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. 

The Motion of the Ocean: Part I

The oceans aren’t static. They’re riddled with currents that cover the entire globe and are one of the most important features in heat distribution across the entire planet. There is more intense heating at the equator than at the poles, as solar radiation is more concentrated here. Water has a higher specific heat capacity than land, and so serves as a major heat sink in the oceans, but also serves as a huge stabiliser for Earth’s climate. 

The general pattern of ocean currents today is shown below:

The modern pattern of ocean circulation

The Physics (no groaning) of this is somewhat mathsy and I’m not going to go into that (but if you do, be prepared for lots of diagrams, differentiation, integration and fairly cool greek letters like ρ, Ω, and η). In short, there are 4 forces that affect the movement of water in the oceans:

• Gravity
• Wind
• Coriolis “force”
• Friction

Wind only affects the surface waters, pushing it around much like when you blow a pile of sand on the beach, but over large scales of many kilometers. This doesn’t so much affect ocean circulation as it just pushes the skin of the ocean around a bit. 

The modern pattern of deep ocean circulation

The more influential aspects of the deep ocean circulation are friction and gravity. Together they act to generate the mixing within the ocean. By density differences in the ocean, horizontal pressure gradient forces move water horizontally in the ocean, as well as vertically where denser water sinks through less dense. This factor is particularly important in the North Atlantic where the Gulf Stream, a warm tropical current that bathes Europe in warm water, sinks and is deflected back south as a cold deep water current. 

Equatorial water is more saline, due to higher evaporation rates in the tropics, than the polar waters which are frequently exposed to meltwater and icebergs. As such, the salty dense Gulf Stream is forced under the less dense North Atlantic as it approaches. The oceanic crustal topography deflects the cool current southward where it eventually becomes the Labrador Current, giving the eastern coasts of Canada and USA bitterly cold winters - even though they lie at the same latitude as Europe and North Africa.  Today, Boston, Massachusetts is at the same latitude as Florence, Italy, but have highs today of 3°C and 14°C respectively. 

This little animation gives a brief overview of the global deepwater circulation pattern, but also focuses on the Gulf Stream in the North Atlantic, so watch it and see what happens.

Next time - The Coriolis Effect and why it's so much more important that you thought.

Extinction

So you've survived the Mayan apocalypse this week - what better time to start talking about something that didn't survive its doom:

What could this little benthic foram tell you?

Orbina universa

Photo: Dr Howard Spero, University of California, Davis

He probably wont be able to tell you the future, remind you of that last item on your shopping list or tell you what to buy this Christmas, but he, and all his friends can tell you alot about climate change in the past. 

He’s a benthic foram and has the biggest habitat in the world - the deep ocean floor. It used to be thought to be a low-diversity and desolate place, but since the 1960s, it has revealed itself to be an incredible diverse habitat. In the deeper depths (>2000m) forams are thought to be the most diverse and constitute over 90% of the biomass. So, in short, they’re well adapted and successful in their habitat!

But 55.5Ma, these little forams experienced their biggest extinction for over 100Ma where 37% of species died out. In some instances this range has expanded to 50%. To put this into a wider perspective, the benthic forams survived the KT extinction relatively unscathed, whereas terrestrially there was huge biological shifts, where the dinosaurs gave way to mammals. Benthic forams have one of the most complete fossil records of marine species and so are used in a variety of paleoclimate reconstructions.

From Thomas, 2007, this figure shows the benthic foram turnover since the Cretaceous.

Note the P/E boundary shows the most significant change in δ¹³C and δ¹⁸O.

The observations across the PETM are unclear, it doesn’t help that the deep sea is still one of the least well known ecosystems on Earth. Many deep-sea species that survived the KT extinction suddenly became extinct at the KT and were replaced by varying taxa with no clear pattern of succession. Other, planktic foraminifera, genera of the dinoflagellates, Apectodinium, appeared to migrate to higher latitudes. There were similar migrations in terrestrial migrations in plants, but more peculiarly, there is a rapid diversification of mammals in this time. It a confusing picture......

So what could affect this, the largest habitat on Earth? 

Ocean Circulation. 

If you’d like to delve more into extinction, and discover more about extinction of other species and its effects on biodiversity today, I recommend this blog here. It provides a particularly detailed insight into a few select species that are at risk of extinction and even what you can do to help! 

The Earth Story

I have been following this on Facebook for some time. It's an excellent little blog that brings some interesting aspects and facts of the Earth right into your lap. I'd like to draw your attention to the post reviewing the data on the most recent global carbon emissions estimates. These were published in the journal Earth Systems Science Data Discussions and subsequently made headlines in the media this week. This of course goes on in a climate-packed week where discussions carry on in Doha on the future of emissions targets and the most accurate data on polar melt is released. It paints a rather dark and disturbing picture of what is to come but still makes for an interesting read, inspiring you to think more about our changing world and comes with this pretty good picture! =)

So, What Do We Know Now?

Today’s post is quite wordy, but I’ve put some nice photos in as a reward! Happy reading and I hope it’s kept you interested. 

Over the past few weeks, we’ve been looking at a series of concepts and processes that have helped us to understand the PETM and its significance. Today, I’m going to review the evidence, put all the pieces together and hopefully draw some conclusions over what exactly caused the PETM.

So, what do we know? We know, from a variety of sources, that 55.5 Ma, there was a large isotopically light excursion of C, of around 2-3‰ into various biomes over the world, resulting in a δ¹³C shift of 2-3‰. Estimates vary quite considerably from 4000-7000Pg of C - thats a window of 75% of the lower limit. In short, its very imprecise! In any case, an excursion of even 4000Pg of C to give a isotopic shift δ¹³C of 2-3‰ points towards new sources of isotopically depleted C. That is to say, C sources that contains more ¹²C than their standard, the Pee Dee Belemnite. After solving the mass balance problem, this cannot be explained by methane hydrate stores alone, and so we’ve come across other potential sources including wildfires, increased volcanic activity and even meteorite impacts.

A fossilised Belemnite

A reconstructed Belemnite

Methane hydrates were first proposed, based on the mass balance arguments, to explain the carbon release across the PETM. Within a decade, this excursion had been linked to various seismic and sedimentological data from the western margins of the Atlantic Ocean, which is thought to have resulted in the release of methane hydrates stored in the sediments. 

As studies probed further into the evidence of the PETM, focus shifted to the early PETM (ePETM), as knowledge of our own modern climate system developed with the advent of concepts such as tipping points and positive feedback. It was here that many researchers have intended to find a localised, intense source of the carbon isotope excursion (CIE). Rather sound data have come from the Vøring and Møre basins of the North Atlantic in support of a link between seismic disturbances and CIE. It is here that large hydrothermal vent complexes were found along seismic reflection profiles, in association with a variety of sill complexes. As has already been explained before, the carbon rich sediments, once in contact with the sills, were metamorphosed into the contact aureoles that we see today, at the expense of releasing at least 7 times as much C as from ordinary basaltic melt. 

Prior to such discoveries, proposals (Katz et al., 2003) from terrestrial sources, notably the burning of peatlands and coal. Such mechanisms too closely resemble the anthropogenic factors that cause modern climate change; ie the burning of fossil fuels and positive feedback mechanisms associated with them. Yet again, the mass balance issue plagued these theories, as it is calculated that with a -26‰ signature of biospheric carbon, for 7000Pg C to be released with a δ¹³C of -2‰ to -3‰, more organic matter would have to be burnt that is present in the total organic carbon reservoir. Basically, there isn’t enough C there to explain it! Whilst the volcanic origins of the ePETM warming solve these problems, there is insufficient resolution on the data to accurately date them. 

Subsequent warming from the ePETM resulted in a positive feedback effect, where it just kept getting warmer and warmer. The largest effect on the biota is seen in benthic foram species, where 35-50% of species became extinct. From evidence from Barite accumulation in sediments, there have been some support of a negative feedback mechanism with planktonic forams. As waters become warmer and richer, the forams are able to fix more CO₂ from the oceans, and thus remove it from the atmosphere, returning the Earth to its original equilibrium. In my personal opinion, this theory is still quite unfounded and is based upon a proposed model by the same people of Barium cycling in the oceans; not to mention that the concentrations involved are far lower than most concentrations of other solutes in the oceans, making the data highly susceptible to tiny perturbations in chemical composition, sometimes not necessarily related to CO₂. Below are examples of forams, exemplifying their morphological diversity, which is extremely useful for dating ancient sedimentary basins. 

Belemnites are extremely diverse

In another proposal, we came across was the extraterrestrial impact theory based upon an iridium anomaly at Zumaya, Spain. The iridium “spike” at Zumaya was dated to be of similar time to the magnetization of minerals within Kaolinite minerals in the sedimentary basins off the eastern coast of North America. Similar observations were made over the KT boundary at the extinction of the dinosaurs. - cue highly inaccurate, but awesome looking KT impact picture: 

The KT Extinction resulted in the demise of the dinosaurs and
wiped out 75% of all species on the Earth

However, these magnetized particle within the rocks were very constrained in their grain sizes; not something you’d expect to see from an event that produced 100 teratonnes of TNT (which is over a billion times the energy that was dropped on Nagasaki and Hiroshima)! Instead, the authors concluded that the particles may have been biogenic is origin, as this is known to occur in nature. But how to explain the iridium anomaly.....

The anomaly itself is small. It is measured in 133-143ppt, which is small, so we’re not looking at another KT event. Other evidence for a KT-style impact is either localised, absent or misleading. The key give-away for the KT extinction was the Yantucan peninsula where the chixuclub crater was the “smoking gun” for the extinction of the dinosaurs. Such signs are absent for the 55.5Ma extinction. Now it is believed that the PETM occurred at a time where there was potentially a large meteor shower. Fossil evidence certainly points towards there being no major terrestrial extinction events; which is an unlikely scenario for an impact theory. 

So, there we have it. I conclude that the PETM was initiated by increased volcanic in the North Atlantic, and after crossing a tipping point, resulted in a series of positive feedback mechanisms, that ultimately led to a runaway greenhouse gas excursion, the most obvious measure of which was the release of methane from methane hydrates due to changes in the  climatic systems, resulting in warmer conditions globally. Primarily, by the feedback mechanisms of the flora of both the oceans and land that CO₂ levels could then again be lowered and returned to a more stable condition. 

Next time I intend to look at life across the PETM and extinctions. Today we are thought to be entering a 6th mass extinction through a combination of climatic change and other disturbances by how we live and develop and grow. We weren’t around then, so were there species shifts across the PETM? Did taxa go extinct? What changed?

Ciao for now.