the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Jan 2025
ESD Ideas: Cenozoic Ice Volume as a Driver of Geomagnetic Events
Abstract. This study investigates the relationship between Cenozoic ice volume changes and geomagnetic events, including reversals and incomplete reversals, revealing that high frequencies of these events responded to ice volume increases over the past 49 million years. Geomagnetic events forming chrons or wiggles shorter than 0.1 Myr are particularly sensitive to ice volume changes. The findings suggest that future global warming could suppress geomagnetic activity, highlighting the impact of climate-driven ice volume changes on Earth's magnetic field dynamics.
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RC1: 'Comment on esd-2024-43', Nicolas Thouveny, 29 Jan 2025
REVIEW OF Cenozoic Ice Volume as a Driver of Geomagnetic Events
By JAISCHENG CHEN
In this very short paper the author attempts to establish a correlation between the Ice volume variations and the frequency of the Earth magnetic field reversals during the Cenozoic Era. A speculative discussion follows linking the increase of ice volume to high frequency reversals particularly short subchrons (. The conclusion forecasts a possible decrease of geomagnetic activity due to the future global warming.
This paper suffers from several and severe defaults:
1) The introduction is a very short summary of distinct fundamental concepts of geophysics and paleoclimatology, each illustrated by one single reference : Gubbins, 2008; Doake 1977; Zachos et al. 2001.
2) The paleomagnetic event serie used for this study is identified only by the refs: Cande and Kent, 1992, Gee and Kent 2007 ( which one is used ?). No graphical presentation, nor description is provided to help the reader understand the data sets and the differences between the studied clusters ( duration < 0.03 Ma or >0.03 Ma , <0.1 Ma or > 0.1 Ma).
3) The Doake (1977) reference provides the basic physical concept of the article: polar ice caps provoke an increase of Earth’s rotation speed (conservation of the angular momentum) and thus an increase of the outer core fluid motion, providing more energy to the dynamo, resulting in stronger geomagnetic field .
However this old and single ref should also be questioned and completed by other references.
- The Doake (1977) hypothesis (in fact firstly introduced in 1975 by Olausson and Svenonius) was weakened by Kent (Nature 1982) who denounced the paleoclimatic biases on the natural remanent magnetization intensity records in sediment cores supposed (at this time) to represent the geomagnetic field intensity). Few experimental studies however supported the idea that the Earth’s rotation speed acts on the geomagnetic field intensity ( e.g. Miyagoshi and Hamano, in Phys. Rev. Lett., 2013).
However, several other studies suggested a causal link on the EMF intensity by orbital forcing ( precession, obliquity and eccentricity): see Fuller 2006, Thouveny et al. 2008, or Zhou et al. 2023 (and references therein).
4) Some studies listed together in line 28 and 29 are presented as if they all agreed. This is wrong: they provided different (sometimes contradictory) observations and proposed different hypotheses: Fuller 2006 draw a relationship between the occurrence of reversals and the obliquity period; Thouveny et al. 2008 observed that the majority of dipole moment lows and excursions of the last 800 000 years occurred within interglacial episodes, in coincidence with obliquity minima. The most recent high resolution studies paleomag and cosmonuclide isotopes along sediment sequences (e.g. Simon et al. 2016, 2018, 2020), as well as high precision Ar/Ar dating of excursional lava flow series allowed to demonstrate that the last reversal occurred during an interglacial and that most excursions of the last 800 ka occurred in narrow relation with interglacials. This totally cancels the hypothesis of Worm (1997) claiming that excursions occurred during glaciations.
5) Macroscopically for the past 50 Ma, the correlation seems to be supported: frequency of reversals (chron durations shorter than 0.1 Ma) and Ice volume seem to present a significant correlation coefficient. However several important discrepancies appear at ca 20 Ma, 10 Ma and mostly for the last Ma.
Moreover, during the 50 - 60 Ma interval, the massive occurrence of reversals pointing at 55 Ma corresponds to an Ice free Greenhouse world. This observation totally kills the initial hypothesis and forces the author (line 69-75) to find another mechanism (catastrophic water distribution and mass transfers…) that are absolutely not explained, neither constrained.
6) The most fundamental criticism is based on an obvious and revealing contradiction in
lines 9-10 and lines 82 – 85.
Indeed, the sentence (line 82): “such accelerated melting would decrease the occurrence of geomagnetic events”, is in complete contradiction with the sentence (line 83): “…relationship between ice sheet melting … and the increase in geomagnetic events”, the later itself contradicting the last sentence of the abstract (line 9 and 10).
This contradiction points the melting confusion of two different concepts: “occurrence of geomagnetic events” and “geomagnetic activity”.
7) Over the last decades, paleomagnetic studies of lava flows and sediments (e.g. Valet, Meynadier, Guyodo, 2005 in Nature), completed by cosmogenic nuclide studies (e.g. Simon et al. 2016, JGR; 2018, 2020 EPSL; Valet et al. 2024, QSR) demonstrate that reversals and excursions are associated (or even triggered by) dipole moment collapses. Therefore, at multi-million years scales, frequent reversals imply more frequent time intervals of weak dipole field.
This article is based on the hypothesis that the increase of the Earth’s speed rotation due to the mass accumulation in the polar regions (formation of ice caps) increases the energy of fluid motion in the outer core. But this mode should increase the geomagnetic dipole moment and thus provide a stronger stability of the dipole field polarity, i.e. a lower frequency of reversals. On the contrary the author claims that the glaciations (resp. deglaciations) of polar regions are responsible for increases (resp. decreases) of reversal frequency.
Conclusion : The author’s observation that higher frequency of reversals correlates with heavier polar ice masses is in contradiction with the principle of conservation of angular momentum that imply a faster Earth rotation and a stronger dipole field.
Note finally that several other studies suggested that on longer time scales (Ga) the control of the geomagnetic reversal frequency (and occurrence of superchrons), is influenced (or driven) by the heat transfert in the mantle) (e.g. Olson and Amit, 2015, 2019 Frontiers in Earth Science; Franco et al. 2019, Nature Sci. Reports).
Citation: https://doi.org/10.5194/esd-2024-43-RC1 -
RC2: 'Comment on esd-2024-43', M.J. Dekkers, 02 Feb 2025
Review Chen “Cenozoic Ice Volume as a Driver of Geomagnetic Events”
For Earth System Dynamics
The manuscript discusses a potential relation between ice volume and what is referred to as ‘geomagnetic events’, with a focus on short duration events lasting < 0.1 Myr (whether this would be short remains an unanswered question). The current lay-out suffers from several serious issues listed below (random order).
The idea seems to be inspired from Doake (1977) which is not a recent publication on this issue. Potential newer ideas (agreeing or disagreeing with Doake) should be reviewed. Referencing stops at the turn of the 21st century, no newer relevant work on the topic?
The data quality should be assessed; at present they are taken at face value from old sources. This applies at least to the geomagnetic events time series.
Are the ice volume and geomagnetic data placed on the same time scale? For frequency analysis this probably does not matter too much but here data seem to be plotted vs time.
The number of short geomagnetic events shorter 0.03 Myr (excursions or reversal excursions) may be grossly underestimated in the geomagnetic time series. Before 2 Ma or so, the existing data base (as off 2025) is very incomplete.
The 0.1 Myr ice age period applies only after the Mid-Pleistocene Transition (1.2-0.6 Ma). It may vary in older periods.
How the data were processed and with which software package(s) remains unclear.
There is no assessment of the contrast between icehouse and greenhouse state. Visually there is a similar amount of variability in Figure 1h. The claim that minimal geomagnetic variation would occur during the icehouse-greenhouse transition is not assessed. It reads as merely postulated.
What about the Jurassic (greenhouse) as an example? Reversal frequency is higher than 10x per Myr; this is full reversals, no information is available about excursions. If ice volume is a driver, what is the reason for geomagnetic variability during greenhouse times?
The implications/inferences should be assessed with the most meaningful data sets (why are the selected references the best?) and not against a few seemingly randomly picked references.
With the geomagnetic event data base being incomplete (short events are bound to be under-represented) inferences on links between future climate and geomagnetic field variability are on thin ice.
Future global warming is sometimes equated to heading to a uni-polar glacial state but whether completely ice-free conditions will be reached is an open question.
Citation: https://doi.org/10.5194/esd-2024-43-RC2
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