The Paleocene‐Eocene Thermal Maximum: How much carbon is enough?

Abstract

The Paleocene-Eocene Thermal Maximum (PETM), ∼55.53 million years before present, was an abrupt warming event that involved profound changes in the carbon cycle and led to major perturbations of marine and terrestrial ecosystems. The PETM was triggered by the release of a massive amount of carbon, and thus, the event provides an analog for future climate and environmental changes given the current anthropogenic CO 2 emissions. Previous attempts to constrain the amount of carbon released have produced widely diverging results, between 2000 and 10,000 gigatons carbon (GtC). Here we use the UVic Earth System Climate Model in conjunction with a recently published compilation of PETM temperatures to constrain the initial atmospheric CO 2 concentration as well as the total mass of carbon released during the event. Thirty-six simulations were initialized with varying ocean alkalinity, river runoff, and ocean sediment cover. Simulating various combinations of pre-PETM CO 2 levels (840, 1680, and 2520 ppm) and total carbon releases (3000, 4500, 7000, and 10,000 GtC), we find that both the 840 ppm plus 7000 GtC and 1680 ppm plus 7000-10,000 GtC scenarios agree best with temperature reconstructions. Bottom waters outside the Arctic and North Atlantic Oceans remain well oxygenated in all of our simulations. While the recovery time and rates are highly dependent on ocean alkalinity and sediment cover, the maximum temperature anomaly, used here to constrain the amount of carbon released, is less dependent on this slow-acting feedback.

The dynamics of global change at the Paleocene-Eocene thermal maximum: A data-model comparison

“…A simple scale analysis of the diffusion processes involved leads to a characteristic time scale on the order of 8 kyr [ Meissner et al ., ], which is consistent with the time scale of 6.7 kyr in the model simulations and between 7 and 16 kyr (#12) in the isotope data. Thus, gradual bottom water warming is fully consistent with sea‐surface warming [e.g., Bice and Marotzke , ; Thomas et al ., , Meissner et al , ] without requiring a change in the source and circulation of deep waters at the onset of the PETM [e.g., Nunes and Norris , ].…”

“…Acidification extended over much of the upper water column and reflects the absorption of excess atmospheric CO 2 by the surface ocean. Model output at Site 690 shows a decrease in oxygen throughout the water column, but not to hypoxic levels as in some locations of intermediate depth in the Southern Ocean and tropical regions (supporting information Figures S2 and S3) [ Meissner et al ., ]. Moreover, global average surface ocean salinity shows an increase in the early part of the event whereas Site 690 and the Southern Ocean region show decreases.…”

Stomatal Function across Temporal and Spatial Scales: Deep-Time Trends, Land-Atmosphere Coupling and Global Models

“…This resource has proven to be a valuable key to unlocking the history of vegetationclimate coevolution through the Phanerozoic because it has enabled reconstruction of the evolution of plant gas-exchange capacity and productivity (Beerling and Woodward, 1997;Franks and Beerling, 2009a) as well as global atmospheric CO 2 concentration (McElwain et al, 1999;Beerling and Royer, 2002;Grein et al, 2011;Franks et al, 2014;Montañez et al, 2016), the latter a critical boundary condition for simulating paleoclimates using global climate models (Kiehl and Shields, 2013;Meissner et al, 2014;Upchurch et al, 2015).…”

“…Results from these studies are revising paleo-CO 2 estimates that were once widely disparate (e.g. 200-2,800 mmol mol 21 for the Paleocene-Eocene thermal maximum; McInerney and Wing, 2011) to well-constrained values that are more compatible with global paleoclimate simulations (Meissner et al, 2014;Upchurch et al, 2015). However, current paleo-CO 2 proxy methods and paleoclimate simulations require refinement and improvement to achieve better agreement across studies and to reduce uncertainty in model outputs.…”

Orbital forcing of terrestrial hydrology, weathering and carbon sequestration during the Palaeocene-Eocene Thermal Maximum

“…Although there is still no confidence on the identity of such a large (>4000 Pg C) and unstable carbon reservoir, its release and oxidation within the ocean-atmosphere system caused rising atmospheric CO 2 concentrations, warming and a range of Earth System perturbations associated with pronounced global warming (Sluijs et al, 2007). 15 Although considerable attention has been paid to constraining the rates of carbon release, based on deep-ocean carbonate dissolution (Panchuk et al, 2008;Zachos et al, 2005;Zeebe et al, 2009), rates of warming (Meissner et al, 2014;Zeebe et al, 2016), carbon isotope profiles (Bowen et al, 2015;Kirtland Turner and Ridgwell, 2016) and surface ocean pH (Gutjahr et al, 2017a) the mechanisms responsible for both the climatic and isotope recovery at the end of this transient event are still not well 20 constrained (Bowen and Zachos, 2010). The timescales of silicate weathering and carbonate burial (~100-200 ka) are suggested to be too long to drive the main phase of CIE recovery, which the best records available to date indicate is an order of magnitude faster (~10-20 ka) (Bowen and Zachos, 2010).…”