Semiquantitative Estimates of Rainfall Variability During the 8.2 kyr Event in California Using Speleothem Calcium Isotope Ratios.

A multiproxy record from a fast‐growing stalagmite reveals variable hydroclimate on the California coast across the 8.2 kyr event and a precursor event likely caused by initial drainage of proglacial Lake Agassiz. Using speleothem δ44Ca, we develop the first semiquantitative estimates of paleorainfall variability for California through calibration with measurements of the modern climate and cave environment. We find that the magnitude of rainfall variability during the 8.2 kyr event approached the multiyear variability observable in the recent past (1950–2019) and the magnitude of variability during the precursor event likely exceeded this range. Additionally, we observe other instances of multidecadal variability comparable in magnitude to the precursor event during the record. Our work suggests that speleothem calcium isotope ratios are a powerful semiquantitative means to reconstruct paleorainfall, although numerous factors must be assessed in each cave system before applying this approach. Plain Language Summary: Modeling of future climate suggests that California may experience increased frequency of both extremely wet and extremely dry periods in the 21st century, leading to the emergence of "climate whiplash" phenomena which would significantly stress the state's water‐sensitive infrastructure. Understanding hydroclimate changes in California's past can help planners prepare for extremes that may be more severe than those of the historical record. However, existing paleoclimate records are often limited to qualitative interpretations of hydroclimate change, restricting their usefulness. We present new calcium isotope measurements from a California stalagmite that grew from 6,900 to 8,600 years ago, revealing variability in rainfall amounts on the California coast during and surrounding the 8.2 kyr event, an abrupt cold snap noted in other global paleoclimate records 8,200 years ago. We generate semiquantitative estimates of annual rainfall rates during the 8.2 kyr event period and compare them with modern annual rainfall amounts, finding that the magnitude of rainfall variability during and surrounding the 8.2 kyr event approaches and in some cases exceeds that of California today. This work indicates that California may have experienced even more intense "climate whiplash" phenomena in the past than during recent decades, suggesting that future planning may need to consider greater wet and dry extremes. Key Points: The first semiquantitative estimates of paleorainfall rates from Ca isotope ratios measured in a California stalagmiteThe magnitude of rainfall variability in coastal California during the 8.2 kyr and precursor event approaches/exceeds that of recent decadesStalagmite Ca isotope ratios facilitate more direct comparison of paleorainfall with modern climate data, but important questions remain [ABSTRACT FROM AUTHOR]