The MMCO‐EOT conundrum: Same benthic δ¹⁸O, different CO₂

Abstract

Knowledge on climate change during the Cenozoic largely stems from benthic δ18O records, which document combined effects of deep‐sea temperature and ice volume. Information on CO2 is expanding but remains uncertain and intermittent. Attempts to reconcile δ18O, sea level, and CO2 by studying proxy data suffer from paucity of data and apparent inconsistencies among different records. One outstanding issue is the difference suggested by proxy CO2 data between the Eocene‐Oligocene boundary (EOT) and the Middle‐Miocene Climatic Optimum (MMCO), while similar levels of δ18O are shown during these times. This conundrum implies changing relations between δ18O, CO2, and temperature over time. Here we use a coupled climate‐ice sheet model, forced by two different benthic δ18O records, to obtain continuous and mutually consistent records of δ18O, CO2, temperature, and sea level over the period 38 to 10 Myr ago. We show that the different CO2 levels between the EOT and MMCO can be explained neither by the standard configuration of our model nor by altering the uncertain ablation parametrization on the East Antarctic Ice Sheet. However, we offer an explanation for the MMCO‐EOT conundrum by considering erosion and/or tectonic movement of Antarctica, letting the topography evolve over time. A decreasing height of the Antarctic continent leads to higher surface temperatures, reducing the CO2 needed to maintain the same ice volume. This also leads to an increasing contribution of ice volume to the δ18O signal. This result is, however, dependent on how the topographic changes are implemented in our ice sheet model.

The influence of ice sheets on temperature during the past 38 million years inferred from a one-dimensional ice sheet–climate model

“…5c, yellow shading). Using a different tuning, our Middle Miocene values are closer to the observations, but then the Early Oligocene values are also much lower (see Stap et al, 2016b, for an analysis). In short, we are not able to simulate a difference in CO 2 between right before the Eocene-Oligocene transition and during the Middle Miocene climatic optimum.…”

“…The tuning targets and consequent values for C abl have been described in Stap et al (2014). The influence of this parameter for inception of the East Antarctic ice sheet has been discussed in Stap et al (2016b), who showed that changing the value from −30 to −5 results in glacial inception at roughly 450 ppm instead of 700 ppm. The ice sheet models calculate the surface height change and ice sheet extent using the shallow ice approximation with 15 to 25 km of resolution depending on the ice sheet (De Boer et al, 2010).…”

“…As a spin-up the model was initialised with a 1500 ppm CO 2 concentration for 50 kyr and thereafter run for 2 Myr between 40 and 38 Myr ago using the inverse routine. This run is an extension of the reference run used in Stap et al (2016b) to include the past 10 Myr. The record of Lisiecki and Raymo (2005) was used to force the 5 Myr run after initialising the model for 100 kyr with 430 ppm CO 2 .…”

“…We recognise that our simulation of CO 2 may be improved in subsequent studies that include geological processes that are still missing in our model set-up. For instance, tectonics leading to mountain uplift (Kutzbach et al, 1993) and closure of sea ways (Kennett, 1977;Toggweiler and Bjornsson, 2000;Hamon et al, 2013), erosion (Wilson et al, 2012;Gasson et al, 2015;Stap et al, 2016b), and vegetation changes (Knorr et al, 2011;Liakka et al, 2014;Hamon et al, 2012) may have affected the climate system during the past 38 Myr. Here, we will purely focus on the influence of ice sheets on the climate, in particular the relation between CO 2 and temperature, during this time.…”

“…The resulting values were used to force the coupled model over the past 5 Myr. In Stap et al (2016b), the model was run over the period 38 to 10 Myr ago, and the influence of Antarctic topographic changes on the simulated CO 2 was investigated. In this study, however, changes in topographic boundary conditions are not included, although their effect is briefly discussed.…”