Middle Miocene climate instability associated with high-amplitude CO2 variability

• Published in: Paleoceanography
• Main Authors: Greenop, Rosanna, Foster, Gavin L., Wilson, Paul A., Lear, Caroline H.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2014
• Subjects: boron isotopes; middle Miocene; CO2; climate change; ANTARCTIC ICE-SHEET; BORON ISOTOPIC COMPOSITION; CARBON-DIOXIDE; PLANKTONIC-FORAMINIFERA; SEAWATER PH; LONG-TERM; SEA-LEVEL; EVOLUTION; RECORD; TEMPERATURE; Antarctic; Antarc*; Ice Sheet; Planktonic foraminifera
• Online Access: https://research-portal.st-andrews.ac.uk/en/publications/be1eca7f-b11b-4c50-9330-aa5b576dff70; https://doi.org/10.1002/2014PA002653

The amplitude of climatic change, as recorded in the benthic oxygen isotope record, has varied throughout geological time. During the late Pleistocene, changes in the atmospheric concentration of carbon dioxide (CO2) are an important control on this amplitude of variability. The contribution of CO2 to climate variability during the pre-Quaternary however is unknown. Here we present a new boron isotope-based CO2 record for the transition into the middle Miocene Climatic Optimum (MCO) between 15.5 and 17Myr that shows pronounced variability between 300ppm and 500ppm on a roughly 100kyr time scale during the MCO. The CO2 changes reconstructed for the Miocene are similar to 2 times larger in absolute terms (300 to 500ppm compared to 180 to 280ppm) than those associated with the late Pleistocene and similar to 15% larger in terms of climate forcing. In contrast, however, variability in the contemporaneous benthic oxygen isotope record (at similar to 1) is approximately two thirds the amplitude of that seen during the late Pleistocene. These observations indicate a lower overall sensitivity to CO2 forcing for Miocene (Antarctic only) ice sheets than their late Pleistocene (Antarctic plus lower latitude northern hemisphere) counterparts. When our Miocene CO2 record is compared to the estimated changes in contemporaneous O-18(sw) (ice volume), they point to the existence of two reservoirs of ice on Antarctica. One of these reservoirs appears stable, while a second reservoir shows a level of dynamism that contradicts the results of coupled climate-ice sheet model experiments given the CO2 concentrations that we reconstruct.