Northern Hemisphere moisture variability during the Last Glacial period

It was previously shown that large oxygen isotope variability related to changing moisture sources in the southwestern United States (SW) match the Greenland ice core temperature record. The variations were attributed to changes in the ratio of winter to summer precipitation delivered to the SW, with lighter winter δ¹⁸O values compared to summer monsoon rainfall, due to meridonial shifts in the position of the polar jet stream, which directs winter storm tracks. Cold stadial δ¹⁸O excursions are associated with strongly negative values, while interstadials have higher than average δ¹⁸O values. Although these data documented moisture source variability to the SW, the question of effective moisture variability remains unanswered. Here we present new high-resolution δ¹⁸O and δ¹³C isotopic data from a precisely dated speleothem, FS-AH1, from Fort Stanton Cave, New Mexico USA. The sample grew continuously between 47.6 and 11.1 kyr. The new chronology is more precise than previous work due to high sample growth rate, new gains in efficiency provided by our upgraded Neptune MC-ICPMS and new more precise determinations of the half-lives of ²³⁰Th and ²³⁴U. The FS-AH1 δ¹⁸O and the Greenland δ¹⁸O data (on the GICC05 time scale) show a remarkable match, both with respect to stadials/interstadial amplitudes and variability, and in the overall long-term trend. Our interpretation of the δ¹⁸O data remains the same, an indicator of moisture source variability. The δ¹⁸O and δ¹³C isotopic data show no correlation (R² <0.0001) because the δ¹⁸O primarily reflects differences in moisture sources and temperature (at least during large-scale excursions), while δ¹³C variability reflects the amount of effective moisture in the soil zone overlying the cave, with low δ¹³C attributed to high soil productivity, high effective moisture, and wet conditions. The stadial and interstadial events are expressed mutely, if at all, in the δ¹³C data, while the secular variation follows the change in Northern Hemisphere summer insolation (insolation), similar to other Northern Hemisphere data, such as the strength of the East Asian summer monsoon as recorded in the Hulu speleothem, although the match to the East Asian monsoon is inverse. The much diminished expression of stadials and interstadials and secular variations in the effective moisture proxy data that match insolation seem to be hemispherical in scale. In humid settings, such as east Asia monsoon regions, warm temperatures lead to northward shift of the ITCZ and increase in the strength of the Asian monsoon, while in the desert SW any increase in the strength in the North American monsoon is counterbalanced by decrease in winter moisture due to the northward shift of the polar jet stream and more importantly, the onset of more evaporative conditions. In contrast to the large and rapid shifts seen in the Greenland ice core data and the apparent shift in position in air masses, as indicated by our δ¹⁸O data, large-scale changes in moisture regimes in the Northern Hemisphere seem to be driven by changes in insolation. Locations that are sensitive to small changes in atmospheric pressure and/or sea surface temperature gradients may be the exception.