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I've already discussed the backstory of the Paleocene-Eocene Thermal Maximum more than once already, so let me shorten this into a list of notes:

  • 56 million years ago
  • 5-9-degree-Celsius rise in temperature in 20-50 millennia
  • A duration of roughly 200 millennia

In an alternate timeline, the only difference in the PETM is that it lasted three to four times longer than in our timeline. It poses so many questions, but because of the SE's "one-at-a-time" policy, this would have to work as a series. The first question concerns a little something called the lysocline. What, exactly, is that? Here is how the Wikipedia article on the PETM described it:

The lysocline marks the depth at which carbonate starts to dissolve (above the lysocline, carbonate is oversaturated): today, this is at about 4 km, comparable to the median depth of the oceans. This depth depends on (among other things) temperature and the amount of CO2 dissolved in the ocean. Adding CO2 initially raises the lysocline, resulting in the dissolution of deep water carbonates. This deep-water acidification can be observed in ocean cores, which show (where bioturbation has not destroyed the signal) an abrupt change from grey carbonate ooze to red clays (followed by a gradual grading back to grey). It is far more pronounced in north Atlantic cores than elsewhere, suggesting that acidification was more concentrated here, related to a greater rise in the level of the lysocline. In parts of the southeast Atlantic, the lysocline rose by 2 km in just a few thousand years.

In that context, would a longer PETM raise the lysocline, and to how much of an extent?

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The lysocline is the depth below which the oceans are undersatured with respect to carbonates (limestone, corals, shells etc.) Below this depth carbonates start to dissolve. The term is closely related to the Carbonate compensation depth or CCD, the depth below which no carbonates remain in the sediments.

As the wikipedia article states, the lysocline (they really mean CCD as that is what remains preserved in the sediment record) initially decreased in depth as the consequence of a large amount of CO2 dissolving in the ocean, accidifying the water. On the long term however the oceans warm, and with increased temperature gas solubility decreases expelling CO2 back into the atmosphere, decreasing ocean acidity.

In addition higher temperatures, higher atmospheric CO2 concentrations and increased precipiation will all conspire to increased weathering of rocks, which will increase the amount of Calcium-ions transported to the oceans, which will lower the lysocline further.

A warm world without icecaps also implies more shallow warm seas which are ideal for calcifying organisms to deposit more carbonates. Due to a strange quirk of chemistry whereby the alkalinity of ocean water changes when carbonates are precipitated the result is a netto release of CO2 to the atmosphere.

Where the ultimate balance lies is always a big question in climate science. It seems to be the case that the earths climate hops from one stable equilibrium to a very different one once a certain treshhold is reached. There certainly isn't an easy linear correlation between CO2, temperature and carbonate compensation depth.

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  • $\begingroup$ I see nothing suggesting an answer to the question. $\endgroup$ – JohnWDailey Jun 20 '20 at 1:40
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    $\begingroup$ @JohnWDailey The answer is "there isn't a[...] linear correlation" and "the earth's climate hops from one stable equilibrium to a very different one". $\endgroup$ – Logan R. Kearsley Jun 26 '20 at 23:19
  • $\begingroup$ @Tantalus'touch: decreased is correct. Here is a reference: en.wikipedia.org/wiki/Lysocline. The point is that at the bottom of the ocean, conditions are suitable to dissolve carbonates, and when CO2 is added, that zone of dissolution expands (i.e., decreases in depth). $\endgroup$ – Joanna Marietti Jul 2 '20 at 23:15
  • $\begingroup$ @JoannaMarietti Ahh, thanks, got it. $\endgroup$ – Rottweiler on market-day. Jul 2 '20 at 23:55
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No. If the only variable that changes is that the duration (not the temperature, carbon saturation, or anything else) of the PETM increases, that will not affect the depth of the lysocline.

Exactly where the lysocline lies is a complex phenomenon, dependent on multiple interacting variables, and not consistent even from one part of the ocean to the next during the same time frame (such as now). I'm not remotely qualified to try and calculate its depth for a given set of conditions. But I can still answer your question, because wherever that equilibrium lies, it is an equilibrium, and it would already have been reached by the time your PETM-extension occurs. Changes in ocean acidity in response to temperature are a thing we're watching unfold right now on a timescale of decades, so after thousands of decades, things would long since have settled down to whatever is stable under those conditions. Keeping the same conditions around for another 600 thousand years won't change anything.

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