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

Abstract

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 (~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 ~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.

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

“…In other cases, the variable proportion of land-derived siliciclastic detritus to carbonate is approximated by analytical proxies (Da Silva et al, 2016, 2018. The LMA cycles may reflect orbital forcing, hypothesized in the Praha Formation by Da Silva et al (2018) LMAs, however, represent a particular challenge for cyclostratigraphy.…”

“…Westphal et al (2010) and Nohl et al (2019) thus explicitly appealed for diagenesis to be taken into consideration whenever LMAs are used for cyclostratigraphy. Da Silva et al (2018) paid no regard to these factors, although such distortion of depth-age relationships could be expected to distort any subprecession cycles.…”

“…It is not possible to reconstruct all the environmental and tectonic factors in the ancient record, but it should be possible to assess the likely lateral variability in, for example, water depth and distance to shoreline, as well as proximity to faults. Such an assessment is, however, missing in Da Silva et al (2018).…”

“…Da Silva et al (2016Silva et al ( , 2018 used proxies for the fraction of siliciclastic components for their spectral analyses. They quoted the immobility of Ti to justify its use in their analyses.…”

“…in Interval 2) would-for the given sedimentation rate and sampling density-include, on average, only two or three data points per cycle, which is barely sufficient to detect such cycles, particularly with the possibility of differential compaction in mind. Surprisingly, those signals apparently showed the largest spectral power in the multitaper method (MTM) spectra in Figure 1 of Da Silva et al (2018), suggesting that it may not be advisable to trust the spectra.…”