Evidence for a Direct Link Between Solar Variability and Holocene Climate in the Southwest USA

We obtained high-resolution (<20 yr sample interval) O and C isotopic data for the past 12,000 years from a well-dated speleothem with continuous growth from the Guadalupe Mountains, New Mexico. The δ ¹⁸O and δ ¹³C data show significant variability (-5.81 to -2.66 and 0.35 to -5.26, respectively). At short time scales variations in δ ¹⁸O and δ ¹³C are incoherent, while large shifts in both isotopic values are fairly coherent over the period of interest. It is unlikely that changes in δ ¹⁸O and δ ¹³C values are driven by kinetic fractionation. Large shifts in δ ¹⁸O and δ ¹³C values coincide with changes in other climate proxies such as tree ring data (although these data cover a short period) and speleothem gray scale data. Summer moisture is primarily derived from the Gulf of Mexico, while isotopically lighter Pacific moisture dominates winter precipitation. Stable isotope variations in our speleothem likely reflect variability in the amount of precipitation and/or balance in moisture source. The most salient feature of our stable isotopic data, and especially the δ ¹⁸O data, is that shifts in isotopic values at centennial time-scales strongly coincide with shifts in the Δ ¹⁴C values. Variations in Δ ¹⁴C values are thought to reflect changes in solar activity and intensity. Residuals of the detrended δ ¹⁸O and Δ ¹⁴C values are negatively correlated. The excellent visual match is supported by preliminary results from cross correlation and spectral analyses. The strong coherence between the stable isotope data and the Δ ¹⁴C data suggests solar forcing may play a role in continental climate variability at centennial to millennial time scales. The forcing may not be necessarily direct however, given the inferred magnitudes in solar variability over the past 12,000 years are not large.