Emplacement of Antarctic ice sheet mass affects circumpolar ocean flow

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Highlights

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    The Antarctic ice sheet mass affects Southern Ocean bathymetry.

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    The induced pressure and density variations affect mean flow and its variability.

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    Eocene-Oligocene realistic bathymetry test case shows strong frontal shifts.

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    Heat transport, nutrient availability, erosion/sedimentation could have been impacted.

Abstract

During the Cenozoic the Antarctic continent experienced large fluctuations in ice-sheet volume. We investigate the effects of Glacial Isostatic Adjustment (GIA) on Southern Ocean circulation for the first continental scale glaciation of Antarctica (~ 34 Myr) by combining solid Earth and ocean dynamic modeling. A newly compiled global early Oligocene topography is used to run a solid Earth model forced by a growing Antarctic ice sheet. A regional Southern Ocean zonal isopycnal adiabatic ocean model is run under ice-free and fully glaciated (GIA) conditions. We find that GIA-induced deformations of the sea bottom on the order of 50 m are large enough to affect the pressure and density variations driving the ocean flow around Antarctica. Throughout the Southern Ocean, frontal patterns are shifted several degrees, velocity changes are regionally more than 100%, and the zonal transport decreases in mean and variability. The model analysis suggests that GIA induced ocean flow variations alone could impact local nutrient variability, erosion and sedimentation rates, or ocean heat transport. These effects may be large enough to require consideration when interpreting the results of Southern Ocean sediment cores.

Section snippets

Motivation

At the Eocene–Oligocene boundary ~ 33 million years ago (Myr), the Southern Hemispheric climate system experienced a rapid transition. The quasi ice-free Antarctic continent glaciated within less than 5 · 105 years, oscillated in orbitally paced glacial cycles between 40% and 140% of its present day volume for about ten million years, almost vanished at the Oligocene–Miocene boundary (23 Myr), but increased again in the mid to late Miocene (Hambrey et al., 1991, Zachos et al., 1997, DeConto et al.,

Reconstruction of early Oligocene topography

We compiled a global early Oligocene topography from different datasets (Fig. 1). Markwick et al. (2000) is used for the continental shape and topography, Wilson et al. (2012) for the Antarctic topography, and Müller et al. (2008) for the deep sea bathymetry. The shelf and coast areas are interpolated between these datasets (Somme et al., 2009), dependent upon how much is known for a region (e.g., Close et al., 2009, for the continental margin along Wilkes Land). Poorly constrained regions were

Results

In the following, we compare the two equilibrated ocean simulations (control and GIA). All plotted fields are 25 year averages following a spin up of 120 years. The mean flow of the control case (Fig. 4a and c) is overall eastward and strong frontal patterns can be seen, e.g. south of Australia. Locally velocities reach up to 1.5 m/s, but are mostly within 0.1 to 0.3 m/s, which compares well to today's ACC velocities (Zambianchi et al., 1999). Since the Drake Passage width is similar to modern

Analysis

To interpret the flow differences between the control and GIA case, we analyze the depth integrated vorticity equation. Here, we use the more transparent Cartesian form, whereas HIM uses spherical coordinates.u¯⋅∇fH=1H2∂χ∂y∂H∂x−∂χ∂x∂H∂y−∂∂yτxHρ0+R,where u¯=h1u¯1+h2u¯2 is the vertically integrated velocity, f is the latitudinally dependent Coriolis parameter, H = h1 + h2 is the sum of the layer depths andχ=∫zρ0∂p∂zdzis a measure of the vertically integrated potential energy. Furthermore, τx is the

Discussion and implications

In the following we discuss time constraints and the uncertainty of the representation of the topography. We link our finding to proxy measures and suggest in which directions further research could follow up.

We stress that the mechanism proposed here holds for all continental configurations with large ice volume fluctuations on Antarctica and open and deep Southern Ocean gateways. Our estimates of possible flow change can be interpreted as conservative for the Eocene–Oligocene transition since

Conclusion

Within a two-layer ocean model, we have shown that GIA bathymetry changes may induce substantial Southern Ocean flow variations. The dominant mechanism causing these flow changes is attributed to the change in the JEBAR effect, which represents the impact of bottom topography on a stratified flow. Locally, however, the vorticity contribution from the eddy stresses is equally strong.

The repositioning of fronts and the changing variability of the velocity can change local nutrient availability,

Acknowledgments

This work was funded by the Netherlands Organization for Scientific Research (NWO), Earth and Life Sciences, through project ALW 802.01.024. The ocean model computations were done on the Cartesius at SURFsara in Amsterdam. Use of the SURFsara computing facilities was sponsored by NWO under the project SH-209-12. We gratefully acknowledge two anonymous reviewers for detailed suggestions and J. Caves for comments on the manuscript.

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