Middle Miocene climate instability associated with high‐amplitude CO₂ variability

Abstract

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 (CO 2 ) are an important control on this amplitude of variability. The contribution of CO 2 to climate variability during the pre-Quaternary however is unknown. Here we present a new boron isotope-based CO 2 record for the transition into the middle Miocene Climatic Optimum (MCO) between 15.5 and 17 Myr that shows pronounced variability between 300 ppm and 500 ppm on a roughly 100 kyr time scale during the MCO. The CO 2 changes reconstructed for the Miocene are~2 times larger in absolute terms (300 to 500 ppm compared to 180 to 280 ppm) than those associated with the late Pleistocene and~15% larger in terms of climate forcing. In contrast, however, variability in the contemporaneous benthic oxygen isotope record (at~1‰) is approximately two thirds the amplitude of that seen during the late Pleistocene. These observations indicate a lower overall sensitivity to CO 2 forcing for Miocene (Antarctic only) ice sheets than their late Pleistocene (Antarctic plus lower latitude northern hemisphere) counterparts. When our Miocene CO 2 record is compared to the estimated changes in contemporaneous δ 18O 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 CO 2 concentrations that we reconstruct.

Boundary conditions for the Middle Miocene Climate Transition (MMCT v1.0)

“…Although somewhat arbitrary, these values are within the range of published estimates. The 400 ppmv MMCO estimate represents a maximum rather than a mean (within the interval ~15-17 Ma) and is in favourable agreement with Foster et al (2012) and Tripati et al (2009), as well as with the most recent alkenone-and boron isotope-based p CO 2 reconstructions for the MMCO by Zhang et al (2013) and Greenop et al (2014). The 200 ppmv MMG estimate represents a minimum rather than a mean (within the interval ~12-15 Ma) and is in good agreement with Foster et al (2012) andPagani et al (2005).…”

The influence of ice sheets on temperature during the past 38 million years inferred from a one-dimensional ice sheet–climate model

“…5 we show the new reference CO 2 results together with proxy data over three periods for which data are relatively abundant: the Late (Pagani et al, 1999(Pagani et al, , 2011Badger et al, 2013) are indicated by orange asterisks. Boron-isotope-based data (Pearson et al, 2009;Foster et al, 2012;Greenop et al, 2014;Martínez-Botí et al, 2015) are indicated by blue plus symbols. Stomata-based data (Van der Burgh et al, 1993;Kürschner, 1996;Kürschner et al, 2008;Retallack, 2009) Pliocene (3.5 to 2.5 Myr ago), the Middle Miocene (18 to 13 Myr ago), and the Early Oligocene (35 to 30 Myr ago).…”

“…Stomata-based data (Van der Burgh et al, 1993;Kürschner, 1996;Kürschner et al, 2008;Retallack, 2009) Pliocene (3.5 to 2.5 Myr ago), the Middle Miocene (18 to 13 Myr ago), and the Early Oligocene (35 to 30 Myr ago). The data are based on three important proxies (see also Beerling and Royer, 2011): alkenones (Pagani et al, 1999(Pagani et al, , 2011Badger et al, 2013), boron isotopes (Pearson et al, 2009;Foster et al, 2012;Greenop et al, 2014;Martínez-Botí et al, 2015), and stomata (Van der Burgh et al, 1993;Kürschner, 1996;Kürschner et al, 2008;Retallack, 2009). During the Late Pliocene, the simulated CO 2 is more variable than the alkenone data from Badger et al (2013) (Fig.…”

Boron Isotopes in Foraminifera: Systematics, Biomineralisation, and CO2 Reconstruction

“…Boron isotope measurements on fossil carbonates thus have the potential to constrain changes in pH and CO 2 over geological time (Pearson and Palmer 2000;Hönisch et al 2009;Rae et al 2014;Penman et al 2014;Greenop et al 2014;Martínez-Botí et al 2015a). The shells, or "tests", of foraminifera, which are single-celled protists, provide a convenient host for the d 11 B-pH proxy, being widely distributed throughout the world's oceans, relatively well-preserved in marine sediment cores, and having chemistry that closely reflects surrounding seawater conditions (Wefer et al 1999;Erez 2003).…”