Millennial-scale climate changes manifest Milankovitch combination tones and Hallstatt solar cycles in the Devonian greenhouse world

摘要

Sub-Milankovitch rhythmic features in sedimentary records have been reported from throughout geological time. However, their origin remains enigmatic, in particular during so-called greenhouse periods in Earth's history. To better understand such short-term climatic changes, we sampled two 3-m-thick intervals of early Devonian hemipelagic carbonate at 1 cm resolution in the Pod Barrandovem section (Czech Republic). Greenhouse conditions prevailed during early Devonian times, and the chosen resolution enables the detection of millennial-scale climate change as recorded by elemental abundances. We used a previously published astrochronology for the section to transform the studied series from the stratigraphic into the time domain. Spectral analysis of the time-calibrated log-transformed Ti records reveal obliquity and precession cycles, confirming the applied astrochronology. Additional spectral peaks with periods of 2.3-2.7, 6-8, and 10-12 k.y. appear in both records. Furthermore, a 1.5 k.y. periodicity, close to the Pleistocene Dansgaard-Oeschger oscillation, is also identified, but only in the record with higher accumulation rate (similar to 3.5 cm/k.y.). Bi-coherence spectra reveal that the 6-8 and 10-12 k.y. periodic components are combination tones of Milankovitch cycles. We infer the shorter similar to 2.5 k.y. periodicity to be the result of solar forcing, related to the Hallstatt cycle. These new observations strengthen the case for an external origin of millennial-scale features.