Reconciling equatorward migration of Southern Ocean fronts with minor ice volume change during Miocene cooling

Abstract

A Miocene phase of gradual climate cooling and CO2 decline was recently shown not to be associated with major ice volume expansion, challenging a fundamental paradigm in the functioning of the Antarctic cryosphere. Here, we explore Miocene ice-ocean-climate interactions by presenting a multi-proxy reconstruction of subtropical front (STF) migration, bottom water temperature (BWT) and global ice volume change, using dinoflagellate cyst biogeography, benthic foraminiferal clumped isotopes, and sea surface temperature (SST) reconstructions from offshore Tasmania. We demonstrate a mid-late Miocene (16–9 Ma) equatorward migration (from ~53°S to ~42°S) and strengthening of the STF, concurrent with SST decline. We expand evidence for strong BWT decline and apparent absence of ice volume change into the late Miocene with new clumped isotope data. To reconcile these counterintuitive findings, we argue based on new, idealized ice sheet model simulations that the Miocene Antarctic ice sheet progressively lowered in height while expanding seawards during the mid-Miocene, to maintain a stable volume. This can only be achieved with rigorous intervention in model precipitation regimes and ice-ocean interactions and requires rethinking the interactions between ice-ocean and climate during Neogene cooling.