Atmospheric co 2 during the Midpiacenzian Warm period and the M2 glaciation Elwyn de la Vega ✉ , thomas B. chalk, Paul A. Wilson, Ratna Priya Bysani & Gavin L. foster 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 CO 2 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 highresolution record of atmospheric CO 2 using the δ 11 B-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 CO 2 ranged from − + 389 8 38 ppm to − + 331 , 11 13 ppm, with CO 2 during the KM5c interglacial being − + 371 29 32 ppm (at 95% confidence). Our findings corroborate the idea that changes in atmospheric CO 2 levels played a distinct role in climate variability during the mPWP. They also facilitate ongoing datamodel comparisons and suggest that, at present rates of human emissions, there will be more CO 2 in Earth's atmosphere by 2025 than at any time in at least the last 3.3 million years.

Coral reefs are important ecosystems that are increasingly negatively impacted by human activities. Understanding which anthropogenic stressors play the most significant role in their decline is vital for the accurate prediction of future trends in coral reef health and for effective mitigation of these threats. Here we present annually resolved boron and carbon isotope measurements of two cores capturing the past 90 years of growth of the tropical reef‐building coral Siderastrea siderea from the Belize Mesoamerican Barrier Reef System. The pairing of these two isotope systems allows us to parse the reconstructed pH change into relative changes in net ecosystem productivity and net ecosystem calcification between the two locations. This approach reveals that the relationship between seawater pH and coral calcification, at both a colony and ecosystem level, is complex and cannot simply be modeled as linear or even positive. This study also underscores both the utility of coupled δ11B‐δ13C measurements in tracing past biogeochemical cycling in coral reefs and the complexity of this cycling relative to the open ocean.

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