Decreasing Atmospheric CO₂ During the Late Miocene Cooling

Abstract

A pronounced late Miocene cooling (LMC) from ~7 to 5.7 Ma has been documented in extratropical and tropical sea surface temperature records, but to date, available proxy evidence has not revealed a significant pCO2 decline over this event. Here, we provide a new, high‐resolution pCO2 proxy record over the LMC based on alkenone carbon isotopic fractionation (εp) measured in sediments from the South Atlantic at Ocean Drilling Program (ODP) Site 1088. We apply a recent proxy calibration derived from a compilation of laboratory cultures, which more accurately reflects the proxy sensitivity to pCO2 changes during late Quaternary glacial‐interglacial cycles, together with new micropaleontological proxies to reconstruct past variations in algal growth rate, an important secondary influence on the εp. Our resulting pCO2 record suggests an approximately twofold to threefold decline over the LMC and confirms a strong coupling between climate and pCO2 through the late Miocene. Within this long‐term trend are pCO2 variations on sub‐myr timescales that may reflect 400‐kyr long‐eccentricity cycles, in which pCO2 minima coincide with several orbital‐scale maxima in published high‐resolution benthic δ18O records. These may correspond to ephemeral glaciations, potentially in the Northern Hemisphere. Our temperature and planktonic δ18O records from Site 1088 are consistent with substantial equatorward movement of Southern Ocean frontal systems during the LMC. This suggests that potential feedbacks between cooling, ocean circulation and deep ocean CO2 storage may warrant further investigation during the LMC.

The Miocene: The Future of the Past

“…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 pCO 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 pCO 2 declined from 600 to below 300 ppm, punctuated by substantial orbital scale variability (Tanner et al, 2020).…”

Hydrologic Changes Drove the Late Miocene Expansion of C4 Grasslands on the Northern Indian Subcontinent

“…Why then were C 4 plants not widespread? No unambiguous mechanism accounts for the timing of the broadly synchronized and near-global expansions, although declining atmospheric pCO 2 is a potential candidate (Cerling et al, 1997;Ehleringer et al, 1997;Herbert et al, 2016;Polissar et al, 2019;Stoll et al, 2019;Tanner et al, 2020). The timing and tempo of C 4 grassland expansion differ by continental region, highlighting the importance of local factors as proximal drivers of ecosystem change (Pagani et al, 1999).…”

“…Pagani et al, 1999;Sosdian et al, 2018). Although accurate reconstruction of past CO 2 change remains a challenge, recent studies suggest CO 2 levels were higher and then declined across the late Miocene (Bolton et al, 2016;Mejia et al, 2017;Stoll et al, 2019;Tanner et al, 2020). The earlier establishment of C 4 ecosystems in the northwest and east Africa has been attributed to declining CO 2 levels (Polissar et al, 2019).…”

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

“…MCO: Miocene Climatic Optimum; MMCT: middle Miocene Climatic Transition; CM6: last δ 13 C maximum within Monterey Excursion; LMCIS: Late Miocene Carbon Isotope Shift. Sources for pCO2 reconstructions: 1. alkenone data from ODP Sites 925 and 929 (Stoll et al, 2019); 2. alkenone data from ODP Site 806 (Stoll et al, 2019); 3. alkenone data from Zhang et al (2013), recalibrated by Stoll et al (2019); 4. alkenone data from Bolton et al (2016), recalibrated by Stoll et al (2019); 5. simulation 6, 50 th percentile (Tanner et al, 2020); 6. alkenone data from…”