Atmospheric CO2 during the Mid-Piacenzian Warm Period and the M2 glaciation

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 [Formula: see text]ppm to [Formula: see text]ppm, with CO₂ during the KM5c interglacial being [Formula: see text]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.

Figure 1

Top panel: Current CO₂ estimates from boron isotopes across the Plio-Pleistocene boundary. G. ruber data in red circles from site 999 (Martinez-Boti et al.), T. sacculifer in red squares from site 999 (Seki et al.), blue squares from site 926 (Sosdian et al.) and pale red squares from site 999 (Bartoli et al.). Bottom Panel: δ¹⁸O from benthic foraminifer Cibicidoides wuellerstorfi at ODP Site 999 (blue circles) with a 5 point running mean (this study and ref. ) compared to the benthic isotope stack of ref. . M2 glacial and early Pleistocene strong glacials are highlighted in blue for context. Interglacial KM5 and KM5c are highlighted in yellow and orange, respectively. Note that there are no estimates for the M2 glacial and very few across the mid-Piacenzian warm period (mPWP), the low resolution of previous studies makes pin-pointing individual interglacials such as the KM5c future analogue difficult. These studies also differ in their estimates of Mid-Piacenzian CO₂^–,.

Figure 2

Top panel: Red circles and lines show δ¹¹B-derived CO₂ data from Globigerinoides ruber at ODP Site 999 (this study and Martinez-Boti et al., Chalk et al.), red squares are Trilobatus sacculifer at ODP 999 (this study and Seki et al.), purple squares are T. sacculifer from ODP 668 (Honisch et al.) and blue squares are T. sacculifer from ODP 926 (Sosdian et al.). Black solid line shows ice core-derived CO₂ from ref. . Left; Late Pleistocene CO₂ from boron isotopes^,,, and ice core data. Also shown are CO₂ projections in line with RCP8.5 at current emission rates to the year 2040 (black broken line). Middle column; MPT CO₂ estimates^, including disturbed ice estimates^, (Note: age adjusted for scale). Right; mPWP estimates of CO₂ (this study combined with Martinez-Boti et al.), new data from T. sacculifer is shown in red squares and shows no offset from G. ruber estimates. Second panel: Time periods as above, LR04 and ODP 999 δ¹⁸O from C. wuellerstorfi^,,. Third panel: Iron mass accumulation rate from the Southern Atlantic ODP Site 1090. Fourth panel: % Northern Component Water (NCW) estimated from δ¹³C in benthic foraminifera (grey) and ɛ_Nd from fish debris (dark green) in the deep North Atlantic (core U1313). Note the lag of ocean circulation and CO₂ relative to the M2 glaciation.

Figure 3

Distribution plots showing the distribution of absolute CO₂ (left), climate forcing from CO₂ (middle), and δ¹⁸O (right) across three time intervals (where CO₂ cycles are consistent); Late Pleistocene 0–250 ka (LP250, bottom), Mid-Pleistocene (MPT, 1050–1500 ka) and Middle Piacenzian including M2 (mPWP, 3–3.3 Ma). The white dots represent the median values with the quartile ranges shown in the thick and thin black lines. The width of the distribution shows the density of data in each range. Climate forcing (ΔR_CO2) is expressed relative to pre-industrial CO₂ value of 278 ppm. The effect of excluding M2 in the mPWP interval has a negligible effect on the distribution of CO₂ and CO₂ forcing but a visible impact on δ¹⁸O.