The Eocene–Oligocene Transition (EOT) marks the onset of modern icehouse conditions when continental-scale ice sheets enveloped Antarctica (‘Oi1,’ ∼33.7 Ma, Coxall et al., 2005; Ehrmann and Mackensen, 1992). Oi1 typifies the non-linearity of global climate, with the emplacement of an ice volume between 60% and 100% of that present today on Antarctica over only ∼400 ka, superimposed on a million-year timescale cooling trend (Coxall et al., 2005; Lear et al., 2008; Zachos et al., 2001). Declining atmospheric pCO2 levels are thought to have driven the accelerated onset of Antarctic glaciation during Oi1, with numerical models suggesting a ‘threshold’ response of the cryosphere to a long-term lowering of pCO2 (DeConto and Pollard, 2003). Recent reconstruction of pCO2 across the late Eocene and early Oligocene (Pagani et al., 2011) documents a decline that began ∼2 Ma prior to geological and stable isotope evidence for Antarctic glaciation, supporting this hypothesis. However, the mechanisms responsible for this gradual pCO2 drawdown remain unclear. The biological carbon pump, in particular associated with increased diatom abundance in the Southern Ocean during the late Eocene, represents a potential candidate (Rabosky and Sorhannus, 2009; Salamy and Zachos, 1999; Scher and Martin, 2006).
Oi1
∼33.7 Ma
Ma
ice volume
present today on Antarctica
between 60% and 100%
%
Oi1
emplacement of an ice volume
∼400 ka
ka
timescale
million-year
year
decline
began
∼2 Ma
Ma