Declining Atmospheric pCO2 During the Late Miocene and Early Pliocene: New Insights from Paired Alkenone and Coccolith Stable Isotope Barometry

Abstract:

The relationship between atmospheric CO₂ concentrations and climate is broadly understood for the Cenozoic era: warmer periods are associated with higher atmospheric carbon dioxide. This understanding is supported by atmospheric samples of the past 800,000 years from ice cores, which suggest CO₂ levels play a key role in regulating global climate on glacial interglacial timescales as well. In this context, the late Miocene poses a challenge: sea-surface temperatures indicate substantial global warmth, though existing data suggest atmospheric CO₂ concentrations were lower than pre-industrial values.

Recent work using the stable carbon and oxygen isotopic composition of coccolith calcite has demonstrated these organisms began actively diverting inorganic carbon away from calcification and to the site of photosynthesis during the late Miocene. This process occurs in culture experiments in response to low aqueous CO₂ concentrations, and suggests decreasing atmospheric pCO₂ values during the late Miocene. Here we present new data from ODP Site 806 in the western equatorial Pacific Ocean that supports declining atmospheric CO₂ across the late Miocene. Carbon isotope values of coccolith calcite from Site 806 demonstrate carbon limitation and re-allocation of inorganic carbon to photosynthesis starting between ~8 and 6 Ma. The timing of this limitation at Site 806 precedes shifts at other ODP sites, reflecting the higher mixed layer temperature and resultant lower CO₂ solubility at Site 806. New measurements of carbon isotope values from alkenones at Site 806 show an increase in photosynthetic carbon fractionation (ε_p) accompanied the carbon limitation evident from coccolith calcite stable isotope data. While higher ε_p is typically interpreted as higher CO₂ concentrations, at Site 806, our data suggest it reflects enhancement of chloroplast CO₂ from active carbon transport by the coccolithophore algae in response to lower CO₂ concentrations.

Our new data from ODP Site 806 combined with previous published measurements suggests atmospheric CO₂ values declined across the late Miocene and early Pliocene. This decline is coincident with decreasing ocean temperatures suggesting the fundamental relationship between atmospheric CO₂ and climate can qualitatively explain late Miocene warmth.