Impact of Dynamic Topography on Late Cenozoic Sea-Level Records: Examples from Australia

Abstract:

Past sea-level estimates derived from paleoshorelines place important constraints on late Cenozoic ice volume changes and are often used to infer the sensitivity of Earth’s climate to different forcings. However, the elevation of sea-level markers reflects the integrated effect of cryospheric and solid Earth processes, and isolating these contributions is both essential and non-trivial. In particular, while uncertainties associated with glacial isostatic adjustment (GIA) can be reduced by concentrating on sites that are located far from ice sheets, the same cannot be said for mantle flow-driven dynamic topography, which is ubiquitous. This latter contribution can generate vertical motions of ~100 m Myr^-1, suggesting it is an important consideration for sea-level studies, even on the timescale of the Last Interglacial period (~125 ka). Better constraints on the spatio-temporal evolution of dynamic topography are therefore required to improve ice volume estimates and enhance our understanding of climate sensitivity.

Reconstructing dynamic topography using mantle flow simulations relies on accurate models of Earth’s internal structure. Importantly, recent models yield present-day predictions that are in good agreement with residual depth measurements. Fits are particularly good around Australia, suggesting that “retrodictions” of vertical motions are more likely to be reliable in this region. Predicted temperatures are also consistent with paleogeotherms inferred from xenolith suites, indicating that lithospheric thickness is well constrained. Finally, Australia’s remoteness from major ice sheets reduces uncertainty in GIA contributions to relative sea-level change. These factors, combined with abundant and well distributed sea-level indicators, make Australia a promising location for separating global mean sea-level change from local variations related to dynamic vertical motions. By back-advecting density perturbations from an ensemble of Earth models, we show that ±250 m relative sea-level changes since the Mid-Pliocene Warm Period (MPWP; ~3 Ma) can be attributed to dynamic topography. Significantly, removing this signal from observed sea-level trends suggests that global mean sea-level during the MPWP was 4–14 m above present, which is towards the lower end of previous estimates.