THE CARBONATE BUFFERING CONSTRAINT ON LONG-TERM GEOLOGIC CHANGES IN THE CARBON CYCLE

A review of the natural processes controlling Earth's carbon cycle suggests that carbon-cycle changes can be classified into two modes depending on the timescales of interest. Over timescales up to a few thousand years, changes occur primarily through redistribution of carbon among the atmosphere, biosphere, and oceans. Over timescales longer than about 10,000 years, additional changes may occur as a result of carbon exchange involving Earth's crust and sediments. From the perspective of the atmosphere-biosphere-ocean system, these relatively short-term and long-term modes of change can be described as “internal” redistribution vs. “external” exchange.

An important control on the “external” mode of change is the effectiveness of buffering by thermodynamically-driven changes in the balance between dissolution and precipitation of marine carbonate sediments. The carbonate buffering constraint implies a robust relationship between long-term changes in global carbon mass balance and oceanic charge balance. Specifically, a long-term change in the total amount of inorganic carbon in the ocean-atmosphere system should be accompanied by an equal molar equivalent change in the total amount of oceanic alkalinity. Using this relationship, it is possible to interpret modern oceanic and atmospheric chemistry as a tie-point that constrains past long-term relationships between the total amounts of inorganic carbon and alkalinity in Earth's atmosphere and oceans.

This constraint has important implications. For example, in principle, it could define past relationships between long-term variations in atmospheric carbon dioxide levels and oceanic pH values. However, its practical application to the geologic record is complicated by a number of factors, including the evidence for relatively abrupt “external” exchange events (e.g., bolide impacts and some methane hydrate releases), the influence of changes in the major-ion chemistry of the oceans, and the likely simultaneous occurrence of both short-term and long-term modes of carbon-cycle change throughout Earth history. These factors imply additional constraints and caveats that must be considered in applying the long-term constraint of carbonate buffering.