Decoupled changes in upwelling and acidity in the eastern equatorial Pacific during the Pliocene

Tuesday, 15 December 2020: 11:34
Madison Shankle1, Natalie Burls2, Alexey V Fedorov1, Matthew David Thomas3, Donald E Penman1, Heather L Ford4, Peter Jacobs5, Noah Planavsky1 and Pincelli M Hull1, (1)Yale University, Department of Earth and Planetary Sciences, New Haven, CT, United States, (2)George Mason University Fairfax, Department of Atmospheric, Oceanic & Earth Sciences, Fairfax, VA, United States, (3)UCAR Geophysical Fluid Dynamics Laboratory, Princeton, NJ, United States, (4)Queen Mary, University of London, School of Geography, London, United Kingdom, (5)George Mason University, Department of Environmental Science and Policy, Fairfax, VA, United States
The Pliocene epoch (5.3-2.6 million years ago) is the last time Earth experienced atmospheric carbon dioxide levels comparable to present day anthropogenic levels. As such, this time interval is a potential analogue for future, warmer Earth system states. One enigmatic feature of the Pliocene is a reduced east-west sea surface temperature gradient in the equatorial Pacific (indicative of reduced equatorial upwelling) coinciding with enhanced biological productivity in the eastern equatorial Pacific at this time. Here we use boron isotopes to investigate these dynamics and to reconstruct the zonal surface pH gradient across the Pliocene equatorial Pacific. We find a strengthened pH gradient relative to modern (with more acidic conditions in the east than the west) despite a reduced temperature gradient at this time. These findings are in contrast to modern-day dynamics in which temperature and acidity co-vary, such that the reduction of the zonal temperature gradient during an El Niño event is accompanied by reduced acidity (as well as reduced upwelling and productivity) in the eastern equatorial Pacific. We show that this decoupling between changes in the pH and temperature gradients is consistent with biogeochemically enabled model simulations of Pliocene climate containing an active Pacific meridional overturning circulation and a weakly stratified equatorial thermocline. This reorganization of Pacific circulation and the onset of north Pacific deep water formation allows old, acidic, more nutrient-rich waters to reach the eastern equatorial Pacific despite weak wind-driven upwelling rates, accounting for the low pH values we observe there as well as previous evidence of enhanced productivity.