Coupling between Grand cycles and Events in Earth’s climate during the past 115 million years

Abstract

Geological sediment archives document a rich periodic series of astronomically driven climate, but record also abrupt, severe climatic changes called events, the multi-Myr boundary conditions of which have generally been ascribed to acyclic processes from Earth’s interior dynamics. These events have rarely been considered together within extended time series for potential correlation with long-term (multi-million year, Myr) cycling. Here I show a coupling between events and multi-Myr cycles in a temperature and ice-volume climatic proxy of the geological past 115 Myr. I use Cenozoic through middle Cretaceous climatic variations, as recorded in benthic foraminifera δ18O, to highlight prominent ~9 and ~36 Myr cyclicities. These cyclicities were previously attributed either to astronomical or tectonic variations. In particular, I point out that most of the well-known events during the past 115 Myr geological interval occur during extremes in the ~9 and ~36 Myr cycling. One exception is the early Cenozoic hyperthermal events including the salient Paleocene-Eocene Thermal Maximum (~56 Ma), which do not match extremes in long-period cyclicities, but to inflection point of these cycles. Specific focus on climatic events, as inferred from δ18O proxy, suggest that some “events”, marked by gradual trends within the ~9 and ~36 Myr cycle extremes, would principally be paced by long-term cycling, while “events”, recorded as abrupt δ18O changes nearby cycle extremes, would be rather induced by acyclic processes. The connection between cyclic and acyclic processes, as triggers or feedbacks, is very likely. Such link between cycling and events in Earth’s past climate provides insight into celestial dynamics governing perturbations in Earth’s surface systems, but also the potential connection between external and Earth’s interior processes.

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Coupling between Grand cycles and
Events in Earth’s climate during the
past 115 million years
Slah Boulila1,2
Geological sediment archives document a rich periodic series of astronomically driven climate, but
record also abrupt, severe climatic changes called events, the multi-Myr boundary conditions of which
have generally been ascribed to acyclic processes from Earth’s interior dynamics. These events have
rarely been considered together within extended time series for potential correlation with long-term
(multi-million year, Myr) cycling. Here I show a coupling between events and multi-Myr cycles in a
temperature and ice-volume climatic proxy of the geological past 115 Myr. I use Cenozoic through
middle Cretaceous climatic variations, as recorded in benthic foraminifera δ18O, to highlight prominent
~9 and ~36 Myr cyclicities. These cyclicities were previously attributed either to astronomical or tectonic
variations. In particular, I point out that most of the well-known events during the past 115 Myr
geological interval occur during extremes in the ~9 and ~36 Myr cycling. One exception is the early
Cenozoic hyperthermal events including the salient Paleocene-Eocene Thermal Maximum (~56 Ma),
which do not match extremes in long-period cyclicities, but to inection point of these cycles. Specic
focus on climatic events, as inferred from δ18O proxy, suggest that some “events”, marked by gradual
trends within the ~9 and ~36 Myr cycle extremes, would principally be paced by long-term cycling, while
“events”, recorded as abrupt δ18O changes nearby cycle extremes, would be rather induced by acyclic
processes. The connection between cyclic and acyclic processes, as triggers or feedbacks, is very likely.
Such link between cycling and events in Earth’s past climate provides insight into celestial dynamics
governing perturbations in Earth’s surface systems, but also the potential connection between external
and Earth’s interior processes.
Earth’s past climate has varied quasi-periodically from hundred to billion years, and these variations were driven
by celestial dynamics over a large frequency band1–3. In particular, Earth’s climate during the Cenozoic-Late
Cretaceous (past 115 million years, Ma) experienced long-term (multi-million year, multi-Myr) changes punc-
tuated by severe, short-lived events4–8. e multi-Myr climatic variations have oen been considered irregular
(acyclic) in nature on the very long term9. is has led to the hypothesis that these changes were mainly driven
by Earth’s interior dynamics and plate tectonic motions assuming that these processes are acyclic in nature9,
although more recent studies point to the cyclic behavior of large plate tectonic motions10–16. e climatic events
have been intensively investigated in terms of their heavy impacts and consequences on Earth’s supercial sys-
tems, in particular the resulting profound modications in ocean and terrestrial environments, expressed as per-
turbations in the sedimentary biogeochemical cycles4,5. Delineating the cyclic versus acyclic nature of multi-Myr
climatic variations and their potential link with the climatic events is of paramount importance in deciphering the
origin of events and thus their causal mechanisms. However, the study of multi-Myr climatic variations requires
the acquisition of high-resolution data covering several million years, hence hampering the characterization of
long-term cycling in previous studies17–19. New advancements in laboratory analytical techniques together with
large collaborative projects allow today the availability of a highly resolved 115-Myr-long climate record from
sedimentary benthic foraminifera δ18O (see Methods). e δ18O record indicates variations in deep-sea temper-
ature as well as changes in ice volume4,7,8.
1Sorbonne Université, CNRS, Institut des Sciences de la Terre Paris, ISTeP, F-75005, Paris, France. 2ASD/IMCCE,
CNRS-UMR8028, Observatoire de Paris, PSL University, Sorbonne Université, 77 Avenue Denfert-Rochereau,
75014, Paris, France. Correspondence and requests for materials should be addressed to S.B. (email: slah.boulila@
sorbonne-universite.fr)
Received: 23 July 2018
Accepted: 20 November 2018
Published: xx xx xxxx
OPEN
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Results
Time-series analysis of δ18O data shows two prominent cyclicities of ~9 and ~36 Myr (Figs1 and 2). Other
low-frequency cyclicities are also present, in particular, 1.3 Myr, 1.6 Myr, a triplet peaks of 2.3, 2.5 and 2.8 Myr,
and a 4.7 Myr peak (Fig.2a), which are close to Milankovitch astronomical periods (Fig.2b). e 1.3 Myr corre-
sponds to the 1.2 Myr obliquity modulation term. e 1.6 Myr matches the eccentricity term. e triplet corre-
sponds to combined eccentricity and obliquity terms of 2, 2.3 and 2.6 Myr (Fig.3 and Supplementary FigureS3).
Finally, the 4.7 Myr is close to the 4.6 Myr eccentricity peak. Even though spectrally well detected and strikingly
close to Milankovitch frequency band (Fig.3) the periods from 1 to 5 Myr could be altered by the stacking pro-
cess of the data. Data from single sites document with high delity some of these periods20–32. In contrast, longer
periods (>5 Myr) are notably preserved in the compiled δ18O data. In the present study, I focus on the prominent
cyclicities of ~9 and ~36 Myr, and their possible link with the geologically well known climatic events. Another
potential low-frequency (16 to 18 Myr) cycle could also be seen in the δ18O power spectra (Fig.3), but will not be
discussed below.
Figure 1. Time-series analysis of benthic foraminifera δ18O data of the past 115 Ma (see Methods). (a) e
raw data with two smoothing ts: a third-order polynomial t and a 14% weighted-average t. (b) Detrended
δ18O data (third-order polynomial t shown in ‘a’ is removed) with two smoothing ts: a 14% weighted-
average t to highlight the 36 Myr cycles, and a 4% weighted-average t to highlight both the ~9 and ~36 Myr
cycles. e well known climatic events are shown: IA: Pliocene-Pleistocenbe Ice Age, MMCO: Mid-Miocene
Climatic Optimum, Mi-1 and Oi-1: Oligocene-Miocene and Eocene-Oligocene major glacial events, MECO:
Mid-Eocene Climatic Optimum, EECO: Early Eocene Climatic Optimum, PETM: Paleocene-Eocene ermal
Maximum, K/T: Cretaceous/Tertiary boundary, EMaC: Early Maastrichtian Cooling episode, OAE2 and
OAE1b: Oceanic Anoxic Events. (c) Bandpass ltering of δ18O data: 36 Myr cycle band (0.03 ±0.01 cycles/
Myr) and 9 Myr cycle band (0.11 ±0.03 cycles/Myr). Ages, P: Pleistocene, Pl.: Pliocene, Oligoc.: Oligocene,
Paleoc.: Paleocene, Ma.: Maastrichtian, Campan.: Campanian, S: Santonian, Co: Coniacian, Tu.: Turonian, Cen.:
Cenomanian, A.: Aptian. Vertical shaded bars are extrema (minima in red and maxima in blue) of the ~36 Myr
δ18O cycles, which coincide with the most known thermal and cooling episodes. Also indicated climatic events
(vertical grey lines), some of them matching extrema in the ~9 Myr cycles. Ox1 to Ox12 are the interpreted
~9 Myr δ18O cycles. Ox1, Ox2, Ox3,… to designate ~9 Myr δ18O oscillations 1, 2, 3,… Ox for oxygen (δ18O),
and increasing numbers indicate the older oscillations. is is used by correlation with Cb1, Cb2, Cb3,… that
designate the ~9 Myr δ13C oscillations 1, 2, 3,… (Supplementary FigureS1) as in Boulila et al.6, Cb. for carbon
(δ13C), and increasing numbers indicate older cycles. Ox1 to Ox4 indicated by asterisks, correlate with their
equivalents Cb1 to Cb4 in δ13C record (ref.6, Supplementary FigureS1). Ox7 and Ox8 indicated by question
marks include one oscillation, equivalent to two ~9 Myr oscillations in the δ13C record (see text for discussion,
and Supplementary FigureS2), Ox12, shown with a question mark, is a poorly constrained cycle, due to low-
resolution data within this time interval.
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Discussion
e ~9 Myr cyclicity was previously detected in carbon-cycle variations6. Comparison of δ18O and δ13C data
shows a coupling between climate and carbon cycle at the ~9 Myr cycle band, especially during icehouse, i.e.
the past 34 Ma (Supplementary FigureS1). A strong decoupling between them is remarkably noted within the
interval from 50 to 65 Ma as follows. While the δ13C document the two strongest ~9 Myr cycles Cb7 and Cb8 of
the early Cenozoic6, the δ18O has exceptionally only one oscillation (Fig.3d, see also Supplementary FigureS2).
e origin of ~9 Myr cyclicity has previously been attributed to the modulation of 2.4 Myr eccentricity cycle
band6. Here I investigate additional statistical tests to support the modulation at the 2.4 Myr cycle band, but with
possible contribution from both eccentricity and obliquity terms (Fig.2b, and Supplementary FiguresS3 and S4).
Prominent ~9 Myr geological oscillations have been detected in carbon-cycle and sedimentological proxies of
Cenozoic and Mesozoic strata6,33–35.
Interestingly, I note four ~9 Myr oscillations grouped in each ~36 Myr cycle (Fig.1b,c), suggesting a relation-
ship between these two climatic cyclicities. Previous studies ascribed the ~36 Myr climatic variability to another
dimension of astronomical forcing36,37. In particular, the ~36 Myr period has been attributed to the vertical pas-
sage of the Solar System through the Galactic midplane that modulates the ux of galactic cosmic rays (GCR) on
Earth36. Despite our limited knowledge on the gravitational potential of the Galaxy, there is some consensus that
the Solar System vertically oscillates across the Galactic midplane, with a half-period of about 36 Myr16,38–40. is
eective 36 Myr half-period corresponds to the motion of the Solar System when it moves down- and upwards
the galactic midplane. is would induce signicant change in GCR ux on Earth, especially when crossing the
galactic midplane. is would favor the formation of cloud layer from the incidental GCR ux on Earth, hence
resulting in variations of Earth’s albedo and the consequent climate change16,36,41,42. Yet, the impact of cosmic ray
on climate remains a controversial subject43. Another possible hypothesis is that Earth’s interior dynamics may
resonate every 36 Myr inducing global climate change, although recent studies pointed to dierent periodicities of
25 to 50 Myr13,14 and 56 Myr15. A combined eect from the aforcited mechanisms on climate is presumably44. e
most intriguing result in time-series analysis of δ18O climatic proxy is the record of both ~9 and ~36 Myr oscilla-
tions, sharing very likely the same forcing process. While the ~9 Myr periodicity may represent a Milankovitch
origin, the ~36 periodicity could be forced by solar system vertical motion, that could in turn modulate Earth’s
incident (Milankovitch) insolation, or may correspond to a not resolved Milankovitch band. Although I draw
Figure 2. 2π-MTM power spectra of benthic foraminifera δ18O data of the interval from 0 to 115 Ma (see
Methods) and of its time-equivalent astronomical variations. (a) Upper panel: Spectrum of the detrended δ18O
data (50% weighted average of the series is removed), lower panel: Spectrum of 1x zero padded δ18O aer the
detrend (50% weighted average of the series is removed). Signicant spectral periods are shown across vertical
grey bars. (b) Upper panel: Spectrum of lowpass ltered (>1 Myr), composite astronomical time series of the
interval 0 to 115 Ma (see Methods), the power axis is a logarithmic scale for comparison with spectra in ‘a’,
lower panel: Spectrum of the same astronomical time series (power in linear scale) but over the interval from
0 to 146 Ma (Cenozoic and Cretaceous time). e most prominent astronomical periods and origins of some
of them are shown across vertical grey bars, periods indicated by asterisk may represent minor components.
Note that the ~9 Myr δ18O cyclicity originates from the interfering astronomical terms 2 and 2.6 Myr (see
Supplementary FigureS3). e 16.3 Myr δ18O period may correspond to a harmonic or may originate from the
couple interfering terms 2 Myr vs 2.3 Myr and 2.3 Myr vs 2.6 Myr.
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attention to climate and astronomy linkage at the scale of ~9 and ~36 Myr periods, we must caution that the eect
of tectonics on climate at this timescale remains well plausible16.
Along climate-proxy variations in the Cenozoic and Mesozoic eras (past 0–250 Ma) were recognized and
widely studied a number of climatic events4,5,8. e studied interval includes some of them (Fig.1b). Features
and nature of Earth’s system responses to these perturbations differ from one event to another5, however,
tectonic-volcanic mechanisms were thought to be the principal, common cause for most of the events4.
Here, I show that most of Cenozoic–Cretaceous climatic events and phases (see Fig.3 for denition of ‘event’
versus ‘phase’) during the past 115 Ma fall into extremes in amplitudes of the ~9 and ~36 Myr cyclicities (Fig.1).
For instance, the so-called Oi-1 and Mi-1 glacial episodes are close to the ~9 Myr cycle extremes (Ox4,
Fig.1c). e Mid-Miocene Climatic Optimum (MMCO), which represents a warming phase, bounds ~9 Myr
Ox2 and Ox3 δ18O oscillations (see Fig.1 caption for ‘Ox’ cycle numbering). Importantly, some of the events
coincide with extremes in the ~36 Myr cycling, thus, they would be with greater magnitudes (Fig.1b). e three
cooling/glacial episodes at Pliocene-Pleistocene Ice Age (IA), Eocene-Oligocene glacial event (Oi-1) and Early
Maastrichtian Cooling phase (EMaC) correspond to maxima of the ~36 Myr δ18O cycles. e two warming phases
of Mid-Miocene Climatic Optimum (MMCO) and Early Eocene Climatic Optimum (EECO) coincide with the
~36 Myr cycle minima. Finally, the globally recognized Oceanic Anoxic Event OAE-2 occurs around a minimum
of the ~36 Myr δ18O cyclicity. In contrast, the early Cenozoic hyperthermals including the most pronounced
Paleocene-Eocene ermal Maximum (PETM) event do not match extremes in the long-term cycling. e rela-
tive variance of the ~36 Myr δ18O cyclicity is more than three times higher than that of the ~9 Myr cyclicity. is
implies that events (and phases) matching only extremes in the ~9 Myr cycles are with lesser intensities compared
to events (and phases) in relation with the ~36 Myr oscillations. However, Earth’s climate response to the forcing
processes at the ~9 and ~36 Myr cycle bands from astronomy, for example, could not be direct, because insolation
(or GCR) change at these very longer timescales should be weak (see below).
The most striking result is the match between ~9 and ~36 Myr cycle extremes and most of the
Cenozoic-Cretaceous events. is result hints at a coupling between Earth’s climate cycling and perturbations.
e ~9 and ~36 Myr cyclicities could have an astronomical and/or a tectonic origin (see above). e events have
been postulated to be caused by tectonics, volcanism, pCO2 trend, etc.
I suggest that among the events in relation with long-period cyclicities some of them may have been mainly
related to cyclic process, others may have been the result of combined eects of acyclic and cyclic processes. e
Figure 3. Expanded views of climatic events and phases within the ~36 Myr cycle extrema. e term ‘phase’
opposes to the term ‘event’: ‘event’ indicates abrupt, severe change, whereas ‘phase’ indicates gradual/progressive
trend in climate change that could reach an optimum (see below). I also used the term ‘episode’ to designate
either event or phase. (a) e gradual entry of the Earth into the Pliocene-Pleistocene ice age (IA) as expressed
in the trend (40% weighted average). Note that there is no trend during the 100-kyr-cycle dominated climate of
the past ca. 800 ka. (b) Gradual variations within the Mid-Miocene Climatic Optimum, MMCO (35% weighted
average). (c) Abrupt change within the Eocene-Oligocene transition (EOT) including the Oi-1 glacial event
(15% weighted average). (d) Gradual variations within the Early Eocene Climatic Optimum, EECO (35%
weighted average). Note that the Paleocene-Eocene ermal Maximum, PETM, corresponds to a deviation
from the long-term cycling within the EECO, and is the most abrupt, severe event in the Cenozoic era.
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IA, MMCO, EECO and possibly EMaC4,8 show gradual variations (Figs1 and 3), thus they may mainly be the
result of a cyclic forcing. e Oi-1 and OAE-24,5 show abrupt changes (Figs1 and 3), thus they may occur acy-
clically, but could also be triggered by a cyclic mechanism. Large-scale (multi-Myr) Milankovitch astronomical
forcing would induce negligible changes in insolation (or GCR) budget on Earth6, hence a nonlinear mechanism
is required to explain the match between the long-period astroclimatic cycles and events. Energy transfer process
from higher to lower frequency driving forces has previously been suggested6, but this process cannot argue the
match between the cycles and events. An astronomically forced gravitational distortion of Earth’s viscose mantle
in boundary conditions may explain the potential link between cycle extremes and events at the multi-Myr times-
cales. Astronomical control of gravitational deformation (e.g., dynamical ellipticity)45 of the mantle at boundary
conditions would result in intensication of mantle convection, and the occurrence of climate events, which have
been argued to be paced by processes from Earth’s interior dynamics5,44. Also, feedback responses of Earth’s inte-
rior dynamics to astronomically driven climate and Earth’s surface processes have equally been suggested even
at the shorter Milankovitch timescales46,47. For instance, glacial cycles have been correlated to oceanic crust pro-
duction47,48. Astroclimatically paced deglaciations would promote mantle decompression, hence increase magma
production and thus volcanic eruptions44,46.
Conclusions and Perspectives
In summary, the detection of a potential link between long-period climatic cyclicities (driven by astronomy and/
or tectonics) and events provides insights into multi-disciplinary studies involving acyclic versus cyclic nature
of Earth’s interior dynamics, astronomical forcing at multi-Myr timescales, and the possible interaction between
astronomical and tectonic forcings. e correspondence between very long climate cycle extremes and events
during the mid-Cretaceous to Cenozoic time interval suggests a highly nonlinear mechanism response, if the
events were responding to insolation, such as astronomical pacing, at boundary conditions, of gravitational per-
turbation of the Earth’s spin axis and shape. is may, in turn, aect the gravitational deformation of the mantle,
stimulate mantle convection and the dynamic topography, hence the occurrence of geodynamically paced cli-
matic events. Cyclic tectonic forcing at the 9 and 36 Myr cycle bands, without evoking the nonlinear astronomical
driving force, remains equally plausible.
Future studies should focus on the cyclic vs acyclic nature of Earth’s interior dynamics (tectonics, volcan-
ism,…), at ten to tens of Myr, and likely on how could Earth’s interior dynamics vary or interact with climate44–51,
and its potential link with the recognized climatic events. Other potential questions remain open, e.g., what would
be the paleoenvironmental implications of the strong decoupling, at the 9 Myr oscillations, between climate and
carbon cycles during the Paleocene-Eocene extreme greenhouse? Future studies should also focus on the origin
of the ~36 Myr climatic cyclicity, by exploring the hypothesis of the possible link between GCR driven climates
from vertical motion of the solar system, and insolation (Milankovitch) forced climates.
Methods
I used compiled benthic foraminifera δ13C and δ18O data from more than 40 deep-sea cores of the Deep Sea
Drilling Project and Ocean Drilling Program4,7,8. These data spanning the Cenozoic to the middle-Late
Cretaceous, i.e. the past ~115 Ma (Fig.1a), indicate global variations in climate and carbon cycle, including the
well known climatic events4. e Cenozoic compilation is from Cramer et al.7, and the Cretaceous compilation
is from Friedrich et al.8. Although the resolution of the stacked data would distort the record of high-frequency
cyclicities, the low-frequency, multi-Myr, if present would emerge in the compiled records6. Here I performed
time-series analysis on the compiled δ18O record to seek for multi-Myr cycling in climate; comparision with
long-term carbon cycling6 was also carried out (Supplementary FigureS1).
I applied spectral analysis to the δ18O data (Fig.2a) and to long-period (>1 Myr) astronomical variations
(Fig.2b). I generated long-term astronomical variations by extracting amplitude maxima in short eccentricity
and amplitude maxima in high-frequency obliquity time series52,53, then summing them aer standardization,
nally I lowpass ltered periods longer than 1 Myr. e astronomical variations from amplitude maxima of the
signals are susceptible to detect lower frequencies, as the conventional amplitude modulation technique. I applied
this procedure to the interval from 0 to 115 Ma (Fig.2b, upper spectrum), and to the interval from 0 to 146 Ma
(Cenozoic and Cretaceous time, Fig.2b, lower spectrum). For spectral analysis, I used the multi-taper method
(MTM) and the robust red noise as implemented in the ‘astrochron’ R packages54. MTM spectral analysis was
conducted, with three 2π tapers (Fig.2), on the linearly resampled (5 kyr) δ18O data. To precisely determine the
long periods of 9 and 36 Myr I additionally applied 1x zero padding to the δ18O time series prior spectral analysis
(Fig.2a). I carried out smoothing and tting long-term δ18O variations using both polynomial method and the
Lowess weighted average method (Fig.1). For the detailed study of δ18O record (expanded views in Fig.3), I have
rather performed only the weighted average method on the uneven spaced raw data.
Bandpass ltering was conducted using conjointly the Taner lter, which is characterized by a steep stop-
band, and the Gaussian lter which has a gentle stopband. Lowpass ltering was conducted using the Taner lter.
Filtering δ18O signal was applied to the 5 kyr linearly resampled (5 kyr) data, while Lowess weighted average
smoothing was applied to the raw data. Finally, in order to highlight the amplitude modulation of the 2.4 Myr
cycle band by the ~9 Myr cycles in the astronomical variations, I used evolutive harmonic analysis (EHA) with a
function computing a running periodogram of a uniformly sampled time series using FFTs of zero-padded seg-
ments, and normalized to the highest amplitudes (Supplementary Figure 3).
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Acknowledgements
I thank O. Friedrich (Heidelberg University), who kindly provided the compiled benthic foraminifera isotope
data. is work was supported by INSU-SYSTER grant. I acknowledge reviews of two anonymous colleagues and
the associate editor that led to important revisions of my manuscript.
Additional Information
Supplementary information accompanies this paper at https://doi.org/10.1038/s41598-018-36509-7.
Competing Interests: e author declares no competing interests.
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