In this paper, published in The Cryosphere, we used Raman spectroscopy to trace carbonaceous material from the shoreline to the furthest reaches of the East Siberian Arctic Shelf.
Raman spectroscopy uses laser light to determine the molecular structure of a wide range of materials. During my PhD, I showed that Raman could be used to probe individual particles of organic carbon to show whether they were disordered, like coal, or crystalline sheets of graphite. This collection of carbon particles, known as carbonaceous materials, are particularly hardwearing and resistant to breaking down during erosion and transport. During my post-doctoral research, I then used a range of organic geochemistry techniques to investigate the transition from terrestrial to marine carbon across the East Siberian Arctic Shelf, showing that coastal erosion and river erosion were both supplying organic matter to the ocean.
This study uses the Raman technique on those same Arctic Shelf sediments to look at the sources and distribution of carbonaceous material on the shelf. The samples used in our paper were collected from close to the mouths of some major rivers, from areas experiencing rapid coastal erosion, and from hundreds of kilometres offshore.
The hardest work, collecting over 1400 Raman spectra, was carried out by two undergraduate students, Melissa Maher and Jerome Blewett, who are co-authors on the paper. The collected spectra were then analysed using an automated peak fitting script, and grouped according to the shape of the fitted peaks. This provides an unbiased method for determining whether a carbon particle is highly graphitised, mildly graphitised, disordered, or somewhere in between. For each of the sites on the shelf we collected spectra from up to 30 particles, and looked at how many fitted into each group. Statistics were then used to spot patterns across the shelf.
Our first finding is that the relative proportion of “disordered” and “intermediate” carbon particles varies, and that there are patches with more of one or the other group. At the coastline these patches align with two of the major rivers (Kolyma and Indigirka) and areas of rapid coastal erosion. Surprisingly, the patches can then be traced all the way across the shelf. We would have expected the currents in the ocean to have mixed the particles together further offshore, and in the biomarker studies we’ve done before we did not spot this kind of pattern. This means that Raman is a great way to trace the different sediment and carbon sources on the shelf.
Our second finding is that the amount of “highly graphitised” carbon particles is highest in the furthest offshore samples. These very crystalline flakes of graphite are behaving differently to all the other carbon particle groups. It’s not clear exactly why this is, but one option is that everything else is breaking down and degrading before getting that far offshore. Or, the graphite particles could be so light that they sink very slowly, floating out to the shelf edge much easier than the other types.
This problem has implications for the global carbon cycle. These carbon particles have been released from permafrost on land and transported for hundreds of kilometres offshore, a trip that has taken thousands of years. If all of the carbonaceous material can survive the journey, it means that this fraction of the organic matter is not at risk of being degraded and released to the atmosphere as greenhouse gases. Burying it in the ocean provides protection from degradation for thousands or millions of years. Future studies should look at just how well the carbon particles can survive erosion and burial.
In summary, carbonaceous material is resilient to degradation and can be used to trace sediment sources across the Arctic shelf.
Update: The paper is now published and can be downloaded from the journal webpage.
The study continues our work on the East Siberian Arctic Shelf, and contains two new datasets. The first is a radiocarbon study, measuring the age of organic matter on the shelf using carbon dating (see map above). By measuring the age, we can determine whether the carbon has come from the ocean (very young), the topsoil (quite young) or the coastal permafrost (thousands of years old). We combined our results with those already measured on the shelf to form the most complete radiocarbon map for this area. The high-resolution map shows that areas close to the shore and away from the major rivers are home to very old carbon, almost certainly sourced by erosion of old permafrost cliffs. Elsewhere on the shelf, the carbon is younger but not as young as modern topsoils or ocean carbon. Therefore the coastal erosion carbon is having an influence right across the shelf.
Our second technique is pyrolysis GCMS, where samples are smashed into small pieces using high temperatures and the small pieces are then analysed using GCMS. This technique generates a large amount of small pieces, too many to analyse each one individually, and so we decided to concentrate our efforts on a few target molecules. These included Phenols, which are probably sourced from lignin, a major component of land plants, and Pyridines, which are nitrogen-containing compounds probably sourced from proteins. We think that a lot of the Pyridines in the Arctic Ocean will come from organisms living in the ocean itself, and therefore the Pyridines are a potential tracer for marine organic matter. By comparing the concentrations of Phenols and Pyridines, we can estimate the amount of terrestrial and marine organic carbon in a sample.
In the map above, red areas are dominated by Phenols and are therefore rich in terrestrial carbon, blue areas are dominated by Pyridines and are therefore rich in marine carbon. This pattern matches very well with our previous work in the region, showing that there is a transition from terrestrial to marine conditions across the Arctic Shelf, and that the transition zone lasts for hundreds of kilometres offshore. This means that there is a lot of terrestrial carbon being deposited, and hopefully buried, on the shelf, rather than all of the eroded carbon being degraded and released as CO2.
06/09/2016 UPDATE: The paper has been accepted and is now published. The final version is available from the journal.
Our international team of East Siberian researchers currently has a paper in open review at Biogeosciences. The discussion paper, and its interactive comments, can be downloaded from the journal website.
The paper studies a group of compounds called “bacteriohopanepolyols” (BHPs for short), which are found in the cell membranes of a range of microbes and are therefore one of the most common organic compounds around. They are found in modern and ancient sediments from all over the world. This study has concentrated on two groups of these. Group 1 is the soil marker compounds. These are only found in soils, and so have been used as tracers for soil material in rivers, lakes and offshore. Here is how they are spread across the East Siberian Artic Shelf:
Note how the soil marker concentrations are highest (orange colours) near to the rivers and coastlines. By measuring the concentration next to the river mouths, and in the sediments being washed away by coastal erosion, we show that it is not just rivers that are delivering the soil markers to the Arctic Ocean.
There is no single compound that is a true tracer for carbon produced in the ocean itself, but the compound bacteriohopanetetrol (BHT) is most abundant in marine settings despite being found in soils as well. Therefore if your sample is rich in BHT, and poor in soil markers, it is likely dominated by carbon from the ocean. Here’s a map of BHT across the East Siberian Arctic Shelf:
The BHT results show a fairly constant amount across the ocean floor. If we compare the soil marker concentrations to the BHT concentrations, we can see which areas are rich in soil carbon (more soil markers than BHT) and which are rich in marine carbon (more BHT than soil markers). This comparison is called the R’soil index, and is shown below:
The R’soil index shows that all along the East Siberian Arctic coastline, offshore sediments are dominated by carbon from the land. As you go further offshore, especially in eastern parts nearer to the Pacific Ocean, marine carbon is more important. This result shows a similar pattern to that seen using stable carbon isotopes, but is different to the pattern shown by the BIT index. Therefore these two indices, both based on microbial biomarkers, are tracing different parts of the carbon cycle.
The Guardian newspaper today reported on an interesting set of photographs today documenting one side effect of warming permafrost. Photographer Amos Chapple travelled to the East Siberian region to mine prehistoric ivory from permafrost. Locals had discovered tusks emerging from the river bank as rising temperatures coupled with erosion to uncover bones and tusks that had been buried for thousands of years.
The photographs are well worth a look, documenting the extreme (and extremely dangerous) lengths that the ivory miners (“tuskers”) will go in order to find ivory worth tens of thousands of dollars per piece. They carve caverns into the permafrost using high pressure hoses, leaving behind a pock-marked hillside and a river full of debris.
A successful find will net more than $50 000 in cash, and the carved products will be sold for millions, likely in China. While the payback for the few who strike lucky can be life changing, the damage to the local ecosystem, and the likely increase in permafrost degradation and carbon release due to this activity, means that the long-term regional and global consequences will far outweigh the local gain.
The paper is a combined effort from a large team of researchers interested in the way that organic carbon is exported from small tropical islands. These islands are very biologically productive, the forest grows quickly in the warm, wet climate. They are also responsible for delivering large amounts of sediment to the ocean. Rocks are weakened by earthquakes and then eroded by the frequent tropical storms that hit the islands. This washes sediment and carbon out to sea via two mechanisms. High sediment concentrations lead to ‘hyperpycnal’ plumes of material flowing along the ocean floor, lower sediment concentrations cause ‘hypopycnal’ flows that stay at the ocean surface. Both systems can spread sediments and carbon a long way offshore.
It had long been thought that hyperpycnal delivery of sediment, which usually only occurs in the most extreme weather conditions when floods deliver vast amounts of sediment to the ocean in a short period of time, were efficient methods of organic carbon preservation. Our data confirmed this hypothesis, showing that little terrestrial organic carbon was lost during transport, and little marine carbon added to the mixture.
However, the dataset also investigated how efficient hypopycnal delivery can be. Sediment and carbon are delivered throughout the year, in smaller floods and less extreme storms, and it had been suspected that this mechanism exposed the carbon to oxidising conditions where it could be degraded and released as CO2. We showed that in Taiwan, where hypopycnal conditions exist but are still receiving and accumulating a large amount of sediment, the burial efficiency of carbon in hypopycnal conditions is still very high. Marine carbon is mixed into the sediments, but at least 70% of the terrestrial carbon survives.
This means that small tropical islands are even better at exporting and burying carbon than was previously thought, and therefore better at sequestering atmospheric CO2. We estimated that more than 8 Teragrams (million tonnes) of carbon could be buried each year throughout Oceania.
This paper was published in Organic Geochemistry, and is available open-access through the journal website and the MMU e-space repository. In the paper we take a detailed look at lipid biomarkers along a transect from the Kolyma River to the Arctic Ocean.
The data used in this paper is a subset of the data from across the East Siberian Artic Shelf (ESAS) published shortly afterwards in Biogeosciences. In this paper we took a closer look at the offshore trends seen in material delivered to the ocean by the Kolyma River, the easternmost of the Great Russian Arctic Rivers. The Kolyma River catchment is entirely underlain by continuous permafrost, which makes this are an extreme endmember in terms of permafrost systems. The main sources of organic matter from the Kolyma region are river erosion, mostly from top few metres of soil, the active layer that freezes and thaws each year, and coastal erosion from the “yedoma” cliffs along the shoreline.
These samples had previously been analysed for bulk properties (total organic carbon content, carbon isotope ratios) and some basic biomarker measurements, but we added complex lipid analyses to the story. We measured both GDGT and BHP lipids, these are found in microbes and can be analysed using LCMS. Amongst the many applications of these lipids, they can be used to trace the amount of soil found in offshore sediments. Each group of molecules has an index associated with it: GDGTs are used in the BIT index and BHPs are used in the R’soil index. Values of 0 would have no soil input and 100% marine carbon, while values towards 1 would be dominated by soil.
Usually these indices show the same offshore trends, which would be expected since they are both supposed to be tracing the same number – the proportion of carbon coming from soil. However, as the figure above illustrates, the two indicies have very different patterns from the river mouth (0 km) across the shelf (500 km). The BIT index drops quickly offshore, making a curved offshore profile, but the R’soil index forms a straight line offshore. Therefore two different techniques, supposedly measuring the same thing, don’t show the same results.
We think that this is due to the source of lipids used to make each index. Branched GDGTs (from soils) are common in sediments close to the river mouth, but their concentration drops quickly offshore. Marine GDGT concentrations increase across the shelf and this combination causes the BIT index to decrease rapidly. Branched GDGT concentrations in soils and lakes on land are high, but they are very rare in the coastal permafrost cliffs. Therefore any coastal erosion is not really affecting the BIT index.
On the other hand, soil marker BHP molecules are found in river sediment and coastal permafrost, and so there are two terrestrial sources. The concentration of soil marker BHPs drops much slower offshore than for the GDGTs. Also different, the concentration of the marine BHP marker doesn’t increase offshore. This combination means that the R’soil index drops much slower than the BIT index.
In the end, what this paper mainly shows is that when using biomarkers as proxy measurements for something else one single result is probably not enough. Proxy measurements are valuable tools, but they depend on measuring one thing to discover another. Combining multiple proxies together adds value and reliability to a study, either by confirming a hypothesis or bringing new insights.
This paper, with Jens Turowski and Bob Hilton, was recently published as an open-access article in Geology and is available from the journal website and via the MMU e-space repository. In it we show that erosion and transport of large woody material (coarse particulate organic carbon; CPOC) is very important in terms of the overall carbon cycle, but is concentrated in very extreme events.
The research is based in the Erlenbach, a small river in the Alps that has been studied by Swiss researchers for several years. They have built a very sophisticated stream sampling station, which can capture everything that flows down past a gauging station. There is a large retention pond that catches the logs/pebbles/sediments and a shopping-trolley sized wire basket that can be moved into the middle of the stream to catch a particular time point (for example the middle of a large storm). The photo below was taken during winter when the river was frozen over, but you can see the v-shaped river channel, the three wire baskets ready to move into position to catch material, and the snow in the foreground covering the retention pond.
This sampling system led to Jens observing that there was a lot of CPOC coming down the river and piling up in the retention pond. A quick calculation suggested that this was a significant portion of the total carbon coming out of the river catchment, but the scientific consensus was that actually more carbon came through as fine particles than CPOC. An experiment was designed to test this.
Across a wide range of river flow speeds, the amount and size of woody debris flowing down the river was measured, both waterlogged and dry material. This allowed a rating curve to be defined – that is for a given river flow speed, how much organic carbon would be expected to flow down the river? The rating curve was very biased towards the high-flow end for CPOC, much more so than for fine carbon (FPOC) or dissolved carbon (DOC). At low flow rates, very little CPOC is moved, but at high flow rates a very large amount is mobilised.
During the 31 years of data collection there were four particularly large storms. Integrating over the rating curve shows two things. Firstly, if the large storms are ignored then the Erlenbach is already a major source of CPOC, about 35% of the total carbon, with CPOC being roughly equal to the FPOC estimate. Thus it is much more important than might previously have been imagined. If the extreme events are included, the CPOC becomes ~80% of the total organic carbon transported by the river.
A majority of the CPOC transported by the river was waterlogged, having sat on the river bank or behind a log jam while waiting for a large storm to wash it downstream. Waterlogging increases the density of the wood and makes it more likely to sink when it reaches a lake or the sea. My contribution to the paper was to provide evidence of this process. My PhD work in the Italian Apennines found CPOC, from millimetre scale up to large tree trunks, that had been preserved in ocean sediments for millions of years. Again, a lot of this CPOC is too large to measure using standard techniques, and suggests that rivers can deliver organic carbon from mountains to the ocean far more efficiently than previously thought.
This paper is a result of collaboration with researchers at the University of Glasgow who I met while interviewing for a position. I didn’t get the job, but I did get talking to Paula Lindgren and we discovered a common interest in using Raman Spectroscopy to study organic carbon. This publication is the first result of that, and is available as an open-access article.
The paper is a comprehensive study of a meteorite collected from Antarctica. Antarctica is a great place to find meteorites because they sit on top of the ice and are easy to spot – sometimes the flow of ice even concentrates them into particular areas to makes things even easier. This meteorite is classified as a “CM carbonaceous chondrite” and has experienced very little change since it was part of the protoplanetary disc billions of years ago. Therefore we can use it like a time capsule to look at what the early solar system was like.
However, some meteorites are better time capsules than others. As they float around the solar system, they can build up ice, which can then be melted by radioactivity and the water released can alter the crystallography of the meteorite. Our paper uses a wide range of techniques to characterise the meteorite and show that it is one of the least altered carbonaceous chondrites ever found.
The techniques used included electron microscopy (both scanning and transmission techniques; SEM and TEM), X-ray analysis, X-ray diffraction, thermogravimetric analysis (TGA), oxygen isotope measurements and Raman Spectroscopy. My contribution was to use my automatic Raman processing technique to determine how crystallised the carbon in this meteorite was in comparison to carbon in other meteorites.
The island of Taiwan, in the South China Sea, has an interesting, yet devastating, combination of climate, biology and geology. It sits on the plate boundary between Eurasia and the Phillipines, which are moving into each other and causing the island to rise out of the ocean. This leads to earthquakes as the land is pushed up, and rapid erosion as the mountains get steeper and taller. The mountain sides collapse as landslides, producing sediment that is prone to being washed away by Taiwan’s many rivers. These rivers may not be the longest in the world, but they carry a lot of water, because Taiwan sits in the tropical zone where the year-round high levels of rainfall are topped up by several typhoons each year. The final piece of this jigsaw is the biology – being in a warm, wet region means that Taiwan is very biologically productive, with extremely fast forest growth rates.
Coupling all of these features together leads to a heavily forested mountainous island on which the hillsides are regularly landsliding and generating woody debris (tree trunks, branches, shrubs etc.) The large typhoons that hit the island each year provide water which washes the woody debris into the rivers and then out to the ocean.
In 2009, Taiwan was struck by a particularly devastating tropical cyclone, Typhoon Morakot. The island received about 4 metres of rainfall in just a couple of days, enough to cause rivers to burst their banks, washing away entire villages and unfortunately leading to several deaths. I visited the island a few months later, and the clear-up operation was still going on. Beside the rivers was up to a metre of chaotic sediment with tree branches sticking out of it, since the waters had carried everything off the hillside and dumped it when the floods receded. A lot of the tree trunks made it all the way through the river, out to the sea. They washed up on the shoreline around Taiwan, and were reported as far away as Japan.
These trees contain a lot of carbon, which has been moved from the hillside to the floodplain and out to the sea. Our study tried to work out just how much carbon, in the form of coarse woody debris, was being transported during this storm. Single river channels, such as the picture above, could contain 40 million tonnes of carbon – how much carbon was washed away by the whole storm?
There were two independent methods used to make the carbon estimates. The first one compared aerial photography before and after the storm to look at how much area was affected by landslides, and how much of the island was covered in forest. If you combine the forest cover data with the landslide map, and correct for areas where landslides do not deliver the woody debris to the river network, an estimate of carbon mobilisation can be made
The second method used reservoirs as sampling facilities. Reservoirs have filters to stop large trees going through their exit pipelines, and so any woody debris reaching the reservoir will be stopped at the dam (see the top picture for an extreme example). Knowing the area of land that drains into the reservoir, and the amount of wood trapped at the dam, you can scale up to the area of the entire river catchment.
Both of these methods produced similar results, they agreed that there was a shocking amount of carbon washed to the ocean during the storm. The storm delivered 3.8 – 8.4 Teragrams of woody debris from Taiwan to the ocean, which represents 1.8 – 4.0 Teragrams of carbon. This is about 1/4 the annual delivery of carbon from the Amazon River, but most of that is as small particles. The woody debris delivery in these few days was over 10 times greater than the annual woody debris delivery from the Amazon. So one single event on a small island was significant from a global point of view, but how much carbon is a Teragram?
One Teragram is equal to one million tonnes; an oil tanker can carry 300 000 tonnes of oil (mostly carbon) and therefore the storm delivered 10 oil tankers worth of carbon to the ocean. Obviously this event is not as disastrous as an oil tanker spill – the woody material will rot down and provide food for ocean-living creatures as well as potentially being buried safely in the sediments.
Our paper shows just how much carbon can be washed away by a single storm, and highlights that large pieces of woody debris, too large to analyse by most techniques, are an important and probably under-studied element of the organic carbon cycle.