Multi-proxy stalagmite records from northern California reveal dynamic patterns of regional hydroclimate over the last glacial cycle
Introduction
Proxy evidence supports a dynamic hydroclimate across western North America during the last glacial period and deglaciation. Regional proxy compilations illustrate the spatial pattern of hydroclimatic variability in response to the climate of the Last Glacial Maximum (LGM) ∼21,000 yrs BP (Thompson et al., 1993; Bartlein et al., 2011; Oster et al., 2015a; Scheff et al., 2017). Comparisons of terrestrial proxy time series reveal hydroclimatic shifts at millennial and longer time scales, providing evidence for coherent but potentially complex changes during the last deglaciation (Oster and Kelley, 2016; Feakins et al., 2019) and earlier during Marine Isotope Stages 3–5 in response to Dansgaard-Oeschger cycles (Benson et al., 2013; Glover et al., 2017). Comparisons of the timing of pluvial lake highstands and other proxy records have revealed complex regional patterns arising from forcings such as the growth and decay of the Laurentide Ice Sheet, variations in Atlantic Meridional Overturning Circulation, interhemisphere temperature gradients, and the position of the Intertropical Convergence Zone that influence the amount and sources of moisture reaching western North America (Oster et al., 2015a; Morrill et al., 2018; Asmerom et al., 2017; McGee et al., 2018; Hudson et al., 2019). The relative importance of these drivers on different time scales remains a source of active investigation.
Presently, winter precipitation in western North America has been shown to exhibit a north/south, wet/dry dipole pattern on interannual to decadal timescales partly in response to ocean-atmosphere interactions across the Pacific and Atlantic Oceans including the El Niño/Southern Oscillation, the Pacific Decadal Oscillation, and the Atlantic Multidecadal Oscillation (Redmond and Koch, 1991; Wise, 2010). This dipole pattern is associated with both the distribution of total rainfall across the region as well as extreme precipitation events (Dettinger et al., 1998; Jiang et al., 2013). Analysis of tree ring records from across western North America suggests that the transition between wet and dry dipole regions has remained consistently centered around 40°N for the past 500 years (Wise, 2016). A geographically similar dipole pattern is evident in regional hydroclimate variations during the LGM in response to the influence of the Laurentide Ice Sheet on atmospheric circulation (Oster et al., 2015a; Lora et al., 2017). At the LGM, climate models and proxy compilations indicate a transition between wet and dry conditions along the west coast occurred near the California-Oregon border (42 °N) (Oster et al., 2015a). There is further evidence that this dipole pattern persisted during millennial-scale climate oscillations of the last deglaciation (Hudson et al., 2019). However, there are comparatively few paleoclimate records that exist close to the location of this dipole transition zone that possess the potential to track changes in the location and behavior of the dipole at different temporal scales.
Here we present a new, U-series dated, multi-proxy speleothem paleoclimate record from Lake Shasta Caverns (LSC) in northern California (40.804°N, 122.304°W). Given its location, LSC is well-situated to track variations in regional patterns of precipitation change and reveal spatial shifts in the dipole transition zone across the last glacial period and deglaciation (Fig. 1A). We present records of oxygen and carbon isotope and trace element variability in two partly overlapping stalagmites that grew collectively between ∼37,000 and 14,000 yrs BP. These combined proxies allow us to explore how changes in moisture source and amount may have influenced the overall hydroclimate at this location and how this variability may have contributed to regional patterns of climate variability during the last glacial period and in response to Dansgaard-Oeschger cycles and Heinrich Events. Our results demonstrate temporal variability in the dipole pattern and highlight the need for multi-proxy records that can parse changes in moisture source versus amount to examine the relative importance of different hydroclimatic drivers for this hydrologically sensitive region.
Section snippets
Site and description
Lake Shasta Caverns (LSC) is a commercial cave located approximately 32 km north of Redding, California, on the east side of the McCloud arm of Shasta Lake (40.804°N, 122.304°W, 725 m a.s.l.) (Fig. 1A; B). The cave is developed within the McCloud limestone, which crops out in a north-south belt in the Shasta Lake area (Fig. 1B). The McCloud Limestone is Early Permian in age and ranges in thickness from less than 30 m to about 800 m. It consists mainly of calcarenite composed of varying amounts
Water collection and analysis
Through a collaboration with the Lake Shasta Caverns staff, we collected drip water samples at seven locations throughout the cave (Fig. 1C). Three sites have been monitored beginning in December 2015, with water collection at the four other sites beginning at later dates. The drip sites chosen are the most consistently wet locations within the cave. Collections occurred regularly during the cool wet season when drip sites within the cave were more active. Water collections during the summer
Water isotopes
Our analysis of δ18O and δ2H in rain and drip waters demonstrates that all waters fall near the Global Meteoric Water Line, with similar calculated local meteoric water lines (Fig. 2A; Table S1). Samples of rainwater collected at NADP site CA76 show a wider range of variability than both the limited measurements of rainwater from Lake Shasta Caverns and the LSC drip water. This discrepancy may reflect a few factors. Rainwater samples from site CA76 are available for a longer period of time and
δ18O and δ13C
Analysis of drip water δ18O and δ2H at LSC demonstrates that drip water isotope values fall along a local meteoric water line (Fig. 2A), suggesting minimal modification of drip waters by evaporation or other epikarst processes. Drip water δ18O analyzed at multiple sites within the cave also shows similar variability. Furthermore, for the period of time over which LSC drip water and measurements of CA76 rainfall data overlap, they show similar isotopic variability, with δ18O values decreasing
Conclusions
In sum, records from LSC stalagmites provide novel insights into the glacial/deglacial hydroclimate of western North American as a result of their location near a long-lived hydroclimatic transition and the information they provide on moisture sources and precipitation amounts through several key paleoclimatic events from ∼35,000–14,000 years BP.
The LSC record largely supports the presence of a north/south dipole in hydroclimatic response to forcings on different timescales, indicating that
Credit author statement
Jessica Oster: Conceptualization, Investigation, Formal Analysis, Funding acquisition, Data Curation, Writing-Original draft Isabelle Weisman: Investigation, Formal Analysis, Writing-Original draft Warren Sharp: Investigation, Resources, Validation, Writing-Review and editing.
Data availability
All speleothem data are archived with the NOAA Paleoclimatology Database (https://www.ncdc.noaa.gov/paleo/study/30254).
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
JLO acknowledges support from the National Science Foundation (AGS-1554998) and the National Geographic Foundation (39815). IEW acknowledges support from the Geological Society of America. We thank David Mundt and the Lake Shasta Caverns staff for logistical help during visits to the cave and for collecting water samples, Dr. Ralf Bennartz (Vanderbilt University) for assistance with HYSPLIT analyses, Dr. Peter Blisniuk (Stanford University) and Dr. Hari Mix (Santa Clara University) for
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