The Carbon Cliff

This is an interesting article from the Guardian in December which discusses that the most recent research suggest we are at the point of no return if we want to remain within a 2°C warming this century. It discusses the social and politic agenda in dealing with the issue to stay within our 42-47Gt window. Don’t know what that is? Read it and find out!

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. 

Say Cheese!

Enjoy this photo, taken by Joshua Holko for the National Geographic photo competition this year. It depicts a group of penguins adrift an iceberg after a snow storm in the Antarctic. If your curiosity takes you, do have a look at the other entries. Some of them are incredible. Happy New Year!

The Year Abnormal Became The New Normal

Ocean circulation is responsible for many weather patterns across the globe, from the variable conditions of temperate climes to the frozen wastes of the Antarctic. This article from the Guardian reviews the weather of 2012 and gives a stark warning to the skeptics of anthropogenically induced climate change.

The Thermohaline Conveyor Belt Shutdown

The IPCCs Third Assessment Report in 2001 provided a comprehensive review of the thermohaline conveyor system in both hemispheres. By analyzing a series of models with varying global warming scenarios they predicted and showed a general weakening of the system.

Simulated water-volume transport change of the Atlantic "conveyor belt" (Atlantic overturning)

 in a range of global warming scenarios computed by different climate research centres.

Observations appear to agree with this. As a reminder, this is what the deep water oceanic circulation looks like:

The Thermohaline Conveyor Belt

What Bryden, Longworth and Cunningham found was that from 1957 to 2004 some more tropical regions around the Bahamas and Florida Straits showed little to no evidence of changes above or beyond the annual variation in Gulf Stream transport. But this wasn’t the case in the North Atlantic where there has been a tendency for a weakening and shallowing of water flow; a 50% decrease in the southward deep ocean flow (3000-5000m depth), but mid-ocean circulation increasing 50% southward. 

Southward water flow is different in the Norwegian-Greenland Sea, where deep water formation appears to have ceased as a direct result of increased melt from nearby ice sheets. More freshwater has lowered the density gradient between the Gulf Stream and the local waters in the area, making it harder for the current to sink. Such changes have altered the structure of deep water circulation. 

Climate models have predicted that this weakening of the Gulf Stream will cool northwestern Europe by 4°C. For perspective, the Gulf Stream is thought to artificially increase European temperatures by up to 10°C. Because of this, some people believe that should the Gulf Stream completely shut down, then it may be enough to push Europe, and subsequently the rest of the world into the another glacial, through rapid glaciation of the continent. 

The IPCC rejects this, by first proposing that the Gulf Stream will only weaken and not shut off completely. However, the UK has now seen a 30% fall in the quantity of warm water it receives. Temperatures are thought to remain artificially high due to radiative effects of atmospheric greenhouse gases. There is a danger that the warming in the North Atlantic may trigger the destabilisation of 2.5Gt of methane trapped in methane clathrates, which as we have seen earlier, is a highly potent greenhouse gas able to rapidly spread throughout the global systems (geologically speaking of course). This is only a fraction (0.2% at most) of the methane required to cause the PETM (2000Gt). Knowledge of the global extent of methane clathrate destabilisation is still largely unknown but likely to be significantly higher than 2.5Gt. So, understanding the PETM is now more important than ever!

Climate Change & the Humboldt Current

Over the past 100 years, the sea surface temperatures have risen between 0.3°C and 0.6°C and is generally believed to be a general trend across the globe. There are deviations from this in isolated cases. 

Again, we’re going back to South America and looking at the strong upwelling in the Humboldt current. Here, observations in the Humboldt Current's large marine ecosystem have shown a cooling over the past 25 years. Although this is not a consistent cooling over 25 years, any long term warming episodes have been attributed to El Niño events where the entire oceanic circulation of the Pacific shifts. The cooling effect has been connected to increased strength of the upwelling occurring across the eastern Pacific, as a direct result of the increasing strength of the trade winds, associated with global warming. The trade winds run in an upwelling-favourable equatorward direction. The increasingly rapid deep water being pulled to the surface draws heat from the air and thus cools the environment.

We have already noted that this region of the world houses the world’s most productive fisheries. Unfortunately, these are in decline due to human impact of overfishing. The change in catch composition has shifted remarkably over the years, particularly in the occurrence of achoveta and sardines. Studies have shown a trophic level collapse, particularly since the 1950s where the sardine, a low-trophic level species, declined rapidly, with fishery explosion occurring simultaneously. It is estimated that today that 80% of stocks are overexploited or have collapsed, and 80% landings are from these collapsed stocks. It’s a shocking statistic.

Mean Trophic Level in the Humboldt Current (Sea Around Us, 2007)

Stock-Catch Status Plots for the Humboldt Current Large Marine Ecosystem, 

showing the proportion of developing (green), fully exploited (yellow), 

overexploited (orange) and collapsed (purple) fisheries by number of stocks (top) 

and by catch biomass (bottom) from 1950 to 2004 (Heinemann al., 2009).

Whilst upwelling could be seen as an opportunity for relief from this detrimental human activity, it is far from it. Upwelling provides nutrients to the surface waters which promote primary productivity. The graph below shows that there has been an increase in primary productivity in the sea, as you would expect. 

Humboldt Current Large Marine Ecosystem trends in chlorophyll a (left) and primary productivity (right) 

1998-2006, from satellite ocean colour imagery. (O’Reilly & Hyde, cited Heinemann et al. 2009)

But the benefits to this bloom (like below) are dwarfed by the relentless harvesting by fisheries, where annual catches may sum to over 18 million tonnes.

Planktonic bloom off the coast of New Zealand due to local upwelling

The Motion of the Ocean: Part II

What does the spinning of water in the toilet when you flush it have to do with fisheries in Peru? 

Both exist because of the Coriolis effect.

The Coriolis “force” isn’t really a force as such, but a consequence of the centrifugal force, which acts outward from the centre of the Earth. This force exists because the Earth is rotating. It has it’s greatest effect at the equator. As it acts outward, at the equator this acts solely against gravity. This means that at the equator, because of the centrifugal force you weigh less than at the equator. This isn’t a great way to lose weight and cheat a diet as it only affects your weight by ~ 0.5% than from the poles when there is no effect from the centrifugal force. 

Anyway, interesting quirks aside, the centrifugal force component that acts against gravity is not constant; it diminishes with latitude, as such the Coriolis effect itself is not constant. Therefore, when things move with increasing latitude, they appear to be deflected. It’s hard to perceive, but take it as this - in the Northern hemisphere, everything deflects to the right, and deflects to the left in the southern hemisphere, with increasing latitude. 

Visualizing the deflection caused by the Coriolis Effect

The Coriolis force is weak and its effect only visible over large scales. The oceans have the size required for this as they cover many degrees of latitude, and form the gigantic oceanic gyres.

The 5 oceanic gyres on Earth. Note the clockwise (right deflection) in the
Northern hemisphere,and anti-clockwise (left deflection) in the Southern
hemisphere, due to the Coriolis effect.

The Coriolis effect is responsible for generating some of the most important features on the planet. Wind stress on the ocean surface moves subsequent layers beneath it, with gradually less momentum as a consequence of friction. However, in the oceans, the Coriolis effect causes a deflection in the motion of the water known as Ekman motion. Do to gravity and the Coriolis “force” acting on the Ekman motion, the water moves in a spiral, called the Ekman spiral. In the Northern hemisphere, Ekman motion is to the right of the wind stress and to the left in the southern hemisphere. 

The Ekman spiral

Like the oceans, the winds are deflected by the Coriolis effect and so create atmospheric gyres. Atmospheric anti clockwise (cyclonic) motion in the Northern hemisphere induces anti-clockwise wind stress on the ocean, which will therefore result in a Ekman motion to the right, out of the atmospheric gyre. This movement is termed Ekman pumping. This will remove water from the gyre. Divergence of this water results in upwelling, as deeper water is drawn to the surface. The opposite, downwelling, is seen when anticyclonic stress is applied to the oceans, and Ekman pumping moves water into the gyre and is forced down. This is summarized in the diagram below. 

Upwelling is particularly important, as it draws up valuable nutrients from the sediments on the sea floor to the surface. Below is another diagram showing all the areas in the world where upwelling occurs. 

The must intense upwelling occurs off the western coast of South America, where the combination of a strong wind stress and the strong cold Humboldt current along the eastern edge of the South Pacific gyre create ideal conditions for very intense upwelling. 

The water that upwells is also very nutrient rich and comes from the nutrient rich Southern Ocean. This combination of factors has led to very rich marine ecosystems. It is estimated that ~20% of the world’s fish supplies are harvested from this area of the world. It's importance is felt during the years of El Nino in the Southern hemisphere, when the system weakens, and the economies of several South American countries who depend on fisheries suffer chaotic and catastrophic losses; El Nino itself is named by Peruvian fishermen who noticed a severe drop in their catches during this period. In 1997-1998, the economic damages were estimated to be somewhere in the order of $3.5bn. Recent articles indicate that Peru will suffer a weak El Nino event into 2013.