Revisiting the Middle Eocene Climatic Optimum “Carbon Cycle Conundrum” With New Estimates of Atmospheric pCO₂ From Boron Isotopes

Abstract

The Middle Eocene Climatic Optimum (MECO) was a gradual warming event and carbon cycle perturbation that occurred between 40.5 and 40.1 Ma. A number of characteristics, including greater-than-expected deep-sea carbonate dissolution, a lack of globally coherent negative δ 13 C excursion in marine carbonates, a duration longer than the characteristic timescale of carbon cycle recovery, and the absence of a clear trigger mechanism, challenge our current understanding of the Earth system and its regulatory feedbacks. This makes the MECO one of the most enigmatic events in the Cenozoic, dubbed a middle Eocene "carbon cycle conundrum." Here we use boron isotopes in planktic foraminifera to better constrain pCO 2 changes over the event. Over the MECO itself, we find that pCO 2 rose by only 0.55-0.75 doublings, thus requiring a much more modest carbon injection than previously indicated by the alkenone δ 13 C-pCO 2 proxy. In addition, this rise in pCO 2 was focused around the peak of the 400 kyr warming trend. Before this, considerable global carbonate δ 18 O change was asynchronous with any coherent ocean pH (and hence pCO 2 ) excursion. This finding suggests that middle Eocene climate (and perhaps a nascent cryosphere) was highly sensitive to small changes in radiative forcing.Plain Language Summary Geoscientists often look to periods of global warming in the geological past to understand how the Earth responds to input of atmospheric CO 2 . However, during the Middle Eocene Climatic Optimum (or MECO) 40 million years ago, the Earth did not respond in the way one would expect, given what we know from these earlier warming events. The MECO poses a number of puzzles for geoscientists relating to what caused it and why the Earth system responded in the way it did. Before we can hope to answer these questions, however, we need to know what atmospheric CO 2 levels were in the middle Eocene and how much they changed over the MECO event. Here we use boron isotope ratios in fossil plankton shells to tell us how ocean pH (which predominantly reflects CO 2 levels) changed over the MECO. We show that relatively little change in CO 2 at this time were associated with large-scale changes in climate. This suggests that during the Eocene, when CO 2 levels were similar to those likely to be reached by the end of this century, the Earth's climate (and possibly ice sheets) was very sensitive to minor disturbances.

Surface-circulation change in the southwest Pacific Ocean across the Middle Eocene Climatic Optimum: inferences from dinoflagellate cysts and biomarker paleothermometry

“…Taken together, these results confirm previous evidence that once a surface-oceanography-tracking plankton community has become established, relative abundance changes within the community correspond closely with changes in SST (Bijl et al, 2011). In the modern ocean, phytoplankton distribution patterns are driven by the interplay of passive transport by surface currents and temperature selection (Thomas et al, 2012;Hellweger et al, 2016). A similar dual selection mechanism seems to have affected the middle Eocene dinocyst assemblages in the region.…”

Proxy evidence for state-dependence of climate sensitivity in the Eocene greenhouse

“…2 , Supplementary Fig. 1 ) overlaps with existing low-resolution δ 11 B -based records from Tanzania 4 , 5 , and records from the Middle Eocene Climatic Optimum (MECO; ~40.1–40.5 Ma) 11 , Eocene Thermal Maximum 2 (ETM2; 54.1 Ma) 9 , and the Paleocene-Eocene Thermal Maximum (PETM; ~56 Ma) 9 , 10 (all re-calculated for consistency, see “Methods” and Supplementary Data 3 ), and demonstrates the validity of our multi-species treatment of δ 11 B in deriving mixed-layer pH and CO 2 concentrations. This continuous view of the evolution of CO 2 confirms that the highest CO 2 levels, outside of the short-lived increase in CO 2 at the PETM 9 , 10 , 17 , occurred during the Early Eocene Climatic Optimum (EECO; 49–53 Ma 18 ).…”

“…Atmospheric CO 2 began to decline from a maximum at ca. 49 Ma, reaching a minimum immediately prior to the MECO 4,11 where it increased to an average of 1240 (+250/ −210) ppm (or 1490 (+290/−240) ppm using the T. sacculifer calibration) 18 . Following the MECO, CO 2 levels remain largely stable at 900 ± 130 ppm (2 s.d.)…”

“…1 ). This record, coupled to existing δ 11 B-CO 2 reconstructions 4 , 5 , 9 – 11 and novel Global Mean Temperature (GMT) estimates, is used to provide proxy evidence of the state dependency in climate sensitivity, with higher sensitivity during the warm period of the early Eocene, and lower towards the transition to the colder, late Eocene.

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New composite bio- and isotope stratigraphies spanning the Middle Eocene Climatic Optimum at tropical ODP Site 865 in the Pacific Ocean

“…Benthic foraminiferal stable isotope (δ 13 C and δ 18 O) data were generated from the epifaunal taxon Nuttallides truempyi following the taxonomic concept of Holbourn et al (2013). Foraminifers were picked from the 250-300 µm sieve size fraction and cleaned by ultrasonication to remove any loose fine material prior to stable isotope analysis.…”