Figure 4: Spectral analysis of speleothem and lake sediment δ¹⁸O records.

(a,b) Power and coherency spectra for (a) OCNM (red) and BCC (blue) Speleothem δ¹⁸O records and (b) Lake Jellybean (red) and Lake Grinnell (blue) sediment δ¹⁸O records. The cross-spectral analysis was performed using computer package SPECTRUM⁴⁸ (see Methods). Periods of the main spectral peaks are labelled. The horizontal dashed lines indicate significance at the 90 and 95% level in the coherence spectra. Grey bars denote significantly strong coherence between paired proxy δ¹⁸O records. (c) Paired speleothem δ¹⁸O time series after 195±5 year band-pass filtering. (d) Paired lake sediment δ¹⁸O time series after 219±5 year band-pass filtering. Insets in c and d are the comparisons of BCC speleothem and Lake Grinnell δ¹⁸O records with independently reconstructed PNA index values¹² (pink line) filtered in the 200±5 year periodicity domain. Note that the δ¹⁸O records are inverted to express their anticorrelation with the PNA index. All filtering used a third-order Butterworth band-pass filter.