INCORPORATING THE EFFECT OF CHANGING ATMOSPHERIC CO2 CONCENTRATION ON TERRESTRIAL CARBON ISOTOPE RECORDS

Numerous studies of Phanerozoic sediments have revealed coincident carbon isotope excursions (CIE) in both marine and terrestrial records. One unresolved and recurring issue is the significant difference in the magnitude of terrestrial versus marine excursions; specifically, multiple researchers have commented on the tendency for the terrestrial excursion to be greater than the marine excursion for a single event. Several arguments have been invoked to explain these differences (including terrestrial wetting/drying, community composition change, etc.), or to argue that one record is a superior reflection of global carbon cycling compared to the other. Here we explain how the difference in magnitude between marine and terrestrial CIEs is caused by a carbon isotope fractionation by C₃ land plants in response to changing atmospheric CO₂ levels (pCO₂). Our recent work growing hundreds of plants under controlled conditions and a wide range of pCO₂ (370-4200 ppm) allowed us to quantify a hyperbolic relationship between pCO₂ and Δδ¹³C that reconciles the wide range of linear relationships previously suggested by experiments across small changes in pCO₂. These results indicate the need to consider changes in pCO₂ together with changes in the δ¹³C value of atmospheric CO₂ when reconstructing the drivers behind terrestrial CIEs. As an application, we discuss the Paleocene-Eocene Thermal Maximum (PETM) for which >150 CIEs have been documented worldwide. In PETM marine substrates (benthic forams, planktic forams, bulk carbonate) the average CIE is -2.6 ± 1.1‰ (±1σ; n = 105); in terrestrial substrates (soil carbonate, plant lipids, bulk soil organic matter, tooth enamel) the average CIE is -4.7 ± 1.5‰ (±1σ; n = 48) (McInerney and Wing, 2011, Ann Rev Earth Plan Sci, 39:489-516). When we consider the effect of changing pCO₂ at the PETM, we can reconcile the different size of the marine versus terrestrial CIE and quantitatively reconstruct Late Paleocene and PETM pCO₂ for any probable excursion source. We conclude that the larger magnitude CIE recorded in terrestrial substrates resulted from the concomitant increase in pCO₂ directly associated with the massive release of carbon into the environment and that this mechanism is likely to explain other Phanerozoic CIEs more simply than evoking changes ancillary to the carbon release.