Eccentricity forcing on Pleistocene carbonate production and biological evolution
Luc Beaufort,
Cerege, CNRS-University Aix-Marseille, France,
Web Lecture on April 21, 2022 at 3:00 PM (Paris Time)
Among the orbital cycles, the eccentricity is the one that has the most mysterious effects on the Earth’s environments: Although the annual mean insolation changes very little between maxima and minima of eccentricity, its ~100 kHz components are dominant in the Glacial/Interglacial cycles since 1Ma and absent before that. The 405 kyr cycle is not a significant component of Pleistocene ice volume and d18O, confirming that eccentricity does not directly affect Pleistocene climates (a non-linearity is necessary). It is therefore extremely intriguing that eccentricity cycles are found in ancient sediments (in absence of large continental ice-sheets) primarily in the carbonate and d13C records. Common explanations rely on changes in hydrological cycles, erosion, monsoon and ocean primary productivity, although all of these are also related to climate.
At the origin of recent work from my laboratory (Beaufort et al., 2021), we aimed to describe biological evolution at high resolution in the Pleistocene (last 2.6 Myr). We developed a new metric that characterize the morphologic evolution of a prolific group a phytoplankton. We found that the their evolution was forced by Earth’s orbital eccentricity with rhythms of 100,000 years and 405,000 years. Surprisingly it presented a distinct spectral signature to that of coeval global climate cycles (e.g presence of a 405 kyr period absent in the climate proxies from the same age). Also precession and obliquity was not present in the biologic evolution series. This difference in the dynamics between global climate variations and biologic evolution implies that eccentricity influence strongly a climate variable that is not at play in the size control of continental ice sheets.
Simulations with an Earth System Model coupled with an ocean biogeochemical model demonstrated strong eccentricity modulation of the seasonal cycle, which we suggest directly impacts the diversity of ecological niches occurring over the annual cycle in the tropical ocean. Reduced (enhanced) seasonality favours species with mid-size (large and small) coccoliths, increasing (decreasing) coccolith carbonate export and burial. We posit that eccentricity pacing of phytoplankton evolution contributed to the strong 405-kyr cyclicity seen in global carbon cycle of older records.
Luc Beaufort, Clara T. Bolton, Anta-Clarisse Sarr, Baptiste Suchéras-Marx, Yair Rosenthal, Yannick Donnadieu, Nicolas Barbarin, Samantha Bova, Pauline Cornuault, Yves Gally, Emmeline Gray, Jean-Charles Mazur & Martin Tetard, Nature, 601, 7891, 74-84, 2022
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