Abstract
AbstractGeological evidence indicates large continental‐scale Antarctic ice volume variations during the early and mid‐Miocene. On million‐year timescales, these variations can largely be explained by equilibrium Antarctic ice sheet (AIS) simulations. In contrast, on shorter orbital timescales, the AIS needs not be in equilibrium with the forcing and ice volume variations may be substantially different. Here, we introduce a conceptual model, based on ice dynamical model results, to investigate the difference between transient variability and equilibrium differences of the Miocene AIS. In our model, an ice sheet will grow (shrink) by a specific rate when it is smaller (larger) than its equilibrium size. We show that phases of concurrent ice volume increase and rising CO2 levels are possible, even though the equilibrium ice volume decreases monotonically with CO2. When the AIS volume is out of equilibrium with the forcing climate, the ice sheet can still be adapting to a relatively large equilibrium size, although CO2 is rising after a phase of decrease. A delayed response of Antarctic ice volume to (covarying) solar insolation and CO2 concentrations can cause discrepancies between Miocene solar insolation and benthic δ18O variability. Increasing forcing frequency leads to a larger disequilibrium and consequently larger CO2‐ice volume phase differences. Furthermore, an amplified forcing amplitude causes larger amplitude ice volume variability, because the growth and decay rates depend on the forcing. It also leads to a reduced average ice volume, resulting from the growth rates generally being smaller than the decay rates.
Highlights
The early and mid-Miocene (23 to 14 Myr ago) is an essential period for studying the dynamics of the Antarctic ice sheet (AIS) in a warmer climate, since it was the last period in Earth's history when extensive continental-scale AIS changes occurred
CO2-Index-V eq Relation To apply our conceptual model to the Miocene AIS, we prescribe a C-V eq relation based on ice dynamical model results we presented in an earlier study (Stap et al, 2019) (Figures 2c and 2d)
Concurrent ice volume increase and CO2 level rise are explained as a consequence of increased precipitation outweighing increased surface melt in a warmer climate (Oerlemans, 1991)
Summary
The early and mid-Miocene (23 to 14 Myr ago) is an essential period for studying the dynamics of the Antarctic ice sheet (AIS) in a warmer climate, since it was the last period in Earth's history when extensive continental-scale (tens of meters sea level equivalent) AIS changes occurred. Geological evidence from sea level records (John et al, 2011; Kominz et al, 2008; Miller et al, 2005) and ice-proximal ocean sediment cores (e.g., Levy et al, 2016) indicates that the AIS ranged from being almost vanished to its modern-day marine-terminating configuration during this time. One factor was the focus of an earlier study (Stap et al, 2019). It concerns to what extent these benthic δ18O fluctuations are caused by deep-sea temperature changes and to what extent by ice volume changes
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