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. 2018 Jul 23;9(1):2877.
doi: 10.1038/s41467-018-05104-9.

Middle Eocene greenhouse warming facilitated by diminished weathering feedback

Robin van der Ploeg  1 , David Selby  2   3 , Marlow Julius Cramwinckel  4 , Yang Li  5   6 , Steven M Bohaty  7 , Jack J Middelburg  4 , Appy Sluijs  4
Affiliations

Middle Eocene greenhouse warming facilitated by diminished weathering feedback

Robin van der Ploeg et al. Nat Commun. .

Abstract

The Middle Eocene Climatic Optimum (MECO) represents a ~500-kyr period of global warming ~40 million years ago and is associated with a rise in atmospheric CO2 concentrations, but the cause of this CO2 rise remains enigmatic. Here we show, based on osmium isotope ratios (187Os/188Os) of marine sediments and published records of the carbonate compensation depth (CCD), that the continental silicate weathering response to the inferred CO2 rise and warming was strongly diminished during the MECO-in contrast to expectations from the silicate weathering thermostat hypothesis. We surmise that global early and middle Eocene warmth gradually diminished the weatherability of continental rocks and hence the strength of the silicate weathering feedback, allowing for the prolonged accumulation of volcanic CO2 in the oceans and atmosphere during the MECO. These results are supported by carbon cycle modeling simulations, which highlight the fundamental importance of a variable weathering feedback strength in climate and carbon cycle interactions in Earth's history.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1
Osi values (in blue) and 192Os concentrations (in red) for the analyzed middle Eocene sediments from the three different sites. a ODP Site 959; b ODP Site 1263; c IODP Site U1333. The MECO interval is defined based on TEX86 values for Site 959 (in black; Cramwinckel et al.) and bulk carbonate stable oxygen isotope ratios (δ18O) for Site 1263 (in black; Bohaty et al.). The MECO is characterized by low carbonate content at Site U1333 (in grey; Westerhold et al.). The error bars indicate fully propagated analytical uncertainties (2σ)
Fig. 2
Fig. 2
Comparison of Osi records from the MECO with Osi records from the PETM and ETM2, shown against the overall Osi evolution of the Cenozoic and the relative weathering feedback strength of the Cenozoic. a MECO data from Site 959 (in red), Site 1263 (in blue) and Site U1333 (in green) plotted against age (GTS2012). See Methods for discussion of the age models for the study sites. b MECO data from Sites 959, 1263, and U1333 (this study); PETM and ETM2 data from DSDP Site 549 (in purple) as published in Peucker-Ehrenbrink & Ravizza; Cenozoic data from ferromanganese crusts D11 and CD29 (in black) as published in Klemm et al. and Burton, respectively, based on the updated age model of Nielsen et al.. c Model estimates of the relative continental weathering feedback strength of the Cenozoic as published in Caves et al., based on their CO2 scenario 1 and a logarithmic expression for the weathering feedback
Fig. 3
Fig. 3
LOSCAR and Os cycle model simulations of the most likely MECO scenario. a Forcing for two scenarios involving a gradual, transient 20% increase in the volcanic CO2 flux over ~500 kyr. The solid lines represent a scenario in which the silicate and carbonate weathering fluxes are allowed to vary in response to CO2 forcing (normal weathering feedback), while the dashed lines represent a scenario in which these weathering fluxes are kept constant (diminished weathering feedback). Only the latter scenario corresponds to all observations. b Model response in the 187Os/188Os composition of the global ocean, shown against smoothed fits to the MECO Osi records from the study sites. c Model CCD response of different ocean basins, shown against carbonate content (wt%) records for different depths in the Atlantic, Indian and Pacific oceans as compiled by Sluijs et al. d Model atmospheric pCO2 response and pH response for the surface Atlantic and Pacific oceans. e Model δ13C response for the DIC of the deep Atlantic and Pacific oceans. For a full description of the LOSCAR model, see Zeebe
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References

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