Atmospheric CO2 during the Late Miocene Cooling

Abstract

<p>Alkenone sea surface temperature records recently observed suggest a substantial long-term and large-magnitude ocean surface cooling during the Late Miocene. At the same time, starting about seven million years ago, both hemispheres on Earth witnessed synchronous cooling and large areas of the continents experienced drying and enhanced seasonality. Coinciding with this climatic shift were significant changes in ecology, including the rise of C<sub>4</sub>-photosynthesizing terrestrial plants and the emergence of so-called "vital effects" in oceanic coccolithophores. These changes are collectively hypothesized to be induced by declining atmospheric CO<sub>2</sub>. However, the sparse proxy data available for this time interval limits our understanding of the link between these changes and atmospheric greenhouse gas fluctuations and has let people to propose a "climate-CO<sub>2</sub> decoupling".<br />In this study, the alkenone based pCO<sub>2</sub> proxy is used to reconstruct atmospheric CO<sub>2</sub> for the time interval between 4.5 and 8.5 Ma. Estimations are based on the carbon isotopic fractionation during photosynthesis (εp) and a new statistical multilinear regression model based on an analysis of culture and sediment data. Past coccolithophore growth rates are reconstructed using foraminiferal isotopic-based proxies, related to water column structure which favour or limit nutrient supply to the photic zone. A thorough sensitivity analysis of modern and past  εp values and its influencing factors in the Southern Ocean yield to a new, high resolution pCO<sub>2</sub> record. Estimated pCO<sub>2</sub> concentrations synchronously decline with the observed long-term cooling (5°C) from 6.8 to 5.9 Ma, periodically decreasing to sufficiently low values of <200 ppm, potentially inducing ephemeral Northern Hemisphere glaciation. CO<sub>2</sub> concentrations during the Late Miocene Cooling Event are thus successfully reproduced in this study and allow a reasonable interpretation of past conditions as has not yet been previously achieved in the relevant literature. </p>

The Miocene: The Future of the Past

“…(b) p CO 2 estimates derived from the phytoplankton proxy. Light turquoise and blue records are derived from alkenone isotopic fractionation in sites in the tropical Atlantic (Bolton et al., 2016; Y. G. Zhang, Pagani, et al., 2013), applying new culture‐based calibrations (Bolton et al., 2016; Stoll et al., 2019; Y. G. Zhang, Pagani, et al., 2013); the dark blue curve to the left is derived from alkenone isotope fractionation in the South Atlantic applying new culture‐based calibrations (Tanner et al., 2020). The purple record is derived from diatom‐bound organic matter from a tropical Pacific site, applying a culture‐based calibration model (Mejia et al., 2018).…”

“…A late Miocene decline in p CO 2 has been estimated from both alkenone (Bolton et al., 2016) and diatom‐derived (Mejía et al., 2017) ε p records; the latter suggest a decrease from 794 ± 233 ppm at ∼11 Ma to 288 ± 25 ppm at ∼6 Ma (Figure 5b). Higher resolution alkenone records suggest between 8 and 5 Ma, average p CO 2 declined from 600 to below 300 ppm, punctuated by substantial orbital scale variability (Tanner et al., 2020).…”

A ∼12 Myr Miocene Record of East Asian Monsoon Variability From the South China Sea

“…(2016), recalibrated by Stoll et al. (2019); (5) simulation 6, 50th percentile (Tanner et al., 2020); (6) alkenone data from Badger et al. (2013); (7) Boron isotope data from Badger et al.…”

“…Between ∼7.7 and 7 Ma, a massive long‐term decrease in δ 13 C of >1‰, corresponding to the global decline known as the late Miocene carbon isotope shift (LMCIS; Keigwin, 1979; Keigwin & Shackleton, 1980), indicates fundamental changes in carbon cycling during the peak phase of the Biogenic Bloom (Figures 3 and 5). This period was characterized by global cooling (Herbert et al., 2016), a marked decline in atmospheric p CO 2 (Tanner et al., 2020; Figure 5), and by widespread continental aridification of the Northern Hemisphere (e.g., Guo et al., 2002; J. Liu et al., 2016; Wan et al., 2007).…”

A genomic approach to the evolution, diversification and domestication of the genus Citrus

“…Assuming that the C. linczangensis fossil represented the last common ancestor of all Citrus, the main Citrus speciation events were estimated to occur in a period no longer than 2 million years, starting 8 Mya (Wu et al, 2018), during the Late Miocene. It is widely accepted that the global climate went through rapid changes in this epoch, producing a worldwide cooling, likely triggered by the reduction of the planetary CO2 levels (Holbourn et al, 2018;Tanner et al, 2020). In South East Asia, this period of cooling was also accompanied with the aridification of the region (Herbert et al, 2016), generating a series of climatic conditions that might have forced Citrus to rapidly diversify and radiate.…”