Cycles in Fossil Diversity Robert A. Rohde & Richard A. Muller Department of Physics and Lawrence Berkeley Laboratory University of California, Berkeley California 94720 USA Report LBNL-56544 20 October 2004 submitted to Nature It is well-known that the diversity of life appears to fluctuate during the course the Phanerozoic, the eon during which hard shells and skeletons left abundant fossils (0-542 Ma). Using Sepkoski's compendium 1 of the first and last stratigraphic appearances of 36380 marine genera, we report a strong 62 ± 3 Myr cycle, which is particularly strong in the shorter-lived genera. The five great extinctions enumerated by Raup and Sepkoski 2 may be an aspect of this cycle. Because of the high stastical significance, we also consider contributing environmental factors and possible causes. Sepkoski's posthumously published Compendium of Fossil Marine Animal Genera , and its earlier versions, has frequently been used in the study of biodiversity and extinction 3-4 . For our purposes, diversity is defined as the number of distinct genera alive at any given time, i.e. those whose first occurrence predates and last occurrence postdates that time. Because Sepkoski references only 295 stratigraphic intervals, the International Commission on Stratigraphy's 2004 time scale 5 is used to translate the stratigraphic records into a record of diversity vs. time, with details given in the supplement. Though Sepkoski's is the most extensive compilation available, it is known to be subject to certain systematic limitations due primarily to the varying availability and quality of geologic sections 6-7 , the implications of this will be discussed where appropriate. Fig. 1A shows diversity vs. time for all 36380 genera in Sepkoski’s compendium. In Fig. 1B we show the 17797 genera that remain when we remove those with uncertain ages (given only at epoch or period level), and those with only a single occurrence. The smooth trend curve through the data is the third-order polynomial that minimizes the variance of the difference between it and the data. The overall shape of 1A and 1B is similar to those previously published for fossil families 2 and for genera 3 . It rises rapidly at the beginning of the Phanerozoic (right side), drops to a nadir near the Permian- Triassic boundary (251 Ma), and then rises steeply until the present. These variations may result from evolutionary and environmental drivers 8 , observational biases 6 , or changes in the number of available geologic sections 7 ; for example, the sharp rise towards the present may be driven by the greater availability and study of recent sections. Our focus is not on the trend but on the short-term variations shown in 1C, obtained by subtracting the trend from 1B. The Fourier spectrum of 1C is shown in 1E. It is dominated by a strong peak with period 62 ± 3 Myr (frequency 0.016 cycles/Myr). The sine wave corresponding to this cycle is also shown in Fig. 1C, where it accounts for 35% of the variance. Note that because steep drops in diversity are often followed by gradual recoveries, the peaks and valleys in the data don’t precisely align with those of the sine curve. Also, some abrupt features may appear more gradual because of incomplete records. 9 We indicate the 5 major extinction events of Raup and Sepkoski 2
The impact of a large extraterrestrial object on the Earth can produce a geomagnetic reversal through the following mechanism: dust from the impact crater and soot from fires trigger a climate change and the beginning of a little ice age. The redistribution of water near the equator to ice at high latitudes alters the rotation rate of the crust and mantle of the Earth. If the sea‐level change is sufficiently large (>10 meters) and rapid (in a few hundred years), then the velocity shear in the liquid core disrupts the convective cells that drive the dynamo. The new convective cells that subsequently form distort and tangle the previous field, reducing the dipole component near to zero while increasing the energy in multipole components. Eventually a dipole is rebuilt by dynamo action, and the event is seen either as a geomagnetic reversal or as an excursion. Sudden climate changes from other causes such as volcanic eruptions could also trigger reversals. This mechanism may not be the sole cause of geomagnetic reversals, but it can account for the rapid drop of the dipole component preceding a reversal, the predominance of multipole components during a transition, the associations of microtektites, temperature drops and extinctions with reversals, and the possible correlation between peaks in the geomagnetic reversal rate and the times of mass extinctions. The model may also account for the long‐term changes in the average rate of reversals. We make several testable predictions.
Spectral analysis of climate data shows a strong narrow peak with period approximately 100 kyr, attributed by the Milankovitch theory to changes in the eccentricity of the earth's orbit. The narrowness of the peak does suggest an astronomical origin; however the shape of the peak is incompatible with both linear and nonlinear models that attribute the cycle to eccentricity or (equivalently) to the envelope of the precession. In contrast, the orbital inclination parameter gives a good match to both the spectrum and bispectrum of the climate data. Extraterrestrial accretion from meteoroids or interplanetary dust is proposed as a mechanism that could link inclination to climate, and experimental tests are described that could prove or disprove this hypothesis.
The partial collapse of topographic structure at the core‐mantle boundary (CMB) in avalanches, slumps or turbidity flows, would cause sudden temperature changes in both the upper core and the lower mantle. Although such collapses are hypothetical, it is interesting to investigate the potential consequences. Downwelling from such events could disrupt core convection cells and trigger geomagnetic excursions and reversals. Buoyant sediment from the freezing of the inner core is hypothesized to rebuild the avalanched structures. Large avalanches could trigger mantle plumes. Oblique extraterrestrial impacts impart high shear to the CMB, and can trigger one or more simultaneous avalanches, yielding observed coincidences between craters, tektite fields and reversals. A triggered avalanche can explain the coincidence between the formation of the largest known volcanic province (the Ontong‐Java Plateau), the start of the 35 Myr Cretaceous geomagnetic quiet period, and reported coincidences between large flood basalts and extinctions.
Composite stacks were constructed by superimposing 6 to 13 benthic foraminiferal δ 18 O records covering the period 0–850 ka. An initial timescale for each core was established using radioisotopic age control points and assuming constant sedimentation rates between these points. The average of these records is our 13‐core “untuned” stack. Next, we matched the 41 kyr component of each record individually to variations in Earth's obliquity. Four of the 13 records produced timescales that were inconsistent with one or more of the known radioisotopic ages. The nine remaining cores were averaged to create a “minimally tuned” stack. Six of the minimally tuned cores were assembled into a “tropical” stack. For each stack we estimated the uncertainty envelope from the standard deviation of the constituents. Spectral analysis of the three stacks indicates that benthic δ 18 O is dominated by a 100 kyr oscillation that has a narrow spectral peak. The contribution of precession to the total variance is small when compared to prior results from planktic stacks.
It is not possible to understand the present or future climate unless scientists can account for the enormous and rapid cycles of glaciation that have taken place over the last million years, and which are expected to continue into the future. A great deal has happened in the theory of the ice ages over the last decade, and it is now widely accepted that ice ages are driven by changes in the Earth's orbit. The study of ice ages is very inter-disciplinary, covering geology, physics, glaciology, oceanography, atmospheric science, planetary orbit calculations astrophysics and statistics.
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