Evaluating Late Pliocene Atlantic Meridional Overturning Circulation dynamics through biogeochemical proxy–model comparison

Abstract. Atlantic Meridional Overturning Circulation (AMOC) is a major feature in the climate system, moving heat and carbon globally. Climate models suggest AMOC’s strength will decrease in the coming decades, with profound implications for regional and global climate patterns. Biogeochemical proxies from benthic foraminifera tests can act as water mass tracers and provide information on deep ocean circulation during past periods of global warmth. The strength of AMOC during the Late Pliocene warm period (∼3.0–3.3 Ma), a pseudo-analogue for future climate change, is poorly constrained, with benthic foraminiferal δ¹³C showing a spatially expanded North Atlantic Deep Water (NADW), whereas models often simulate a spatially retracted NADW relative to Pre-Industrial. Here we compare δ¹³C and δ¹⁸O proxies in the Atlantic for the warm mid-Piacenzian KM5c interval (3.205 ± 0.02 Ma) with four isotope-enabled climate model simulations allowing for a direct comparison between proxy data and climate models. Proxy data generally show a more expansive NADW with Antarctic Bottom Water limited to the greatest depths. Although climate models resolve key intermediate and deep water masses, differences in spatial patterns between the models are attributed to model parameterisations and boundary conditions. The models and proxy data show different deep ocean structures, which limits our ability to understand deep ocean circulation during the warm mid-Piacenzian KM5c interval and its relevance to future warming scenarios.