We have been measuring PH for about 80 years but we have only been measuring in an accurate repeatable manner since the 90s,that is why so many graphs start there.

Using small words please explain to me how we make direct measurement of the ocean's acidity level by measuring air pockets tapped in fresh water? Here is a hint we don't, we measure the carbon and extrapolate the acidity from that. It probably close but since it is only one of many factors that affect acidity and our understanding of ocean acidity is in its early stage calling that anything but a swag is an overstep.

Sorry but the ice caps are about 1.5 million years we do estimate the acidity level of the oceans pH by looking at old sea floors to make estimates on ocean acidification.

Here is a nice link show the issues in a none sensational where they talk about the concerns but also about the uncertainties. Oh and they talk about looking at rock to estimate acidity.