Eos, Vol. 94, No. 12, 19 March 2013
VOLUME 94
NUMBER 12
19 MARCH 2013
EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION
PAGES 113–120
Are Mantle Plumes Periodic?
The number of LIPs used in this analysis
trails off prior to 250 million years ago, as
older Paleozoic LIPs are more difficult to
recognize.
The wavelet analysis of the fossil diversity
data showed two strong periods of around
62 million years and 140 million years, in
agreement with the results of Rohde and
Muller’s [2005] earlier spectral analysis
(Figure 2a). An additional period of roughly
35 million years was also detected in these
data, especially over the last 135 million
years (since the early Cretaceous). In the
case of the LIP ages, wavelet analysis by
Prokoph et al. [2013] similarly detected a
cycle of about 62 million years and a
somewhat weaker 140‐million‐year cycle. A
shorter 30‐ to 35‐million‐year period (more
pronounced during the last 135 million
years) was also detected in the ages of LIPs
(Figure 2b). These wavelet analysis results
agree with Fourier analyses reported in
Prokoph et al. [2013].
The strength and the simultaneous onset
of the 30‐ to 35‐million‐year cycle 135 million
years ago in both data sets suggest that these
cycles are real and not edge effects of time
PAGES 113 –114
In the past few years, researchers have
uncovered evidence that several kinds of
geological and biological events seem to
show regular cycles of similar lengths. For
example, Rohde and Muller [2005] looked at
the record of diversity of marine organisms
over the past 540 million years and found
evidence for two cycles in the data—a
roughly 62‐million‐year cycle and a longer
cycle of about 140 million years. This was
followed by reports of an approximately
56‐million‐year cycle in long‐term stratigraphic sequences in sedimentary basins
[Meyers and Peters, 2011] and a 59‐million‐
year period in the marine strontium‐isotope
record [Melott et al., 2012]. A similar period
may even exist in atmospheric carbon
dioxide over the past 542 million years of the
Phanerozoic [Franks et al., 2012]. A cycle of
about 140 million years was reported by
Veizer et al. [2000] and Mayhew et al. [2008]
in long‐term fluctuations in global climate.
These regular cycles that hover around
60 and 140 million years in length, if real,
suggest an underlying cause in large‐scale
Earth processes. But what? Because they are
much longer than the well‐known cycles of
fluctuations in Earth’s orbit and axial tilt,
scientists have increasingly looked inward
for answers, to phenomena such as mantle
convection and plume activity.
One important result of upwelling plumes
is the eruption of large igneous provinces
(LIPs), which are large‐volume, short‐
duration, primarily iron‐rich eruptions not
associated with typical plate boundary
processes. LIPs are supposedly formed by
the initiation of hot spots by the upwelling
of mantle plume heads impinging on the
continental or oceanic lithosphere [Richards
et al., 1989]. LIPs have been linked to
biological mass extinctions through various
effects on the environment [Wignall, 2001;
Arens and West, 2008]. Could they also be
linked to changes seen in other records?
Rather than isolated events, does their
occurrence follow some sort of pattern? A
look at correlating events and underlying
BY M. R. R AMPINO AND A. PROKOPH
causes that might tie LIPs to cycles seen on
Earth’s surface may answer these questions.
Correlating Cycles in Geologic Records
With Mantle Plume Activity
To test the idea that mantle plume activity
follows cycles, Prokoph et al. [2013] chose to
search for regular periods in LIP eruptions.
They applied continuous wavelet transform
analysis to the fossil diversity data of Rohde
and Muller [2005] and also to data on the
probability distribution of the ages of known
LIP events [Courtillot and Renne, 2003]
(Figure 1). Wavelet analysis allows the
transformation of a time series into wavelet
coefficients in both “time” and “scale”
(or frequency) domains; it screens out the
chaotic values and simultaneously transforms these domains (by using various
shapes and sizes of short filtering functions
called wavelets) to search for cycles in the
data [Grossman and Morlet, 1984] (Figure 2).
Fig. 1. (a) Detrended marine genera for the past 540 million years, including the removal of the
mean value, resulting in negative genera numbers for times when diversity was below the mean.
(b) Probability weights (p) for the initiation ages of large igneous provinces (LIPs), containing
age and uncertainty values for each event. These have been filtered through a normal distribution and transformed to an equidistant time series by using the Gaussian probability function to
determine the probability (0–1) of each event occurring in a specific 1-million-year interval (after
Prokoph et al., 2013).
© 2013. American Geophysical Union. All Rights Reserved.
Eos, Vol. 94, No. 12, 19 March 2013
in mantle plume activity exist [Hannisdal
and Peters, 2011; Smith and McGowan, 2005].
Such efforts would be helped by further
modeling of mantle plume generation. A
growing body of evidence suggests that
periodic mantle plume activity leads to hot
spots, LIP eruptions, regional uplift, and
rifting, and plumes may act as a pacemaker
for changes in sea level, climate, and
biodiversity. These new results point to the
possibility of a unification of geologic
processes, related in part to changes in the
deep mantle.
References
Fig. 2. Continuous wavelet transform (CWT) scalograms for (a) the detrended marine genera
record and (b) the probability-weighted initiation ages of large igneous province (LIP) eruptions.
The matrix of the wavelet coefficients—the scalogram—was coded in color scale (orange is the
highest; blue is the lowest) for better graphical interpretation. Yellow dashed lines mark the upper
border of the region where edge effects due to truncation of the time series are significant. The red
vertical dashed line marks the onset of roughly 28- to 35-million-year cyclicity patterns at around
135 million years ago. Red horizontal dashed lines mark the rough cycles of 28 to 35 million,
62 million, and 140 million years (after Prokoph et al., 2013). The 28- to 35-million-year cycle is
not affected by edge effects, except potentially for the Neogene. Edge effects are inaccurate wavelet coefficients due to partial CWT window coverage at the start and end of the analyzed records.
series truncation. In addition, the discovery
of the 62‐million‐year cycle in the marine
strontium‐ and sulfur‐isotope records [Melott
et al., 2012; Prokoph et al., 2013] supports the
conclusion that the period is not the result of
sampling, as proposed by Smith and
McGowan [2005]. The three cycles seen in
the LIP eruptions and fossil diversity are
negatively correlated; that is, times of high
production of lava are generally times of low
fossil diversity.
From Correlation to Cause
A possible driver for the biodiversity and
other geologic cycles may be found in the
cyclic activity of mantle plumes that directly
produce the LIPs [Richards et al., 1989].
However, before reaching that conclusion, a
fundamental question must be asked: Why
would mantle plumes exhibit regular
periodic activity?
There are several ways in which regular
periodic or quasiperiodic plumes may be
generated in the mantle. Deep mantle
plumes may arise by repeated generation of
instabilities in the thermal‐boundary layer—
the D" layer—just above the core‐mantle
boundary [Olson et al., 1987]. The D" layer is
marked by a flux of heat from the core into
the lower mantle, where the more buoyant
material accumulates, becomes unstable,
and eventually rises as narrow plumes
[Loper and Stacey, 1983]. It is estimated that
such boundary layer instabilities can reach a
critical threshold and can grow into plumes
in about 50 million to 100 million years
[Olson et al., 1987].
Periodic mantle activity could also be
triggered by subduction, where sinking
cold plate material accumulates at the
670‐kilometer discontinuity in the mantle
[Solheim and Peltier, 1994]. This unstable
buildup may result in periodic avalanches of
upper mantle material into the lower mantle,
where the disturbance can create a return
flow from the deeper mantle [Machetel and
Humler, 2003].
Periodicity in plumes and mantle
convection may also derive from plate
geometry. Imagine a supercontinent acting
like a blanket over the mantle, causing a
buildup of heat beneath the continental
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and LIP eruptions [O’Neill et al., 2009]. Thus,
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and Jarvis, 1993] and physical models
[Zhang and Libchaber, 2000] where a
supercontinent is present. Past findings also
suggest that time‐dependent nonlinear
convection in the Earth’s mantle may go
through periodic or quasiperiodic behavior
prior to more chaotic flow [Hansen and Ebel,
1988].
A Search for More Information
Further statistical analyses of possible
periodic components in other events in the
geologic record need to be pursued before
the scientific community can come to a
consensus on whether or not regular cycles
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—MICHAEL R. RAMPINO, Environmental Studies
Program and Department of Biology, New York
University, N.Y.; E-mail: [email protected]; and ANDREAS
PROKOPH, Department of Earth Sciences and
Ottawa Geoscience Center, Carleton University,
Ottawa, Ontario, Canada