Calcium Carbonate Dissolution Rates and the Alkalinity Cycle of the Ocean

Atmospheric CO2 is largely buffered by reaction with calcium carbonate in the ocean.  This carbonate compensation is thought to happen at the seafloor where increased pressure makes the mineral calcite more soluble.  However, the details of how this process works, and its role in the overall cycle of alkalinity in the ocean, are dependent on the rate of carbonate dissolution.  We have been studying the kinetics of dissolution in the lab, in the water column, and very recently at the seafloor.  Strong ‘kinks’ in the relation between dissolution rate and thermodynamic undersaturation characterize the rate law in all three cases and these accelerations in rate are predicted from basic theory.  These kinks help explain several enigmatic features of the alkalinity distribution in the ocean and force us to conclude that there is an active precipitation and dissolution cycle of CaCO₃ in the upper ocean.  Given that the waters are mostly super saturated, dissolution driven by organic matter respiration.  The importance of organic matter carries down to the sediments where a new in situ instrument has allowed us to see the same ‘kinks’ at work during early diagenesis.  A shallow alkalinity cycle driven by organic matter respiration leads to a few cool new insights about the history of CO₂ on several timescales and provides a framework for understanding why the relative stability of CO₂ in the atmosphere might be a simple consequence of this reaction.

About this Series: The Department Lecture Series at EAPS at MIT is a series of weekly talks given by leading thinkers in the areas of geology, geophysics, geobiology, geochemistry, atmospheric science, oceanography, climatology, and planetary science. For more information and Zoom password please contact Madelyn Musick: mmusick@mit.edu