Hydroclimate response of spring ecosystems to a two-stage Younger Dryas event in western North America

The Younger Dryas (YD) climate event is the preeminent example of abrupt climate change in the recent geologic past. Climate conditions during the YD were spatially complex, and high-resolution sediment cores in the North Atlantic, western Europe, and East Asia have revealed it unfolded in two distinct stages, including an initial stable climatic period between ~ 12.9 and 12.2 ka associated with a weakened Atlantic Meridional Overturning Circulation (AMOC) and a second phase characterized by variable conditions until 11.7 ka as the AMOC recovered. Decades of investigations into the climate of western North America during the YD have failed to identify this stepped phenomenon. Here we present hydroclimate data from paleospring deposits in Death Valley National Park (California, USA) that demonstrate unequivocal evidence of two-stage partitioning within the YD event. High groundwater levels supported persistent and long-lived spring ecosystems between ~ 13.0 and 12.2 ka, which were immediately replaced by alternating wet and dry environments until ~ 11.8 ka. These results establish the mid-YD climate transition extended into western North America at approximately the same time it was recorded by hydrologic systems elsewhere in the Northern Hemisphere and show that even short-lived changes in the AMOC can have profound consequences for ecosystems worldwide.

Bonsai Lake, Alaska Summary: Wilcox et al. (2020) used radiocarbon dating and measured pollen concentrations, organic content, C/N, δ 13 C, grain size, and magnetic susceptibility in sediments from a core taken at Bonsai Lake in southeastern Alaska to elucidate the nature and expression of the Younger Dryas (YD) climate event in the Pacific Northwest. They found the YD event consisted of at least two, and possibly three, distinct stages. The first stage was characterized by cooler and drier than modern conditions that prevailed between ~12.9 and 12.6 ka based on decreases in the percentages of pine and mountain hemlock pollen that were accompanied by increases in alder pollen and fern spores. The beginning of the second stage was marked by an increase in the percentages of pine and Sitka spruce which they interpreted as reflecting slightly warmer temperatures during the latter portion of the YD, between ~12.6 and 12.2 ka. Continued increasing temperatures and/or precipitation is indicated by a further increase in Sitka spruce and a concomitant decrease in pine between 12.2 and 11.7 ka.
Assessment: The pollen data from Bonsai Lake exhibit possible evidence of two stages within the YD climate event in southeastern Alaska with a transition at ~12.6, although we note that none of the other proxies show a marked change at this depth interval (their Figure 5). Although the data from Bonsai Lake data suggest a possible two-staged (or even three-staged) YD event, the lack of consistency between the pollen and all of the other proxies makes it difficult to determine the significance and magnitude of the inferred hydroclimate changes.

Discovery Pond, Alaska
Summary: Kaufman et al. (2010) examined sediments from multiple cores taken from a small pond in southern Alaska that span the YD climate event. They used radiocarbon dating and measured concentrations of several climate proxies, including magnetic susceptibility, organic matter content, biogenic silica, pollen, and some microfossils, and found that temperatures in the region increased throughout the YD, reaching a maximum sometime around 11 ka. The data also exhibit a pronounced increase in the abundance of green algae at ~12.2 ka, which they attributed to a shift from wetland to open-water conditions driven by an increase in temperature and effective moisture during the second half of the YD.

Assessment:
The proxy data from the Discovery Pond sediment cores clearly show an increase in green algae within their unit 2b, which they have bracketed in time between ~12.8 and 11.0 ka (their Figure 4). The authors interpret this increase to reflect warmer/wetter conditions beginning at ~12.2 ka, the timing of which was determined based on extrapolation of ages above and below unit 2b and assuming a constant sedimentation rate despite the climatic transition into and out of the YD climate event. Notably, none of the other proxies show a marked change at this depth interval. Although the data from Discovery Pond suggest the possibility of a two-staged YD event in this part of southern Alaska, the timing of the transition between stages is tenuous and the lack of consistency between the different proxies makes it difficult to determine the significance and magnitude of the inferred hydroclimate changes.

Mount Waskey, Alaska
Summary: Young et al. (2019) used cosmogenic 10 Be to determine surface exposure ages of glacial moraine boulders (n=15), inboard erratic boulders (n=3), and an upvalley erratic boulder to determine the age of a late Pleistocene glacial advance near Mount Waskey in the Ahklun Mountains of southern Alaska. The ages indicate the glacier reached its maximum extent at 12.52±0.24 ka, early within the YD stadial, before retreating ~1 km during the remainder of the YD. The ages also showed the glacier remained at this upvalley position until 11.66±0.23 ka.
Assessment: The cosmogenic 10 Be ages show the glacier near Mount Waskey advanced during the early part of the YD, which was followed by glacial retreat through the remainder of the climate event.
Although cosmogenic ages can be systematically offset from the true age because of issues related to production rates, exhumation, and episodic snow cover, the authors were careful in selecting boulders when sampling so they could avoid these issues as much as possible. Overall, the data presented by Young et al. indicate the two-stage character of the YD climate event extended into southern Alaska, although the exact timing of the transition between the cool/wet conditions required for glacial advancement and the warmer/drier conditions that led to glacial retreat can only be constrained to between ~12.5 and 11.7 ka.

Dove Springs Wash, Little Dixie Wash, and Mesquite Springs, California
Summary: Pigati et al. (2019) evaluated paleospring deposits exposed in outcrop at three sites in the Mojave Desert (Dove Springs Wash, Little Dixie Wash, and Mesquite Springs) as part of an effort to determine how fast spring ecosystems responded to past episodes of abrupt climate change. They used radiocarbon dating, grain size, redox properties of the sediments, organic content, and microfauna (terrestrial gastropods) to show that these ecosystems responded to climate change faster than they could delineate with 14 C dating.
Assessment: Hydrographs showing relative groundwater levels at each of the three sites (their Figure 5) clearly show conditions fluctuated between periods of high water table levels and active groundwater discharge represented by organic-rich black mats and times of low water table levels represented by aeolian, alluvial, or colluvial sediments that were deposited under dry conditions. Although the data demonstrate that unstable climate conditions prevailed during the YD event, the records do not contain evidence of two distinct stages within the event itself.

Fallen Leaf Lake, California
Summary: Ball et al. (2018) examined core sediments dated by radiocarbon from Fallen Leaf Lake, California to reconstruct the response of the region's aquatic and terrestrial ecosystems to climatic changes that accompanied the Younger Dryas, the end of the Pleistocene, and early Holocene warming. Lignin phenols indicate expansion of angiosperms at the Pleistocene-Holocene transition, whereas pollen shows closed canopy forests became more open and grasses and aster colonized meadows at this time. Within the YD event, a pulse of woody gymnosperm lignin was recorded at 13.0-12.6 ka, followed by an abrupt transition to a nonwoody lignin source at 12.3 ka. After 12.3 ka, lignins gradually decreased until they stabilized at ~10.9 ka.
Assessment: The proxy data from the Fallen Leaf Lake sediment cores show some ecosystem changes occurred within the YD chronozone, but the details are uncertain because of the coarse sampling resolution (4 samples that span the entire YD climate event). Although there appears to be at least some evidence for intra-YD climate variability in this part of the Sierra Nevada, unequivocal evidence of a twostage YD event is not present in this record.

Lake Barrett and Starkweather Lake, California
Summary: MacDonald et al. (2008) evaluated lacustrine sediments from cores taken at Lake Barrett and Starkweather Lake in the east-central Sierra Nevada, California to examine hydroclimate conditions in the region during the YD climate event. They used radiocarbon dating and measured a number of different hydroclimate proxies, including loss-on-ignition (LOI), chironomid temperature reconstructions, bulk organic δ 13 C, cellulose-inferred lake water δ 18 O, diatom salinity and depth reconstructions, planktonic diatom percentages, and pollen percentages, constrained cluster analysis (CONISS) pollen zonation based upon terrestrial pollen, and charcoal. The authors concluded there was an initial increase in effective moisture during the early part of the YD that was followed by drier conditions later in the YD, but did not determine the precise timing of the transition.

Assessment:
The proxy data presented (their Figure 4) clearly show variability throughout the YD climate event and there is some evidence of a wet to dry transition during the YD, particularly in the diatominferred lake data from Lake Barrett. However, the significance and timing of the transition is difficult to decipher because many of the proxies do not show the same wet-to-dry transition as the diatoms and only one of the seven calibrated 14 C ages actually falls within the YD chronozone (their Table 1). Although the data suggest a possible two-staged YD event in this part of the Sierra Nevada, unequivocal evidence of such an event is not present in this record.

Moaning Cave, California
Summary: Oster et al. (2009) used uranium-series dating and measured stable isotopes (δ 18 O, δ 13 C), trace elements, and strontium isotopes in a speleothem from Moaning Cave, California that spans much of the late glacial period. Following an initial decrease in δ 18 O and δ 13 C and trace element values at 12.4 ka, which they correlate to the beginning of the YD, the isotopic and trace element values increase slightly at ~12.2 ka. 87 Sr/ 86 Sr values also increase slightly at this time. The authors interpret these changes as suggesting "a decrease in the contribution of soil-respired CO2 to seepage waters and an increase in prior calcite precipitation coupled with increased water-host-rock interactions, consistent with a slight mid-YD increase in aridity and possibly temperature in this region." An additional shift in isotopic values occurred at ~12.1 ka, which was attributed to movement of the stalagmite drip center.
Assessment: Although the Moaning Cave proxy data exhibit variability within the YD (their Figure 3), the significance and timing of the mid-YD variations is not well defined. Specifically, it is unclear if (1) the change in values observed at ~12.4 truly reflect the beginning of the YD (this is ~500 years after the beginning of the YD event as defined in the Greenland ice cores), (2) the changes at ~12.2 ka are the result of a change in local hydroclimate conditions (the authors do not elaborate on how a slight increase in aridity would affect these proxy systems), and (3) the changes at ~12.1 ka reflect movement of the stalagmite drip center (and not a change in climate) as hypothesized. Moreover, uncertainties in the age dating of the speleothem within the YD timeframe are on the order of ±450 years, which precludes robust comparison with other proxy records. Although the Moaning Cave data indicate unstable conditions likely prevailed in this part of the Sierra Nevada during the YD climate event, they do not provide unequivocal evidence of two distinct stages within the event itself.

Swamp Lake, California
Summary: Street et al. (2012) investigated sediment cores taken at Swamp Lake in the central Sierra Nevada, California to reconstruct climatic changes on millennial and centennial timescales over the past ~20,000 years. They used radiocarbon dating and measured total organic carbon (TOC), total nitrogen, C/N, δ 13 Corg, δ 15 N, biogenic silica, magnetic susceptibility, and sediment lithology at high resolution, and found that climate fluctuated between relatively warm/dry intervals with high TOC and cold/wet intervals characterized by low TOC and high detrital input. Their unit SL-7 dates to 13.1-11.7 ka, which correlates temporally with the YD climate event, and consists of several distinct sedimentary units, including a major sand/gravel layer (13.1-12.8 ka), low TOC laminated and massive gyttja (12.8-12.5 ka), high TOC peat (12.5-12.2 ka), and interbedded clay/gyttja/sand (12.1-11.6 ka), suggesting a complex structure for the YD interval at this site. The authors interpret the lacustrine sediments and chemical/biological proxies to indicate depressed lake productivity and cold, wet conditions during the early part of the YD, followed by partial warming and drying after ~12.5 ka.
Assessment: The sediments and proxy data from the Swamp Lake cores (their Figure 6) clearly show tremendous variability within the YD climate event. The peat layer that dates to ~12.5-12.2 ka is particularly interesting as it indicates a wetland, rather than a lake, prevailed at this time, likely as a result of warmer/drier conditions compared to those prior to 12.5 ka. However, the presence of clay/gyttja/sand above the peat layer suggests a return to cold, wet conditions at the end of the YD, analogous to the interpretations of cold, wet conditions based on the sand and gyttja layers just below the peat. Although the data clearly show unstable climate conditions prevailed during the YD event, unequivocal evidence of two distinct stages within the event itself is not present in this record.

Chihuahuaños Bog, New Mexico
Summary: Cisneros-Dozal et al. (2010) reconstructed hydroclimate conditions over the past 15,000 years based on sediments from a core taken from Chihuahuaños Bog in northern New Mexico. Although evidence of the YD climate event as a whole is equivocal in the bog's pollen record (Anderson et al., 2008), the δ 13 C, δ 15 N, C/N, organic carbon profiles and radiocarbon ages clearly demarcate the YD, which was characterized by low terrestrial productivity typical of cold/wet conditions. In addition, high concentrations of algae during the second half of the YD was interpreted as signifying an increase in aquatic productivity.

Assessment:
The proxy data from Chihuahuaños Bog (their Figure 3) exhibit variability during the YD climate event, but only the concentrations of algae increased in the latter part of the YD. Moreover, it is unclear exactly when this increase took place as none of the seven 14 C ages reported fall within the YD chronozone. Although there appears to be at least some intra-YD climate variability in this part of New Mexico, unequivocal evidence of a two-stage YD event is not present in this record.