The volume of the ocean basin is something that I haven’t been able to find data for one way or the other. The timeline is different — there is a lot of data an

The volume of the ocean basin is something that I haven’t been able to find data for one way or the other. The timeline is different — there is a lot of data and of course it doesn’t all line up perfectly. The generally accepted reference timeline, which is based on a single south Atlantic core sample, is calculated using an assumption about the annual rate of sediment deposition. There is evidence that mid ocean sediment deposition dropped during PETM while coastal deposition increased. This is likely because the oceans become inhospitable due to increasing acidity and warming caused the same thing we see today — increased evaporation rate and warmer air being able to hold more moisture as water vapor leads to increased rainfall and therefore river flows increased generally. There have been attempts to try and correct for this factor using iron deposition variability which may correlate with planetary precession and other techniques, some of which change the timeline fairly drastically at the scale we’re talking about. Long story short — yes this is an oddity but there’s not a well accepted explanation for it and there is reason to question whether we have the PETM timeline correct for the area of interest. (the onset is quite sudden, geologically speaking, although not as sudden as the data we are producing right now!) I would encourage you to do more reading and build an opinion based on the sum of evidence rather than that one paper, which I linked to simply because it has an accessible graph for sea level variability compared to today.