POWER SPECTRA ANALYSIS OF ANNUALLY LAMINATED STALAGMITES FROM PACIFIC ISLAND CAVES

Power spectral analysis of annually laminated stalagmites offers an important tool in the search for controlling mechanisms of regional and global climate. Stalagmite growth bands in tropical caves typically form as a function of precipitation such as during periods of low rainfall the annual couplets are relatively thin whereas during periods of high rainfall the growth bands thicken. Therefore application of power spectral analysis to these growth band measurements may reveal rainfall periodicities and controlling factors. Actively growing stalagmites from two adjacent caves on Niue Island have been studied and their lamina thickness was measured accurately from thin sections. One stalagmite spans a record of 2,691± 13 yrs based on U/Th dating. The other actively growing stalagmite was not radiometrically dated but annual couplets counting suggests a 200 yrs duration of growth. Power spectra density estimates via Welch’s method were used to unravel the periodicities in the data. The cycles that were revealed from these stalagmites are similar to the periodicities occurring in modern climatic indices. Power spectral analysis reveals statistically significant cycles of 40, 17, 10.5, 7.5, 6.5, 2.5, 2 and 1 yrs that are consistent with periodicities of Sunspot Activity (SA, 11 years cycle and multiples), El Nino/Southern Oscillation (ENSO, 3 to 8 yrs periodicity) and the Quasi-Biennial Cycle (QBC, about 2 yrs. periodicity). On the basis of our study we suggest that SA, ENSO and QBC are likely the principal controlling mechanisms of rainfall variability in the tropical Pacific sector.