Very large release of mostly volcanic carbon during the Palaeocene–Eocene Thermal Maximum

Abstract

Global warming during the Palaeocene-Eocene Thermal Maximum1,2 (PETM, ~56 Ma) is commonly interpreted as being primarily driven by the destabilization of carbon from surficial sedimentary reservoirs such as methane hydrates3. However, the source(s) of carbon remain controversial1,3–5. Resolving this is key to understanding the proximal cause, as well as quantifying the roles of triggers versus feedbacks in driving the event. Here we present new boron isotope data – a proxy for seawater pH – that demonstrate the occurrence of persistently suppressed surface ocean pH across the PETM. Our pH data, alongside a paired carbon isotope record, are assimilated in an Earth system model to reconstruct the unfolding carbon cycle dynamics across the event6,7. We find strong evidence for a much larger (>10,000 PgC) and on average isotopically heavier carbon source than considered previously8,9. This leads us to identify volcanism associated with the North Atlantic Igneous Province, rather than carbon from a surficial reservoir, as the main driver of the PETM10,11. We also find that, although amplifying organic carbon feedbacks with climate likely played only a subordinate role in driving the event, enhanced organic matter burial was important in ultimately sequestering the released carbon and accelerating the recovery of the Earth system12.

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

“…In all experiments, the majority of C input occurs in the first 20 ka with a low rate of C input (<0.05 Pg C yr-1) required to maintain low isotope values until 78 ka. Total modelled C input is broadly comparable to previous model-based estimates constrained by deep-ocean carbonate dissolution that exceed ~4000 Pg C (Bowen et al, 2015;Cui et Gutjahr et al, 2017a;Panchuk et al, 2008). In this case the lower bound of carbon input (4154 PgC) is generated by an inversion of the CIE isotopic profile assuming a pure biogenic methane carbon source with isotope composition of -60‰.…”

“…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).…”

Global Extent of Early Eocene Hyperthermal Events: A New Pacific Benthic Foraminiferal Isotope Record From Shatsky Rise (ODP Site 1209)

“…In summary, the new Site 1209 data support the hypothesis of a direct orbital forcing of the carbon cycle in the Eocene (Kirtland Turner et al, ; Lourens et al, ) with infrequent additional carbon released from slowly recharged reservoirs (Dickens, ). This might not be the case for the PETM because its relation to orbital cycles and much larger magnitude compared to all other hyperthermal events requires additional sources of carbon and other causal mechanisms (e.g., Dickens, ; Gutjahr et al, ; Littler et al, ; Zachos et al, ).…”

Linking Marine Plankton Ecosystems and Climate: A New Modeling Approach to the Warm Early Eocene Climate

“…We employ the carbon‐centric version of the GENIE Earth system model of intermediate complexity: cGENIE. This model has been used successfully to explore the interactions between marine biogeochemistry and climate for a range of timescales and time periods including the Eocene and the PETM (e.g., Gibbs et al, ; Gutjahr et al, ; John et al, ; Norris et al, ; Ridgwell & Schmidt, ).…”