Putting Everything Together: Why was it so hard for deep sea benthic forams at the PETM?

Being a benthic foram 55.5Ma would not have been a good thing. Up to 50% of your friends, family, relatives and estranged acquaintances wouldn’t make it to the other side. What is difficult to explain is that the loss at this boundary far exceeded that of the explosive KT extinction 10 millions years previous. This is a time where other taxa are experiencing massive proliferation, even as closely related as the planktic forams who enjoy basking in the warm sunlit waters above. So what’s going on?!

1. Low Oxygenation - All Life Needs Oxygen!

All complex life needs oxygen to respire, giving you the energy to go about your daily life. Where there is no oxygen, there tends to be little life. In the sedimentological record along coastal margins, more laminated clay materials represent periods of low oxygen due to their absence of bioturbation. The sediments are also filled with large amounts of organic matter indicating that it was not broken down conventionally by other organisms before sedimentation. This is supported by multiple observations of intense erosion along the continental margins, which, coincidentally has also been a candidate for the methane release. But why this drop in oxygen?

Oceanic circulation is believed to have been a lot more sensitive to climate perturbations at the PETM. The lack of an ice sheet in the northern hemisphere meant that there was a much shallower temperature and salinity gradient, which weakened the downwelling of the northward Atlantic current, much like the shrinking Arctic ice sheet is showing a weakening of the Gulf Stream. Models have shown that increasing that increasing [CO₂] results in greater stratification of the water column. Cue colourful diagram:

Sections of idealised age of water masses for Palaeocene-Eocene Thermal Maximum 

(100 year zonal mean). A: 4xCO₂ Pacific. B: 4xCO₂ Atlantic. C:8xCO₂ Pacific. D: 8xCO₂ Atlantic.

E: 16xCO₂ Pacific. F: 16xCO₂ Atlantic. (Winguth et al., 2012)

Stratification is a disaster and crucial in understanding extinction. Everything stagnates; there is no mixing of water and no recycling of nutrients. If there is no recycling, then life will exhaust its supplies and collapse. Models have predicted that there would have been a 42% reduction in nutrient supply.

Benthic foram ecosystems are reliant on the organic snow of particulates from the surface waters for sustenance. If the surface waters don’t mix enough then they will fail and the snow will stop and so the benthos will also collapse. However, planktic organisms do not suffer the same collapse as the benthic forams, so they can’t be a result of the same cause? 

     2. Ocean Acidification - No-one Likes to Lose Their Body

With increasing atmospheric [CO₂] there’s increasing ocean acidification, as the CO₂ reacts with water to produce carbonic acid ... which is an acid! The diagram below shows the mechanisms for oceanic acidification in its simplest form. The site I got it from does provide a useful schematic walkthrough of other effect associated with climate change and is well worth a quick read!

Ocean Acidification

Analysis into ostracods has shown that over the PETM they shrank! Coupled with data from foram oxygen isotope records, it is thought that this happened because of restricted carbon fluxes in the benthic ecosystems as a direct result in increased temperature. Other sites in Spain have shown a dwarfing in other benthos, including the forams. The corrosiveness of the increasingly acidic water reacts with the calcite shells and skeletons of benthic organisms, making it increasingly difficult for them to grow. The acidification of the water, as we have already discussed, shoaled the CCD (carbonate compensation depth) by 2km, which (if you’ve forgotten) is the depth at which carbonate rain is dissolved faster than it is supplied. So, in this critical region of shoaling, the benthic fauna were no longer able to form their calcitic shells. There is some disagreement as any species that already lay below the CCD would be unaffected, however we also see extinction in these species too. So ocean acidification doesn’t appear to be the leading factor, though not one we should ignore. 

There is a huge issue currently with ocean acidification as it is proving to be detrimental to coral ecosystems. The effect is known as coral bleaching and occurs when the zooxanthellae symbiont of the coral can no longer be supplied with nutrients and dies. It’s a widespread problem that seems to have accelerated since the 1970s. To show you how widespread this issue is, here a diagram of NOAAs Coral Bleaching Alert status from 3rd January 2013.

There is, of course, controversy - as there is with everything. The evidence is not consistent across the globe. Whilst continental margins may point toward hypoxia and even anoxia, the story is different in pelagic sediments. Sediments from Spain show no geochemical or sedimentological evidence to support the hypoxia theory of benthic foram extinction. The clay beds were red in colour, indicative of oxic environments. They’re red because of the oxide compounds formed from a reaction with iron - much like rust! The evidence from Spain does support low O₂ levels. 

Models have proved valuable in predicting the effects of climatic shifts across the PETM, however they should be taken with a pinch of salt because not everything was the same. For one, there were no ice sheets present at the poles. This is particularly important in understanding ocean circulation as any downwelling in the North Atlantic would have been much weaker than today due to a lack of the thermohaline conveyor system which enhances the natural movement of the water. Modern benthic forams themselves are not truly indicative of those at the time of PETM. The naturally warmer conditions meant that any benthic forams would have been evolved to live in an environment 10°C warmer. 

Models aren’t all bad though. They have thrown up some interesting support for some field evidence. They have shown less deep sea ventilation, enhanced vertical nutrient and oxygen gradients; with a marked concentration decrease in the deep sea, a global export reduction of 42% to the sea floor and increased erosion to maintain the productivity in the coastal environments. Which, if you’ve been reading this with bated breath for the past 3 months, incorporates most of the observations in the literature we have come across! So it might be safe to say that the benthic extinction is a combination of all factors, but with heavy influence from a weakening oceanic circulation system. The difference in response from this to the KT event is most likely due to a decoupling effect of the sea surface and deep sea systems. The mechanisms behind this are still largely unknown.