Ocean Acidification

Abstract

Ocean acidification due to anthropogenic CO₂ emission 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 atmospheric pCO₂ led 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.

Key Points

• Pelagic carbonate production during the warm Middle Miocene was approximately doubled relative to present-day values
• Net pelagic carbonate burial of the Middle Miocene was likely ∼30%–45% higher than modern values
• The decreases in []_sw and carbonate production toward the present kept Neogene dissolved inorganic carbon and total alkalinity nearly constant despite a global pCO₂ decrease

Si W., Herbert T., Wu M. & Rosenthal Y., 2023. Increased biogenic calcification and burial under elevated pCO2 during the Miocene: a model-data comparison. Global Biogeochemical Cycles 37: e2022GB007541. doi: 10.1029/2022GB007541. Article.