Atmospheric CO2 Amplification of Orbitally Forced Changes in the Hydrological Cycle in the Early Mesozoic

Abstract:

Models of increasing atmospheric CO₂ predict an intensification of the hydrological cycle coupled with warming, possibly amplifying effects of orbitally-forced fluctuations. While there is some Pleistocene evidence of this, CO₂ concentrations were much lower than projected for the future. For the potentially more relevant Early Mesozoic, with CO₂ >1000 ppm, we observe that both the soil carbonate and stomatal proxies for CO₂ strongly and positively correlate with climatic-precession variance in correlative continental and marine strata of both eastern North America and Europe with temporal correlation robustly supported by magneto-, astro-, and U-Pb zircon geochronology. Eastern North American lacustrine and paleosol strata are generally characterized by >3000 ppm CO₂ over most of the Norian (228-207 Ma) dropping to ~1000-3000 ppm during the succeeding latest Norian to late Rhaetian (207 to 201.6 Ma) correlative with a dramatic drop in the amplitude of the response to orbital forcing. This is followed by an extraordinary doubling to nearly tripling of CO₂ (~2000-5000 ppm) in the latest Rhaetian to Early Jurassic (201.6 to 200.6 Ma) and a concurrent profound increase in the amplitude of the apparent climatic-precession variance during the eruption of the massive Central Atlantic Magmatic Province. Decreasing CO₂ (~1000-2000 ppm) afterward is tracked by decreasing amplitude in the orbitally-paced cyclicity. Likewise, in the UK, high amplitude cyclicity in the lacustrine to paralic Twyning Md. Fm. gives way upward into the paralic Blue Anchor and marine Rhaetian Westbury fms in which lithological cyclicity is muted. Again, the amplitude of the orbitially-paced lithological cyclicity dramatically increases into the paralic to marine late Rhaetian Lilstock Fm. and marine latest Rhaetian to Early Jurassic Blue Lias. Parallel and correlative transitions are seen in at least western Germany. The agreement between the continental eastern US and paralic to marine European records on two continents and in different paleoclimate belts over a timespan greater than the Neogene, strongly supports the hypothesis that atmospheric CO₂ modulates the hydrological cycle on very large spatial and temporal scales with high CO₂ tending to amplify the hydrological response to orbital forcing. This a contribution to IGCP 632.