How is todays change in ocean chemistry different from those of previous geological periods?
Present conditions differ from the past largely because the rate of change of atmospheric CO2 does not match the rate of mitigating geological processes. If CO2 is added slowly over hundreds of thousands of years, as it was during the Ordovician by volcanic and plate tectonic activity, the CO2 that enters the ocean has time to mix throughout the ocean from top to bottom. As a result, even though the amount of CO2 that is taken up by the ocean is large, it is spread out over a very great volume of water and the resulting decrease in pH is small. At the same time, as the CO2 level in deep oceans increases over millennia, carbonate sediments lying on the seafloor begin to dissolve and release carbonate ions that neutralize some of the acidity, further minimizing the decrease in pH. Past oceans also contained higher calcium and magnesium ion concentrations, which helped stabilize calcium carbonate minerals in marine animals’ skeletons. Today, the CO2 in the atmosphere is increasing much fa
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