AUSTRALIAN FRONTIERS OF SCIENCE, 2003
Canberra, 31 July to 1 August 2003
Novel biomarker
distributions and their stable isotopes in Permian/Triassic sediments
by Dr Kliti Grice
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Kliti Grice is a recipient of an ARC Queen Elizabeth II Fellowship. She received her PhD in 1995 (University of Bristol-UK) on stable isotopes of biomarkers for reconstructing (palaeo)environments. She held a postdoctoral position at the Royal Netherlands Institute for Sea Research. In 1998 she joined Curtin University of Technology, Perth. Since then she has established and leads a stable isotope research group within WA Centre for Excellence in Applied Organic Geochemistry (director Professor Robert Kagi). The stable isotope group carry out fundamental research applied to petroleum, water, (palaeo)climate and the environment. Kliti has received a number of awards including the international Pieter Schenck Award, the Australian Academy of Science's J G Russell award, and the inaugural WA Premier's Science Award. She has published around 30 papers and 60 conference presentations and is a member of PESA, EAOG, ACS and RACI. |
I will give you a brief overview of biomarkers, explaining what they are and how we use their stable isotopic compositions in ancient sediments. This particularly focuses on the Permian/Triassic boundary. I would like to point out that this is a collaborative effort with researchers from around the world: for example Richard Twitchett, Clinton Foster, Paul Greenwood, Evelyn Krull, Graham Logan and Cindy Barber.
I will give you a very brief introduction on the Permian/Triassic boundary, this was the largest mass extinction event, and I will introduce the proposed causes for this particular extinction boundary. I will give you some features of this Permian/Triassic boundary and introduce you to this study, which is using a fairly novel approach using molecular fossils, biomarkers and their stable isotope geochemistry to establish environmental changes which may have led to this devastating event.
The Permian/Triassic extinction has now been dated to have occurred 251 million years ago, when there was the most extensive loss of marine and terrestrial fauna or flora in the last 600 million years. A variety of mechanisms have been invoked for this extinction event, in particular the formation of a supercontinent called Pangaea at that time, the impact of a meteorite, the release of CO2 from massive volcanism it is believed that the release of massive volcanics from the Siberian Traps in the Late Permian was one of the causes which led to the eventual decline in the species at the Permian/Triassic boundary and also the release from the melting of hydrates, which I will describe during my talk, the overturn of stagnant deep-ocean waters and the development of anoxia, the lack of oxygen in the oceans.
More recently, Benton and Twitchett have indicated that most of these mechanisms are the cause of the mass extinction, but there is less favour in relation to the impact of meteorites.
Click on image for a larger version of figure 1
This study involves the analysis of organic carbon-rich samples. These are sample sets from sediment profiles from Greenland, China, Canada and Western Australia (figure 1). The organic carbon contents are around 1-2 per cent, which is particularly high, with the exception of the Chinese section [Heping] at 0.4 per cent.
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Figure 2 shows the location of these samples in the ancient continent 251 million years ago.
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The general features of the Permian/Triassic is that this event straddles an interval of rapid sea rise, and many of the Triassic sediments are characterised by oxygen-restricted facies. The Triassic has actually been dated, based on the presence of a phosphatic macrofossil called a conodont, which has been identified in a reference section from China. In the Triassic, the bivalve called Claraia dominated the low diversity of benthic macrofauna. This is very similar to the types of shells we find on the beach today, but this one is particularly associated to the Triassic (figure 3).
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Other features include the spinose acritarchs. Acritarchs are microfossils. Figure 4 shows an acritarch which is around 25 micrometres in length. These acritarchs are extinct. They are thought to be like algae, and these appeared in abundance in the Triassic as a consequence of the sea level rise. Other features of the Permian/Triassic are an isotopic excursion which occurs at the boundary.
Before I introduce the isotopic excursion I will explain what we mean by stable isotopes of carbon. Carbon exists as two stable isotopes, 12C and 13C. 12C is around 98.88 per cent of the total global carbon pool, while 13C is 1.12 per cent. We can actually determine the ratio of these isotopes in organic materials and in organic carbonates by this notation, where we determine the ratio of the heavy to the light isotope 13C to 12C in a sample and a standard. And because we are looking at a small difference between the 13C and the 12C composition, we multiply this difference by 1000 to get this per mil unit [also shown as 0/00]. The stable isotopes ratio changes and is controlled predominantly by biochemistry and the environment.
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Just to give you an indication of the variation in stable isotopes, figure 5 is a plot of the 13C per mil. As you go from 20 to -100 you are essentially containing relatively more 12C, the more negative. The arbitrary standard, which is marine carbonate, is 0 per mil.
Carbon dioxide today is around -8 per mil, relatively more 12C than the arbitrary standard. We can go to soil carbonates, marine bicarbonates and, in particular, algae, phytoplankton and the various land plants the grasses and the trees, wood, peat and coal. These are all negative, falling between -15 and -40 per mil.
Crude oils, which derive from the organic inputs from land plants and algae, and bacteria, fall between -20 and -40 per mil. Natural gas has quite a large variation, and bacterial methane is the most depleted or has the most negative isotopic composition.
So we know that the natural components on Earth, compared with our arbitrary standard, have very negative values. That is because they contain relatively more 12C than the standard.
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At the Permian/Triassic boundary there has been an isotopic anomaly reported. In figure 6 we have the older samples, Permian, and the younger samples, Triassic, and a plot of the 13C/12C composition of carbonates, going from 1 to -2 per mil. As you see, there is a dramatic change in the isotopic composition of the carbonates at the boundary. Likewise, there is a change in the 13C/12C composition of the organic matter, from -28 to -32 per mil. This implies that there was light carbon which was introduced into the atmosphere at the Permian/Triassic boundary.
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Just to show you a synthesis of curves of the 13C/12C composition of carbonates from a variety of sections, from western to eastern Tethys (figure 7). Going from the Permian into the Triassic we see this dramatic change. Also in the Chinese section there are some fluctuations but generally there is an isotopic change at the Permian/Triassic boundary.
The preferred model for this isotopic anomaly is related to global warming. It is thought that the Siberian Traps caused an increase in global temperature, maybe an increase of around 6°C, and this caused gas hydrates, which are found beneath the ocean, to melt and introduce light carbon into the ocean and atmospheres from the gas hydrates.
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Essentially, we have this analogy. Methane in the hydrates today is about 1x104 gigatons of carbon, and this is around twice that amount trapped as fossil fuels. There are found in permafrost regions and the outer continental margins 300-500 metres deep. When there is a global warming, these gas hydrates start to melt and they release burps of methane containing this 12C signature into the sediments, and then a small flux of that methane into the oxic oceans. Eventually some methane escapes into the atmosphere and is oxidised to CO2 to cause a 'runaway greenhouse' climate (figure 8).
Biomarkers are hydrocarbon compounds which are preserved in sedimentary organic matter and oils. They actually derive from biochemicals in photosynthetic organisms such as our land plants and algae, archaea and organisms that may feed on process carbon such as methane. Some are specific structurally and some are non‑specific.
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Figure 9 shows a biochemical, which is a bacteriohopanotetrol, found in prokaryotic bacterial membranes. Here is the living cell. The living cell contains a membrane which is lipid-rich, and part of this lipid membrane contains these types of biochemicals.
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When the organism dies, this biochemical becomes incorporated into the sediments, and over time becomes defunctionalised it loses these hydroxyl groups and we end up with a biomarker. These biomarkers retain the carbon signature and also the stable carbon isotopic signature in sediments over millions of years old (figure 10).
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How do we get biomarkers out of the rocks? We actually grind the rock and extract with organic solvents. Extracts contain thousands of biomarkers, and we can separate those biomarkers into simple classes which all contain hundreds of biomarkers saturate, aromatic and polar biomarkers with liquid chromatography, using solvents of increasing polarity (figure 11).
We can then identify individual biomarkers in these complex mixtures by special GCMS techniques. So we can identify the structure of the biomarkers. And we can take it one step further and obtain the isotopic composition of the 13C/12C ratio of these biomarkers to identify the organisms which once lived and establish any environmental changes which may have occurred, such as release of methane, which may result in a change in their isotopic signatures.
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We actually have some biomarkers of methane hydrates, which makes this interesting for the Permian/Triassic. The biomarker crocetane, which is shown in figure 12, is derived from organisms called archaea. They are pretty smart, in that they anaerobically oxidise methane, using sulfate. Very recently, crocetane was accidentally found in recent sediments near gas hydrate vents, based on its light isotopic signature, which is around -150 per mil very negative. Crocetane analysis, however, is particularly challenging, because this compound, by conventional chromatographic techniques, correlates with another biomarker, phytane, which derives from chlorophyll.
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However, looking at some samples from China we have tentatively identified the presence of crocetane in the Permian/Triassic boundary samples, in particular at the boundary and in the Permian samples before the large change in isotopic signature at the boundary (figure 13). This is the first molecular evidence for methane release which occurred at the Permian/Triassic extinction event.
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In the Triassic samples we have identified some novel biomarkers. Figure 14 shows a series of biomarkers this is an Australian sample and this biomarker here, C33 alkyl cyclohexane (ACH), has been identified in the Basal Triassic. This, incidentally, has been a unique feature of Triassic rocks which generate petroleum in the Perth Basin for many years. However, we now identify this component in the Basal Triassic of Greenland, which may indicate that this biomarker is in fact a novel feature of the Basal Triassic.
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We also find similar components this is phytanyl toluene in the Australian sample and also in Greenland (figure 15). This biomarker has been associated with saline environments, and again has been identified in Triassic holes in source-rocks of the Perth Basin.
So where did these biomarkers come from? We suspect that they may be derived from spinose acritarchs, which are extinct species, and algal blooms off these acritarchs occurred in abundance in the Triassic when the sea level rose. Alternatively, there may be other sources, but this is ongoing work.
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What do the isotopic signatures of some of the biomarkers in the Greenland and Australian samples show? Figure 16 shows a plot of the 13C/12C signature versus a carbon number of each of the biomarkers. These are the Permian samples from both Greenland and Australia, holding very tightly together between -27 and -29 per mil. Here are the Triassic biomarkers, which are much more negative, much more 12C, and they appear to carry the light isotopic signature which is evident in the carbonates and the total organic carbon which occurs at the Permian/Triassic boundary.
The stable isotopes of the biomarkers and their distributions appear to be similar in both the Permian and Triassic samples about the globe. This is quite remarkable, given that these two sample sets are so far away from each other during the Late Permian. So I suggest that the conditions in these oceans were pretty similar in both the northern part of Pangaea and the southern part of Pangaea.
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We are also searching for biomarkers derived from green sulfur bacteria. Green sulfur bacteria are strictly anaerobic they do not like oxygen but they require light to photosynthesise, so they live in the photic zone (figure 17). In their photosynthesis, unlike most photosynthetic organisms like algae, which use water in photosynthesis, these organisms use hydrogen sulfide as an electron donor to fix their CO2 in the presence of light. To enable them to capture those longer wavelengths of light, they actually make specific pigments for photosynthesis which are different from all the other photosynthetic organisms in the water column. They make this carotenoid, called isorenieratene, which has these unsaturated groups here. The biomarker which results from this is isorenieratene, which is saturated. It loses these unsaturations.
Click on image for a larger version of figure 18
It also makes specific bacteriochlorophylls, and this gives rise to specific biomarkers. Figure 18 shows the bacteriochlorophyll C of the green sulfur bacteria, and this part of the macrocycle of the chlorophyll gives rise to these maleimides, which have quite an unusual group here which is from the original bacteriochlorophyll.
The presence of green sulfur bacteria indicates there were periods of photic zone euxinia, the presence of hydrogen sulfide, that these organisms lived in the photic zone. Also, the presence of these markers indicates that the water column was very stratified that there was very little mixing and there was little oxygen being replenished into the basin. This leads to excellent anoxic conditions for the preservation of organic matter, which can lead to rocks which can generate petroleum.
In conclusion, we have evidence for the release of methane from biomarkers, from the presence of crocetane found at the Permian/Triassic boundary in China. We also identify novel biomarkers in the Basal Triassic in both Greenland and Australia, and these novel biomarkers the C33 alkyl cyclohexane and the phytanyl toluene may be related to spinose acritarchs, which evolved as a result of the Permian/Triassic event when the sea level rose.
The biomarker and isotope profiles in these sections and other sections I have not shown all the sections I have looked at but they are actually quite similar, indicating that there were similar environmental conditions, at least for the microbes to live, during the Triassic and the Permian.
I would like to acknowledge the ARC for funding for the isotope equipment, and technical assistance from Sue Wang, Geoff Chidlow and Christian Thun.
