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

Abstract

Da Silva et al. (2018) reported sub-precessional cycles in carbonaceous hemipelagites in the Praha Formation (Czech Republic). The studied sediments belong to limestone-marl alternations (LMAs), which show rhythmic bedding visible to the naked eye in partly weathered outcrops, where the LMA couplets can be counted (Chlupáč, 2000). 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. The precursor deposits of LMAs are mixtures of calcite, aragonite, organic matter, and siliciclastic detritus. Of these, carbonates and organic matter can undergo post-depositional diagenesis, which leads to transfer of carbonate to nodules or to calcite-enriched (limestone) beds. Active discussion of the influence of diagenesis on LMAs over many decades (Westphal et al., 2010; Brett et al., 2012; Westphal et al., 2015, and references therein) demands that careful tests should be applied before deciding that climatic signal is the only cause of the observed bedding (Westphal et al., 2010). Vertical transfer and recrystallization of carbonates result in vertically non-homogeneous compaction (Westphal et al. 2010; Brett et al., 2012; Nohl et al., 2019), which would distort original depth-age relations. Thermodynamic and kinetic models of diagenetic reactions can even produce bedding not related to the original climatic signal (Böhm et al., 2003; L’Heureux, 2018). 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. There are several models of how LMA could record orbital cycles (Colombié et al., 2012; Eldrett et al., 2015). Most of them include proximity of land, discontinuous deposition by turbidites, and sensitivity of carbonate production to environmental or paleoceanographic disturbances, which represent a complex interplay of factors (Colombié et al. 2012; Brett et al., 2012). The depositional environment of the Praha Formation was also affected by synsedimentary tectonics, which resulted in different thickness and stratigraphic completeness in sites over distances of a few to several tens of kilometers (Chlupáč, 2000; Da Silva et al., 2016; Bábek et al., 2018). 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. (2016, 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. Diagenesis, however, affects the carbonate matrix, inevitably also affecting the immobile components. Westphal et al. (2010) thus advocated the use of element ratios rather than raw element concentrations. These could definitely show whether the nature (grain size, provenance, mineralogy) of the clastic components changed periodically, as would be expected for a climatic signal. In an optimal case, it could exclude possible (expected) diagenetic influence on the raw concentration of a single element. Last but not least, the shortest cycles shown by Da Silva et al. (2018) in their Figure 1 (1.5 k.y. in Interval 1 and 0.8 k.y. 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. LMAs remain a challenge for cyclostratigraphy. The conclusions by Da Silva et al. (2018) would be more robust if the phenomena indicated above had been addressed. Particular care is needed because sedimentary records of sub-precessional cycles, including solar cycles, have not been widely claimed, even in younger, diagenetically less-impacted sediments.