Sediment description and LOI

Since finishing the 2015 fieldwork season in mid-November, the (not so) cold Winter days have been spent locked away in the lab working on the cores. In total, we collected just over 150 metre-long sediment cores from our 10 study sites this year… we’ve certainly got plenty of work to do!

Once the cores are offloaded back at the university, the first job is to saw the cores open using a small circular saw. The exposed face of the sediments within is then cleaned: the surface of the sediment which was in contact with the plastic tube is usually a little disturbed, so this is carefully peeled away with a scalpel. This reveals all the fine details: the subtle changes in texture, colour, and any layering or ‘structure’ all of which can help us to understand how the deposits used to make the mound were formed.

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The cores are carefully cleaned with a scalpel to reveal the fine details – here are some very fine wavy laminations, possibly a ‘trample’ layer, with some waterlogged wood at the base of a core.

Once opened and cleaned, the next job is to ‘log’ the cores – the physical characteristics of each layer is described and noted. The colour of each layer is determined using a Munsell Soil Color Chart, the predominant particle size (silt/clay, fine sand, gravel etc), inclusions, structure (bedding, lamination etc), and the characteristics of the boundaries between layers (grading, diffuse or sharp) are all recorded.

Prior to subsampling for further assessment, the cores are then photographed. High-resolution digital photos are taken of each core using a copy stand and a colour rendition chart. Four photographs are taken of each core. The photographs are then stitched together and the background removed using photo editing software.

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Once the cores have been cleaned they are then carefully photographed.

The next stage of assessment is to carry out ‘loss-on-ignition’ (LOI) – this is a basic sedimentological technique which is used to determine the moisture (which is often closely related to grain size and organic content), organic carbon and carbonate content of the sediments. In particular, identifying layers with higher levels of organic material may help identify buried soil horizons and layers in which palaeoenvironmental indicators (e.g. pollen and plant macrofossils) might be preserved.

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Small subsamples of sediment are collected at regular intervals for “loss-on-ignition”. Small pieces of clingfilm are then placed in the gaps where the samples were taken.

The loss-on-ignition method is relatively simple: first small subsamples of sediment are collected at regular intervals from each sediment unit. These are removed using a clean spatula and placed in weighed porcelain crucibles.

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Each subsample is weighed using a sensitive balance (to the nearest 0.0001g)

The crucibles containing the subsamples are then weighed and placed in an oven set at 105°C for 12 hours to dry. The crucibles are then removed and re-weighed. The difference in mass is then used to calculate the moisture content of the sample.

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The crucibles containing the subsamples are first dried in an oven set to 105 degrees, re-weighed and then heated in a furnace at 550 degrees.

Next, the crucibles are transferred to a furnace set at 550°C for 2 hours. This burns off all the organic matter in the subsamples. Again the samples are re-weighed, and the mass lost is used to provide an estimate of the organic carbon content of the sediment.

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The red hot furnace is cracked open…

Finally, the crucibles are once again returned to the furnace, this time set to 950°C. Heating to this temperature converts carbonates to oxides. The amount lost is then used to calculate the original carbonate content of the sediment (the amount is multiplied by 1.36 to account for the difference in molecular weights of CO2 and CO3).

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The same subsamples after being heated to 950 degrees.

These basic data (sediment descriptions and LOI data) are then used to formulate a sampling strategy for further analysis: key layers are targeted for wet-sieving to recover material suitable for dating the construction of the mound, and for palaeoenvironmental analyses (pollen analysis, plant macrofossils etc) in order to understand the environmental conditions at the site during and prior to the construction of each mound.

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All the data (sediment descriptions, photographs, and loss-on-ignition data) can then be combined to understand how the sediments in the cores formed. The black unit at the base of these cores is a layer of peat beneath the mound. The peat layer is likely to have a high potential for reconstructing environmental conditions at the site prior to the construction of the mound…

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One thought on “Sediment description and LOI

  1. Pingback: Radiocarbon dates from 10 castle mounds – results of year 1 | The Round Mounds Project

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