The astronomical theory of climate and the age of the Brunhes-Matuyama magnetic reversal

Below oxygen isotope stage 16, the orbitally derived time-scale developed by Shackleton et al. [1] from ODP site 677 in the equatorial Pacific differs significantly from previous ones [e.g., 2–5], yielding estimated ages for the last Earth magnetic reversals that are 5–7% older than the K Ar values [6–8] but are in good agreement with recent Ar Ar dating [9–11]. These results suggest that in the lower Brunhes and upper Matuyama chronozones most deep-sea climatic records retrieved so far apparently missed or misinterpreted several oscillations predicted by the astronomical theory of climate. To test this hypothesis, we studied a high-resolution oxygen isotope record from giant piston core MD900963 (Maldives area, tropical Indian Ocean) in which precession-related oscillations in δ 18O are particularly well expressed, owing to the superimposition of a local salinity signal on the global ice volume signal [12]. Three additional precession-related cycles are observed in oxygen isotope stages 17 and 18 of core MD900963, compared to the specmap composite curves [4,13], and stage 21 clearly presents three precession oscillations, as predicted by Shackleton et al. [1]. The precession peaks found in the δ 18O record from core MD900963 are in excellent agreement with climatic oscillations predicted by the astronomical theory of climate. Our δ 18O record therefore permits the development of an accurate astronomical time-scale. Based on our age model, the Brunhes-Matuyama reversal is dated at 775 ± 10 ka, in good agreement with the age estimate of 780 ka obtained by Shackleton et al. [1] and recent radiochronological Ar Ar datings on lavas [9–11]. We developed a new low-latitude, Upper Pleistocene δ 18O reference record by stacking and tuning the δ 18O records from core MD900963 and site 677 to orbital forcing functions.