Interglacials between 1 and 2.7 Ma

Franco Zavatti
Abstract:
Data in the range 0-2.7 million years (Ma) BP, as reported in the Tzedakis et al. (2017) paper, have been used in order to verify if, going back in time, the separation among interglacial periods from 41 to 100 ka (also known as the Mid-Pleistocene Transition) can be put in evidence. Such separation could not be observed with the 0-800 ka data used in the two earlier post of the series. It is observed that the spectrum of the δ18O within the range 1.5-2.7 Ma, shows the main spectral peak at 41 ka, while the ones at ~100, 72, 51 ka have a very low spectral power, also if with a high 99% significance level (white noise). The spectrum computed over the entire range of 0-2.7 Ma shows the main periods are both 41 and 100 ka. The spectrum of the intermediate period 0.6-1.5 Ma outlines a half-way structure.
The fact that the 100 ka spectral maximum can be some kind of mix of other peaks and not the direct influence of the orbital eccentricity should be also considered.

Introduction and analysis
After the publication at CM of two earlier posts (here and here) I noted that at the end of February, 2017 a work by Tzedakis et al., 2017 (hereafter T2017) was published, concerning the same argument, with the analysis extended through 2.7 Ma BP (Ma=millions years; ka=thousands years).

T2017 is paywalled, but I found a possibility, described in 02readme.html, I would like to share with the readers of the post.

The work by Tzedakis and colleagues is really important and noticeable: in a simple way they can separate Interglacials (IG) of the whole Pleistocene from Interstadials (IS) and continuous Interglacials (CIG, IGs apparently ending but then revitalized by some new strength in a sort of continuity with the preceding IG. By definition, they are located near the IGs and ISs border).

By the way, I show in fig.1 the series δ18O between 0.6 and 2.7 Ma with IGs labels (T2017; from Lisiecky and Raymo, 2015).

Fig.1: Plot of 0.6-2.7 Ma IGs from benthic δ18O. The data has a variable step of 2, 2.4 and 2.5 ka along the series. Red labels in the upper part of the plots are the MIS IGs coding. The figure is similar to the figure 2 of T2017, where the IG labeled (105) does not appear. Note as the δ18O range is wider after (on the left side of) 1.5 Ma BP, also the date before which T2017 computed the detrended section of the series.

The 0-800 ka series is partially shown because it has been already used in the two earlier posts. I outline the presence of an IG, labeled (105), at about 6.63 Ma, which does not appear in T2017, fig.2.

From the figure, we note that the overall range of glacial-interglacial evolutions between 1.5 and 2.7 Ma appears to be shallower than the one between 0-1.5 Ma. Relatively to the same period, T2017 decided to compute a detrendig of data.

Authors’ model is based on the “effective” energy, derived from the summer peak insolation (Gj/m2); it is defined as

E(Ipeak,Δt)=Ipeak+bΔt           (1)

where Ipeak is the summer insolation peak at 65°N; Δt the time (in ka) lasted from the preceding glacial and b the slope of the line (GJ/m2•ka) reported in their figure 4.

Data of T2017 are available at the site of one of the authors, Michel Crucifix, whose interest here was also data analysis, software and plotting. I downloaded the data and use them to reproduce their results, so the reference to “Crucifix data” used in the support site must be intended as “T2017 data”. From that data I can plot again T2017’s figure 5 into fig.2 which

Fig.2: Rebuilt of T2017 fig.5. Model (1) capability to separate different “warm situations”. Dashed line has been computed from the model maximum a posteriori probability. The diagonal section, the “ramp”, is the Medium Pleistocene Transition. Two IGs (red, 59 and 63) and one CIG (black, 7a) have been marked with the respective MIS code. No IS (aquamarine)
can be found above the dashed line. MIS 1 and MIS 5e (left top) are the Holocene and the Eemian, labelled only as reference.

shows in a very precise way the model’s (1) capability to distinguish among the glacial-interglacial sequences of the last 2.7 million years, the whole Pleistocene. Three cases “out of the choir” are indicated by their respective MIS (Marine Isotope Stage) code, while MIS 1 and MIS 5e, Holocene and Eemian, are identified only as reference.

The δ18O complete series from T2017 is plotted in fig.3, along with the detrended series from 1.5 to 2.7 Ma.

Fig.3: Run of δ18O between 0 and 2.7 Ma. (black) Original data is here indicated as “smoothed” because it is sampled in three steps, from 2 to 2.5 ka, compared to the 1 ka used in the earlier posts. (red) Detrended series which begins at 1.5 Ma BP. With respect to fig.1, here the different amplitude of glacial-interglacial exchanges after 1.5 Ma BP is better evidenced.

A slow but constant decline of the isotopic ratio from the beginning of Pleistocene to about 0.6-0.7 Ma, followed by a weak raise -or maybe a constant phase which follows a “break-point” at about 0.6-0.7 Ma- can be noted as an overall behaviour of the plot.

LOMB spectrum of both fig.3 data and data used in the previous posts are shown in fig.4 where two main maxima at ~100 ka and at ~41 ka clearly appear.

Fig.4: The spectrum of δ18O between 0 and 2.7 Ma, compared to the ones (LOMB and MEM) of the same series in the range 0-800 ka. Lomb power (cyan line) has been divided by 2. We know that 0-2.7 Ma data has a minimum 2 ka step, while the step of 0-800 ka data is 1 ka, so it is unclear the origin of the apparent major resolution of the T2017 data.

A couple of comments about the data used here:

  1. Detrended data, required by the Lomb method, used in place of the original ones does not produce noticeable differences in the spectrum.
  2. 0-2.7 Ma has 2, 2.4, 2.5 ka step, while the 0-800 ka ones, used in the comparison, have a 1 ka step. It is unclear why the spectra of T2017 data could show so much more details than more resolved data, mainly a double peak across 100 ka which does not appear in other spectra.

Given that also in this case (as in the previous posts) the spectrum over the whole range does not allow for checking the existence of a climatic regime change during the Pleistocene, the spectrum between 1.5 and 2.7 Ma has been also computed (fig.5).

Fig.5: δ18O spectrum between 1.5 and 2.7 Ma. (black) Original data. (red) Detrended data in the range 1.5-2.7 Ma. Only the peak at 41 ka appears, which confirms the hypothesis of a change around 1.5 Ma BP. Spectral maxima at about 50, 70, 90 ka are weak but their significance is as high as 99% (white noise).
To be noted that the spectrum of the detrended data (required by the Lomb’smethod) is in practice the same as the spectrum of the original data. The maximun at the extreme left side (period ~2.5 ka) is strong and also visible in fig.4. Its nature is not discussed here.

I don’t think the choice of 1.5 Ma must be considered as a cherry picking: it is the time when the δ18O amplitudes change with respect to nearby ages and also the beginning of the “ramp” of fig.2, namely the Transition of Medium Pleistocene which terminates at 0.6-0.7 Ma BP.

Fig.5 shows as in the first (ancient) section of the Pleistocene the main astronomical influence is the orbital obliquity (period 41 ka), with some possible, weak, contribution by other (one or more) orbital parameters.

A confirmation of that hypothesis comes from fig.6, showing the δ18O spectrum between 0.6 and 1.5 Ma (the “ramp” of fig.2).

Fig.6: δ18O spectrum between 0.6 and 1.5 Ma. The 41 ka peak is again the main spectral behaviour, but spectral maxima appear (at ~80 and ~122 ka) which in nearby ages could merge each other to give a ~100 ka peak. As a whole, the spectrum is less defined when compared to those of adiacent periods.

Here the spectrum is less defined than the ones in figs.4 and 5: the 41 ka maximum is again the dominant feature, but less powerful than in the previous time range; the 100 ka maximun is not yet present but noticeable peaks at 80 and 122 ka appear, which perhaps “promise” to combine themselves and became a 100 ka maximum after 0.6 Ma BP.

Final comments
Tzedakis and colleagues not only shows how it can be possible to separate the different climatic phases of the Pleistocene but also how to restore the position, with a 41 ka step, of all the more than 100 pleistocenic interglacials.
Figs. 5 and 6 show that the main maximum directly depends on orbital obliquity and allow the hypothesis that the 100 ka maximum be only the result of a combination among other different peaks and not a direct influence of changes in orbital eccentricity.

All plots and original/derived data concerning this post are available at the support site here, mainly in the last botton section, referred to as Crucifix’s data

References

  • L. E. Lisiecki, M. E. Raymo: A Pliocene-Pleistocene stack of 57 globally distributed benthic δ18O records. , Paleoceanography, 20, PA1003, 2005. doi: 10.1029/2004PA001071
  • P. C. Tzedakis, M. Crucifix, T. Mitsui & E. W. Wolff: A simple rule to determine which insolation cycles lead to interglacials, Nature, 542, 527-544, 2017. doi:10.1038/nature21364