The first two months

Published:December 2024

This work complements deeper sediment coring described in Sediment Coring at Last, providing a modern reference for interpreting the longer sediment record.

Linking modern lake conditions to the sediment record

One of the first tasks in the project was to obtain some surface sediments that I could examine and begin extracting the fossils of interest.

Surface sediments generally comprise the top 20–30 cm of sediment from a lake bottom, representing the most recently deposited material. These recent deposits can be dated using Lead-210 (210Pb) dating.

This method measures the decay of a radioactive lead isotope deposited into sediments from the atmosphere. The isotope originates from uranium in the Earth’s crust (particularly in igneous rocks), which decays to radon gas. This gas enters the atmosphere, where it further decays to lead-210. This isotope is then washed into lake sediments, and its decay — with a half-life of about 22 years — can be measured to estimate sediment age.

Lead-210 dating is relatively quick and cost-effective, allowing multiple samples to be dated. However, because of its short half-life, it is only useful for sediments from the last 150–200 years.

Choosing the Lakes

Three Lakes is a lowland basin, just over 100 m above sea level, surrounded by woodland and farmland. Archaeological evidence suggests that farming has taken place here for at least 2000 years.

This raises an important question: has human activity influenced the sediment record, and therefore the insect and micro-organism populations preserved within it?

To provide a comparison, we selected a second lake at a higher elevation, less likely to have been affected by human activity. This is a small, unnamed lake in Glandart townland, above Castledonovan — which we have named Glandart Lake.

More details about Glandart Lake can be found in other posts on this site.

Collecting the Sediment

Although both lakes are relatively shallow (around 3 metres deep), extracting an undisturbed column of sediment is a specialist task.

For surface sampling, we use a gravity corer. This device relies on its own weight and the softness of the sediment to penetrate the lakebed.

The corer consists of a perspex tube (around 50 cm long) attached to a weighted frame. It is deployed from a boat, requiring two people:

• one to steady the boat
• the other to handle the corer

Three Lakes, October 2024. The weather is not always conducive to boating and coring

The corer is lowered gently until it rests on the sediment. It is then released, allowing its weight to push the tube down into the sediment. Once it has settled, it is carefully retrieved.

As the corer is lifted, the sediment remains inside the tube. Before it leaves the water, a bung is inserted at the bottom to retain the sample. The tube must be kept upright and handled very carefully — the boundary between water and sediment is extremely delicate.

Keeping the core tube upright while returning to shore is a delicate operation.

The aim is to recover a column of sediment exactly as it lay on the lakebed, including the thin layer of water above it.

Sectioning the Core

Back on shore, the sediment is processed using a sectioning kit. The sediment is gently pushed upwards through the tube, expelling the overlying water.

Once the sediment reaches the top, a tray is fitted, and the core is sliced into 0.5 cm sections. Each section is:

• placed in a labelled bag
• sealed
• stored for later analysis

The upper layers are often very watery and require special care. Once back in the lab, they are allowed to settle and excess water is removed — but only after checking that no microscopic fossils are lost in the process.

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.

Lower layers tend to be more compact and contain less water.

Processing the Samples

Once collected, the samples can be examined for chironomids. The Three Lakes samples were placed directly into a freezer to preserve them for future DNA analysis, while the Glandart samples were stored at 4°C in a cold room at UCC.

I began processing small subsamples to assess how many chironomid remains were present. Initially, I used 1 cc samples, but these contained too many specimens and were time-consuming to process. I reduced this to 0.5 cc, and eventually to 0.25 cc.

At this size, it became possible to extract and mount the chironomid head capsules from a single sample in about a day.

This work took place during October and November.

Building the Bigger Picture

Alongside the laboratory work, I began compiling a database of published studies from Ireland that use environmental proxies, including:

• chironomids
• testate amoebae
• diatoms
• pollen
• cladocera
• tephra

The aim is to understand:

• where studies have been carried out
• which time periods they cover
• and, crucially, where the gaps are

This involves identifying relevant papers, assessing their usefulness, and extracting key information — a time-consuming but essential process.

In doing so, I have come across a number of particularly interesting and relevant studies. One recent open-access book on Irish lakes includes a valuable chapter on lake formation in the early post-glacial period.

Looking Ahead

This opens up another important strand of the project:

• How did the Three Lakes form?
• Why are they located where they are?
• Why does the gorge hold the water back?
• Were they once a single larger lake?

These questions lie at the heart of the site’s physical geography — a subject that has received relatively little recent attention in Ireland.

This will be an important direction for future work.

More on that next time.