Having already undertaken some investigation at the Three Lakes site (starting in 2018 with a peatland core, and a lake coring in 2021 - see here, and here), I started this project 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 at the site dates back to the last glaciation.

The silt indicates low organic matter — in other words, it was deposited in an environment with very few plants. This was probably shortly after the ice had retreated, when the landscape consisted largely of bare sediment. This is relatively unusual for Ireland, and certainly for the south-west. Sites further north often underwent a minor re-glaciation during the ~1000-year cold period known in Ireland as the Nahanagan Stadial, or more widely in Europe as the Younger Dryas. This re-glaciation would have removed any earlier organic deposits, effectively resetting the sedimentary record.

The Nahanagan Stadial lasted from about 12,900 to 11,700 years ago. At Three Lakes, we identified a second layer of silt dating to this period. As with the basal silt, this suggests that the landscape surface was exposed to erosion and weathering, with little if any plant cover. Plants normally act as a protective layer, with roots helping to stabilise soil and sediment.

What was probably happening — based on prior knowledge and reasonable assumptions — is that the climate became colder, plant life declined significantly, and water movement increased. This may have been due to heavier precipitation, seasonal meltwater, or a combination of both.

The Aim of the Project

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?
• How did biodiversity respond to those changes?

This last question is particularly important. During the ice age, very little life existed across Ireland. As the ice retreated, exposing land and forming new rivers, lakes, and landscapes, plants and animals gradually recolonised these new environments.

Can we reconstruct that process? Can we understand the sequence, the timing, and even the successes and failures of recolonisation? That is what this project aims to explore.

How the Project Works

In simple terms, the project involves sampling sediment by taking full-depth cores from both the peatland and the lake. Within these sediments we look for fossils that can help us reconstruct past environments.

Certain organisms act as proxies — indicators of environmental conditions. By identifying which species were present at different times, we can infer how climate and environmental conditions changed. This will be explained in more detail in later posts.

Once this groundwork is complete, we will move on to analysing sedimentary ancient DNA (sedaDNA). But before that, the first task is to become familiar with the fossil material itself — learning how to extract it, recognise it, and identify it as accurately as possible.

Testate Amoebae

From the peatland, we use testate amoebae as indicators of water table depth. What we find are their tests — the shells that remain after the organisms die.

These tests are often distinctive in shape, although some species are very similar and require careful identification. Extraction involves taking a small sediment sample (around 1 cc), and filtering it:

• first through a 350 micron sieve to remove large particles
• then through a 10 micron sieve to remove very fine material

This is a slow process, but necessary. A micron is one thousandth of a millimetre, so we are dealing with very small material indeed.

After filtering, a drop of the remaining sediment is placed on a microscope slide and examined at x200 to x400 magnification. The slide is searched systematically, and all tests are:

• identified
• measured
• photographed (where possible)
• recorded

The method itself is straightforward, but recognising the different species requires training and experience.

Our progress with testate amoebae so far is discussed in a separate blog post here , and this is something we will return to in the future.

Chironomids

The second organism used is a midge — more specifically, the head capsule from the shed skin of its larval stage.

These are non-biting midges (family Chironomidae). The adult insects do not bite and often do not feed at all, existing primarily to mate and reproduce.

The larvae go through four growth stages (instars), shedding their skin at each stage. The head capsules are preserved in the sediment and can often be identified to species level.

Different assemblages of chironomid species are strongly influenced by summer air temperature, making them valuable indicators of past climate change.

Extraction involves:

• gently heating the sediment in a 10% potassium hydroxide (KOH) solution to break down organic clumps
• washing the sediment through a 90 micron sieve

The residue contains the head capsules along with other material. These are then sorted under a stereo microscope, which allows easier manipulation. A stereo microscope is essential here, as it avoids the visual reversal effect seen in standard microscopes, making fine movements much easier.

Micro-tweezers are used to pick out the head capsules and place them on slides for identification.

Where Things Began

At the start of the project, I was learning these methods of processing and identification using the older sediment cores from Three Lakes, one from the peatland and one from the lakebed. These provided valuable training material before moving on to the main core and the main objectives of the project.