4. Where'd all the Carbonate Go?

Calcium Carbonate (CaCO₃)is possibly one of the most well known chemical compounds. At school, you see that it fizzes when you put it in acid and it disappears. Funnily enough, this same process occurs in the oceans! Admittedly, there isn’t any fizzing, as I’m sure many people have noticed that the sea exuding gas when you go for a swim. However, CaCO₃ does disappear. Not by magic, but by dissolution. - i.e. it dissolves!

The diagram below shows how CaCO₃ in our oceans is linked to CO₂ in our atmosphere; the most prominent greenhouse gas. 

The oceans are the largest reservoir of C - 60 times more than
in the atmosphere. Most C in the surface waters is stored as CaCO₃

There is a rain of CaCO₃ that falls from the high productive surface waters of the ocean to the sea floor. CaCO₃ forms the basis of most shells of aquatic organisms and skeletons of foraminifera. This link provides a few extra details of forams, and their significance in the paleo-record as well as some excellent SEM images of their intricate shells. 

As the rain falls, the CaCO₃ re-dissolves until a point known as the carbonate dissolution depth (CCD) where the supply of solid CaCO₃ rain is less that the rate of dissolution - meaning none reaches the ocean floor. The depth of the CCD is dependent on many factors and so varies considerably. The CCD is influenced by ocean pH, rain rate, ocean circulation, and the intensity of remineralisation. Today, there is a 2km difference in the depth of the CCD in the Pacific and Atlantic oceans. 

Coincidentally, this was also the change in the CCD observed over the PETM. The telltale signs of the CCD are marked by low-carbonate or clay layers in ocean cores, known as carbonate dissolution horizons. By tracking where the horizons are in the sediments, scientists can accurate determine where the CCD lay throughout time. In the Southern Atlantic, studies have suggested that the CCD shoaled by over 2km in 10 000 years, taking 100 000 years to return to pre-excursion depths. The sensitivity of this system is still highly debated, as ocean composition is thought to play an important role. Alarmingly, the variations in CCD shoaling are not consistent with the levels of methane released under the methane hydrate hypothesis (which by the way is a rather hefty 2000Gt of C). Arguments now fight for a larger carbon input than the methane hydrate hypothesis provides!