Yes, there is much more than enough limestone, by several orders of magnitude, to neutralize the acidity that we are creating - so much so that I am not even going to bother with the back of an envelope calculation. Given enough time (many millennia and possibly several million years) this will happen naturally. However,the process of re-equilibration with the introduced CO2 is sluggish because of both the reaction kinetics and, more particularly the deep ocean water mixing time. The current rate of CO2 emissions far exceeds both the rate of neutralization and the rate with which we could seed the oceans with CaCO3. Consider that, for limestone neutralization to succeed we would have to micronize entire hills and mountain ranges of limestone, and then distribute it, reasonably uniformly, over the world's oceans. Technically do-able, perhaps, but mind-bogglingly expensive and probably politically divisive. For many reasons it would be cheaper, easier and more ecologically appropriate to convert to renewable energy ASAP.
The above comment that the ocean's pH is temperature-controlled is wrong. There is a temperature effect upon pH - see for example http://www.aslo.org/lo/toc/vol_14/issue_5/0679.pdf . However, this is insufficient to counter the oceanic acidification. Already the pH has decreased by between 0.06 and 0.10, with a global average decrease of about 0.075 (to 1990). See https://en.wikipedia.org/wiki/Ocean_acidification. The last two decades have seen an acceleration of this trend. By century end most models project a mean pH decrease from 8.2 to 7.8 in the upper few hundred metres of ocean, where biological productivity is greatest. This is faster than at any time in the last 300 million years. To be effective, any neutralization with limestone would have to be achieved within a few decades, at a rate of neutralizing about 100 ppm CO2 over the entire ocean surface, and to a depth of several hundred metres.
