Search for: All records

Abstract Peak Neogene warmth and minimal polar ice volumes occurred during the Miocene Climatic Optimum (MCO, ca. 16.95–13.95 Ma) followed by cooling and ice sheet expansion during the Middle Miocene Climate Transition (MMCT, ca. 13.95–12.8 Ma). Previous records of northern high-latitude sea surface temperatures (SSTs) during these global climatic transitions are limited to Atlantic sites, and none resolve orbital-scale variability. Here, we present an orbital-resolution alkenone SST proxy record from the subpolar North Pacific that establishes a local maximum of SSTs during the MCO as much as 16 °C warmer than modern with rapid warming initiating the MCO, cooling synchronous with Antarctic ice sheet expansion during the MMCT, and high variability on orbital time scales. Persistently cooler North Pacific SST anomalies than in the Atlantic at equivalent latitudes throughout the Miocene suggest enhanced Atlantic northward heat transport under a globally warm climate. We conclude that a global forcing mechanism, likely elevated greenhouse gas concentrations, is the most parsimonious explanation for synchronous global high-latitude warmth during the Miocene. 

Abstract Ocean acidification due to anthropogenic CO₂emission reduces ocean pH and carbonate saturation, with the projection that marine calcifiers and associated ecosystems will be negatively affected in the future. On longer time scale, however, recent studies of deep‐sea carbonate sediments suggest significantly increased carbonate production and burial in the open ocean during the warm Middle Miocene. Here, we present new model simulations in comparison to published Miocene carbonate accumulation rates to show that global biogenic carbonate production in the pelagic environment was approximately doubled relative to present‐day values when elevated atmosphericpCO₂led to substantial global warming ∼13–15 million years ago. Our analysis also finds that although high carbonate production was associated with high dissolution in the deep‐sea, net pelagic carbonate burial was approximately 30%–45% higher than modern. At the steady state of the long‐term carbon cycle, this requires an equivalent increase in riverine carbonate alkalinity influx during the Middle Miocene, attributable to enhanced chemical weathering under a warmer climate. Elevated biogenic carbonate production resulted in a Miocene ocean that had carbon (dissolved inorganic carbon) and alkalinity (total alkalinity) inventories similar to modern values but was poorly buffered and less saturated in both the surface and the deep ocean relative to modern. 

Alkenones are long-chain ketones produced by phytoplankton of the order Isochrysidales. They are widely used in reconstructing past sea surface temperatures, benefiting from their ubiquitous occurrence in the Cenozoic ocean. Carbon isotope fractionation (εp) between alkenones and dissolved inorganic carbon may also be used as a proxy for past atmospheric pCO2 and has provided continuous pCO2 estimates back to ca. 45 Ma. Here, an extended occurrence of alkenones from ca. 130 Ma is reported. We characterize the molecular structure and distribution of these Mesozoic alkenones and evaluate their potential phylogenetic relationship with Cenozoic alkenones. Using δ13C values of the C37 methyl alkenone (C37:2Me), the first alkenone-based pCO2 estimates for the Mesozoic are derived. These estimates suggest elevated pCO2 with a range of 548−4090 ppm (908 ppm median) during the super-greenhouse climate of the Early Cretaceous, in agreement with phytane-based pCO2 reconstructions. Finally, insights into the identity of the Cretaceous coccolithophores that possibly synthesized alkenones are also offered.