 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 (Reference Mensing and Southon1999). 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 (Reference Mensing and Southon1999).
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. Reference Evans, Bergfeld, McGeehin, King and Heasler2010). 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 Reference Doher1980). 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 Reference de Vries and Barendsen1954) and bulk sediments were treated with 2.7N HCl (McNichol et al. Reference McNichol, Osborne, Gagnon, Fry and Jones1994). 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. (Reference Roberts, Elder, Jenkins, Gagnon, Xu, Hlavenka and Longworth2019). 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 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. Reference Stuiver, Reimer and Reimer2019) utilizing the IntCal13 calibration curve (Reimer et al. Reference Reimer, Bard, Bayliss, Beck, Blackwell, Ramsey, Buck, Cheng, Edwards, Friedrich, Grootes, Guilderson, Haflidason, Hajdas, Hatté, Heaton, Hoffmann, Hogg, Hughen, Kaiser, Kromer, Manning, Niu, Reimer, Richards, Scott, Southon, Staff, Turney and van der Plicht2013). 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 18.104.22.168, Blaauw and Christen Reference Blaauw and Christen2011) 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 Reference Tiller1995; 17–10 yr cm–1, Johnson et al. Reference Johnson, Stephenson, Morgan, Shanks and Pierce2003). 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.
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 Reference Romme and Despain1989; Higuera et al. Reference Higuera, Whitlock and Gage2010), and the geochemically fingerprinted Mazama ash (Table 2). All three lake-core tephra are geochemically identical to the Mazama ash (Jensen and Beaudoin Reference Jensen and Beaudoin2016) 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. Reference Zdanowicz, Zielinski and Germani1999) and by thorough radiocarbon dating efforts (7682–7584 cal BP, 2-σ range; Egan et al. Reference Egan, Staff and Blackford2015). 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.
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.
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).
Paired bulk-sediment and picked-pollen sample ages were calibrated utilizing Intcal13 (Reimer et al. Reference Reimer, Bard, Bayliss, Beck, Blackwell, Ramsey, Buck, Cheng, Edwards, Friedrich, Grootes, Guilderson, Haflidason, Hajdas, Hatté, Heaton, Hoffmann, Hogg, Hughen, Kaiser, Kromer, Manning, Niu, Reimer, Richards, Scott, Southon, Staff, Turney and van der Plicht2013) and plotted with CALIB (version 7.1, Stuiver et al. Reference Stuiver, Reimer and Reimer2019, 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 above Mazama 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 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 Reference Reimer and Reimer2004, Figure 4) with the NHZ1 post-bomb calibration (Hua et al. Reference Hua, Barbetti and Rakowski2013) and IntCal13 pre-bomb calibration (Reimer et al. Reference Reimer, Bard, Bayliss, Beck, Blackwell, Ramsey, Buck, Cheng, Edwards, Friedrich, Grootes, Guilderson, Haflidason, Hajdas, Hatté, Heaton, Hoffmann, Hogg, Hughen, Kaiser, Kromer, Manning, Niu, Reimer, Richards, Scott, Southon, Staff, Turney and van der Plicht2013).
δ 13C 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 Reference Smith and Epstein1971). 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).
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. Reference Egan, Staff and Blackford2015) 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 Reference Whitlock1993; Lu et al. Reference Lu, Fritz, Stone, Krause, Whitlock, Brown and Benes2017). Ages from living Pinus contorta pollen are virtually modern by comparison (in equilibrium with atmospheric CO2).
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 Reference Krause and Whitlock2013; Krause et al. Reference Krause, Lu, Whitlock, Fritz and Pierce2015; Nanavati et al. Reference Nanavati, Whitlock, Iglesias and de Porras2019), 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 Reference Regnéll1992; Richardson and Hall Reference Richardson and Hall1994), 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 Reference Friedman1970), but does not crop out near or upstream of Yellowstone Lake nor does any other significant carbonate formation (U.S. Geological Survey Reference Geological Survey1972). Moreover, lake waters are relatively depleted in dissolved carbonate (<0.8 mM ΣCO2, Balistrieri et al. Reference Balistrieri, Shanks, Cuhel, Aguilar and Val Klump2007). Lake sediments themselves are not reported to contain carbonate mud (e.g. Tiller Reference Tiller1995), contain < 1 wt% CaO (Shanks et al. Reference Shanks, Alt and Morgan2007), 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 Reference Love and Good1970; Clifton et al. Reference Clifton, Walters and Simoneit1990) and probably occur in hydrothermally altered sediments of Yellowstone Lake (Shanks et al. Reference Shanks, Alt and Morgan2007). 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.
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 Reference Mensing and Southon1999; Zimmerman et al. Reference Zimmerman, Brown, Hassel and Heck2019) or redeposition occurs from glacial melt (Neulieb et al. Reference Neulieb, Levac, Southon, Lewis, Pendea and Chmura2013; Zhang et al. Reference Zhang, Xu, Turner, Zhou, Gao, Lü and Nesje2017). 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 Reference Licciardi and Pierce2018). Remaining exposures of ancient lake sediments occur upstream (Baker Reference Baker1986) and downstream (Love et al. Reference Love, Good and Browne2007) but are patchy and very small. Ancient shoreline facies also occur north of Yellowstone Lake (Pierce et al. Reference Pierce, Cannon, Meyer, Trebesch and Watts2007), 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 Reference Tweddle and Edwards2010; Howarth et al. Reference Howarth, Fitzsimons, Jacobsen, Vandergoes and Norris2013), 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 Reference Baker1976; Whitlock Reference Whitlock1993; Whitlock et al. Reference Whitlock, Bartlein and Van Norman1995; Theriot et al. Reference Theriot, Fritz, Whitlock and Conley2006), 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. Reference Balistrieri, Shanks, Cuhel, Aguilar and Val Klump2007); in addition, a small, subaerial thermal area exists upslope of Cub Creek Pond (Vaughan et al. Reference Vaughan, Heasler, Jaworowski, Lowenstern and Keszthelyi2014). 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. Reference Vaughan, Heasler, Jaworowski, Lowenstern and Keszthelyi2014). δ13C values of pollen from Yellowstone Lake and nearby Cub Creek Pond (Lu et al. Reference Lu, Fritz, Stone, Krause, Whitlock, Brown and Benes2017) 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 Reference Craig1963; Werner and Brantley Reference Werner and Brantley2003). 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 Reference Werner and Brantley2003). 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., Reference Roberts, Reed, Leng, Kuzucuoğlu, Fontugne, Bertaux, Woldring, Bottema, Black and Hunt2001; Jones Reference Jones2004). 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. Reference Pasquier-Cardin, Allard, Ferreira, Hatte, Coutinho, Fontugne and Jaudon1999; Cook et al. Reference Cook, Hainsworth, Sorey, Evans and Southon2001; Evans et al. Reference Evans, Bergfeld, McGeehin, King and Heasler2010; Lewicki and Hilley Reference Lewicki and Hilley2014) and industrial areas (Baydoun et al., Reference Baydoun, El Samad, Nsouli and Younes2015), but generally do not occur more than 1 km away from the old-carbon source (Pasquier-Cardin et al. Reference Pasquier-Cardin, Allard, Ferreira, Hatte, Coutinho, Fontugne and Jaudon1999; Evans et al. Reference Evans, Bergfeld, McGeehin, King and Heasler2010; Baydoun et al. Reference Baydoun, El Samad, Nsouli and Younes2015). This observation is consistent with dates from modern pollen near Yellowstone Lake (Figure 4), which suggest minimal uptake of volcanic CO2 through photosynthesis.
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 Reference Brooks and Shaw1978). 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.
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.
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.