INTRODUCTION
The late Pleistocene was characterized by dramatic changes in climate. This period covers both glacial (Marine Isotope Stages [MIS] 4 and 2) and interglacial periods (MIS 5 and 3). The late-glacial period (~16–12 ka; ka = thousands of years before present [BP]; 0 years BP = AD 1950) marks the transition from the last glacial episode (115–11.7 ka) to the interglacial Holocene (past 11.7 ka) (Walker et al., Reference Walker, Johnsen, Rasmussen, Popp, Steffensen, Gibbard and Hoek2009). With the onset of the warming Holocene epoch, organisms had to adapt, migrate, or go extinct. The late-glacial transitional period was characterized by cold stadial and warm interstadial phases, as seen in Greenland ice core records (Seierstad et al., Reference Seierstad, Abbott, Bigler, Blunier, Bourne, Brook and Buchardt2014). Two well-studied stadial episodes during the late glacial are the so-called Oldest Dryas (16–15 ka) and Younger Dryas (12.9–11.7 ka) (Broecker et al., Reference Broecker, Denton, Edwards, Cheng, Alley and Putnam2010).
The Younger Dryas (12.9–11.7 ka) was a reversal of a warming period (Bølling–Allerød) (14.7–12.9 ka) and a temporary return to glacial like conditions (Broecker et al., Reference Broecker, Denton, Edwards, Cheng, Alley and Putnam2010). Climate during the Younger Dryas was complex and varied spatially (Kaufman et al., Reference Kaufman, Scott Anderson, Hu, Berg and Werner2010). Evidence for the Younger Dryas is strongest in the North Atlantic region but is observable in many areas across the world. Emerging evidence from sediment cores in the North Atlantic, western Europe, east Asia, and western North America suggest the Younger Dryas can be grouped into two distinct stages (Brauer et al., Reference Brauer, Endres, Günter, Litt, Stebich and Negendank1999; Bakke et al., Reference Bakke, Heegaard, Haug, Birks, Nilsen, Lie, Dulski and Dokken2009; Kaufman et al., Reference Kaufman, Scott Anderson, Hu, Berg and Werner2010; Schlolaut et al., Reference Schlolaut, Brauer, Nakagawa, Lamb, Tyler, Staff, Marshall, Bronk Ramsey, Bryant and Tarasov2017; Pigati and Springer, Reference Pigati and Springer2022). The first stage, the early Younger Dryas (~12.9–12.3 ka), is characterized by stable conditions due to a weakened Atlantic Meridional Overturning Circulation (AMOC), and the second stage, the late Younger Dryas (12.3–11.7 ka), is defined by variable conditions as the AMOC began to strengthen (Pigati and Springer, Reference Pigati and Springer2022). The AMOC is an important component of the Earth's climate system that transports warm water from the tropics to the North Atlantic Ocean, where it cools and sinks before returning southward. The AMOC helps regulate temperatures and influence weather patterns globally. As noted, at the start of the Younger Dryas, the AMOC weakened, leading to a decrease in the amount of warm water transported to the North Atlantic Ocean. This reduction in the poleward transport of warm ocean water likely played a role in triggering a cold period lasting from 12.9 to 11.7 ka and had significant impacts on global climate and ecosystems (Alley et al., Reference Alley, Meese, Shuman, Gow, Taylor, Grootes and White1993). Understanding the behavior of climatic responses to the AMOC is critical for forecasting future climate changes, as recent observations suggest the AMOC has been gradually weakening over the past several decades (Boers, Reference Boers2021).
While the effects of climate change are often manifested globally, polar regions have exhibited greater vulnerability to current rapid global warming (Stewart et al., Reference Stewart, Kunkel, Stevens, Sun and Walsh2013). Changes in Arctic climate have global repercussions that are forecast to intensify in the future. Alaska specifically is warming three to four times as fast as the rest of the United States (Taylor et al., Reference Taylor, Maslowski, Perlwitz and Wuebbles2017; Walsh and Brettschneider, Reference Walsh and Brettschneider2019). Since the 1950s, Alaska's average annual air temperatures have increased by 1.5°C, and average winter temperatures have increased by 4°C (Walsh and Brettschneider, Reference Walsh and Brettschneider2019). Paleoclimate records from climate-sensitive regions are thus critical for developing a better understanding of the biological impacts of climate change at both organismal and ecosystem scales.
Despite the significance of these regions in past and present climate change, oxygen and carbon isotopic paleoclimate records in interior Alaska are rare. A few studies have looked at δ18O records of diatom silica from Alaska lake sediments (Hu and Shemesh, Reference Hu and Shemesh2003; Kaufman et al., Reference Kaufman, Axford, Anderson, Lamoureux, Schindler, Walker and Werner2012). Nevertheless, using other terrestrial climate proxies is essential to deepen our understanding of climate transitions in the past, which also provides clues for future climate change. Here, we investigate the utility of land snails as a new paleoclimate proxy for interior Alaska across a range of sedimentary and archaeological settings, to develop a better understanding of late-glacial climatic conditions in interior Alaska.
Land snails are useful indicators of the environment due to their widespread occurrence and abundance and the persistence of their shells after they die. In North America alone, there are around 1200 species of land snails (Nekola, Reference Nekola2014). While the greatest variety of species is found at tropical latitudes, land snails can be found in all terrestrial environments except extremely dry deserts (e.g., Yanes et al., Reference Yanes, Al-Qattan, Rech, Pigati, Dodd and Nekola2019). Their shells are made of aragonite (CaCO3) (Balakrishnan and Yapp, Reference Balakrishnan and Yapp2004) and can commonly be found in Quaternary sediment archives worldwide (Goodfriend, Reference Goodfriend1992).
Land snail shell δ13C has been studied in both natural (Goodfriend and Ellis, Reference Goodfriend and Ellis2000; Yanes et al., Reference Yanes, Delgado, Castillo, Alonso, Ibáñez, De la Nuez and Kowalewski2008) and laboratory settings (Stott, Reference Stott2002; Metref et al., Reference Metref, Rousseau, Bentaleb, Labonne and Vianey-Liaud2003; Zhang et al., Reference Zhang, Yamada and Yoshida2018). A model developed by Goodfriend and Hood (Reference Goodfriend and Hood1983) proposed that land snail carbon is derived from three sources: (1) snail diet, (2) atmospheric CO2, and (3) ingested carbonates (including limestone and recycled snail shell). However, recent studies have found that ingested carbonates do not seem to affect the shell δ13C of small (<5 mm) Succineidae shells (Pigati et al., Reference Pigati, Quade, Shahanan and Haynes2004, Reference Pigati, McGeehin, Muhs and Bettis2013). Hence, stable carbon isotope values of land snail shells are most often used to infer the carbon isotope compositions of local vegetation and, where present, calculate the relative proportions of C3, C4, and CAM plants in a given setting (Goodfriend and Stipp, Reference Goodfriend and Stipp1983; Goodfriend, Reference Goodfriend1990; Yanes et al., Reference Yanes, Delgado, Castillo, Alonso, Ibáñez, De la Nuez and Kowalewski2008, Reference Yanes, Romanek, Delgado, Brant, Noakes, Alonso and Ibáñez2009).
While snail shell δ18O compositions are most commonly used to reconstruct δ18O values from precipitation experienced during a snail's active period (Goodfriend, Reference Goodfriend1991; Zanchetta et al., Reference Zanchetta, Leone, Fallick and Bonadonna2005; Yanes et al., Reference Yanes, Yapp, Ibáñez, Alonso, De-la-Nuez, Quesada, Castillo and Delgado2011), Balakrishnan and Yapp (Reference Balakrishnan and Yapp2004) showed that the oxygen isotope composition of land snail shells is also controlled by relative humidity (RH) and temperature, as well as by the δ18O composition of local precipitation and water vapor. Because land snails are mainly active during or after rain events (Ward and Slotow, Reference Ward and Slotow1992), most of the water absorbed by a snail and in a snail's body fluid is derived from rainwater (Riddle, Reference Riddle and Russell-Hunter1983). The flux balance mixing model by Balakrishnan and Yapp (Reference Balakrishnan and Yapp2004) demonstrates that shells are precipitated in oxygen isotopic equilibrium with the snail body fluid. While snails are forming their shells, the body fluid and shell experience 18O enrichment due to evaporation, by about 2‰ on average, compared with that of the source precipitation (Lécolle, Reference Lécolle1985; Goodfriend, Reference Goodfriend1992; Balakrishnan and Yapp, Reference Balakrishnan and Yapp2004).
Here, we report the first oxygen and carbon isotope values of fossil land snail shells from interior Alaska, retrieved from an array of Quaternary depositional settings. The shells are constrained chronologically by stratigraphy and/or radiocarbon dating. The stable isotope results are interpreted in the context of a published flux balance mixing model and compared with other published climate proxy records to elucidate the complex late-glacial climate history of interior Alaska.
BACKGROUND
Modern climate and environment in interior Alaska
Crucial to interpretations of stable isotopic compositions of fossil land snails is an understanding of how modern snails are conditioned to today's climate. Nield et al. (Reference Nield, Yanes, Pigati, Rech, von Proschwitz and Nekola2022) found that the δ18O values of modern Succinea strigata and Euconulus fulvus shells are correlated with temperature and precipitation δ18O between April and October; they referred to this as the “extended growing season.” Interior Alaska can be described as having a continental climate that is largely driven by the distance to oceanic moisture and the precipitation shadows formed by both the Brooks Range to the north and the Alaska Range to the south (Pewe, Reference Pewe1975). This climatic setting supports a boreal forest biome in most of interior Alaska. Typical trees in the Alaskan boreal forest include white spruce (Picea glauca), black spruce (Picea mariana), larch (Larix), balsam poplar (Populus balsamifera), aspen (Populus tremuloides), Alaska birch (Betula neoalaskana), willow (Salix polaris), and alder (Alnus) (Viereck and Little, Reference Viereck and Little1994). Interior Alaska receives, on average, 305 mm of rain per year and 1550 mm of snow per year (statesummaries.sncics.org, accessed January 2023). Precipitation in interior Alaska is derived mainly from North Pacific Ocean moisture (Abbott et al., Reference Abbott, Finney, Edwards and Kelts2000). Based on modeled outputs (IAEA/WMO, 2015; Bowen et al., Reference Bowen, Wassenaar and Hobson2005), the average δ18O value for precipitation during the snail extended growing season is −17.1‰ (Bowen, Reference Bowen2020). Average extended summer RH is 60% (www.ncdc.noaa.gov, accessed August 2020). Average high and low temperatures during the extended growing season are 14°C and 2°C, respectively (www.usclimatedata.org, accessed August 2020).
Late-glacial climate and environment in interior Alaska
Based on interpretations of lake and peat pollen records throughout Alaska, climate conditions during the Younger Dryas were spatially complex (Kokorowski et al., Reference Kokorowski, Anderson, Mock and Lozhkin2008). Nevertheless, despite high spatial variability, these proxy records consistently indicate that Younger Dryas temperature changes in interior Alaska were muted compared with the North Atlantic region (Graf and Bigelow, Reference Graf and Bigelow2011). Most records from interior Alaska do not record a sharp temperature reversal at the onset of the Younger Dryas, unlike those that can be seen in Greenland ice core records (Alley et al., Reference Alley, Meese, Shuman, Gow, Taylor, Grootes and White1993). Pollen records from Windmill Lake (interior Alaska) do not show evidence for RH changes during the Younger Dryas (Bigelow and Edwards, Reference Bigelow and Edwards2001). In contrast, these records indicate cool and dry conditions during the Bølling–Allerød that were replaced by high lake levels at the onset of the Holocene (Bigelow and Edwards, Reference Bigelow and Edwards2001). Similarly, using branched glycerol dialkyl glycerol tetraethers (brGDGT) from loess–paleosol sequences as a temperature proxy, Kielhofer and colleagues (2023) found no dramatic temperature change at the onset or during the Younger Dryas.
Shifts in microfloral fossils indicate mild changes in climate during this time as well. During the late glacial (~16–11.7 ka), interior Alaska was characterized by herb-rich tundra vegetation (Bigelow and Powers, Reference Bigelow and Powers2001; Anderson, Reference Anderson2003). In the Tanana River valley, vegetation shifted from herb tundra to shrub tundra dominated by birch around ~14 ka. This switch from herb to birch tundra points to a warming air temperature and increased precipitation at ~14 ka (Anderson, Reference Anderson2003). Land snails were collected from deposits dated to these time periods in order to test these hypotheses about paleoclimate shifts in interior Alaska.
MATERIALS AND METHODS
Modern land snail assemblages
Modern snails were collected live from seven locations in the boreal forest in interior Alaska for two summer seasons, 2015 and 2022 (Fig. 1). Sampling took place for 1 hour at each field location where all living and recently dead snails found were collected within a 20 m radius. Field locations were chosen based primarily on their accessibility; however, all localities are representative of a modern, undisturbed boreal forest. Sampled modern sites in interior Alaska contained up to five land snail species, including S. strigata, E. fulvus, Columella simplex, Discus whitneyi, and Punctum sp. However, in this study, isotopic analyses were conducted on the most abundant and widespread genus, Succinea, to taxonomically standardize our comparisons through time, thus removing isotopic variability associated with different species’ vital rates, body sizes, or ecologies (Yanes et al., Reference Yanes, Nekola, Rech and Pigati2017). The family Succineidae exhibits a broad spatial distribution, with shells present on almost every continent. Succineids are associated with highly humid swamp and flooded areas and typically live for 1–2 yr (Oerstan, Reference Oerstan2010). However, recent field analysis has observed succineids in drier areas as well, and they are frequently recovered from Quaternary loess deposits of North America (Pigati et al., Reference Pigati, Rech and Nekola2010, Reference Pigati, McGeehin, Muhs and Bettis2013) and Europe (Moine et al., Reference Moine, Rousseau and Antoine2005).
Figure 1. (A) Quaternary depositional settings (circles) where fossil shells and modern snails were sampled (this study). Studied sites include permafrost soils (Cold Regions Research and Engineering Laboratory [CRREL] tunnel), loess deposits (Dalton highway), and archaeological sites (Mead, Bachner, North Gerstle, and Upward Sun River [USRS]). Published paleoclimate proxy sites (white stars), including marine cores, lake cores, and paleosols (see text). (B) Map of Greenland showing locations of two previously published paleoclimate proxies from ice cores; North Greenland Ice Core Project (NGRIP; north location) and North Greenland Eemian Ice Drilling (NEEM; south location). Locations of paleoclimate proxies in the published literature: (1) Grebmeier et al., Reference Grebmeier, Cooper and DeNiro1990; (2) Hu et al., Reference Hu, Lee, Kaufman, Yoneji, Nelson and Henne2002; (3) Hu et al., Reference Hu, Nelson, Clarke, Rühland, Huang, Kaufman and Smol2006; (4) Kaufman et al., Reference Kaufman, Axford, Anderson, Lamoureux, Schindler, Walker and Werner2012; (5) Kaufman et al., Reference Kaufman, Schneider, Mckay, Ammann, Bradley, Briffa, Miller, Otto-Bliesner, Overpeck and Vinther2009; (6) Jones et al., Reference Jones, Peteet, Kurdyla and Guilderson2009; (7) Kurek et al., Reference Kurek, Cwynar, Ager, Abbott and Edwards2009; (8) Abbott et al., Reference Abbott, Finney, Edwards and Kelts2000; (9) Kielhofer et al., Reference Kielhofer, Tierney, Reuther, Potter, Holmes, Lanoë, Esdale, Wooller and Bigelow2023; (10) NGRIP Members, 2004; (11) Masson-Delmotte et al., Reference Masson-Delmotte, Steen-Larsen, Ortega, Swingedouw, Popp, Vinther and Oerter2015.
Late-glacial snail assemblages
Six sites dating to the late-glacial period containing pristine fossil snail shells were investigated in this study (Figs. 1 and 2). In sharp contrast to modern assemblages, most late glacial–age sites contained only one genus, Succinea (Fig. 3). However, the permafrost tunnel site, described below, contained three genera, including Succinea, Columella, and Euconulus. A total of 121 fossil shells were selected for isotope analysis (Table 1). Snail shells collected from all paleontological sites were handpicked and sieved from bulk sediment.
Figure 2. Photographs where late-glacial snail shells were collected. (A) Mead archaeological site. (B) Cold Regions Research and Engineering Laboratory (CRREL) permafrost research tunnel. (C) Upward Sun River archaeological site. (D) Yukon-Tanana Upland loess deposits at Dalton highway (adapted from Muhs et al., Reference Muhs, Pigati, Budahn, Skipp, Bettis and Jensen2018).
Figure 3. Photographs of modern and fossil Succinea specimens. Pictures depict (A and B) late-glacial (~14,000 cal yr BP) specimens retrieved from the Cold Regions Research and Engineering Laboratory (CRREL) permafrost tunnel and (C and D) living Succinea specimens found in the forest near Fairbanks, Alaska. Scales are in millimeters.
Table 1. Carbon and oxygen isotope compositions of modern and fossil Succinea shells from central Alaska with median calibrated ages (BP).a
a Modern snails were live collected in 2015/2022.
The Cold Regions Research and Engineering Laboratory (CRREL) permafrost tunnel is a human-made tunnel that penetrates sediments of the Goldstream Creek valley near Fairbanks, Alaska (Figs. 1 and 2). The tunnel exposes fossiliferous reworked loess and alluvial sediments. Much has been published on the permafrost stratigraphy, ice formations, and preserved fossils at this site (e.g., Hamilton et al., Reference Hamilton, Craig and Sellmann1988; Shur et al., Reference Shur, French, Bray and Anderson2004; Kanevskiy et al., Reference Kanevskiy, Shur, Bigelow, Bjella, Douglas, Fortier, Jones and Jorgenson2022). However, analyses have yet to be done on the snails in these deposits. Ages for loess within the tunnel range from ~34 to ~9.5 ka (Hamilton et al., Reference Hamilton, Craig and Sellmann1988). Snails from the tunnel were collected via handpicking or sieving of bulk sediment. Snails were found in two areas within the tunnel, referred to as facies A and facies B. Facies B is an organic-rich area on the ceiling of the tunnel located at the top of the winze (for more details, see Hamilton et al., Reference Hamilton, Craig and Sellmann1988). Facies A is poorly sorted loess to large cobble–sized sediment located on the floor of the tunnel 12–13 m from the tunnel entrance. The tunnel was constrained chronologically for each facies by several radiocarbon ages from shells and plant remains (Table 2). The isotopic values of 13 Succinea shells were determined from these deposits, dating to ~14 ka.
Table 2. Sample identifications, radiocarbon ages, and calibrated ages for Succinea shells from the Cold Regions Research and Engineering Laboratory (CRREL) permafrost tunnel, the Bachner, and North Gerstle Point sites, and organic matter in the CRREL tunnel.
Snails were also collected from last glacial loess road outcrops along the Dalton Highway. Snails were found in two locations sampled by Muhs et al. (Reference Muhs, Pigati, Budahn, Skipp, Bettis and Jensen2018), at milepost 45.9 (AK-650A) and milepost 54.7 (AK-855). While Quaternary loess is common throughout Alaska, loess dating to the last glacial period in Alaska is rare (Muhs et al., Reference Muhs, Pigati, Budahn, Skipp, Bettis and Jensen2018). The Dalton Highway sites present unusual occurrences of last glacial loess deposits dating from 19 to 12 ka in the Yukon-Tanana Upland (Figs. 1 and 2). At these sites, Succinea shells were found in the upper parts of loess sections. Calibrated radiocarbon ages of snails span an interval between ~13.4 and ~12.7 ka (Muhs et al., Reference Muhs, Pigati, Budahn, Skipp, Bettis and Jensen2018). At these sites, 36 Succinea shells were selected for isotopic analysis.
To complement sampling at paleontological sites, we also retrieved land snail shells from four Alaskan archaeological sites: Mead, Upward Sun River (USRS), North Gerstle, and Bachner (Figs. 1 and 2). Each site has loess deposits that are greater than 1 m in thickness. The Mead archaeological site is located along Shaw Creek Bluff near Delta Junction (Dilley, Reference Dilley1998) and is interpreted as a human base camp (Potter et al., Reference Potter, Holmes, Yesner, Graf, Ketron and Waters2013). Succinea shells were found in a trench away from the main archaeological excavation in stratigraphic zones dated to ~12.6 to 11.8 ka (Kielhofer et al., Reference Kielhofer, Miller, Reuther, Holmes, Potter, Lanoë, Esdale and Crass2020). Here, 10 Succinea shells were analyzed for isotopes. The Upward Sun River site is located on a loess-covered dune near the Tanana River valley (Reuther, Reference Reuther2013). Cultural remains come from repeated occupations of the dune dating back to 13.3 ka and include a late-glacial residential structure and the cremated and buried remains of a child (Potter et al., Reference Potter, Irish, Reuther, Gelvin-Reymiller and Holliday2011, Reference Potter, Irish, Reuther and McKinney2014). 20 Succinea shells were analyzed for isotopes from horizons dated to ~13.1–12.6 ka. The North Gerstle site is also in the Tanana River valley on a bluff overlooking the Tanana River. Succinea shells were collected from horizons dated to 12.9–12.8 ka (Vanderhoek et al., Reference Vanderhoek, Dilley and Holmes1997). Here, 18 Succinea shells were selected for analyses. The Bachner site overlooks Quartz Lake and is adjacent to Shaw Creek Flats, also within the Tanana River valley, within 10 km of the Mead and North Gerstle sites (Reuther, Reference Reuther2013). Succinea shells were found at layers dated to ~12.8–11.9 ka, and 25 Succinea shells were analyzed for isotopes.
Radiocarbon dating
For this study, fossil Succinea shells were either radiocarbon dated directly or collected from a horizon where the chronology is well constrained by radiocarbon dating of other shells, plant macrofossils, wood, or charcoal. Three snail samples collected from the CRREL permafrost tunnel were selected for paired graphite-target and carbonate-target radiocarbon dating analysis (Bright et al., Reference Bright, Ebert, Kosnik, Southon, Whitacre, Albano and Flores2021; Fig. 4). Historically, land snail shells have been avoided for radiocarbon dating because of the potential incorporation of 14C from limestone during shell formation (Pigati et al., Reference Pigati, Quade, Shahanan and Haynes2004). However, several genera of small gastropods, including Succineidae, have been shown to provide reliable radiocarbon ages (Pigati et al., Reference Pigati, Quade, Shahanan and Haynes2004). To further verify the age of snail shells from the permafrost tunnel, two plant remains samples taken from the same horizon as the snail assemblage were radiocarbon dated and yielded results similar to those of the shells (Table 2).
Figure 4. (A) Scatter plot comparing carbonate-target vs. graphite-target radiocarbon-dated Succinea shells collected from the Cold Regions Research and Engineering Laboratory (CRREL) permafrost tunnel with error bars for age uncertainty. (B) Data from this study (filled red dots) plotted on figure from Bright et al. (Reference Bright, Ebert, Kosnik, Southon, Whitacre, Albano and Flores2021). Reduced major axis regression of paired direct carbonate and graphite pMC (percent modern carbon) determinations.
Late-glacial Succinea shells from loess deposits and archaeological sites have been radiocarbon dated in prior work (Potter et al., Reference Potter, Irish, Reuther, Gelvin-Reymiller and Holliday2011, 2013; Reuther, Reference Reuther2013; Muhs et al., Reference Muhs, Pigati, Budahn, Skipp, Bettis and Jensen2018; Kielhofer et al., Reference Kielhofer, Miller, Reuther, Holmes, Potter, Lanoë, Esdale and Crass2020). Sections at the Bachner and North Gerstle site were previously dated using charcoal and collagen from animal bones (Reuther, Reference Reuther2013). For the present study, shells from these deposits were radiocarbon dated (Table 2) at the Center for Applied Isotope Studies at the University of Georgia, following procedures outlined in their laboratory protocols (https://cais.uga.edu/service/radiocarbon-dating-by-ams). Shell and plant remains from the CRREL permafrost tunnel were radiocarbon dated at the W.M. Keck Carbon Cycle Accelerator Mass Spectrometry Laboratory at the University of California–Irvine following the procedures outlined in their laboratory protocols (https://www.ess.uci.edu/group/ams/protocols). Shells were dated using the carbonate-target and graphite-target radiocarbon methods, and detailed methodology for both dating methods can be found in Bright et al. (Reference Bright, Ebert, Kosnik, Southon, Whitacre, Albano and Flores2021). The resultant radiocarbon ages were all calibrated to calendar years using the IntCal20 data set and CALIB 8.20 (Reimer et al., Reference Reimer, Austin, Bard, Bayliss, Blackwell, Bronk Ramsey and Butzin2020).
Stable isotope analyses
In total, 174 individual shells were selected for isotope analysis by choosing the cleanest and best-preserved shells. Shells were mechanically cleaned with deionized water to remove any remaining debris and air-dried before laboratory analysis. Land snail shells were analyzed at the University of Florida using a Finnigan-MAT 252 Isotope Ratio Mass Spectrometer coupled with a Kiel III automated carbonate preparation device. Additional details of the methodology can be found in Nield et al. (Reference Nield, Yanes, Pigati, Rech, von Proschwitz and Nekola2022). Analytical precision was better than ±0.1‰, as shown by repeated measurements of samples and standards. Results were calibrated against the international standards NBS-18 and NBS-19.
Statistical analyses
All statistical analyses were done using R v. 2.5.1 software (R Core Team, 2023). All data were treated as nonnormally distributed. The Kruskal-Wallis test was used to determine whether the multiple groups exhibited statistically equivalent median values. The Mann-Whitney test was used when comparing the median value between two groups only. Statistical significance was accepted when the P value was less than 0.05. Pearson correlation analysis was conducted to assess the significance of monotonic relationships between two variables. The coefficient of determination (R 2) was used to identify the strength of the model and relationship.
RESULTS
Radiocarbon results
Results from the paired graphite-target and carbonate-target radiocarbon dating analysis (Bright et al., Reference Bright, Ebert, Kosnik, Southon, Whitacre, Albano and Flores2021) of shells from the CRREL permafrost tunnel show the two dating methods are not statistically different (R 2 = 0.98, P = 0.7; Fig. 4, Table 2). Both methods yield equivalent ages, although the graphite-target method (±35 yr) is more precise than the carbonate-target method (±140 yr). The carbonate-target method is less expensive and uses a far smaller sample, <1 mg compared with 20+ mg, which makes it an attractive option for land snail shell radiocarbon dating. All analyzed snails from the permafrost tunnel date to ~14 ka, in agreement with plant remains samples that were taken from the same horizon (Table 2). Ages of snail shells from the tunnel range from 14.0 to 13.9 ka, and plant remains ages range from 13.9 to 13.6 ka. Other snails sampled from the same assemblage of the CRREL tunnel are provisionally assumed to be the same age. Additional traditional graphite carbon radiocarbon analysis was performed on shells from the Bachner and North Gerstle sites (Table 2). Snails from Bachner were dated to 12.6 and 11.9 ka, and snails from North Gerstle were dated to 12.9 and 12.8 ka.
Carbon isotopic composition
Carbon isotope analyses were performed on 121 fossil and 53 modern Succinea shells, and the results are shown as box plots in 250 yr time bins (Fig. 5). Late-glacial shell δ13C values average −7.7‰ and range between −9.9‰ and −5.9‰ across the time intervals (Fig. 5, Table 1). Median δ13C values of shells from the early Bølling–Allerød (~14 ka), early Younger Dryas (12.9–12.3 ka), and late Younger Dryas time (12.3–11.7 ka) are −7.5‰, −7.8‰, and −8.2‰, respectively (χ2 = 9.9792, df = 4, P = 0.0408). Median δ13C values in late-glacial time bins vary by only 0.7‰. Modern shell δ13C values have a median value of −10.9‰, ranging from −12.7‰ to −7.9‰ (Fig. 5). All late-glacial time bins differ significantly from values in modern shells, being on average 2.8‰ higher than modern shells (W = 6048, P < 0.001).
Figure 5. Succinea shell δ13C values (in PBD, Pee Dee Belemnite) binned into 250 yr time intervals. Box extremes represent lower and upper quartiles. Whiskers depict the range of values. The solid line inside the plot depicts the median δ13C value. Number in parentheses is the number of samples. The dots represent outliers that were outside the interquartile range. The three shaded regions mark the Bølling–Allerød (BA) (14.7–12.9 ka), the early Younger Dryas (YD) (12.9–12.3 ka), and the late Younger Dryas (12.3–11.7 ka).
Oxygen isotopic composition
Oxygen isotope analyses were performed on the same 121 fossil and 53 modern Succinea shells as the δ13C analysis. The results are illustrated again as box plots and separated into 250 yr bins (Fig. 6). Late-glacial (14–11.5 ka) shell δ18O values have a median value of −11.9‰ and range from −13.7‰ to −8.3‰ (Fig. 6B, Table 1). Within this time period, early Bølling–Allerød (~14 ka), early Younger Dryas (12.9–12.3 ka), and late Younger Dryas (12.3–11.7 ka) have median shell δ18O values of −10.6‰, −12.1‰, and −11.5‰, respectively, and differ significantly from each other (χ2 = 29.636, df = 4, P < 0.001). Modern shell δ18O has a median value of −10.8‰ and ranges from −12.8‰ to −8.9‰ (Fig. 6). Late-glacial shell values differ significantly from values in modern shells (W = 778.5, P < 0.001).
Figure 6. δ18O values binned into 250 yr time intervals. The three shaded regions mark the Bølling–Allerød (BA) (14.7–12.9 ka), the early Younger Dryas (YD) (12.9–12.3 ka), and the late Younger Dryas (12.3–11.7 ka). (A) Seawater δ18O values (in SMOW) calculated from benthic foraminifera off the coast of Alaska (Grebmeier et al. Reference Grebmeier, Cooper and DeNiro1990; Praetorius et al., Reference Praetorius, Mix, Walczak, Wolhowe, Addison and Prahl2015, Reference Praetorius, Mix, Jensen, Froese, Milne, Wolhowe, Addison and Prahl2016); (B) snail shell δ18O values (in PDB) of Succinea shells from central Alaska; and (C) North Greenland Ice Core Project (NGRIP Members, 2004) and North Greenland Eemian Ice Drilling (NEEM; Masson-Delmotte et al., 2015) Greenland ice core δ18O values in SMOW. Box extremes represent lower and upper quartiles. Whiskers depict the range of values. The solid line inside the plot depicts the median δ18O value. The dots represent outliers that were outside the interquartile range. The number in the parentheses is the number of samples.
DISCUSSION
Paleo-vegetation inferences
Median modern shell δ13C values are 2.8‰ lower than median late-glacial δ13C values (Fig. 5). This offset between modern and late-glacial values is likely best explained by shifts in the carbon isotope composition of one or more of the three main components affecting the carbon isotope values of land snail shells: atmospheric carbon, diet, and ingested carbonate (Goodfriend and Ellis, Reference Goodfriend and Ellis2002). Of these, changes in atmospheric carbon and diet may be most likely to have produced differences in shell δ13C. While land snails may consume limestone as a source of calcium to build their shells, and limestone ingestion may be capable of positively shifting (by several per mil) shell δ13C values, recent studies have found that small-bodied Succineidae gastropods may consume only minimal amounts of carbonate sediments (Pigati et al., Reference Pigati, Quade, Shahanan and Haynes2004, Reference Pigati, McGeehin, Muhs and Bettis2013).
Since the Industrial Revolution, increased burning of fossil fuels has not only increased CO2 concentrations but also affected carbon isotopic composition of atmospheric CO2. Overall, δ13C values of atmospheric CO2 have become more negative following the Industrial Revolution (Friedli et al., Reference Friedli, Loetscher, Oeschger, Siegenthaler and Stauffer1986). This decline in atmospheric CO2 δ13C has caused a decline in plant (Dawson et al., Reference Dawson, Mambelli, Plamboeck, Templer and Tu2002), and therefore snail shell, δ13C values. This phenomenon is known as the Suess effect (Keeling, Reference Keeling1979). However, the Suess effect can only account for a 1–1.5‰ negative shift in δ13C values (Wang et al., Reference Wang, Surge and Walker2011). If we apply the correction to the snail shells analyzed here, there would still be an ~1–2‰ decrease in shell δ13C composition from the late glacial to today.
The remaining offset between modern and late-glacial shell δ13C values is most likely due to differences in the δ13C values of the snail diet. Generally, modern tundra, boreal forest, and coastal forest biomes in Alaska are all dominated by C3 plants (average δ13C value of −26‰), which include trees and cool-season grasses and are depleted in 13C compared with C4 plants (average δ13C value of −12‰) (O'Leary, Reference O'Leary1988; Muhs et al., Reference Muhs, Aleinikoff, Stafford, Kihl, Been, Mahan and Cowherd1999, 2000). C3 and C4 plants fractionate carbon isotopes in different ways. Soil organic matter in most Alaskan biomes shows δ13C values of −25‰ (Muhs et al., Reference Muhs, Ager, Been, Rosenbaum and Reynolds2000). Additionally, none of the δ13C analyses from samples taken from the CRREL permafrost tunnel or from nearby Quartz and Goldbottom Creeks are in the range of δ13C values of C4 grasses (Wooller et al., Reference Wooller, Zazula, Edwards, Froese, Boone, Parker and Bennett2018). Laboratory studies have found aragonite carbon of land snails shells to be positively fractionated with respect to the bicarbonate carbon from which it precipitated (Rubinson and Clayton, Reference Rubinson and Clayton1969). This can in part explain why shell δ13C values are more positive than consumed plant δ13C.
C4 plants are supported by lower atmospheric CO2 concentrations (Wang et al., Reference Wang, Gao, Yong, Wang, Ma and He2020). In theory, this could suggest higher concentrations of C4 plants in Alaska during the last glacial period, when atmospheric CO2 concentrations were much lower than during interglacial times (Sigman and Boyle, Reference Sigman and Boyle2000). However, it is likely that regional temperatures override this factor, with lower last glacial temperatures being more important than CO2 in determining the relative abundance of C3 and C4 plants, as seen in Muhs et al. (Reference Muhs, Aleinikoff, Stafford, Kihl, Been, Mahan and Cowherd1999).
C4 plant occurrences are mainly found in mid-latitudes and tropical grasslands. Hence, a negative shift in shell δ13C values is most likely explained by lower δ13C values in the vegetation composition of C3 plants today, compared with the late glacial. This may be caused by changes in the water-use efficiency in C3 plants in slightly drier conditions during the late glacial compared with today (Policy et al., Reference Policy, Johnson, Marino and Mayeux1993). Water-use efficiency of plants refers to the amount of CO2 uptake through photosynthesis versus water loss to transpiration. Carbon isotope discrimination occurs during photosynthesis, when plants preferentially take up the lighter 12C isotope (Hubrick et al., 1986). When water is more limited, a plant's ability to discriminate between carbon isotopes is reduced, leaving plants with higher 13C concentrations and more positive δ13C values at drier conditions (Hubrick et al., 1986).
Paleo-temperature and precipitation inferences
Shell δ18O values were compared with the oxygen isotope values of two well-studied paleoclimate proxies: (1) Recent and Pleistocene seawater δ18O and (2) Greenland ice core δ18O values (Fig. 6). For seawater, we can evaluate time series of δ18O values of benthic foraminifera collected from the southwest coast of Alaska (Grebmeier et al., Reference Grebmeier, Cooper and DeNiro1990; Fig. 1). Late-glacial seawater δ18O averaged 1‰, with low variability (<1.5‰). Modern seawater δ18O off the coast of Alaska averages −1.1‰, which is ~2‰ lower than late-glacial seawater δ18O (Fig. 6A). Late-glacial δ18O values were reported from the North Greenland Ice Core Project ice core (NGRIP Members, 2004) and modern δ18O values were reported from the North Greenland Eemian Ice Drilling (NEEM) ice core (Masson-Delmotte et al., Reference Masson-Delmotte, Steen-Larsen, Ortega, Swingedouw, Popp, Vinther and Oerter2015). Greenland ice core δ18O compositions vary through the late glacial, averaging −39‰ during the early Bølling–Allerød (~14 ka) and shifting to −40.4‰ during both the early Younger Dryas (12.9–12.3 ka) and the late Younger Dryas (12.3–11.7 ka). Modern Greenland ice core δ18O values are the highest, averaging −32.4‰ (Fig. 6C).
When comparing snail shell δ18O values with those of seawater and Greenland ice core records, it is important to recognize that they are recording different periods of time: seawater and ice core records generally average inputs across annual timescales (Grebmeier et al., Reference Grebmeier, Cooper and DeNiro1990; Masson-Delmotte et al., Reference Masson-Delmotte, Steen-Larsen, Ortega, Swingedouw, Popp, Vinther and Oerter2015), while snail values record signatures of their growing seasons (Yanes et al., Reference Yanes, Al-Qattan, Rech, Pigati, Dodd and Nekola2019). Despite the differences in temporal resolutions, comparing snail shell δ18O values to these well-established paleoclimatic proxies provides context that aids in the interpretation of the respective roles of precipitation and temperature in determining fossil snail shell δ18O values in interior Alaska. Fluctuations in snail shell δ18O values do not mimic either Alaska seawater δ18O or Greenland ice core δ18O trends, as indicated by their nonsignificant Pearson correlation. Changes in shell δ18O values through the late glacial likely tracked a combination of several atmospheric conditions, including precipitation and water vapor δ18O composition, air temperature, and local RH (Balakrishnan and Yapp, Reference Balakrishnan and Yapp2004). Similar to this study, other temperature proxies from interior Alaska show results inconsistent with Greenland ice core records. The brGDGT temperature records from interior Alaska do not indicate warmer temperatures during the Bølling–Allerød or consistently cooler temperatures during the Younger Dryas, in contrast to Greenland ice core records (Kielhofer et al., Reference Kielhofer, Tierney, Reuther, Potter, Holmes, Lanoë, Esdale, Wooller and Bigelow2023).
Flux balance model
Measured shell δ18O values from this study, along with local temperature and precipitation δ18O estimates gathered from previously published data (Rozanski et al., Reference Rozanski, Araguás-Araguás and Gonfiantini1993; Kielhofer et al., Reference Kielhofer, Tierney, Reuther, Potter, Holmes, Lanoë, Esdale, Wooller and Bigelow2023), were used to apply the Balakrishnan and Yapp (Reference Balakrishnan and Yapp2004) evaporative steady-state flux balance mixing model to estimate changes in late-glacial RH in interior Alaska. The model assumes (1) snail body water is lost through evaporation and (2) precipitation δ18O and water vapor δ18O values are in isotopic equilibrium. Despite the model's reliance on various unknown variables and assumptions, it can still provide a valuable nominal estimate of the RH at which snails grow. The flux balance model was used in this study to assess how RH changes from the early Bølling–Allerød to the early and late Younger Dryas with respect to the modern. Each period was analyzed under two temperature scenarios.
The model was applied to three late-glacial time periods: the early Bølling–Allerød (~14 ka), with an average shell δ18O value of −10.3‰; the early Younger Dryas (12.9–12.3 ka), with an average shell δ18O value of −12.1‰; and the late Younger Dryas (12.3–11.7 ka), with an average shell δ18O value of −11.2‰. Scenario 1 assumes snails were active only above 10°C (Cowie, Reference Cowie1984; Thompson, Reference Thompson1996). Precipitation δ18O was calculated from the Rozanski et al. (Reference Rozanski, Araguás-Araguás and Gonfiantini1993) isotope–temperature coefficient at high latitudes. These investigations reported that precipitation δ18O decreases by 0.58‰ for every 1°C of temperature decrease. Therefore, if temperatures decreased by ~2°C during the last glacial period (Kielhofer et al., Reference Kielhofer, Tierney, Reuther, Potter, Holmes, Lanoë, Esdale, Wooller and Bigelow2023) and modern rainwater δ18O is −17.1‰, paleontological rainwater δ18O would have been −18.2‰ (Standard Mean Ocean Water [SMOW]) at this site. If this scenario is valid, then interior Alaska snails dating to the early Bølling–Allerød (~14 ka), with an average shell δ18O value of −10.3‰, grew at times when snails were active at 10°C, precipitation δ18O was −18.2‰, and RH was 85% (Fig. 7B). Shells dating to the early Younger Dryas (12.9–12.3 ka PB), with an average shell δ18O value of −12.1‰, grew at times when snails were active at 10°C, precipitation δ18O was −18.2‰, and RH was 90% (Fig. 7B). Shells dating to the late Younger Dryas (12.3–11.7 ka), with an average shell δ18O value of −11.2‰, grew at times when snails were active at 10°C, precipitation δ18O was −18.2‰, and RH was 87% (Fig. 7B).
Figure 7. Calculated Succinea shell δ18O values as a function of relative humidity (RH) using the evaporative steady-state flux balance mixing model by Balakrishnan and Yapp (Reference Balakrishnan and Yapp2004). (A) Calculations for modern (live-collected) land snail shells from central Alaska assuming snails were active for an extended growing season, months of April to October (Nield et al., Reference Nield, Yanes, Pigati, Rech, von Proschwitz and Nekola2022. With rain δ18O value of −17.1‰ SMOW, two possible temperature scenarios, 8°C (dashed line) and 10°C (solid line) (see text). Filled dots represents the average shell δ18O value for modern shells. (B) Paleoclimatic scenario 1. Calculations were performed assuming snails were active only above 10°C (Cowie, Reference Cowie1984; Thompson, Reference Thompson1996) and rain δ18O values (−18.2‰ SMOW) were estimated using the isotope–temperature coefficient at high latitudes (see text) (Rozanski et al., Reference Rozanski, Araguás-Araguás and Gonfiantini1993). Filled dots represent the average shell δ18O value for the early Bølling–Allerød (BA) (~14 ka), early Younger Dryas (YD), late YD, and modern. (C) Paleoclimate scenario 2. Calculations were performed using glacial temperature (6°C) estimated from a branched glycerol dialkyl glycerol tetraethers (brGDGT) record from a loess–paleosol sequence in interior Alaska (Kielhofer et al., Reference Kielhofer, Tierney, Reuther, Potter, Holmes, Lanoë, Esdale, Wooller and Bigelow2023) and rain δ18O value of −18.2‰ SMOW (Rozanski et al., Reference Rozanski, Araguás-Araguás and Gonfiantini1993) (see text). Filled dots represent the average shell δ18O value for the early BA (~14 ka) (−10.3‰), early YD (−12.1‰), late YD (−11.2‰), and modern (−10.7‰).
Because snail activity has not been studied in detail in high-latitude regions, and some previous work has documented that snails track extended summer (April–October) atmospheric conditions (Nield et al., Reference Nield, Yanes, Pigati, Rech, von Proschwitz and Nekola2022), it is possible that Succinea snails could be active at temperatures below 10°C. Hence, scenario 2 assumes modern snails are active at 8°C. This scenario further assumes that late-glacial snails were active at 6°C, as inferred from a nearby brGDGT record, which found growing season temperatures during the Younger Dryas were approximately ~2°C colder than modern temperatures in interior Alaska, on average 8°C during the snail active period (Kielhofer et al., Reference Kielhofer, Tierney, Reuther, Potter, Holmes, Lanoë, Esdale, Wooller and Bigelow2023). Precipitation δ18O was estimated at −18.2‰ (SMOW), based on the Rozanski et al. (Reference Rozanski, Araguás-Araguás and Gonfiantini1993) equation, as calculated earlier. If this scenario is valid, then interior Alaska snails dating to the early Bølling–Allerød (~14 ka), with an average shell δ18O value of −10.3‰, grew at times when temperatures were 6°C, precipitation δ18O was −18.2‰, and RH was 88% (Fig. 7C). Shells dating to the early Younger Dryas (12.9–12.3 ka PB), with an average shell δ18O value of −12.1‰, grew at times when temperatures were 6°C, precipitation δ18O was −18.2‰, and RH was 92% (Fig. 7C). Shells dating to the late Younger Dryas (12.3–11.7 ka), with an average shell δ18O value of −11.2‰, grew at times when temperatures were 6°C, precipitation δ18O was −18.2‰, and RH was 90% (Fig. 7C).
Calculations using the flux balance model for modern (live-collected) land snails from interior Alaska were made assuming snails were active for an extended growing season, April– October, as suggested by prior work (Nield et al., Reference Nield, Yanes, Pigati, Rech, von Proschwitz and Nekola2022). A precipitation δ18O value of −17.1‰ (SMOW) was assumed based on modeled results from IAEA/WMO (2015) by Bowen et al. (Reference Bowen, Wassenaar and Hobson2005) (Fig. 7A). Model calculations were again performed at two possible temperature scenarios, 8°C, average extended growing season temperature in interior Alaska, and 10°C (Fig. 7A). If the assumptions behind these two scenarios are valid, then modern snails with an average shell δ18O value of −10.7‰, grew at times when RH was ~89–90% (Fig. 7A).
In both plausible situations, modern RH is predicted to be higher than in the early Bølling–Allerød, lower than in the early Younger Dryas, and similar to or slightly higher than in the late Younger Dryas (Fig. 7). Both scenarios predict high RH values (>85%) at all time intervals studied. This interpretation is plausible, as snails are mainly active during or directly after rain events, when RH is the highest (Ward and Slotow, Reference Ward and Slotow1992).
Paleo-humidity fluctuations during the Younger Dryas
As inferred from the Balakrishnan and Yapp (Reference Balakrishnan and Yapp2004) flux balance model, RH varied through the late-glacial period. In this study (14–11.5 ka), the lowest RH was found during the early Bølling–Allerød (~14 ka). Similar to previously discussed paleoclimate proxies (e.g., Bigelow and Edwards, Reference Bigelow and Edwards2001; Kielhofer et al., Reference Kielhofer, Tierney, Reuther, Potter, Holmes, Lanoë, Esdale, Wooller and Bigelow2023), the data presented here do not show a dramatic climate shift at the start of the Younger Dryas. Shell δ18O values from 13.5 to 12.8 ka have values similar to those of the early Younger Dryas, suggesting similar RH levels (Fig. 6A). Some paleoclimate studies in central and northern Alaska reported a prolonged dry period leading up to the Younger Dryas (Abbott et al., Reference Abbott, Finney, Edwards and Kelts2000; Edwards et al., Reference Edwards, Mock, Finney, Barber and Bartlein2001; Rabanus-Wallace et al., Reference Rabanus-Wallace, Wooller, Zazula, Shute, Jahren, Kosintsev, Burns, Breen, Llamas and Cooper2017), which we observed in the early Bølling–Allerød (~14 ka). This is likely due to a combination of factors, including the presence of the continental ice sheets, increased summer insolation, cool sea-surface temperatures, and lower sea levels during the late-glacial (Abbott et al., Reference Abbott, Finney, Edwards and Kelts2000). During the last glacial period, the Cordilleran and Laurentide Ice Sheets have been modeled as deflecting the westerlies south of their modern track, causing lower moisture in interior Alaska compared with today (COHMAP Members, 1988). Additionally, reduced North Atlantic Ocean sea-surface temperatures could have affected westerly flow and the strength of the Aleutian low air-pressure system (Kokorowski et al., Reference Kokorowski, Anderson, Mock and Lozhkin2008). Sea level was ~100 to 120 m lower than today, leaving portions of the Bering shelf exposed, increasing the distance for moisture transport to interior Alaska (Clark et al., Reference Clark, Mitrovica and Alder2014).
Our data suggest an increase in RH after the drier early Bølling–Allerød. RH through the Younger Dryas does not seem to be constant, as our data imply a two-stage climate pattern. The offset in snail shell oxygen isotope values between the early Younger Dryas (12.9–12.3 ka) and the late Younger Dryas (12.3–11.7 ka) is ~1‰. According to results from the flux balance model (Balakrishnan and Yapp Reference Balakrishnan and Yapp2004), this implies a wetter early Younger Dryas and subsequent drier late Younger Dryas. This two-step Younger Dryas phenomenon has been observed in other locations in Alaska (Kaufman et al., Reference Kaufman, Scott Anderson, Hu, Berg and Werner2010) and globally (Brauer et al., Reference Brauer, Endres, Günter, Litt, Stebich and Negendank1999; Bakke et al., Reference Bakke, Heegaard, Haug, Birks, Nilsen, Lie, Dulski and Dokken2009; Schlolaut et al., Reference Schlolaut, Brauer, Nakagawa, Lamb, Tyler, Staff, Marshall, Bronk Ramsey, Bryant and Tarasov2017; Pigati and Springer, Reference Pigati and Springer2022).
The Younger Dryas is well documented in southern and coastal Alaska (Kokorowski et al., Reference Kokorowski, Anderson, Mock and Lozhkin2008). In these regions, the results contradict the findings reported in this study, pointing to a mild early Younger Dryas and subsequently wetter late Younger Dryas. Jones et al. (Reference Jones, Peteet, Kurdyla and Guilderson2009) observed an increase in Polypodiaceae (fern) spores from a peat core in Swanson Fen (southwestern Alaska), indicating an increase in moisture around 12.2 ka. This agrees with paleoclimatic inferences from diatom assemblages at Arolik Lake (Hu et al., Reference Hu, Nelson, Clarke, Rühland, Huang, Kaufman and Smol2006), pollen records in nearby Nimgun Lake (Hu et al., Reference Hu, Lee, Kaufman, Yoneji, Nelson and Henne2002), and pollen and microfossil assemblages from Discovery Pond (Kaufman et al., Reference Kaufman, Scott Anderson, Hu, Berg and Werner2010). However, all these sites are closer to coastal moisture sources compared with interior Alaska (this study).
Late-glacial paleoclimate records in interior Alaska, although not as abundant as those in southern Alaska, can be compared with our results. Abbott et al. (Reference Abbott, Finney, Edwards and Kelts2000) studied lake-level history from seismic profiles and sediment cores of Birch Lake, located ~50 km from three of the archaeological sites analyzed in this study: Mead, Bachner, and North Gerstle. Lake systems in interior Alaska are sensitive to climate change and respond rapidly to moisture changes. In agreement with snail shell δ18O values, the lake levels at Birch Lake suggest there was an increase in humidity at the beginning of the Younger Dryas, with lake levels rising dramatically (by 18 m) between 12.7 and 12.2 ka and then stabilizing during the late Younger Dryas.
Today, the moisture in interior Alaska is mainly obtained from a westerly flow from the North Pacific Ocean and is related to large-scale atmospheric circulation patterns (Abbott et al., Reference Abbott, Finney, Edwards and Kelts2000; Kokorowski et al., Reference Kokorowski, Anderson, Mock and Lozhkin2008; Chipman et al., Reference Chipman, Clegg and Hu2012). At the start of the Younger Dryas, interior Alaskan paleoclimate proxies suggest an increase in moisture. This change in RH could be caused by the change in sea level, which rose 30 m from 12.5 to 11.6 ka (Clark et al., Reference Clark, Mitrovica and Alder2014), or the decrease in size of the Cordilleran and Laurentide Ice Sheets, which affected circulation. As the ice sheets decreased, their impact on circulation would have diminished, making circulation patterns more like those of today (Bartlein et al., Reference Bartlein, Anderson, Edwards and Mcdowell1991).
CONCLUSIONS
The δ13C values of Succinea shells in interior Alaska suggest that δ13C values of late-glacial vegetation were consistently more positive than those of modern shells. Part of this difference is probably due to changes in the δ13C value of CO2 because of the increased burning of fossil fuels (Suess effect), which affects the δ13C values of consumed plants. The rest of the offset is likely explained by changes in the δ13C composition of C3 plants consumed by snails. This is possibly explained by changes in the water-use efficiency of C3 plants during the slightly drier late-glacial period compared with today, which would leave plants, and therefore snail shells, more enriched in 13C due to changes in isotope discrimination during slightly drier conditions.
The δ18O values of late-glacial Succinea shells in interior Alaska appear to record changes in RH over the late glacial. RH was modeled using the Balakrishnan and Yapp (Reference Balakrishnan and Yapp2004) flux balance mixing model. The results show RH increased from ~14 ka to the early Younger Dryas (~12.9–12.3 ka), then decreased in the latter half of the Younger Dryas (~12.3–11.7 ka), with modern conditions being similar to those in the late Younger Dryas. Shell δ18O values from interior Alaska highlight a two-stage humidity pattern during the Younger Dryas, which other studies have found evidence of at other sites in interior Alaska and elsewhere in North America. Published climate and humidity records in Alaska during the Younger Dryas are somewhat complex; however, lake proxies from interior Alaska agree with our findings and show evidence for a wetter early Younger Dryas compared with other periods of the late glacial.
This study illustrates that ancient land snails may be useful for clarifying the complex climatic history of the late glacial in interior Alaska. Isotope records dating to the late glacial in interior Alaska are rare. Additional isotope records dating to the Younger Dryas may help solidify emerging patterns about late-glacial humidity in interior Alaska.
Acknowledgments
This project was funded by National Science Foundation grants EAR–1529133 and 1802153 to YY. Additional funding was awarded to CBN from the Geological Society of America (GSA), Paleontological Society, Western Interior Paleontological Society (WIPS), Society for Sedimentary Geology (SEPM), and Sigma Xi (Scientific Research Society). Special thanks go to Jason Curtis (University of Florida) for assistance with isotope analyses; John Southon (University of California–Irvine) for assistance with radiocarbon analyses; Ezekiel King Phillips for field assistance collecting modern and fossil samples; Julie Esdale for archaeological site field assistance; Briana Doering (University of Wyoming) for samples from Bachner; Ben Potter, Gerad Smith, Chuck Holmes, and Barbara Crass (University of Alaska Fairbanks) for support in sampling Upward Sun River site and Mead and North Gerstle sites; George Harrison, William Wright, John Scot, and Thomas Sniezak for sampling shells from the permafrost tunnel; the U.S. Army Corps of Engineers, CRREL and Gary Larsen for tunnel access; and the U.S. Army for land access. Funding for JSP and DRM was provided by the U.S. Geological Survey's Climate and Land Use Change Research and Development Program through the Quaternary Hydroclimate Records of Spring Ecosystems project. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. government.
Data Availability Statement
Downloadable files of the data presented in Tables 1–3 can be found at https://doi.org/10.5066/P953H59T.
Table 3. Institutional abbreviations and codes for snails collected from the Cold Regions Research and Engineering Laboratory (CRREL) permafrost tunnel.a
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