Understanding d13C Cycles in the Geological Record : an Attempt with a Simple Conceptual Model.

To model future climate evolution on large timescales, a correct understanding of the human perturbation on the carbon cycle is needed. However, the « organic » part of the carbon cycle is barely taken into account in classical carbon cycle theories, that are not able to reproduce past d13C data. Sediment records reveal the presence of cycles in d13C. A 400 kyr cycle is noticeable at different time periods, from the Eocene to present [1-3]. But also longer cycles have been observed : 2.4, 4.6 and 9 Myr [4-7]. The 9 Myr cycle is present since the start of the Mesozoic. All these periodicities seem linked to eccentricity periods. The net organic matter burial has a key role on d13C, as terrestrial and marine biology preferentially use 12C during photosynthesis. Therefore, a more important burial of (12C rich) organic matter will increase the d13C of the superficial system. With a conceptual carbon cycle model in which the net organic matter burial is astronomically forced, Paillard [8] reproduced 400 kyr and 2.4 Myr cycles in d13C. However, this conceptual model did not produce longer term cycles at 4.6 and 9 Myr. In this work, we develop a new conceptual model based on Paillard [8], which includes the role of oxygen, as it also influences the organic matter burial. With this new conceptual model, coupling carbon and oxygen cycle, we are able to obtain 400 kyr, 2.4 Myr, but also longer cycles. References : [1] Sexton et al, 2011, Eocene global warming events driven by ventilation of oceanic dissolved organic carbon [2] Palike et al, 2006 The Heartbeat of the Oligocene Climate System [3] Billups et al, 2004 Astronomic calibration of the late Oligocene through early Miocene geomagnetic polarity time scale [4] Boulila et al, 2012, A ~9 myr cycle in Cenozoic 13C record and long-term orbital eccentricity modulation: Is there a link? [5] Ikeda et al, 2014, 70 million year astronomical time scale for the deep-sea bedded chert sequence (Inuyama, Japan): Implications for TriassicJurassic geochronology. [6] Martinez et al, 2015, Orbital pacing of carbon fluxes by a 9-My eccentricity cycle during the Mesozoic [7] Sprovieri M, et al. (2013) Late Cretaceous orbitally-paced carbon isotope stratigraphy from the Bottaccione Gorge (Italy). [8] Paillard, 2017, The Plio-Pleistocene climatic evolution as a consequence of orbital forcing on the carbon cycle.