Southern Ocean control on atmospheric CO2 changes across late-Pliocene Marine Isotope Stage M2

Abstract. During the Pliocene, atmospheric CO₂ concentrations (pCO₂) were similar to today’s and global average temperature was ~3 °C higher. However, the relationships and phasing between variability in climate and pCO₂ on orbital time scales are not well understood. Specifically, questions remain about the nature of a lag of pCO₂ relative to benthic foraminiferal δ¹⁸O in the late-Pliocene Marine Isotope Stage M2 (3300 kiloannum ago, ka), which was longer than during the Pleistocene. Here, we present a multi-proxy paleoceanographic reconstruction of the late-Pliocene subantarctic zone, which is today one of the major ocean sinks of atmospheric CO₂. New dinoflagellate cyst assemblage data is combined with previously published sea surface temperature reconstructions, to reveal past surface conditions, including latitudinal migrations of the subtropical front (STF) over the late-Pliocene at ODP Site 1168, offshore west Tasmania. We observe strong oceanographic variability at the STF over glacial-interglacial timescales, especially across the M2 (3320–3260 ka). By providing tight and independent age constraints from benthic foraminiferal δ¹⁸O, we find that, much more than benthic δ¹⁸O or local SST, latitudinal migrations of the STF are tightly coupled to pCO₂ variations across the M2. Specifically, a northerly position of the STF during M2 deglaciation coincides with generally low pCO₂. We postulate that the efficiency of the Southern Ocean carbon outgassing varied strongly with migrations of the STF, and that is in part accounted for the variability in pCO₂ across M2.