Orbit - Round and Round in a Not So Circular Fashion

I came across this article released in April that details a new proposal for the origin of the PETM. It has long been known that the Earth's orbit is not circular, but varies over time, creating an Orbital Forcing effect on Earth's climate system. This article puts forward the idea that the Earth's orbit may have been both highly eccentric and oblique at the start of the PETM, affecting permafrost in the Arctic and the Antarctic and caused a large release of carbon dioxide and methane!

5. Barium's Mystery Associations

Barium (Ba) is a typical rare Group 2 element in the periodic table. It is rare and therefore very low in abundance. It's most commonly found as the mineral Barite and this is what it looks like: 

Barite

The flux of Ba_biogenic has been shown to have a reasonably good correlation with carbon export in the modern ocean to the sea floor. The graph below shows a nice association with Barium and the δ¹³C and δ¹⁸O across the PETM. 

Bains et al., 2000

The key mineral used in these measurements is Barite. The exact mechanism for its biogenic origin is still largely debated, however it is though to form in the surface waters of the ocean in microenvironments associated with decomposing organic matter - such as foraminifera. The mass accumulation rates over the PETM appear to show a positive shift in response to carbon cycle and climate changes. That is to say, more Barite was being deposited in the sediments at this time. But what does this mean?

More Ba_biogenic in the sediments implies that more organic matter was being produced in the surface waters, so the Barite rain became more intense. Unlike the coverage of our current climate crisis, the PETM appeared to do well for surface water species. Scientists have linked this to other phenomena observed at the time which include: 

• increased continental run off providing nutrients Nitrogen, Potassium and Phosphorous, which limit productivity in the modern oceans.
• increased volcanism, particularly in the North Atlantic, further increased ocean fertilisation
• rising global temperature by 5-7°C and increased CO₂ concentrations by 2000Gt. These are known to increase the rate of photosynthesis in modern plants

Other research has linked the Barite shift to the release of methane. As we have discovered before the methane in hydrates is created by microorganisms that live in the sediment. It has been noted in this research that Barite had a mass balance problem in previous studies - i.e. there was too much Barite in the sediments for this to be caused by productivity. In a bitter sting back, the supporters of the biogenic Barite explained that their cores had come from the continental shelves of the oceans - areas of high productivity, with dynamic equilibria associated with carbon fluxes and organic-carbon burial due. Basically, the continental shelves are very productive regions because, in part, due to their proximity to the continents and the run-off they receive. 

What the biogenic Barite theory also shows is a feedback mechanism. An increase in productivity acts as a counter-process to the release of methane to reduce the levels of greenhouse gases. It has been calculated that, globally, this could have been achieved in 60 000 years. This is critical today when our climate is changing faster than it has ever done so in the history of the planet. I guess it provides an unnerving reassurance that there is a possibility that everything will sort itself out. But, I ask, at what cost? 

4. Where'd all the Carbonate Go?

Calcium Carbonate (CaCO₃)is possibly one of the most well known chemical compounds. At school, you see that it fizzes when you put it in acid and it disappears. Funnily enough, this same process occurs in the oceans! Admittedly, there isn’t any fizzing, as I’m sure many people have noticed that the sea exuding gas when you go for a swim. However, CaCO₃ does disappear. Not by magic, but by dissolution. - i.e. it dissolves!

The diagram below shows how CaCO₃ in our oceans is linked to CO₂ in our atmosphere; the most prominent greenhouse gas. 

The oceans are the largest reservoir of C - 60 times more than
in the atmosphere. Most C in the surface waters is stored as CaCO₃

There is a rain of CaCO₃ that falls from the high productive surface waters of the ocean to the sea floor. CaCO₃ forms the basis of most shells of aquatic organisms and skeletons of foraminifera. This link provides a few extra details of forams, and their significance in the paleo-record as well as some excellent SEM images of their intricate shells. 

As the rain falls, the CaCO₃ re-dissolves until a point known as the carbonate dissolution depth (CCD) where the supply of solid CaCO₃ rain is less that the rate of dissolution - meaning none reaches the ocean floor. The depth of the CCD is dependent on many factors and so varies considerably. The CCD is influenced by ocean pH, rain rate, ocean circulation, and the intensity of remineralisation. Today, there is a 2km difference in the depth of the CCD in the Pacific and Atlantic oceans. 

Coincidentally, this was also the change in the CCD observed over the PETM. The telltale signs of the CCD are marked by low-carbonate or clay layers in ocean cores, known as carbonate dissolution horizons. By tracking where the horizons are in the sediments, scientists can accurate determine where the CCD lay throughout time. In the Southern Atlantic, studies have suggested that the CCD shoaled by over 2km in 10 000 years, taking 100 000 years to return to pre-excursion depths. The sensitivity of this system is still highly debated, as ocean composition is thought to play an important role. Alarmingly, the variations in CCD shoaling are not consistent with the levels of methane released under the methane hydrate hypothesis (which by the way is a rather hefty 2000Gt of C). Arguments now fight for a larger carbon input than the methane hydrate hypothesis provides!

3. Hydrothermal Vents, Sills and the Great Methane Escape

Everyone knows hydrothermal vents. They’re underwater fissures that spew out, amongst other things, superheated water. In the deep sea they play host to whole ecosystems that are completely independent of light - perhaps even the only ecosystem. 

The world's hottest hydrothermal vents - Two Boats and Sister's Peak.
Here water reaches up to 464°C

But, as with most things in life, they’re only good some of the time. Huge hydrothermal vent and sill complexes have been studied in the Vøring and Møre basins in the North Atlantic covering an area of over 80 000km². This is the area here - between Greenland and Norway under the Norwegian Sea. 

Associated with these hydrothermal vent complexes are a series of sills. These horizontal intrusions run for hundreds of kilometers, so likely formed very quickly. The sills, which also fueled the hydrothermal vents with hot magma, intruded into organic-rich Cretaceous and Paleocene mudstones. 

A Mudstone

The contact aureoles, a region where the hot magma of the sill has metamorphosed the surrounding rock, has been shown to have an organic carbon content with a δ¹³C of up to -50‰ which is very isotopically depleted indeed. It’s almost like it’s methane - and it is! There’s alot of it too! 1m³ of magma can release 3.6kg of CO₂ whereas the same volume but intruded through organic sediments can produce anywhere between 25 and 100kg of CO₂. So, it makes a pretty big difference.

Igneous intrusions across the whole of the North Atlantic Volcanic Province have been calculated to have produced enough methane through this process to create the -2.5‰ excursion seen across the early parts of the PETM.

Previously, I referred to methane as a source for the carbon excursion observed over the PETM - now it’s also a cause. 

2. Meteorite or No Meteorite? Iridium Anomalies

Iridium (Symbol, Ir) is a chemical element, atomic number 77 and looks like this:

It is an incredibly rare element that is the second densest around. That coupled with its tendency to bind to iron makes it a very low abundant element in the crust. Most of it sank into the mantle and core when the Earth was young. O dear. 

Its abundance in meteorites however is somewhat different. Chondritic meteorites and asteroids have Iridium concentrations of about 455 parts per billion (ppb) compared to typically 0.3ppb in the Earth’s crust. 

An Iridium anomaly or “spike” in the sedimentary layers across the the Cretaceous-Paleocene boundary 65 million years ago is thought to be a key indicator of a meteorite impact. The spike appears as a sudden and stark few parts per billion above the average 0.8ppb. The exact value varies across the Earth. This crude little graph below shows that it reached 6.5ppb. 

Spot the Spike

The Iridium anomaly at Zumaya, Spain, has been the source of a debated second such event, dated at 55Ma - the time of the PETM. The anomaly is reproducible but differs from the magnitude of the KT extinction by being noticeably less dramatic but most importantly coincides with a decrease in δ¹³C which, as previously mentioned, coincides with the PETM. 

So, is it a seal of approval for a meteorite impact? Well, we're still looking for the impact crater…

5 Geology-Related Things You Should Know About This Week

After a quiet week fighting illnesses, this week I bring you 5 things you should know about to understand some of the mechanisms proposed in causing the PETM. 

1. Kaolinite: Dull as Dishwater but Causing a Stir

Kaolinite is the clay mineral Al₂Si₂O₅(OH)₄ and looks like this:

Kaolinite

It’s not a particularly impressive mineral - it doesn’t dazzle your eyes with its dull earthy lustre and it won’t keep you entertained as much as other minerals like the beautifully named Cummingtonite. If your curiosity takes your fancy there, find out more about it here. 

It is extensively used when pure as a cheap, general-purpose filler and coating material for paper, in ceramics, and also in chemicals and paints. Yet again - nothing too special. It is formed from extensive continental weathering, particularly in hot, humid environments around the tropics, like in tropical rainforest areas. 

What is does do though is provide a convenient measure of paleoenvironments in the past. The Carbon Isotope Excursion (CIE) coincides with higher proportion of Kaolinite in soils across the world from New Zealand, the Southern Ocean and all around the ancient Tethys. This intriguing point is the Kaolinite rich sediments here occur very close to sediments with the minimum isotope value during the PETM, suggesting that deposition occurred within 1 000 - 10 000 years.

Within these sediments, scientist have detected and measured various magnetic readings from magnetic minerals such as magnetite, goethite and hematite. There is evidence that the minerals in the rocks are highly magnetised. 

Magnetite

Goethite

Hematite

Similar observations were made over the K/T boundary at the end of the Cretaceous where there is strong evidence for a meteorite impact which is all too well known for a likely cause for the demise of the dinosaurs. There is a hot debate amongst scientists that a similar event may have occurred at the end of the Paleocene. There is a hot debate amongst scientists that a similar event may have occurred at the end of the Paleocene. So did a meteorite trigger the PETM?

Until next time ...