West Cork Palaeoecology - Blog
The Environment of West Cork through Time
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These webpages are a series of bimonthly blog posts that will document my work on the Three Lakes Biodiversity project. The original blogsite for West Cork Palaeo, in which I looked at palaeo- (and current) ecology and geography related subjects across West Cork, has now moved to a separate page. Those blogs will continue to be updated when - and if - I have time. They can be accessed here.
The Three Lakes Biodiversity project is a PhD research project that I am undertaking at UCC - University College Cork. I am a postgrad researcher in the School of the Human Environment, in the Department of Geography. My supervisor is Dr Michelle McKeown, and advisor is Dr Aaron Lim.
I have been very generously funded in this project by an Irish Research Council grant for the four year duration of this work.
As part of the dissemination and record of work undertaken, these webpages will document, through a bimonthly blog, the course of the work undertaken and the results that are arrived at.
Having already undertaken some investigation at the Three Lakes site (see here, and here) I started this project off with the prior knowledge that the sediment in the valley basin appears to date back as far as 16,000 years. The sediment at the bottom, underlying the organic (peat) deposits, is a silt. The likelihood is that deposition of sediment on the site dates back to the last glaciation. The silt indicates low organic matter, that is, it was deposited in an environment with few plants. Probably just after the ice had left and the land was largely bare sediment. This is pretty unique for Ireland, and certainly for the south west. Sites further north often underwent a small, minor re-glaciation during the c.1000 year long cold spell known in Ireland as the Nahanagan Stadial, or more widely in Europe as the Younger Dryas. This re-glaciation would have cleared out any organic deposits that might have accumulated, and set the sedimentary clock back to zero.
The Nahanagan stadial lasted from about 12,900 until 11,700 years ago. Here at three lakes we found a second layer of silt that dates to around that time. As with the basal silt, this suggests that the surface of the landscape was exposed to erosion and weathering, with little, if any, plant life. Plants generally act as a protective covering, and roots help to hold the sediment or soil in place. What was probably happening here, making use of prior knowledge and making some assumptions, is that the climate turned cold, most plant life died off, and the amount of water increased - either as more heavy precipitation falling more often, or as seasonal meltwater, or both.
The aim of this project is to understand exactly what happened at Three Lakes. How did the site form, what changes occurred as the climate fluctuated, with a primary focus on the changes in biodiversity that occurred. This is of particular interest. During the ice age, few if any living things existed on the landscape of Ireland. As the ice reduced and melted away, exposing the land and creating new river courses, lakes, plains, and hills, nature moved in and repopulated the new habitats. Can we understand the sequence, the timing, the successes and failures? That is what we might achieve in this project.
As a brief summary of the project plan, we will be sampling sediment, by taking full depth cores, from both the peatland and the lake. We will look for various fossils within the sediments to help us create a picture of the changing climate. Some species of organisms are used as proxies for certain factors in the environment. By discovering which ones of these were present at various times in the past, we can see how the climate changed. This will be explained in more detail later. Once we have done this we will sample the sediment for DNA, SEDimentary Ancient DNA - SEDADNA. But the first job is to become familiar with the fossils themselves, learn how to find them in the sediment, and learn to identify them to species level.
Two fossil organisms are used.
From the peatland, testate amoebae are used as an indicator of water table depth. The fossils we find are the tests, or shells, of these micro-organisms, which remain behind when the organisms die. These tests are generally quite distinctive in shape, although some are very similar to others, so it is not always a simple matter to identify the species. They are quite easily extracted from the sediment by taking a small sample of say 1cc, and filtering first at 350 microns to remove any large particles, and then again at 10 microns, to get rid of the very small particles. This is a lengthy business, but necessary. Testate amoebae tests are of variable size, but within this size range. A micron is one thousandth of a millimetre, or one millionth of a metre, so we are dealing with microscopic material. After filtering out the larger and the smaller particles, a single drop of the sediment is placed, in water, on a slide under a microscope at x200 or x400 magnification. The slide is searched systematically and all tests found are identified, measured, photographed if possible, and a record is kept of all those found. It is quite a simple process, but requires training to recognise the tests.
How we have fared in our search for fossil tests in this project so far is dealt with in a separate blog here posted a while ago. This is something we need to revisit at a later date.
The other organism used is a midge, or more accurately, the head case from the cast off skin of the larva of a midge. These midges are non-biting midges, so named because the adult midges that emerge and fly around do not bite. In fact most of the species do not feed once the adult emerges. The adult form as flying insects purely to mate and lay eggs, though some do seek nectar as a sort of energy drink. The head cases can be from any one of the four stages (called instars) that the larvae grow through. At each change they shed the old skin including the head case. The midges are of the family Chironomidae, otherwise known as Chironomids. Again, the task is to learn how to find these head cases, and how to identify the species. The different assemblages of species have been found to vary largely based upon the summer air temperature of the environment, so changing assemblages over time can suggest changing temperatures. The process in extracting these involves a short warming in an alkali (potassium hydroxide at 10% strength) to break down any clumps of organic matter. This is followed by jet washing (using a wash bottle of deionised water) of the sediment through a 90 micron sieve. The head cases - along with quite a lot of other fossils and bits and pieces - remain, and these need to be sorted through, and the chironomid head cases picked out and placed on microscope slides for later identification. The sorting is done under a stereo microscope to enable easier picking and recognition. Using micro tweezers under a microscope is very difficult - because of the microscope lenses all movements appear reversed. This is not the case with a stereomicroscope because they have a simpler arrangement of lenses, and movements do not appear reversed.
When the project started I had got to this stage. I was using some five year old sediment cores from Three Lakes, one from the peatland and one from the lakebed at the Middle Lake, for training purposes.
One of the first tasks was to obtain some surface sediments which I can examine, and start to extract the fossils of interest.
Surface sediments generally comprise the top 20 to 30 cm of sediment from a lake bottom, and thus the most recently deposited sediment. Very recent organic deposits are more accurately dated using Lead210 (210Pb) dating. This method of dating measures the decay of the lead isotope which is deposited into sediment from the atmosphere. The origin of this radioactive isotope is uranium in the earth's crust, principally in igneous rocks (i.e. those formed from magma or lava). The uranium decays to the radioactive gas radium that is released to the atmosphere. The radium decays to lead210 in the atmosphere, and it is this lead isotope that is washed into sediment and the decay of this - it has a half life of about 22 years - can be measured and an age of the sediment reached. Lead210 dating is cheaper, quicker to perform, and thus it is possible to get more samples dated relatively quickly and cheaper. Because of the short half life the use of Lead210 for dating really only goes back about 150 to 200 years. That is why 210Pb dating can only be used for relatively recent sediments.
Three Lakes is a lowland lake basin, at just over 100m above sea level, surrounded by woodland and farmland. Local archaeology indicates that farming has been going on here for at least 2000 years. There is a possibility that such activities by humans has affected the sediment in the lake, and this might have had an influence on the insect and micro-organism populations. So we found a second lake nearby that is at a higher elevation that is less likely to have been affected in such ways. This is another favourite lake of mine, in Glandart townland on the hills above Castledonovan. It appears to be unnamed, so we shall call it Glandart lake. You can read more about Glandart Lake at some other blogs on this site - here, and here.
The water in both these lakes is not deep, about 3 metres at most, but still it is a specialist task to remove a column of UNDISTURBED sediment from the bottom of the lake. Some ingenious coring devices have been developed. For surface sediment samples from a lakebed we use a gravity corer. This relies on it's own weight, and the unconsolidated nature of the sediment, to penetrate into the sediment to the depth required. The corer has to be deployed from a boat - unless there is a quay or jetty projecting into the lake, which there usually isn't. Such a thing would indicate use of the lake by boats which would disturb sediment and it would probably be too disturbed to be worth sampling. Two people are required, one to row the boat and try and keep it steady, the other to deploy the corer.
Three Lakes, October 2024. The weather is not always conducive to boating and coring
The corer has a perspex tube of maybe 50cm length projecting downwards. The whole thing weighs about 15 pounds. It is dangled down from the boat until it rests on the sediment. The core tube is then unlocked which will allow the water that is in it to flow out of the top, and the corer released on the end of a string. The tube is forced to penetrate straight down into the sediment by the weight of the corer above it. Once it has settled and stopped sinking, the whole thing is pulled up. The sediment generally stays in the core tube as it is pulled up, but as soon as the bottom of the tube is within reach, still under the water, a bung is pushed in. The tube must be kept upright at all times, and must not be shaken. The sediment water interface within the tube is very delicate and easily disturbed. Obviously the object is to retrieve a 30, 40 or 50 cm long sample of sediment exactly as it existed in the bottom of the lake, and get it to shore without disrupting it. There will be water sitting on top of the sediment.
Keeping the core tube upright while returning to shore is a delicate operation.
The tube with sediment (and water) is brought to shore where a sectioning kit is used to gently push the sediment up the tube, expelling the water that is on top. When the top of the sediment reaches the top of the tube, we fit a perspex tray and then 0.5 cm of sediment at a time is scraped off and into a bag. Each bag is sealed and numbered. The top few cm are very watery and need to be treated with care, and once back at the lab they are put in a pot and allowed to settle, and the water removed. This should be checked for microscopic fossils to ensure they do not get discarded. The lower samples are generally more solid, containing less water.
The plunger is slowly moved up, pushing the sediment up the tube and expelling water from the top.
With a tray fitted to the top of the tube we can take off 0.5 cm of the core at a time.
Having obtained the samples, and put each 0.5 cm of the core into separate bags, we can think about sampling them and finding chironomids. The sample bags from Three Lakes went straight into the freezer - they need to be preserved as quickly as possible because the sediment will also be used for DNA analysis. The Glandart lake samples went into the cold room at UCC where they are stored at 4 degrees C. I have taken a small part of some of the samples to process and examine. I found that 1cc of the sediment had too many chironomids and took a long time to search through, so I settled for 0.5 cc and then decided on 0.25 cc. I could pick the chironomid head cases out of one of these 0.25 cc samples in a day, and put the chironomids I found onto microscope slides.
That was October and November.
In addition I have been searching the papers that have been published to pull out all those that report on studies undertaken in Ireland making use of environmental proxies. Chironomids, testate amoebae, diatoms, pollen, cladocera, and tephra. By examining where the samples were taken from, and what period of the past each study covered, as well as other data about these studies, we can determine a picture of what has been done up to now in Ireland. And, more importantly, where the gaps are. This is a lengthy process of finding the papers, determining if they are relevant and then, if they are, scan reading them to get the data out of them. In doing this I have come across some particularly interesting papers that I had not seen before. Some of them are not directly relevant to this project's aims, so I refer to them and my comments on them are elsewhere. But some of them are incredibly relevant, and serendipitous. For example, a recent open access book publication about Lakes in Ireland has a very relevant and useful chapter on the formation of lakes in early post glacial times.
That will be another, and to me an essential, strand to these studies. How did the lakes at Three Lakes form? Why are they there? Why is the drainage at the gorge holding the lakes back? Were they once just one big lake? This to me is the hard core and fundamental physical geography that I grew up with at school. Not much work seems to be done in the field of physical geography and landforms in Ireland today, certainly not in UCC. So this will be an interesting and very important strand to the project. More on that next time.
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