The Bog Blog - Humification Analysis
Peatlands provide an archive of past climate; for example by measuring changes in peat decomposition, otherwise known as humification, changes in the past water table and rainfall can be reconstructed. In December 2023, PhD student Cathal Ryan conducted humification analysis on the upper metre of his first Irish blanket bog core. The analysis can be subdivided into two sections; preparation and measurement. The preparation for humification analysis was completed at Maynooth University in the sediment laboratory of the Geography Department. The prepared samples were then taken to Trinity College Dublin, where they underwent a chemical treatment and the humification measurement.
By Cathal Ryan (4/2024)
Figure 1: The samples in preparation. The dark solution is a mixture of grounded peat, NaOH and deionised water
What is humification analysis?
As flora at the peat surface grow and die over centuries and millennia, the vertical accumulation of plant matter results in the development of peat soils. Growth must outpace decomposition for a constant accumulation of dead plant matter, which occurs in areas with a high water table where oxygen struggles to penetrate the waterlogged soil unit. This results in the peatland being divided into two units: the upper subaerial unit called the acrotelm, which is in contact with oxygen and above the water table where peat decomposes rapidly, and the lower unit called the catotelm, which is saturated and has little to no interaction with atmospheric oxygen, where peat decomposes very slowly.
By looking at the changes in peat decomposition along peat cores taken from ombrotrophic (rain-fed) bogs, one can use this to establish changes in past rainfall. Consider a period where rainfall was unusually low, resulting in the peatland having a low water table. As the depth of the water table decreases, more peat becomes exposed to oxygen, causing higher decomposition rates. Suppose the opposite occurs and there is heavy rainfall. In that case, the water table may reach the surface, submerging and saturating the whole bog and preventing oxygen-based decomposition of the plant matter. Therefore, periods with high decomposition result from low rainfall, and periods with low decomposition result from high rainfall. So, by measuring peat decomposition (humification), past climate changes may be inferred.
Figure 2: Laboratory set-up, the volumetric flasks are empty and will be used for filtration of the peat solution
Humification preparation
The preparation of the humification samples was completed in the sedimentology laboratory, Geography Department, at Maynooth University. The uppermost metre was used (zero to one-metre depth) and sampled at a 2-centimetre resolution (0-2, 3-4 centimetres, etc.). Once the samples were extracted from the core, they were placed in separate weighing boats and dried at 50ËšC for 24 hours. Once dried each sample was placed into a clear plastic bag, labelled and packed. The catalogued samples were brought to Trinity College Dublin and underwent further analysis.
Figure 3: Heating the solution of ground peat, NaOH, and deionised water, being heated on hotplates in beakers
Humification measurement:
When measuring humification, a spectrophotometer records the absorbance and percentage of light transmission. To do this, the peat sample must undergo chemical treatment and filtration. The prepared peat samples were ground using a pestle and mortar. Once the peat samples were finely ground, there was a treatment of Sodium Hydroxide (NaOH) applied. The solution of NaOH and finely grounded peat was topped up with deionised water and heated (Figure 3). There were challenges with bringing the solution to a simmer and maintaining a stable temperature. If the solution became too hot, it risked drying out and if it became too cold, then there was a risk of improper heating.
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Once the samples had been brought to a boil and then simmered, filtration must be conducted. The cooled samples were first placed into a 200 ml flask. This was followed by filtering each sample’s solution into new flasks, this time 50 ml. To ensure that the solution and peat were separated, filter paper was used, as shown in figure 4. Each solution had to be filtered multiple times into volumetric flasks of varying sizes. This required a steady hand and even more patience! The flow rate of the solution through the filter paper was always less than my pouring wait. This resulted in a large part of the lab work involving sitting and watching the solution drip slowly as I had to wait to pour more in.
Once all the samples were ready, the spectrophotometer measures the percentage of light transmission. Each sample’s solution wa measured 3 times to find an average. When a sample is dark in colour, it transmits a low percentage of light; this is a sign that the peat sample is highly humified. When the solution’s colour is light and clear, the peat sample is either not decomposed or has a low degree of humification.
Figure 4 The solution is being prepared for filtration using different volumetric flasks