Two massive, rapid releases of carbon during the onset of the Palaeocene–Eocene thermal maximum

Abstract

The Earth's climate abruptly warmed by 5-8 • C during the Palaeocene-Eocene thermal maximum (PETM), about 55.5 million years ago 1,2 . This warming was associated with a massive addition of carbon to the ocean-atmosphere system, but estimates of the Earth system response to this perturbation are complicated by widely varying estimates of the duration of carbon release, which range from less than a year to tens of thousands of years. In addition the source of the carbon, and whether it was released as a single injection or in several pulses, remains the subject of debate 2-4 . Here we present a new high-resolution carbon isotope record from terrestrial deposits in the Bighorn Basin (Wyoming, USA) spanning the PETM, and interpret the record using a carbon-cycle box model of the ocean-atmosphere-biosphere system. Our record shows that the beginning of the PETM is characterized by not one but two distinct carbon release events, separated by a recovery to background values. To reproduce this pattern, our model requires two discrete pulses of carbon released directly to the atmosphere, at average rates exceeding 0.9 Pg C yr −1 , with the first pulse lasting fewer than 2,000 years. We thus conclude that the PETM involved one or more reservoirs capable of repeated, catastrophic carbon release, and that rates of carbon release during the PETM were more similar to those associated with modern anthropogenic emissions 5 than previously suggested 3,4 .Global climate warming from the late Palaeocene through early Eocene was punctuated by several transient hyperthermal events 2 . The most prominent of these events, the PETM, was characterized by a 5-8 • C temperature increase, a 3-6 negative carbon isotope excursion (CIE), and profound shifts in biotic communities 2 . The PETM CIE reflects a massive release of 13 C-depleted carbon to the ocean-atmosphere system at the event's onset. The source(s) and mechanism(s) of carbon release remain controversial, with hypotheses including thermal dissociation of marine methane hydrates, widespread oxidation of organic carbon, igneous intrusion into organic-rich sediments, and bolide impact 2 . Improved constraints on the pattern and pace of PETM carbon-cycle change, particularly during the event's onset, are needed to further evaluate these mechanisms. However, recent publications paint widely divergent pictures of the CIE onset, ranging from slow, protracted (20,000 year) carbon release 4 to a single, massive sub-decadal carbon burst 3 .We recovered new sedimentological and isotopic records from approximately 375 m of strata spanning the Palaeocene-Eocene boundary in two boreholes at Polecat Bench (PCB, Bighorn Basin, Wyoming; Fig. 1) as part of the Bighorn Basin Coring Project (BBCP). Fluvial deposits of the Willwood Formation, which contains the PETM at this site, consist of mudrocks with moderate to strong pedogenic overprinting, heterolithic deposits with weak pedogenic overprinting, and channel and sheet sandstones 6 (Fig. 1). Authigenic palaeosol carbonate is present as semi-round...

Anthropogenic carbon release rate unprecedented during the past 66 million years

“…2a). Our results do not support a 2-step carbon release 32 for which the effect of bioturbation and mixing with our estimated sedimentation rate at Millville would only damp, not obliterate, a prominent δ 13 C reversal midway through the onset. We note that a previous study determined leads/lags between climatic/biotic events at one PETM site 33 .…”

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‰.…”

“…Total carbon input masses, constrained by the most detailed surface ocean pH records of the PETM available to date, are modelled to be >10,000 Pg C 15 (Gutjahr et al, 2017c), but the dynamics of this release must be considerably quicker than assumed within the deep-ocean records recovered from Site DSDP Site 401, in order to satisfy the rate of CIE onset observed at Zumaia, which is ~3.3 ‰ over less than 5 ka. We argue that this CIE onset rate is robust, given that it is observed in both terrestrial (Bowen et al, 2015) and marine (this study) sections where the onset of the CIE is expanded and set within a robust intra-PETM cyclostratigraphic age 20 model. The CIE recorded at Zumaia, and within the Big Horn Basin (Bowen et al, 2015), of 4 ‰ or more, is also greater than that assumed in the favoured model simulations of Gutjahr et al (2017) of 2.6 ‰, and is closer to their alternate run assuming a CIE of ~4 ‰, which reconstructs a source carbon source with a δ 13 C composition of ~ -17 ‰.…”

“…The most consistent explanation for these coupled perturbations is the release of carbon from a large shallow lithospheric reservoir, with a depleted carbon isotopic (δ 13 C) signature (~ -20 to -60 ‰), on a multi-millenial timescale (Bowen et al, 2015;Dickens et al, 1995;Gutjahr et al, 2017b;10 Kirtland Turner and Ridgwell, 2016;Zeebe et al, 2016). 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).…”

“…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).…”