Abstract
AbstractCurrent atmospheric CO2 concentration is known to be higher than it has been during the past ∼800 k.y. of Earth history, based on direct measurement of CO2 within ice cores. A comparison to the more ancient past is complicated by a deficit of CO2 proxies that may be applied across very long spans of geologic time. Here, we present a new CO2 record across the past 23 m.y. of Earth history based on the δ13C value of terrestrial C3 plant remains, using a method applicable to the entire ∼400 m.y. history of C3 photosynthesis on land. Across the past 23 m.y., CO2 likely ranged between ∼230 ppmv and 350 ppmv (68% confidence interval: ∼170–540 ppm). CO2 was found to be highest during the early and middle Miocene and likely below present-day levels during the middle Pliocene (84th percentile: ∼400 ppmv). These data suggest present-day CO2 (412 ppmv) exceeds the highest levels that Earth experienced at least since the Miocene, further highlighting the present-day disruption of long-established CO2 trends within Earth’s atmosphere.
Highlights
Knowledge of atmospheric CO2 concentration is vital for understanding Earth’s climate system because it imparts a controlling effect on global temperatures across recent (Hegerl et al, 2006) and geologic (Foster et al, 2017) time scales
Direct measurement of CO2 has been performed at the Mauna Loa Observatory (Hawaii, USA) for the past 60+ yr, and historical CO2 has been sampled continuously from ice-core bubbles recording the past 800 k.y. (Petit et al, 1999; Lüthi et al, 2008), allowing for trends in CO2 during the latter portion of the Quaternary to be evaluated in detail
Using a compilation of 700 δ13C measurements gathered from 12 previously published studies of terrestrial organic matter (TOM; n = 441) and plant lipids (n = 259) that spanned at least 1 m.y. of the Neogene (Table S1 in the Supplemental Material1)
Summary
Knowledge of atmospheric CO2 concentration is vital for understanding Earth’s climate system because it imparts a controlling effect on global temperatures across recent (Hegerl et al, 2006) and geologic (Foster et al, 2017) time scales. Proxies (Breecker et al, 2010) and models (Royer et al, 2014) indicate that CO2 has varied widely during the geologic past. For time periods older than the Pleistocene, many CO2 proxies have been applied, including the proportion of epidermal cells that are stomatal pores (Kürschner et al, 1996, 2008; Beerling et al, 2009; Grein et al, 2013; Wang et al, 2015; Reichgelt et al, 2016); the stable carbon isotope composition of paleosol c arbonate We illustrate its efficacy by presenting a novel, high-resolution record of CO2 for the Neogene through the Quaternary (i.e., the past 23 m.y.), a period that lacks a continuous record of CO2 from any single proxy
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