Elsevier

Global and Planetary Change

Volume 170, November 2018, Pages 120-125
Global and Planetary Change

Invited research article
Towards determination of the source and magnitude of atmospheric pCO2 change across the early Paleogene hyperthermals

https://doi.org/10.1016/j.gloplacha.2018.08.011Get rights and content

Highlights

  • We report a nearly continuous record of pCO2 for the early Paleogene.

  • Background pCO2 averaged 569 + 250/−146 ppmv, higher than most existing proxy estimates.

  • Maximum pCO2 occurred during the PETM, followed by H1, I1, and H2.

  • Results suggest that methane hydrate may have played no more than a minority role in the CO2 rise.

  • ESS for the PETM, H1, H2 and I1 is 1.4, 1.0, 1.6 and 0.8 KW−1 m2 respectively.

Abstract

The early Paleogene greenhouse climate is punctuated by a series of extreme global warming events known as hyperthermals that are associated with massive additions of carbon to the ocean-atmosphere system. However, no existing proxies have suitable resolution to capture the change in atmospheric carbon dioxide (pCO2) across these events. Here, we reconstruct a nearly continuous record of pCO2 during the early Paleogene based on changes in terrestrial carbon isotope discrimination calculated from published high-resolution marine and terrestrial carbon isotope records. We calculate relatively stable baseline pCO2 = 569 + 250/−146 ppmv with significant increases in pCO2 at each of four hyperthermals. These background levels are significantly higher than most existing proxy estimates, but still lower than levels commonly assumed within carbon cycle models. Based on the pCO2 levels we calculate across each hyperthermal, we show that these events are associated with carbon additions most likely dominated by terrestrial organic matter oxidation or mantle-derived CO2. By matching the new high-resolution pCO2 data with global temperature data we calculate Earth-system sensitivity of ~0.8 to 1.6 KW−1 m2 across these hyperthermals. The slightly elevated ESS during the PETM and H2 suggests positive feedbacks through other greenhouse gases, changes in vegetation and/or oxidation of organic matter/methane may have amplified the temperature response to CO2 addition.

Introduction

Climate sensitivity (CS; the equilibrium temperature increase due to a doubling of CO2) has important implications for policy makers (Knutti and Hegerl, 2008; Rohling et al., 2012; Rogelj et al., 2014; Knutti et al., 2017; Cox et al., 2018). Current understanding of pre-Quaternary climate sensitivity (or Earth-system sensitivity) is based on individual estimates of pCO2 from at least six different proxies (Royer, 2006; Park and Royer, 2011; Martínez-Botí et al., 2015; Anagnostou et al., 2016; Royer, 2016). Taken together, these data reveal Earth-system sensitivity of 1.6 to 9.6 °C during the Cenozoic (Hoffert and Covey, 1992; Hansen et al., 1993; Covey et al., 1996; Bijl et al., 2010; Lunt et al., 2010; Pagani et al., 2010; Royer, 2016), which includes both fast and slow feedbacks. The data used for these estimates are generally based on intervals of Earth history with stable levels of pCO2; the temperature response to a rapid CO2 increase is perhaps fundamentally different from long-term equilibrium Earth-system sensitivity (Zachos et al., 2008; Royer, 2016). Although some workers have studied past intervals of rapid pCO2 and temperature increases as analogs for anthropogenic climate change (e.g., the Paleocene-Eocene Thermal Maximum, PETM; Zachos et al., 2008), existing proxies are generally unable to resolve the shape or magnitude of pCO2 change across these events. The source of these events is also widely debated (Dickens, 2000; Kurtz et al., 2003; Higgins and Schrag, 2006), which leads to large uncertainty when modeling the pCO2 change associated with these events (Panchuk et al., 2008; Zeebe et al., 2009; Cui et al., 2011). For this reason, a new high-resolution pCO2 proxy capable of resolving pCO2 across sub-million year timescales is needed.

The early Paleogene contains at least four such intervals of significant carbon release between 56 and 53.5 Ma, marked by significant negative carbon isotope excursions (CIEs) identified within both marine and terrestrial substrates (Cramer et al., 2003; Nicolo et al., 2007; Abels et al., 2016; Lauretano et al., 2016). High-resolution oxygen isotope measurements on foraminifera preserved within marine sediments suggest global deep sea temperature increases of as much as 11 °C associated with the largest one of these events (Thomas et al., 2002; Zachos et al., 2003; Tripati and Elderfield, 2004; McCarren et al., 2008; Zachos et al., 2008; Dunkley Jones et al., 2013; Hansen et al., 2013; Lauretano et al., 2015). Existing pCO2 proxies, however, generally fail to precisely resolve the pCO2 rise associated with these events (e.g., Gehler et al., 2016), which makes comparison to present-day anthropogenic CO2 release difficult (Zeebe et al., 2016). Furthermore, modeling efforts to simulate pCO2 levels across these events commonly set background pCO2 = ~750 to 1000 ppmv (Panchuk et al., 2008; Zeebe et al., 2009; Cui et al., 2011; Zeebe et al., 2017), higher than any existing proxy-based value in order to simulate reasonable late Paleocene deep ocean temperature; therefore these simulations may result in overestimations of peak pCO2.

In this study, we present a high-resolution pCO2 record across the early Paleogene hyperthermals based on changes in carbon isotope discrimination between the δ13C value of terrestrial organic matter and that of atmospheric CO213C = (δ13CCO2 – δ13Corg)/(1 + δ13Corg/1000) (Schubert and Jahren, 2012). The effect of pCO2 on carbon isotope discrimination has shown potential for reconstructing pCO2 across geologically short (<1 Myr) timescales (Schubert and Jahren, 2015; Breecker, 2017; Hare et al., 2018), including the determination of background and peak levels of pCO2 during the Paleogene hyperthermals (Schubert and Jahren, 2013; Abels et al., 2016; Cui and Schubert, 2017). Here, we demonstrate a new application of this approach towards generating a nearly continuous record of pCO2 change across these events. These results allow for improved understanding of climate sensitivity in response to geologically rapid (i.e., <1 Myr) pCO2 rise, and comparison of pCO2 proxy results with model predictions.

Section snippets

Methods

Schubert and Jahren (2015) showed how pCO2 can be quantified based on a relative change in carbon isotope discrimination between some time t and a reference time (t = 0), designated as Δ(Δ13C):ΔΔ13C=ABpCO2t+C/[A+BpCO2t+CABpCO2t=0+C/[A+BpCO2t=0+CwhereΔΔ13C=Δ13CtΔ13Ct=0which can be expanded as:ΔΔ13C=δ13CCO2tδ13Corgt/1+δ13Corgt/1000δ13CCO2t=0δ13Corgt=0/1+δ13Corgt=0/1000

By rearranging Eq. (1), one can therefore quantify pCO2(t) using the following equation:ΔΔ13C×A2+ΔΔ13C×A×B×ρCO2t=0+2×ΔΔ13C×A×B

Results

Consistent with previous studies comparing the size of the marine and terrestrial CIEs (Bowen et al., 2004; McInerney and Wing, 2011; Abels et al., 2012, 2016), we observed smaller magnitude changes in δ13C value inferred from the marine record (Fig. 1A) compared with the terrestrial record (Fig. 1B) across each event. After correcting for changes in δ13CCO2, we observed an increase in carbon isotope discrimination (Δ13C) across each of the four CIEs (Fig. 1C), consistent with elevated levels

Discussion

Available proxy data suggest relatively low pCO2 (348 + 112/−76 ppmv) during the early Paleogene (i.e., similar to 20th and 21st century levels) while model simulations generally set background pCO2 during this interval ~2 to 3 × higher than these proxies suggest (Cui et al., 2011; Panchuk et al., 2008; Zeebe et al., 2009, 2017). The lowest pCO2 estimates (<300 ppmv) are primarily based on paleosol carbonate (Cerling, 1992; Sinha and Stott, 1994; Royer et al., 2001) and the revised stomatal

Conclusions

These data represent the first high-resolution pCO2 record across the early Paleogene hyperthermals and show that pCO2 tracks temperature changes across the entirety of this interval. Background pCO2 was likely ~1.5× higher than that determined using other proxies; we assert that these new estimates are more in line with global surface temperatures that are >10 °C warmer than today (Hansen et al., 2013). Revision of the background pCO2 used in model simulations of these events will serve to

Acknowledgement

This research was supported by NSF EAR award #1603051. Y.C. thanks D. Royer for helpful discussions and an Obering postdoctoral fellowship from the Department of Earth Sciences at Dartmouth College.

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