Decreasing Atmospheric CO 2 During the Late Miocene Cooling

• Published in: Paleoceanography and paleoclimatology 2020-12, Vol.35 (12)
• Main Authors: Tanner, Thomas, Hernández‐Almeida, Iván, Drury, Anna Joy, Guitián, José, Stoll, Heather
• Format: Article
• Language: English
• ISSN: 2572-4517; 2572-4525
• DOI: 10.1029/2020PA003925

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 p CO 2 decline over this event. Here, we provide a new, high‐resolution p CO 2 proxy record over the LMC based on alkenone carbon isotopic fractionation (ε p ) 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 p CO 2 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 ε p. Our resulting p CO 2 record suggests an approximately twofold to threefold decline over the LMC and confirms a strong coupling between climate and p CO 2 through the late Miocene. Within this long‐term trend are p CO 2 variations on sub‐myr timescales that may reflect 400‐kyr long‐eccentricity cycles, in which p CO 2 minima coincide with several orbital‐scale maxima in published high‐resolution benthic δ 18 O records. These may correspond to ephemeral glaciations, potentially in the Northern Hemisphere. Our temperature and planktonic δ 18 O 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 CO 2 storage may warrant further investigation during the LMC. The alkenone‐based proxy for reconstructing atmospheric CO 2 levels shows a coupling of temperature and CO 2 during the late Miocene cooling Reconstruction of sea‐water δ 18 O shows a freshening and northward shift of the Antarctic Polar Front during the late Miocene cooling Frontal movements and biological pump in the Southern Ocean may have driven CO 2 drawdown and storage in deep ocean