Tropical climate dynamics across the onset of the Miocene Climatic Optimum: assessing premature surface warming in the equatorial Atlantic Ocean with implications for polar ampli?cation factors

Abstract

Past evaluations of tropical and polar temperature evolution revealed linear scaling during the global ice-free conditions of the Eocene which signi?cantly improved our understanding of the role of pCO2 in "greenhouse" climates. Since these polar ampli?cation factors directly affect ice sheet dynamics, it would be reasonable to assume an altered relationship with the presence of major ice sheets on either poles. Deep-ocean temperature records have demonstrated an interruption in the long-term cooling trend of the Cenozoic, known as the Miocene Climatic Optimum (MCO, 14 Ma - 17 Ma), with a well-established Antarctic ice sheet which could serve as a possible analogue for present-day global climate conditions and dynamics. More speci?cally, will the Eocene linear relationship hold up during glaciated MCO warm climate conditions. Here I present the initial, tropical sea surface temperature increase at the onset of the MCO, based on lipid biomarker paleothermometry, reconstructed from equatorial Atlantic Ocean sediments at unprecedented resolution. In addition, I identity an altered position of the tropical MCO onset based on stable isotopic reconstructions. Furthermore, this record is augmented with bulk sediment geochemistry dynamics and biotic response assessment of dinoagellate communities. The resulting records indicate an average sea surface temperature increase from 28 degreesC to 31 degreesC (17.5 Ma). This seems to coincide with indications of heavily reduced regional upwelling, almost 500 kyrs prior to the traditionally de?ned deep-ocean MCO (17 Ma). This early warming trend possibly caused harmful algal blooms and an increasingly stressed primary production community based on the occurrences of elevated Polyshearidium zoharyi dinocyst and reduced Peridinoid-cyst. However, the origin of this early warming trend remains unclear with evidence for volcanic triggered impact on the regional hydrological cycle. Similarly, the extent of this early, tropical warming remains unique for the equatorial Atlantic Ocean based on available data. Comparative analysis of tropical sea surface temperatures with deep-ocean temperatures revealed an asynchronous temperature evolution between tropical and polar regions during the early Miocene. This is in stark contrast with the synchronous temperature evolution established for the Eocene. This suggest the early warming phase of the tropical MCO was caused by variation in oceanic circulation with an unknown extent, rather than increased pCO2, acting as a potential equatorial precursor to the global climatic event.