Abstract
The Piacenzian stage of the Pliocene (2.6 to 3.6 Ma) is the most recent past interval of sustained global warmth with mean global temperatures markedly higher (by ~2–3 °C) than today. Quantifying CO2 levels during the mid-Piacenzian Warm Period (mPWP) provides a means, therefore, to deepen our understanding of Earth System behaviour in a warm climate state. Here we present a new high-resolution record of atmospheric CO2 using the δ11B-pH proxy from 3.35 to 3.15 million years ago (Ma) at a temporal resolution of 1 sample per 3–6 thousand years (kyrs). Our study interval covers both the coolest marine isotope stage of the mPWP, M2 (~3.3 Ma) and the transition into its warmest phase including interglacial KM5c (centered on ~3.205 Ma) which has a similar orbital configuration to present. We find that CO2 ranged from {{bf{394}}}_{{boldsymbol{-}}{bf{9}}}^{{boldsymbol{+}}{bf{34}}} ppm to {{bf{330}}}_{{boldsymbol{-}}{bf{21}}}^{{boldsymbol{+}}{bf{14}}} ppm: with CO2 during the KM5c interglacial being {{bf{391}}}_{{boldsymbol{-}}{bf{28}}}^{{boldsymbol{+}}{bf{30}}} ppm (at 95% confidence). Our findings corroborate the idea that changes in atmospheric CO2 levels played a distinct role in climate variability during the mPWP. They also facilitate ongoing data-model comparisons and suggest that, at present rates of human emissions, there will be more CO2 in Earth’s atmosphere by 2025 than at any time in at least the last 3.3 million years.
Highlights
The Piacenzian stage of the Pliocene (2.6 to 3.6 million years ago (Ma)) is the most recent past interval of sustained global warmth with mean global temperatures markedly higher than today
Phase 1 of the Pliocene Model Intercomparison Project (PlioMIP), found an overall agreement between climate model simulations of mid-Piacenzian warm period (mPWP) surface temperatures and the available data when run with a CO2 of 405 ppm[1]
Our new high-resolution CO2 record is shown in Fig. 2 and is consistent with earlier studies[12,13,14] in showing that CO2 was higher than the pre-industrial during the mPWP
Summary
The Piacenzian stage of the Pliocene (2.6 to 3.6 Ma) is the most recent past interval of sustained global warmth with mean global temperatures markedly higher (by ~2–3 °C) than today. Data from other CO2 proxy systems such as stomata and palaeosol δ13C, are lacking in this interval and it has recently been shown that the marine-based alkenone-δ13C-CO2 proxy underestimates CO2 levels in the Pliocene[15,16] limiting its usefulness to providing a minimum CO2 during the mPWP of >270 ppm[16] To address this data deficiency, we developed δ11B-based CO2 estimates from Ocean Drilling Program (ODP) Site 999 in the Caribbean (Supplementary Fig. 1) at a resolution of 1 sample per 3–6 kyr for the time interval 3.15 to 3.35 Ma, encompassing the M2 glaciation and the KM5 interglacial (including KM5c). Focusing predominantly on the mixed layer dwelling planktic species Globigerinoides ruber (45 new data points, 63 in total), we present new measurements of Trilobus sacculifer (2 new data points, 5 total) on the same samples to provide a check on the consistency of the δ11B-pH calibration for G. ruber which has been recently called into question[17]
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