Which marine organisms would survive this longer Paleocene-Eocene Thermal Maximum?

Question

56 million years ago, Earth underwent the greatest rise in temperature in the last 100 million years. In just 20 to 50 millennia, the global temperature had risen by five to eight degrees Celsius (nine to 14.4 degrees Fahrenheit), and this spike would plateau for 200,000 years. This still-mysterious phenomenon, known as the “Paleocene-Eocene Thermal Maximum”, or “PETM” for short, was a double-edged sword in evolution. Whereas the global jungles spiked the diversity of certain modern clades of mammals, including the primates, this same heat wave warmed up the oceans, weakening their abilities to hold oxygen and rendering them vulnerable to ocean acidification. Deep-sea variations of single-celled organisms called foraminiferans suffered a species loss no greater than half. Coral reefs were cut down, as the greater acidities of the water stunted the production of calcium carbonate. Dinoflagellate cells ballooned, resulting in more common occurrences of harmful algal blooms. Ocean currents took opposite directions, transporting warm water to the depths, reducing the overall temperature gradient and thus worsening the issue. It was so hot that sea levels rose back to mid-Mesozoic levels. The lysocline, the point in which carbonates can dissolve, had risen to a shallow depth of two kilometers, or almost a mile and a quarter. In short, the Paleocene-Eocene Thermal Maximum was both a godsend for life on land and absolute hell for life in the oceans.

On this alternate Earth, the PETM lasted a lot longer than back home. Three to four times longer, as a matter of fact. Another difference is that this was the point in which the entire global supply of methane hydrates, or frozen natural gas—all two trillion metric tons of it—were released into the atmosphere, turning the hothouse into an even hotter house. This didn’t happen all at once, as there wasn’t any evidence of a terrestrial mass extinction. What we may be looking at instead was a gentle upward slope spanning the entirety of the PETM—600 to 950 millennia. Sea levels rose to the extent that the amount of land relative to the rest of the planet’s surface to between 12 and 17 percent. As higher temperatures hold more water vapor than lower temperatures, such a spike would mean that so much moisture would be retained that constant rainfalls would destroy mountains. Which ranges had survived such outcomes and which had suffered big time would be explored in due time. It was so hot that the lysocline had climbed up to an even shallower depth of one kilometer, almost two-thirds of a mile. A band of water spanning 15 degrees from both sides of the equator would have been an impenetrable barrier of sickly purple water, inhabited only by anti-oxygen purple bacteria. Beyond that, the temperate seas were mottled with large masses of marine heatwaves, whereas polar seas had smaller but more numerous heatwaves. Between each heatwave, the waters would suffer repeated short-term episodes of harmful algal blooms, which would rob the waters of their precious oxygen. Life on land was about to get harder, but life in the oceans would suffer another major mass extinction—the absolute worst since the Great Dying, and right in the middle of the process of recovering from the fall of the dinosaur empire one million decades prior! 99% of all marine species were gone. Which marine organisms would make up the surviving one percent?

Answer 1

Nothing that relies on mineralizing calcium carbonate, due to acidity, as you mentioned. That probably rules out cephalopods due to internal shells as well.

Nothing that's too quick -- burning energy requires oxygen, and that's going to be a limited resource except for air breathers (which we'll come back to).

Nothing that's too slow, either -- the rapid rate of change (compared to geologic averages) means that truly sessile and sedentary organisms are unlikely to be able to keep up with their preferred habitats rate of movement. Corals are probably out.

Nothing that's on the top of a tall food pyramid, since the effects of disruptions hit the highest rungs hardest.

At the lowest level, as you mention, forams lose out and algae win.

So likely, at first, very familiar small bony fish like the clown fish, that sit at the bottom of the food pyramid and eat algae. Fast enough to follow food, efficient enough to try to hide from predators instead of outrunning them. Other small bony fish like sardines that school will lose out in low oxygen conditions because they both use more oxygen individually, and school together, depleting a region.

On top of this, the usual predators will decrease in number, and possibly in size, but continue to stick around. Some sharks won't make it through this disaster, but plenty will.

Large marine mammals are probably the most interesting question. Baleen whales are not terribly sensitive to acidity, not sensitive to dissolved oxygen, and can filter feed on pretty much anything small and organic -- but due to square/cube effects the increased temperature itself is a challenge. I'd expect thriving communities of baleen whales near the poles, traveling as close to the equator as they can during each hemisphere's winter to get fat on the krill blooms at the edge of habitability, then return poleward to breed in the winter.

The reduction in dissolved oxygen also raises opportunities for other marine mammals to adapt, or for other air-breathing animals to return to the water. Over a longer timeframe, you'd expect to see semi- and mostly-aquatic airbreathers like iguanas and otters expand to wider niches. Iguanas in particular spend a good amount of effort dealing with thermoregulation -- a sufficiently warm sea is a benefit to them, not a cost.

Answer 2

Non-calcifying organisms with photosymbionts.

As noted, free swimming water breathers will die. Because of difficulties making calcium shells or tests, those organisms will go too.

What remains are shell-less organisms with photosymbionts. The photosymbiotic algal partners can provide oxygen as well as reduced carbon. Organisms with minimal hard parts do not fossilize well but there are modern organisms from several phyla that fit the bill: there exist modern marine worms, sponges, bryozoans, tunicates, sea slugs, medusoids and jellyfish with photosymbionts. Low O2 environments will favor sessile lifestyles and a lack of free swimming competitors will mean these organisms will have more access to photosynthetic plankton prey to get their nitrogen and nutrients.

I posit "soft reefs" built of shell-less organisms from the above groups, thriving in the sun, un-nibbled by fish, molluscs and echinoderms. Perhaps there could be massive floating colonies of photosynthetic jellies forming pelagic reeflike structures that become the home of other animals.

Answer 3

The scenario you put forward looks to be a perfect environment for a prolonged Azola Event where the increased rainfall and erosion adding fresh water and minerals to the ocean, the fern would colonize a greater part of the ocean and could alter the food web over a period of time.

Reptiles during this time thrived, like titan-boa, and other exothermic organisms would have an advantage. Herbivorous land/sea reptiles could take advantage of the Azola blooms growing and evolving in the standard predator prey dynamic, starting a new era of sea monsters like was seen in the Mesozoic. More diversity in animals adapting to feeding on the Azola plant, the animals and plant itself evolving and diversifying over time could arrive at any number of ecological endpoints. Not to mention the sequestration of great amounts of carbon as stated in the wiki article.