Fig. 4.

Spectral analysis by MTM of (A) magnetic susceptibility of unit 3 of the Xiamaling Formation in the depth interval of 261.50–278.73 m and (B) greyscale trace of outcrop photograph in the depth interval of 278–285 m. MTM periodogram (black line) of the band-pass-filtered series computed using two tapers. Median fitted first-order autoregressive noise model (red line) and 85%, 90%, 95%, and 99% confidence level estimates (red dashed lines) following refs. 55–57. Interpreted Milankovitch orbital periodicities are long and short eccentricity (E1, E2), obliquity (O) and precession (P) based on average formation sedimentation rates derived from dated volcanic beds. Note that magnetic susceptibility reflects geochemical changes such as variations in magnetic minerals gregite−magnetite expected to vary relative to ferrouginous−euxinic condition. (C−F) Spectral analyses of the short high-resolution geochemical records are presented: (C) magnetic susceptibility, (D) greyscale, (E) Cu/Al, and (F) K/Al.