Introduction
The western United States has recently experienced the worst drought in the last 1200 years (Williams et al., Reference Williams, Cook and Smerdon2022). Highly resolved paleorecords of drought from the western United States are vital for contextualizing contemporary hydroclimate and providing an archive of the potential limnological changes that accompany them. High-resolution paleoclimate proxy records are also potentially useful in assessing climate models in order to better plan for future environmental variability (e.g., Braconnot et al., Reference Braconnot, Harrison, Kageyama, Bartlein, Masson-Delmotte, Abe-Ouchi, Otto-Bliesner and Zhao2012; Kaufman et al., Reference Kaufman, McKay, Routson, Erb, Davis, Heiri and Jaccard2020). This is particularly true in California, a state with a population of ~40 million people and an agricultural sector that earned ~$51.1 billion in 2021 (https://cdfa.ca.gov/statistics, accessed June 19, 2023). Much of this population is served by water sources from the Sierra Nevada snowpack (e.g., Huang et al., Reference Huang, Hall and Berg2018; Sun et al., Reference Sun, Berg, Hall, Schwartz and Walton2019), which is at risk from hazards like extreme droughts, floods, and wildfires (e.g., Diffenbaugh et al., Reference Diffenbaugh, Swain and Touma2015; Ullrich et al., Reference Ullrich, Xu, Rhoades, Dettinger, Mount, Jones and Vahmani2018; Goss et al., Reference Goss, Swain, Abatzoglou, Sarhadi, Kolden, Williams and Diffenbaugh2020). With these issues in mind, we seek to establish a robust record of Late Holocene hydroclimate variability for the eastern Sierra Nevada using carbonates from June Lake (Fig. 1).
Figure 1. (A) Regional map showing location of study area (white box) and locations of other records discussed in the text (yellow circles): ML, Mono Lake; SM, Stonehouse Meadow; PL, Pahranagat Lake. (B) Lakes and streams of the study area, which are found along CA State Route 158 (the June Lake Loop). Water isotope values for locations indicated by white squares (oxygen, deuterium). (C) June Lake bathymetric map (after Lyon et al., Reference Lyon, McGlue, Woolery, Kim, Stone and Zimmerman2019). White circles indicate locations of the cores discussed in this study. Base images for (A) and (B) are from Google Earth®.
A key objective of this study is to resolve the paleohydrological conditions associated with the Late Holocene Dry Period (LHDP) and Medieval Climate Anomaly (MCA) in the eastern Sierra Nevada. Evidence of the LHDP has been observed in paleoclimatic and paleoecological archives across the Great Basin (e.g., Mensing et al., Reference Mensing, Sharpe, Tunno, Sada, Thomas, Starratt and Smith2013, Reference Mensing, Wang, Rhode, Kennett, Csank, Thomas and Briem2023; Thiessen et al., Reference Theissen, Hickson, Brundrett, Horns and Lachniet2019), including Mono Lake (Fig. 1; Zimmerman et al., Reference Zimmerman, Hemming, Starratt, Starratt and Rosen2021). Those sediment proxy records are relatively low resolution, and define the LHDP as a multicentury drought. Because the sediments of June Lake have relatively high accumulation rates, their proxy data sets, including carbonate stable isotopes (δ18Ocarb and δ13Ccarb, which we hereafter refer to as carbonate isotopes) and X-ray fluorescence (XRF) geochemistry, have the potential to show discrete decadal to centennial-scale droughts within this broader LHDP window, as well as other parts of the Late Holocene. Thus, we seek to apply these data to determine whether the LHDP was a single multicentury dry period or multiple shorter droughts.
Study area
June Lake (Mono County California, 37°47′17″N, 119°4′23″W) occupies a glacial scour basin at the foot of the eastern Sierra Nevada at a surface elevation of 2324 meters above sea level. It is an oligotrophic to mesotrophic lake that is seasonally stratified in warmer months (Lyon et al., Reference Lyon, McGlue, Woolery, Kim, Stone and Zimmerman2019). Seismic stratigraphic analysis revealed that June Lake has two deepwater depocenters separated by a bedrock shoal; the deeper of the two has a maximum depth of 42 m (Fig. 1; Lyon et al., Reference Lyon, McGlue, Woolery, Kim, Stone and Zimmerman2019). Sediments accumulating in these depocenters are organic-matter-rich laminated oozes with variable carbonate concentrations (Lyon et al., Reference Lyon, McGlue, Woolery, Kim, Stone and Zimmerman2019).
The paleoproduction and environmental history of June Lake were first described using low-resolution organic matter geochemical records (Lyon et al., Reference Lyon, McGlue, Erhardt, Kim, Stone and Zimmerman2020). That study defined four chronological zones based on paleoproduction and inferred hydroclimate variation within a 4600-year-long sediment sequence collected from the southwestern depocenter. Using a core from the northeastern depocenter, Streib et al. (Reference Streib, Stone, Lyon, Quang, Yeager, Zimmerman and McGlue2021) established a diatom paleoecological record of lake stratification and nutrient cycling over the last two millennia. Streib et al. (Reference Streib, Stone, Lyon, Quang, Yeager, Zimmerman and McGlue2021) found that the algal community has become both more diverse and less productive over the last ~150 years, owing to the influence of climate warming on water-column stratification.
June Lake is fed by seasonal snowmelt runoff and groundwater discharge from subaqueous springs (Fig. 1). At present, there are no surface outlets, though a low, marshy terrain at the southwestern end of the basin is only a few meters higher than neighboring Gull Lake (Lopera-Congote et al., Reference Lopera-Congote, Westover, McGlue, Yeager, Streib and Stone2024) (Fig. 1). Lyon et al. (Reference Lyon, McGlue, Erhardt, Kim, Stone and Zimmerman2020) and Streib et al. (Reference Streib, Stone, Lyon, Quang, Yeager, Zimmerman and McGlue2021) proposed that during some high stands, June Lake likely spilled over and connected the two basins during wetter time periods, resulting in an open lake system that flowed out through Reversed Creek. Reversed Creek, to our knowledge the only westward-directed stream east of the Sierran crest, joins Rush Creek prior to entering Silver Lake, which is open at its downstream end (Fig. 1). Rush Creek flows into Grant Lake reservoir before flowing out over a dam and into endorheic Mono Lake from the south. This chain of waterbodies parallels California Highway 158 (i.e., the June Lake Loop) (Fig. 1).
Regional climate
The study region has a Mediterranean climate; summers are typically dry and warm (highs in the mid-20s °C), and most of the ~40 cm of total annual precipitation falls as snow (~180 cm, or about 30 cm calculated snow water equivalent) in the winter months, which have average low temperatures around −11°C (National Oceanic and Atmospheric Administration, https://www.ncei.noaa.gov/; accessed May 2024). Most of this winter precipitation is derived from a North Pacific Ocean source, as moist air masses move inland over the Sierra Nevada (e.g., Pyke, Reference Pyke1972; Cayan and Peterson, Reference Cayan, Peterson and Peterson1989). However, an increasing percentage of annual precipitation in California is derived from atmospheric rivers (AR) from the subtropical Pacific (Dettinger, Reference Dettinger2011; Ralph and Dettinger, Reference Ralph and Dettinger2011), accounting for up to 50% of west coast precipitation in recent years (Dettinger, Reference Dettinger2013), contributing to 22–73% of winter snowpack (Guan et al., Reference Guan, Molotch, Waliser, Fetzer and Neiman2010, Reference Guan, Molotch, Waliser, Fetzer and Neiman2013). A recent modeling study proposed that ARs will have an even greater role as the century continues, with overall precipitation and the potential for hazardous extreme events increasing (Rhoades et al., Reference Rhoades, Jones, Srivastava, Huang, O'Brien, Patricola, Ullrich, Wehner and Zhou2020). A consequence of this shift is that ARs will transition from largely beneficial to largely hazardous, especially as back-to-back AR events become more common (Rhoades et al., Reference Rhoades, Jones, Srivastava, Huang, O'Brien, Patricola, Ullrich, Wehner and Zhou2020). These shifts in precipitation source could lead to shifts in isotope values for the water entering June Lake, which could be captured in the carbonate geochemistry. Studies suggest that AR precipitation is isotopically depleted compared to non-AR storms (e.g., Peltier, Reference Peltier2023; Greenblat et al., Reference Greenblat, Allen and Hahm2024).
A number of data sets measured on different sample types were used in this study, including modern water samples, surface sediment samples, and sediments of a composite sediment core. Oxygen (δ18O) and hydrogen (δ2H) isotopes of water were measured to establish June Lake's position on the local evaporation line (LEL), and to track present moisture sources. These data were also needed to establish whether carbonate preserved in surface samples precipitates in equilibrium with modern lake water. The carbonate record from the sediment core was used to track hydroclimate changes and drought frequency over the Late Holocene, which we define here as the last ~4600 years, or the full span of our core record.
Methods
Surface water samples (n = 10) were collected from the shorelines of June Lake, Gull Lake, Reversed Creek, Silver Lake, and Rush Creek in September 2019 (Fig. 1). Sample containers were rinsed with lake or river water prior to sample collection, submerged, and capped underwater to minimize headspace and prevent in situ evaporation. δ18O and δ2H values for water samples were determined at the University of Kentucky's Stable Isotope Geochemistry Laboratory (KSIGL) using a Los Gatos Research T-LWIA-45-EP liquid water isotope analyzer. Raw data were normalized using USGS49 and USGS50 standards. In-session standard deviations for these standards were both <0.1‰. Duplicate measurements of each sample produced an average difference of 0.4‰ for δ2H and 0.1‰ for δ18O. Final δ2H and δ18O values are expressed in ‰ relative to Vienna Standard Mean Ocean Water (VSMOW). Data from Waterisotopes.org were used for comparison with sampled waters and to develop global and local meteoric water lines (Bowen and Revenaugh, Reference Bowen and Revenaugh2003; Bowen, Reference Bowen2011).
To determine if carbonate isotope values had variability within modern June Lake sediments, we collected surface sediment (upper ~1 cm) samples from a grid covering the lake floor (n = 36; Supplementary Fig. 1) using a Ponar grab sampler. To study changes in geochemistry through time, a pair of overlapping sediment cores, JUNE-JNE16-11A/B, were collected from a UWITEC percussion piston coring platform in 2016; details of the core collection and initial sampling procedures can be found in Lyon et al. (Reference Lyon, McGlue, Erhardt, Kim, Stone and Zimmerman2020). Here, we increased the bulk-carbonate sample resolution in the composite core from the previous 5 cm resolution data set from Lyon et al. (Reference Lyon, McGlue, Erhardt, Kim, Stone and Zimmerman2020) to ~2–3 cm, using the earlier data alongside the new.
Initial sediment characterization was based on smear slides, which were made following standard Continental Scientific Drilling (CSD) Facility procedures (Schnurrenberger et al., Reference Schnurrenberger, Russell and Kelts2003; Myrbo, Reference Myrbo2013) for all surface samples and a representative selection of core samples. Both sets of samples were measured for total inorganic carbon (%TIC) on a UIC Coulometrics CM5130 coulometer and those concentrations were used to determine the appropriate mass for isotopic measurements. Precision based on repeated measurements of a pure CaCO3 standard was ≤0.11%. Samples with <0.17% TIC were omitted from gas bench measurements, as reliable isotopic data were not possible at such low carbonate concentrations.
The age model for core JUNE-JNE16-11A/B (Fig. 2) is from Lyon et al. (Reference Lyon, McGlue, Erhardt, Kim, Stone and Zimmerman2020) and was based on 19 radiocarbon ages from both the JUNE-JNE16-11A/B and JUNE-JNE16-16 cores (Table 1; hereafter core 11 and core 16, respectively), correlated based on lithology and peaks in magnetic susceptibility, and the North Mono Tephra (Bursik and Sieh, Reference Bursik and Sieh2013). Materials selected for radiocarbon dating were limited to plant macrofossils and charcoal, owing to the challenges in achieving accurate dates on other carbon-bearing materials (e.g., bulk sediment, carbonates) (e.g., Zimmerman and Wahl, Reference Zimmerman and Wahl2020). The Bayesian statistical program “rbacon” (version 2.5.7; Blaauw and Christen, Reference Blaauw and Christen2011) was used to model the age–depth relationship for the entire core at 0.1 and 0.5 cm, and at the bulk sampling resolution (2–3 cm) (Fig. 2).
Figure 2. Left: Bacon age model results showing the width of the 95% uncertainty window for JUNE-JNE16-11A/B. Gray bands are tephras, which we interpret to represent near-instantaneous sedimentation. Right: The sedimentation rate in cm/yr for the same core. Note that the same y-axis is used for this panel and that the uppermost 50 cm are not included due to high water content, which would suggest an artificially high sedimentation rate.
Table 1. Radiocarbon dates used to construct the age model for June Lake core 11. The model includes dates from another core in the same lake (core 16), which were interpolated for core 11 based on correlation using lithologic units and magnetic susceptibility, as described in Lyon et al. (Reference Lyon, McGlue, Erhardt, Kim, Stone and Zimmerman2020)
a Depth in cm is for core 11.
To determine the relative abundance of different elements through time, core 11, which spanned a greater time interval than core 16, was analyzed on the University of Minnesota-Duluth's ITRAX XRF core scanner. Samples were measured at 5 mm intervals with a Cr-HE source, and at 1 mm intervals on a Cr source; both data sets were run with a 15 s dwell time. The analysis yielded semiquantitative major and trace elemental data for 36 elements in units of counts per second (cps). To further characterize mineralogy, dried and powdered samples were scanned using a Diano 2100-E X-ray diffraction (XRD) instrument with a copper target. Peaks of the important carbonate minerals were identified by comparison with known minerals from the RRUFF database (Lafuente et al., Reference Lafuente, Downs, Yang, Stone, Armbruster and Danisi2016).
Sediment samples for carbonate isotope analyses were pretreated with 2.5% bleach followed by multiple rinses with deionized water to remove organic carbon using the CSD standard operating procedure for lake sediment decarbonation (https://cse.umn.edu/csd/subsample-preparation-and-analysis; accessed July 2024). Samples were then dried in either a low temperature oven (45°C for 48 hours) or a freeze-dryer. Following the KSIGL gas bench standard operating procedure, dried carbonate samples were loaded into acid-cleaned 12 mL Exetainers and dried overnight at 70°C to remove adsorbed water. Each Exetainer was then sealed with a septum cap and flushed with ultra-high purity (99.999% pure) He for 12 minutes to remove atmospheric N2 and CO2. Following the He flush, 0.1 mL of dewatered H3PO4 (final concentration 104%) was injected through the septum and allowed to react with the sample for at least 24 hours at 25°C to produce CO2. Seven consecutive aliquots of headspace gas from each Exetainer were transferred via autosampler to a Thermo GasBench II interfaced with a Thermo Finnigan DELTAplus XP isotope ratio mass spectrometer for measurement. Uncorrected oxygen and carbon isotope values were normalized to Vienna Pee Dee Belemnite (VPDB) using at least two internationally certified reference materials with contrasting δ18O and δ13C values: NIST 915b (δ18O = −23.4, δ13C = −8.6), NBS-18 (δ18O = −23.2, δ13C = −5.01), and NBS-19 (δ18O = −2.2, δ13C = 1.95) (after Coplen et al., Reference Coplen, Brand, Gehre, Gröning, Meijer, Toman and Verkouteren2006). Blind standards were analyzed to assess precision and accuracy. Duplicates of unknowns were also analyzed to assess the reproducibility of measurements. The precision and accuracy of the δ18O data were assessed with multiple in-session measurements of NIST-915b; both values were <0.4‰. The precision and accuracy of the δ13C data, assessed in the same manner, were both 0.2‰. The reproducibility of δ18O and δ13C data, assessed with sample duplicates, were 0.2 and 0.1‰, respectively.
We matched horizons from the 5 mm Ca/Ti data set to the lower resolution (2–3 cm) %TIC data sets by applying a five-point running average to the 5 mm XRF data and only compared those depths for which we also had bulk geochemical data. We used the program PAST (Paleontological Statistics Software; Hammer et al., Reference Hammer, Harper and Ryan2001) to apply LOESS filters to the carbonate isotope chemostratigraphic data, which is a line of best fit through a data distribution that allows for visualization of relationships between measured variables and time. We use the resulting LOESS curves to interpret non-parametric temporal trends. We also analyzed correlations among measured indicators using linear regression analysis with PAST.
Results
Water isotopes
June Lake surface waters have δ18O values of −2.7 to −3.9‰ and δ2H values of −55.1 to −53.9‰, similar to published values of −3.3‰ and −59.6‰ for June Lake water collected in September 2007 (Brooks et al., Reference Brooks, Gibson, Birks, Weber, Rodecap and Stoddard2014). Isotope values for surface waters in the study area were lowest in the most downstream reaches (Reversed and Rush creeks) and highest in June Lake, the only closed lake basin in our transect (Table 2, Fig. 1). These water isotope values were plotted against the global meteoric water line (Fig. 3). June and Gull lakes are isotopically heavier than average regional precipitation, but the streams and Silver Lake are lighter (Fig. 3). Our water samples were collected in September 2019, a time of year when isotopic values for precipitation (−9.5‰ and −66‰ for δ18O and δ2H, respectively; Bowen and Revenaugh, Reference Bowen and Revenaugh2003; Bowen, Reference Bowen2011) are greater than average (avg = −12.2‰ and −88.7‰ for δ18O and δ2H, respectively) due to warm late summer water temperatures in the source area and higher condensation temperatures (e.g., Dansgaard, Reference Dansgaard1964).
Table 2. Water isotope values from waterbodies in the study area. All are measured in per mil, relative to Vienna Standard Mean Ocean Water
Figure 3. Cross plot of average water isotope values in per mil for waterbodies in the study area (light blue dots) and selected global and regional lakes (gray dots and polygons). Standard deviations for these values are 0.45 and 2.32 for June Lake, 0.15 and 0.19 for Gull Lake, and 0.39 and 3.20 for δ18O and δ2H, respectively. The global meteoric water line (GMWL) is also shown for reference (after Jasechko et al., Reference Jasechko, Sharp, Gibson, Birks, Yi and Fawcett2013). Large, dark blue dots on the GMWL mark values for precipitation in the June Lake region. An evaporation line for the study area is shown in light blue with a shallower slope than the GMWL. The blue horizontal bar along the x-axis shows the range of surface sediment δ18O values in June Lake. *Mono Lake values (Friedman et al., Reference Friedman, Redfield, Schoen and Harris1964; Li et al., Reference Li, Ku, Stott and Anderson1997) represented by dark gray circle within Aral Sea polygon.
Snowmelt charged streams entering Mono Lake from the Sierras range from −15.8 to −13.8‰ for δ18O (Li et al., Reference Li, Ku, Stott and Anderson1997). Mono Lake water was measured at −0.1‰ (δ18O) at Navy Beach along the southern shore (Li et al., Reference Li, Ku, Stott and Anderson1997). An early study by Friedman et al. (Reference Friedman, Redfield, Schoen and Harris1964) recorded a δ2H of −6.2‰ for Mono Lake, but more recent δ2H values for Mono Lake water are unavailable.
Sediment composition and sedimentation rate
Modern surface sediments
Surface sediments from June Lake are primarily diatomaceous oozes, particularly in the two depocenters (Fig. 1; Lyon et al., Reference Lyon, McGlue, Woolery, Kim, Stone and Zimmerman2019). δ18Ocarb and δ13Ccarb values (Table 3) are generally lower in the two deepwater depocenters than in shallower areas of the lake (Lyon et al., Reference Lyon, McGlue, Woolery, Kim, Stone and Zimmerman2019). In particular, δ18Ocarb values are lowest in areas where water depth exceeds 30 m (Fig. 4), and values are highest closest to the shoreline. For δ13Ccarb, values are highest (>2.0‰) at the shallow, northeastern beach (Fig. 4). In contrast, δ13Ccarb values are lowest (<1.0‰) in the deepwater just offshore of this beach, and in the southwestern depocenter. Carbonate isotope values for surface sediments are weakly correlated (R 2 = 0.18; Supplementary Fig. 2).
Table 3. Surface sediment carbonate isotope values. All are measured in per mil, relative to Vienna Pee Dee Belemnite
Figure 4. Carbonate isotope values for surface sediments in June Lake. Each black dot represents a sample location on the lake floor. Left: δ18Ocarb values. Right: δ13Ccarb values.
Core sediments
Lyon et al. (Reference Lyon, McGlue, Erhardt, Kim, Stone and Zimmerman2020) established the sedimentology and stratigraphy of the June Lake cores as algae-rich oozes, with intervals of muddy ooze and carbonate mud (Fig. 2). Seven distinct tephras punctuate the stratigraphy, and in some cases, provide distinct chronological controls. Smear slide analysis allowed us to establish the dominant carbonate mineralogy as calcite. Most of the calcite was small (<10 mm), sub-equant crystals (Lyon et al., Reference Lyon, McGlue, Erhardt, Kim, Stone and Zimmerman2020). Ostracods were observed in some horizons while subsampling, but smear slide analysis reveals them to be a minor component overall, so we were largely able to avoid them in bulk sampling. Further, their shells are composed of low Mg-calcite (e.g., Holmes and De Deckker, Reference Holmes and De Deckker2012), which is consistent with the dominant carbonate mineralogy established in smear slide analysis (Lyon et al., Reference Lyon, McGlue, Erhardt, Kim, Stone and Zimmerman2020). Dolomite and aragonite were both very rare in the smear slide analysis. XRD peaks for horizons with high %TIC were identified at 29.4, 47.6, and 48.6 degrees 2-theta (Supplementary Fig. 3). We focus on further interpreting the mineralogy of the carbonate sediments and their utility as hydroclimate indicators in the “Discussion”.
Excluding the tephras, which represent near-instantaneous deposition, the sedimentation rate in June Lake is relatively constant (~1 mm/yr) throughout the core, with a higher apparent rate in the uppermost, waterlogged part of core 11 (Fig. 2). The 95% uncertainty envelope for most of the age–depth model produces an uncertainty range of less than 200 years around any given depth in the core. However, some areas, including the middle and bottom sections of the core, have uncertainties closer to 240 years (Table 4). Age model results are described in greater detail in Lyon et al. (Reference Lyon, McGlue, Erhardt, Kim, Stone and Zimmerman2020). Both the %TIC and carbonate isotope values (sampled every 2–3 cm) are lower resolution than the high-resolution (0.1 cm) XRF record. The sampling for carbonate isotope analysis used ~1 cm3, and thus each sample integrates ~10 years based on the overall sedimentation rate, whereas each XRF data point represents ~1 year based on the same rate. Thus, we feel confident that our record provides centennial-scale resolution or better.
Table 4. Times of increasing carbonate precipitation from the June Lake carbonate archive as designated from rising limbs of Ca/Ti curve. Age model 95% uncertainty is reported for each drought depth. Drought midpoint reported as median years in cal yr BP, rounded to the nearest 10. The average time between intervals of increasing carbonate accumulation is ~230 years
a MCA, Medieval Climate Anomaly; LHDP, Late Holocene Dry Period.
The sediment geochemistry record over the last 4600 years includes several centennial intervals of elevated %TIC (Fig. 5). Six prominent %TIC peaks appear, centered on ~3700, 3200, 2500, 2000, 1100, and 70 cal yr BP. Alongside many of these broad peaks are numerous discrete multidecadal peaks in the 1 mm Ca/Ti data set (1–18 in Fig. 5). For example, two %TIC peaks at ~2500 and 2670 cal yr BP occur within an extended period of elevated carbonate accumulation from 2900 to 2440 cal yr BP; the Ca/Ti curve defines two distinct peaks from 2740 to 2670 and 2530 to 2430 cal yr BP, after which time Ca/Ti values decline, but remain relatively high until about 2330 cal BP. We designated Ca/Ti values greater than 56 (2-sigma deviation) as peaks in that indicator. The exception to this is the most recent part of the record, in which Ca/Ti count values approach, but do not exceed, 56. The Ca/Ti curve shows higher frequency variability throughout the core than the %TIC data, albeit with similar overall structure.
Figure 5. Plot of changes in core geochemistry with time. From left to right: (A) total inorganic carbon (%TIC), sampled every 2–3 cm; (B) ratio of calcium to titanium (Ca/Ti), sampled every 1 mm; (C) oxygen isotope values in per mil (δ18Ocarb), sampled every 2–3 cm where %TIC was high enough to permit measurement; (D) carbon isotope values in per mil (δ13Ccarb), sampled every 2–3 cm where %TIC was high enough to permit measurement. A LOESS smoother applied to the δ18O and δ13C data sets (heavy lines) shows broader changes through time. Here, the six dry intervals described in the text are particularly prominent as increases in oxygen isotope values—these are denoted by horizontal yellow or red bars. The horizontal blue bars indicate the wetter intervals, including the pluvial between the two peaks of the Medieval Climate Anomaly (MCA) megadrought and the Little Ice Age (LIA) (~500–100 cal yr BP, as defined in IPCC, 2021). LHDP, Late Holocene Dry Period.
Values for δ18Ocarb and δ13Ccarb range from −13.6 to −4.4‰ and −1.2 to 2.7‰, respectively. Although this data set is lower resolution than the scanning XRF-derived Ca/Ti data, the long-term trends share many similarities. Higher carbonate isotope values tend to co-occur with each other and occur at the same times as the peaks in %TIC and Ca/Ti, though with offsets due to the different sample resolution. One example of this disparity occurs at ~3200 cal yr BP. In the Ca/Ti data, this interval of elevated values is separated into two prominent peaks centered at 3170 cal yr BP and 3110 cal yr BP, respectively (Table 4).
Discussion
Water isotopes
To determine the viability of δ18O as a proxy for the ratio of precipitation to evaporation (P/E), we considered the difference between the isotopic values of lake water and meteoric water in the study area, including the presumed termination of the flow path at Mono Lake. We applied the water isotope values (δ18O and δ2H) for June Lake and the other waterbodies in Figure 1 to determine June Lake's position on the LEL (Fig. 3). Although June Lake water is isotopically heavier than regional precipitation, Rush Creek, Reversed Creek, Silver Lake, and Gull Lake are lighter likely due to the “catchment effect” wherein the water in the streams is lighter due to the direct contributions of upstream snowmelt and precipitation (Friedman et al., Reference Friedman, Smith, Gleason, Warden and Harris1992). These data are valuable for discriminating isometer type at June Lake: a precipitation isometer (recording precipitation δ18O), a P/E isometer (where δ18O values reflect both precipitation and fractionation by evaporation), or a terminal basin, where all water loss is by evaporation and the δ18O of lake water is most strongly influenced by evaporative fractionation and is therefore very high compared to the other two systems (after the model of Anderson et al., Reference Anderson, Berkelhammer, Barron, Steinman, Finney and Abbott2016).
The likeliest terminal basin under the Anderson et al. (Reference Anderson, Berkelhammer, Barron, Steinman, Finney and Abbott2016) model in the study area is Mono Lake (Fig. 1). However, published water isotope values for Mono Lake (δ18O = −0.1‰ and δ2H = −6.2‰; Friedman et al., Reference Friedman, Redfield, Schoen and Harris1964; Li et al., Reference Li, Ku, Stott and Anderson1997) suggest that it is not along the same evaporation trend as the other waterbodies and thus not the endmember of the LEL defined by the waters in the study area (Fig. 3). This is not entirely unexpected, since Mono Lake has a much larger and geologically more variable watershed (John et al. Reference John, duBray, Blakely, Fleck, Vikre, Box and Moring2012) than the study area, and has many faults and springs through which hydrothermal water or deep ground water likely enters the lake, disconnecting it geochemically from meteoric waters (Oremland et al. Reference Oremland, Miller and Whiticar1987; Tomascak et al. Reference Tomascak, Hemming and Hemming2003). Based on our water isotope data, June Lake would thus seem to be the endmember of the LEL for the waterbodies studied, which would make it a terminal basin as opposed to a P/E isometer in the Anderson et al. (Reference Anderson, Berkelhammer, Barron, Steinman, Finney and Abbott2016) model. However, there remains a question of whether June Lake is hydrologically open in the subsurface, and by consequence a P/E isometer.
June Lake has the highest water isotope values of all the waterbodies we measured (averages: δ18O −3.0‰ and δ2H −55.3‰; Table 2, Fig. 1), which is interpreted to be a consequence of seasonal evaporative concentration. Gull Lake, despite having a surface outlet, has notably higher isotope values (averages: δ18O −9.7‰ and δ2H −91.4‰; Table 2, Fig. 1) than the other open lakes and streams we measured. One potential explanation for this unexpected isotopic composition is subsurface seepage of water from June Lake into Gull Lake, notionally following flow paths through the marsh that separates the basins (Fig. 1). We hypothesize that Gull Lake's intermediate isotopic composition results from a mixture from water sources with varied isotopic composition: direct precipitation, seasonal runoff from snowmelt, and contributions of seepage from June Lake, suggesting that June Lake is not entirely hydrologically closed. This interpretation supports the idea that June Lake is a P/E isometer rather than a terminal basin. Carbonate isotope values from core sediments described below provide additional context for our ability to interpret June Lake's status as a P/E isometer.
Carbonate mineralogy and its utility as a hydroclimate indicator
Establishing dominant carbonate mineralogy
Smear slide analysis (Lyon et al., Reference Lyon, McGlue, Erhardt, Kim, Stone and Zimmerman2020) suggests the dominant carbonate mineralogy in the core is calcite. New XRD data from this study confirm that interpretation; the dominant peak for most samples is at approximately 29.4° 2-theta, consistent with diffractograms of calcite reference materials, and smaller peaks at 47.6° and 48.6° 2-theta are also indicative of calcite (Supplementary Fig. 3). This is important to note, as other carbonate phases, such as dolomite, require correction with a different fractionation factor for δ18Ocarb and δ13Ccarb values if present (e.g., McCormack and Kwiecien, Reference McCormack and Kwiecien2021).
Determining whether calcite precipitates in equilibrium with lake water
To determine whether June Lake carbonates precipitate in equilibrium with lake water, we applied Leng and Marshall's (Reference Leng and Marshall2004) equation to values for oxygen isotopes in both June Lake water (δw) and surface sediment samples (δc):
Using a value of −2.8‰ for δw (June Lake δ18Owater) and a T of 11.5°C (surface water temperature when samples collected in early fall), we calculated δc (δ18Ocarb) = −5.0‰, which is approximately the average value for surface sediments (−4.9‰; Table 3). Thus, it is likely that carbonates precipitate at or near equilibrium conditions with the lake water. While some researchers (e.g., Daeron et al., Reference Daëron, Drysdale, Peral, Huyghe, Blamart, Coplen, Lartaud and Zanchetta2019) suggest such a validation step is superfluous given the likelihood that most calcites are in disequilibrium from the host waters, this does not appear to be the case at June Lake. This validation of equilibrium precipitation adds confidence to our interpretation of the δ18Ocarb record as one of hydroclimatic change.
Establishing the Ca/Ti record as one of carbonate accumulation
There is a relatively strong correlation between %TIC and Ca/Ti (Fig. 6; R 2 = 0.68; P value < 0.00001 at 0.05 significance threshold); it is important to note that the peak heights are greater relative to baseline values for the Ca/Ti record than for the %TIC, which may make this correlation weaker than the depth match in peaks would suggest. Nonetheless, the correlation suggests that most Ca is bonded to the carbonate ion as calcite, rather than another mineral phase such as gypsum or anhydrite, neither of which were observed on smear slides or on X-ray powder diffractograms. From these tests we conclude that Ca/Ti reliably approximates CaCO3 in Late Holocene-age June Lake sediments. This application is consistent with several other lake sediment studies (e.g., Kylander et al., Reference Kylander, Ampel, Wohlfarth and Veres2011; Liu et al., Reference Liu, Colman, Brown, Minor and Li2013; Davies et al., Reference Davies, Lamb, Roberts, Croudace and Rothwell2015).
Figure 6. (A) Scatterplot of carbonate isotope values for core sediments from June Lake. The strength of this correlation is often used as an indicator of hydrologic closure (e.g., Talbot and Kelts, Reference Talbot, Kelts and Katz1990). P value < 0.00001 at 0.05 significance threshold. (B) A strong positive correlation between %TIC and Ca/Ti for core sediments suggests most calcium is associated with an authigenic carbonate phase rather than detrital input, and may therefore be used as a proxy for %TIC in our high-resolution XRF data set. Owing to the differences in sampling resolution, we used a five-point running average on the Ca/Ti data set to ensure comparison of approximately the same strata for the two data sets. P value < 0.00001. (C) Scatterplot of δ18O values and %TIC throughout the core. A logarithmic curve defines the relationship between these two indicators. P value < 0.00001. At %TIC values < ~1%, a range of low δ18O values are possible (−13.5 to −6.5). However, when carbonate precipitation is greater (%TIC > 1), oxygen isotope values, though higher, are less variable (−6.5 to −4.4).
Using carbonate isotope geochemistry to determine June Lake hydrology
Large, hydrologically closed lakes tend to have strong correlations between δ13Ccarb and δ18Ocarb (R 2 > 0.7), whereas open lakes have a weaker correlation and a narrower range of δ18O values (Talbot, Reference Talbot1990; Talbot and Kelts, Reference Talbot, Kelts and Katz1990). Despite high covariance between δ18O and δ13C when carbonate content is high (e.g., R 2 = 0.83, P < 0.05 during peaks of Zone D of Figure 5, discussed below), for the June Lake core record, the overall R 2 is 0.32, which is consistent with an open hydrology, whereas times of higher correlation are interpreted as more closed to surface outflow. Further, the low correlation in modern June Lake sediments (R 2 = 0.18) supports open hydrology following the Talbot (Reference Talbot1990) and Talbot and Kelts (Reference Talbot, Kelts and Katz1990) model.
This lack of isotopic covariance may be due to the subsurface seepage out of June Lake, so we cannot reliably use the core record of isotopic covariation (Fig. 6A) as a proxy for changes in the surface connectivity between June and Gull lakes over the Late Holocene. Instead, we focus on interpreting hydroclimate changes over time using Ca/Ti and δ18Ocarb.
Reconstructing hydroclimate with carbonate core sediments
We interpret both the bulk sedimentary %TIC and high-resolution Ca/Ti data as a response to changes in P/E and lake level: When lake levels fall in response to decreasing precipitation or higher evaporation, calcite precipitates in response to the higher saturation state (e.g., Kelts and Hsü, Reference Kelts, Hsü and Lerman1978).
Variations in δ18O in sediment records are also often interpreted as changes in the balance between precipitation and evaporation (e.g., Horton et al., Reference Horton, Defliese, Tripati and Oze2015; Anderson et al., Reference Anderson, Berkelhammer, Barron, Steinman, Finney and Abbott2016). When evaporation is higher, the lighter 16O preferentially evaporates, leaving water with a higher ratio of 18/16O available during carbonate formation. We interpret such intervals in June Lake sediments as episodes of drought and resulting lake level low stands. In contrast, when effective moisture is higher, the P/E ratio is greater, lake levels are higher, and δ18Ocarb values are lower. The June Lake δ18Ocarb range is almost 10‰ over the period of record; this would represent at least a 30°C temperature change, which is highly unlikely, lending further support to the interpretation that δ18O values are tracking P/E.
By applying a LOESS filter to the carbonate isotope data set, six broad dry periods interpreted from the rising limb of the δ18O LOESS curve are apparent (Zones A–F in Fig. 5). In each of those dry periods, all other carbonate indicators are elevated (Fig. 5). Those six dry periods are described in greater detail below.
Whereas the bulk geochemical indicators (%TIC and δ18Ocarb) reveal broad intervals of drought, the Ca/Ti data set has the capacity to resolve more discrete drought intervals either within or in addition to these periods (Table 4). We designated Ca/Ti values greater than 56 (2-sigma deviation from the average value) as peaks in carbonate accumulation corresponding to drought. The exception to this is the most recent part of the record, in which Ca/Ti values approach, but do not exceed, 56. For example, we find two consecutive Ca/Ti peaks suggesting short-duration droughts around the end of Zone A (peaks 3 and 4, Fig. 5), as well as two droughts separated by about a hundred years at the end of Zone B. Further, the Ca/Ti data set makes it clear that the response of the carbonate system tends to be abrupt, with carbonate precipitation taking place over a matter of years to decades following drought onset. During the LHDP, Ca/Ti peaks suggest the potential for seven episodes of drought, each separated by at least 50 years, with an average of 110 years between droughts (Table 4, peaks 6–14 in Fig. 5). In the intervening years, Ca/Ti counts return to values observed in wetter parts of the record.
Regional paleoclimate as interpreted from the June Lake carbonate record
The four hydroclimate zones established in Lyon et al. (Reference Lyon, McGlue, Erhardt, Kim, Stone and Zimmerman2020) are described here to offer contrast with the findings of this study (Supplementary Fig. 4). The earliest of these zones (~4600–3600 cal yr BP) was a relatively wet interval, as demonstrated by high values for organic geochemical indicators, low %TIC, and several sand beds that suggested enhanced runoff. The second zone (~3600–1700 cal yr BP), which overlaps with the LHDP described by Mensing et al. (Reference Mensing, Sharpe, Tunno, Sada, Thomas, Starratt and Smith2013), is characterized by elevated carbonate precipitation and low algal production as indicated by organic carbon and biogenic silica. The third zone (~1700 cal yr BP to ~130 years ago) roughly corresponds with the relatively cold and wet Little Ice Age (LIA), though the early part also includes the MCA, which is characterized by two centennial-scale droughts interrupted by a pluvial interval. The youngest zone encompasses the last ~130 years (Current Warm Period) and is characterized by an abrupt decline in algal production, high %TIC, and a dramatic change in the diatom community that is consistent with a shift to enhanced stratification and warmer lake water (Streib et al., Reference Streib, Stone, Lyon, Quang, Yeager, Zimmerman and McGlue2021). In the following sections, we describe the dry intervals that are better elucidated by our new carbonate data sets.
The Late Holocene Dry Period (~3200–1800 cal yr BP)
The most striking examples of drought in this record are the three zones (B–D in Fig. 5) exhibiting pronounced peaks in Ca/Ti between ~3500 and 2000 cal yr BP (Fig. 5). These zones likely represent the LHDP first described in Mensing et al. (Reference Mensing, Sharpe, Tunno, Sada, Thomas, Starratt and Smith2013) from Stonehouse Meadow (eastern Nevada) pollen and geochemistry records. The LHDP is partially correlative to the Marina Lowstand in Mono Lake (Stine, Reference Stine1990; Zimmerman et al., Reference Zimmerman, Hemming, Starratt, Starratt and Rosen2021), and has been recognized in other areas of the Great Basin (e.g., Mensing et al., Reference Mensing, Benson, Kashgarian and Lund2004, Reference Mensing, Wang, Rhode, Kennett, Csank, Thomas and Briem2023; Theissen et al., Reference Theissen, Hickson, Brundrett, Horns and Lachniet2019), the transitional area between the Great Basin and Colorado Plateau (Anderson et al., Reference Anderson, Skipp, Strickland, Honke, Havens and VanSistine2023), and southern California (e.g., Kirby et al., Reference Kirby, Feakins, Hiner, Fantozzi, Zimmerman, Dingemans and Mensing2014, Reference Kirby, Patterson, Lachniet, Noblet, Anderson, Nichols and Avila2019), showing that the climatic drivers of the LHDP affected a broad region. A Great Basin drought beginning at ~3100 cal yr BP was identified in Millar et al. (Reference Millar, Charlet, Delany, King and Westfall2019) and was also identified as the initiation of the LHDP in Mensing et al. (Reference Mensing, Wang, Rhode, Kennett, Csank, Thomas and Briem2023). Our ~3200 cal yr BP droughts (peaks 6 and 7 in Fig. 5) are likely the same event, as the discrepancy is within the uncertainty of our radiocarbon age model (as well as those of the other sediment records). However, the June Lake record is more highly resolved than many of those other records (e.g., Mensing et al., Reference Mensing, Sharpe, Tunno, Sada, Thomas, Starratt and Smith2013; Theissen et al., Reference Theissen, Hickson, Brundrett, Horns and Lachniet2019), suggesting higher frequency drought variations within the dry “period” (Fig. 7).
Figure 7. Great Basin regional records of hydroclimate change. (A) June Lake Ca/Ti data from core 11 (1 mm resolution—this study). (B) Ratio of Lindavia:Stephanodiscus diatoms from June Lake (Streib et al., Reference Streib, Stone, Lyon, Quang, Yeager, Zimmerman and McGlue2021). (C) Mono Lake shoreline elevation (Stine, Reference Stine1990). (D) δ18O data for Pahranagat Lake from Thiessen et al. (Reference Theissen, Hickson, Brundrett, Horns and Lachniet2019). (E) %Carbonate from Stonehouse Meadow from Mensing et al. (Reference Mensing, Sharpe, Tunno, Sada, Thomas, Starratt and Smith2013). Horizontal bars defined using Mensing et al. (Reference Mensing, Wang, Rhode, Kennett, Csank, Thomas and Briem2023) (Late Holocene Dry Period [LHDP]), Stine (Reference Stine1994) (Medieval Climate Anomaly [MCA]), IPCC (2021) (Litte Ice Age [LIA]), and Streib et al. (Reference Streib, Stone, Lyon, Quang, Yeager, Zimmerman and McGlue2021) (modern drought).
Our record also shows the LHDP was likely not one continuous dry period, consistent with the observation of Mensing et al. (Reference Mensing, Wang, Rhode, Kennett, Csank, Thomas and Briem2023) that there were two droughts separated by a pluvial. Our record further resolves the LHDP into a series of discrete multidecadal drought episodes with decades to centuries separating each peak (peaks 5–14 in Fig. 5). Comparing the Ca/Ti data to the LOESS curve in Figure 5 shows this difference in resolution: Whereas the δ18O LOESS curve shows broad intervals of low lake level and dry paleoclimate, the Ca/Ti curve resolves several discrete droughts over the same time periods. Thus, our record is distinct from many other LHDP records (and hydroclimate records from the region more generally) in that we can resolve many discrete droughts within the broader dry period. Wetter intervals may be present in other basins but partially obscured by the proxy resolution, as in the Lyon et al. (Reference Lyon, McGlue, Erhardt, Kim, Stone and Zimmerman2020) records—for example, Lake Pahranagat records a wet interval within the LHDP centered on 2350 cal yr BP (Theissen et al., Reference Theissen, Hickson, Brundrett, Horns and Lachniet2019). The LHDP is characterized by drier than average conditions from ~3100 to 1800 cal yr BP (Millar et al., Reference Millar, Charlet, Delany, King and Westfall2019; Mensing et al., Reference Mensing, Wang, Rhode, Kennett, Csank, Thomas and Briem2023), but our record shows that there were also wet phases, in evidence from the high variability in carbonate abundance. It is possible that these wet phases were not sufficiently long enough to allow recovery of the ecosystems responding to the prolonged drought (e.g., Mensing et al., Reference Mensing, Sharpe, Tunno, Sada, Thomas, Starratt and Smith2013, Reference Mensing, Wang, Rhode, Kennett, Csank, Thomas and Briem2023; Millar et al., Reference Millar, Charlet, Delany, King and Westfall2019), but were sufficiently wet to raise lake levels and curtail carbonate accumulation. This variability among systems may also be due to the difference between our small alpine lake fed directly by the Sierra Nevada snowpack, and those lower elevation spring-fed meadows near smaller ranges much farther from the Pacific moisture source. The different sensitivities of geochemical and biological proxies to the immediate effects of high-frequency droughts versus the cumulative effects of long-term drought with pronounced ecological effects provides a more nuanced view of the impacts of climate on the landscape and deserves further study through a variety of proxies in multiple locations and basin types.
The Medieval Climate Anomaly (~1350–900 cal yr BP)
Two increases in carbonate accumulation in June Lake at 1220 and 890 cal yr BP (peaks 18 and 19 in Fig. 5) likely represent the dry conditions of the MCA, which appear in our records at ~1350–900 cal yr BP. First described in the region from tree ring records (Graumlich, Reference Graumlich1993) and the age of submerged tree stumps (Stine, Reference Stine1994), the MCA drought is interrupted by a pluvial interval, which is also captured in our record as a time of low carbonate accumulation and low carbonate isotope values. The drought phases of the MCA have been detected in lake records across the region (e.g., Stine, Reference Stine1990; Adams et al., Reference Adams, Negrini, Cook and Rajagopal2015; Zimmerman et al., Reference Zimmerman, Hemming, Starratt, Starratt and Rosen2021), though the intervening pluvial is not always well resolved (e.g., Benson et al., Reference Benson, Kashgarian, Rye, Lund, Paillet, Smoot, Kester, Mensing, Meko and Lindström2002). Owing to the combined uncertainties of our age–depth model (+/− 220 and 180 years for 1220 and 890 cal yr BP, respectively) and the radiocarbon dating of the tree stumps, the dates given here for the two droughts are offset somewhat from those first reported by Stine (Reference Stine1994). The MCA is known to be very dry regionally (e.g., Stine, Reference Stine1990; Cook et al., Reference Cook, Seager, Heim, Vose, Herweijer and Woodhouse2010; Kirby et al. Reference Kirby, Zimmerman, Patterson and Rivera2012), but as described below (see “Comparing Droughts Through Time”), %TIC and other drought indicators were less pronounced at this time than in some earlier droughts.
Recent warming and drying (~100 cal yr BP–present)
As established in Lyon et al. (Reference Lyon, McGlue, Erhardt, Kim, Stone and Zimmerman2020) and Streib et al. (Reference Streib, Stone, Lyon, Quang, Yeager, Zimmerman and McGlue2021), the last ~150 years at June Lake are characterized by environmental changes likely resulting from anthropogenic warming. Diatom assemblages record an increase in lake stratification (Fig. 7), likely due to shorter winters and warmer summers (Streib et al., Reference Streib, Stone, Lyon, Quang, Yeager, Zimmerman and McGlue2021). Further, the decline in primary productivity indicators noted in Lyon et al. (Reference Lyon, McGlue, Erhardt, Kim, Stone and Zimmerman2020) supports a warming/drying-induced stress to the algal communities and a potential shift favoring diazotrophic cyanobacteria in June Lake over some diatoms. We note an increase in both δ18Ocarb and δ13Ccarb values over this period, which we interpret as further evidence of enhanced stratification in response to warming, despite the lack of a prominent coeval peak in Ca/Ti. However, as discussed in Lyon et al. (Reference Lyon, McGlue, Erhardt, Kim, Stone and Zimmerman2020) and Streib et al. (Reference Streib, Stone, Lyon, Quang, Yeager, Zimmerman and McGlue2021), the limnology (including hydrologic closure) of the June Lake–Gull Lake system may have changed in the historical period due to anthropogenic warming, which could account for the decoupled geochemical signal. Drummond et al. (Reference Drummond, Patterson and Walker1995) noted that when lake waters warm and stratify for longer portions of the year, the dissolved inorganic carbon in the epilimnion becomes isotopically heavier, leading to higher δ13Ccarb values in authigenic carbonates. Elevated δ13C values in the recent record are also consistent with increased vapor exchange with atmospheric CO2 during dry periods (e.g., Li and Ku, Reference Li and Ku1997).
Comparing droughts through time: Differences in the MCA and LHDP
The MCA is often attributed to an increase in solar output and a decrease in low-latitude volcanic activity (e.g., Ammann et al., Reference Ammann, Joos, Schimel, Otto-Bliesner and Tomas2007; Mann et al., Reference Mann, Zhang, Rutherford, Bradley, Hughes, Shindell, Ammann, Faluvegi and Ni2009), though some models also point to changes in land use (Goosse et al., Reference Goosse, Arzel, Luterbacher, Mann, Renssen, Riedwyl, Timmermann, Xoplaki and Wanner2006). In contrast, the mechanisms responsible for the LHDP are less well understood. Mensing et al. (Reference Mensing, Sharpe, Tunno, Sada, Thomas, Starratt and Smith2013) pointed to the influence of the Pacific Decadal Oscillation and its manifestation as a north–south precipitation dipole in western North America with a boundary zone at about 40–42°N (see also Wise, Reference Wise2010, Reference Wise2016). This would produce similar effects to the El Niño Southern Oscillation in the region, albeit at multidecadal to centennial timescales. This interpretation aligns with our observations of centennial-scale shifts from droughts to pluvials in the June Lake record throughout the LHDP.
Although several Late Holocene studies point to the MCA as a time of extreme drought (e.g., Stine, Reference Stine1994; Cook et al., Reference Cook, Seager, Heim, Vose, Herweijer and Woodhouse2010), in some records (e.g., Stine, Reference Stine1990; Mensing et al., Reference Mensing, Sharpe, Tunno, Sada, Thomas, Starratt and Smith2013, Reference Mensing, Wang, Rhode, Kennett, Csank, Thomas and Briem2023; Zimmerman et al., Reference Zimmerman, Hemming, Starratt, Starratt and Rosen2021; this study), the MCA appears to be a more modest dry interval when compared to the LHDP. At June Lake, both the TIC and Ca/Ti curves show that carbonate accumulation was lower during the MCA than during the LHDP (Fig. 5). However, the LHDP droughts are characterized by only marginally greater δ18Ocarb and δ13Ccarb values than the MCA. This poses the question: Is there a limit to 18O enrichment in June Lake carbonates? A mechanism for limiting isotopic enrichment may be the seepage of ground water from June Lake to Gull Lake. This subsurface connection is suggested by the intermediate water isotope values for Gull Lake (Fig. 3). If June Lake were closed in the subsurface, we might anticipate higher δ18Ocarb during droughts. Indeed, Steinman and Abbott (Reference Steinman and Abbott2013) find that closed lake seepage acts to control longer-term hydrologically forced isotopic response: When outflow seepage is lower, lake isotopic sensitivity to hydrologic forcing declines.
Thus, although carbonate precipitation was much greater during the LHDP than the MCA, the carbonate isotope record does not exhibit a proportional response to drought conditions. Although they are positively correlated, a linear relationship between %TIC and δ18Ocarb is not obvious (R 2 = 0.36; P value < 0.00001 at 0.05 significance threshold). Instead, a logarithmic function would seem to better describe the relationship between these two indicators (Fig. 6C; R 2 = 0.65; P value < 0.00001): At %TIC values < ~1.0%, a range of low δ18Ocarb values are possible. However, when carbonate precipitation is greater (%TIC > 1.0%), δ18O values are higher and less variable, reaching a maximum of −4.4‰. We suggest that this is due to the aforementioned closed lake seepage model (Steinman and Abbott, Reference Steinman and Abbott2013).
Although calcium limitation in some lakes may produce peaks in carbonate accumulation discordant with the expected model (e.g., Shapley et al., Reference Shapley, Ito and Donovan2005; Zimmerman et al., Reference Zimmerman, Hemming, Hemming, Tomascak and Pearl2011), June Lake has relatively high dissolved Ca2+ (~24.2 mg/L in 2016; Lyon et al., Reference Lyon, McGlue, Woolery, Kim, Stone and Zimmerman2019) and is supersaturated with respect to calcium carbonate. We also note lower pH at depth, at least during summer stratification (pH = 7.49 at 23.4 m near the coring site versus 8.7 at the surface in May 2016; Lyon et al., Reference Lyon, McGlue, Woolery, Kim, Stone and Zimmerman2019). This may lead to some dissolution of carbonate at depth (e.g., Dean, Reference Dean1999), particularly during warm/dry intervals when seasonal stratification is intense, as at present and during the MCA (Streib et al., Reference Streib, Stone, Lyon, Quang, Yeager, Zimmerman and McGlue2021). This may explain some of the decoupling between the δ18O and TIC data sets. However, additional hydrologic characterization of the system would be needed to fully test these ideas, which is beyond the scope of this study.
The LIA and other pluvials
We interpret times of low carbonate accumulation as the wettest periods, and thus as the most likely times that June Lake overflowed and connected with Gull Lake along surface flow paths (Fig. 1). One notable example is the LIA (~500–100 cal yr BP; IPCC, 2021), a known interval of glacial re-advance and higher lake levels in the region (e.g., Stine, Reference Stine1990; Kirby et al., Reference Kirby, Lund, Patterson, Anderson, Bird, Ivanovici, Monarrez and Nielsen2010, Reference Kirby, Zimmerman, Patterson and Rivera2012; Bowerman and Clark, Reference Bowerman and Clark2011; Bacon et al, Reference Bacon, Lancaster, Stine, Rhodes and Holder2018; Thiessen et al., Reference Theissen, Hickson, Brundrett, Horns and Lachniet2019). Further, the Lindavia:Stephanodiscus ratio for June Lake is low during the LIA, indicating a well-mixed lake (Fig. 7; Streib et al., Reference Streib, Stone, Lyon, Quang, Yeager, Zimmerman and McGlue2021), consistent with cool conditions that would inhibit stratification and evaporative concentration of lake water. Low carbonate accumulation also occurred between ~1800 and 1400 cal yr BP, and at ~4300 cal yr BP in the June Lake sediment archive. Thus, although this carbonate accumulation record allows us to recognize discrete droughts over the last 4600 years, it also illuminates times of high effective moisture (Fig. 5).
Conclusions
The sedimentary record of carbonate accumulation presented here shows changes in P/E and lake level over the Late Holocene for June Lake, which lies at the eastern edge of the Sierra Nevada. When %TIC and Ca/Ti are high, so are values of δ13Ccarb and δ18Ocarb, supporting the interpretation that carbonate accumulation is higher during times of drought. A lack of correlation between δ13Ccarb and δ18Ocarb indicates that June Lake is hydrologically open, likely due to groundwater seepage in the subsurface. Although surficially closed at present, June Lake may have been open at different points in the Late Holocene, including most of the LIA (~800–300 cal yr BP), from ~1800 to 1400 cal yr BP, and around 4300 cal yr BP, based on very low carbonate content in the sediments. Despite its closure at the surface, June Lake likely experienced subsurface outflow over most of the last 4600 years, as suggested by carbonate isotope values that do not increase linearly with carbonate precipitation. This idea has implications for paleoclimatic reconstructions that is ripe for exploration in future research.
Using the high-resolution Ca/Ti record as an archive of abrupt changes in hydroclimate over the last 4600 years, we find at least six broad intervals of centennial-scale drought, many of which can be resolved into a series of decadal to centennial-scale droughts (Table 4, Fig. 5). The most prominent episodes of carbonate accumulation and drought occur during the LHDP, during which there are at least seven droughts separated by several decades to ~250 years. We also find evidence of drought as early as ~3200 cal yr BP in June Lake, which is likely the earliest phase of the LHDP (Fig. 7). The LHDP has heretofore been described mainly for the eastern and central Great Basin, though it is also described in some southern California records (e.g., Kirby et al., Reference Kirby, Feakins, Hiner, Fantozzi, Zimmerman, Dingemans and Mensing2014); this study adds new evidence for the timing and magnitude of the LHDP droughts in the eastern Sierra Nevada, and adds additional information to the LHDP records from the Mono Lake basin (Stine, Reference Stine1990; Zimmerman et al. Reference Zimmerman, Hemming, Starratt, Starratt and Rosen2021).
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/qua.2024.38
Acknowledgments
A portion of this work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344; this is LLNL-JRNL-844307. Research at June Lake was permitted through the Inyo National Forest (USFS). Fieldwork was supported in part by the Overcash Fund for Field Research and UK alumni donors to the UK Field Geology Fund for fieldwork support. We also thank John and Mickey Frederickson at the June Lake Marina for generously allowing us to use their equipment and facilities. We are grateful to the staff of the Continental Scientific Drilling Facility for their help with core preparation, sampling, and curation. Thorough comments by several anonymous reviewers, as well as QR editors L. Anderson and N. Lancaster, improved the quality of the manuscript.
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