A symmetrical CO2 peak and asymmetrical climate change during the middle Miocene

Abstract Understanding the future trajectory of Earth's climate requires knowledge of shifts in atmospheric CO 2 concentrations during past warm episodes. The Miocene Climatic Optimum (MCO, ∼17–14 Ma) was likely the warmest episode of the past 25 Myr, and thus atmospheric CO 2 concentrations during this interval are of particular interest. However, CO 2 records across the middle Miocene are rather scattered and data are notably sparse for the latter part of the MCO. Here we present a paleosol-based CO 2 record from the Tianshui Basin, northern China, spanning 17–7 Ma. Our results show elevated mean CO 2 during the second half of the MCO corresponding with some of the lowest benthic δ 18 O values and highest benthic δ 13 C values, as part of the “Monterey excursion”, published for the Neogene. This result supports the idea that the broader Monterey excursion was primarily associated with a CO 2 maximum, not carbon burial and CO 2 minima as previously interpreted. The new CO 2 record, along with previous CO 2 records based on paleosols, stomata and foraminiferal boron isotope compositions, also suggests that mean CO 2 across the MCO was elevated compared with the immediately following (post-MCO, 14–11 Ma, >80% probability) and immediately preceding (pre-MCO, 20–17 Ma, 70% probability) time periods. The most probable magnitude of the MCO CO 2 peak is 20% higher than post-MCO and 12.5% higher than pre-MCO levels. Larger factors, of perhaps 50% higher CO 2, likely apply in narrower ( 2 records from each proxy individually support the conclusion of modestly elevated MCO CO 2 , although large temporal gaps exist in records from any one proxy. Using all proxies together, we estimate average MCO CO 2 of 375+150/−100 (84th and 16th percentile) ppm. Although mean MCO CO 2 was elevated, the MCO was also characterized by highly variable CO 2 . In addition, determinations from all three proxies suggest that at times during the MCO, CO 2 levels were as low as they were following the ice sheet expansion of the Miocene Climate Transition. Furthermore, pre-MCO CO 2 levels are indistinguishable from post-MCO CO 2 levels (60% probability of pre-MCO CO 2 > post-MCO CO 2 ), despite significantly lower benthic δ 18 O values during the former. We conclude that 1) the MCO was a period of slightly elevated and highly variable CO 2 compared with the immediately preceding and following intervals, and 2) neither CO 2 decrease, orbitally-controlled seasonality over Antarctica nor the confluence of these factors was sufficient to cause Miocene Climate Transition ice sheet expansion. Rather strengthening of the Antarctic Circumpolar Current and Southern Ocean cooling related to closure of the eastern Tethys was a necessary first step.