In #paleoclimate time series amplitude of spectral peaks usually varies with time. Evolutionary power spectral analysis such as the FFT-based spectrogram and wavelet power spectral analysis helps. These methods, however, require #interpolation of the time series to a grid of evenly-spaced times. Instead we can use the Lomb-Scargle Method for unevenly-spaced spectral analysis, computed for a sliding window, to map changes of the #cyclicities through time.

http://mres.uni-potsdam.de/index.php/2020/06/26/evolutionary-lomb-scargle-power-spectral-analysis-with-matlab/