Pollen analysis

Pollen analysis is underway on the sequence from Red Loch and further updates will follow on the pattern of vegetation change being seen in the levels being investigated. Those levels currently being analysed show good preservation of pollen and non-pollen palynomorphs (NPPs), such as fungal spores. Photographs taken from the pollen slides showing some of the types of fossil pollen and spores present are provided here.

Radiocarbon dating

The radiocarbon dates have been returned from the peat samples sent off for dating at the Scottish Universities Environmental Research Centre (SUERC) labs. Two dates, one from the base of the core taken at 7.48m and one taken at 2.48m have added important information to the chronology of the peat deposit sequence. These dates compliment the two previous dates we have from the site, which were taken during the assessment stage.

All four of the dates we currently have are shown in the table. The dates returned from the peat samples show that peat accumulation began at the site during the Early Holocene at approximately 8709-8488 cal BC (SUERC-37141) and continued to accumulate, through into the Post Medieval period at cal AD 1420-1620 (SUERC-32308) and given the depth this sample was taken from is probably still continuing to accumulate today. 

Radiocarbon dating results from Red Loch, Isle of Bute

The radiocarbon dates from Red Loch can be plotted against peat depth to make an age/depth graph for the sequence (see graph). This graph assumes the accumulation of peat at Red Loch has been uniform over time between the radiocarbon dated levels in order to allow for a rough calculation of which periods the non-dated areas of the core represent. There is clearly a margin of error made based on this assumption, which is illustrated by the two bottom dates as they lie some distance from the overall trend line (linear) for peat accumulation. Further radiocarbon dating of the core in the future would aid in refining this chronology even further and help to narrow the margin of error in locating archaeological periods within the core. For now though this is evidently a useful chronological guide in showing which periods are represented by the Red Loch sequence.

Team

Abby Mynett                                             Scott Timpany

Gary Edmondson                                            Paul Madden

Sub-sampling

Once back in the lab the cores are recorded in order determine any changes in stratigraphy, such as changed from a wood peat, signalled by the presence of frequent wood fragments within the peat to a sedge peat, signalled by the presence of monocotyledon plant fragments (such as grasses and sedges) within the peat.

The cores are then sub-sampled for pollen analysis. Pollen samples are taken in small cube blocks (c.1cm3), cut into the peat and placed into small plastic bags (see photo). These will then be sent to The University of Aberdeen for preparation, once prepared the samples will be sent back to us ready for pollen counting. Any wood fragments that can be identified were also removed from the cores. Identification of the wood to taxa will provide further information on the local vegetation communities growing on the bog surface and help to indicate what tree pollen represents local woodland as opposed to more regional woodland.

Pollen samples

Lastly the cores have been sub-sampled for radiocarbon dating material. The peat itself has been used for dating from the Red Loch sequence and it is hoped these dates will ties in with the previous work done at the Loch and help to overlap the two sequences.

Laboratory work

The majority of the work carried out will be done in the lab with sub-sampling and pollen counting all taking place here. Regular updates to this work will be posted on this blog.

Fieldwork

We went back to Red Loch on the 7th October to take a new core from the Red Loch site that would hopefully allow us to overlap our results from the previous work and provide more material for pollen study. We were helped out on our fieldwork by volunteers who again braved the long trek to the site. Special thanks to Mr Gary Edmondson and Mr Paul Madden who not only completed the climb but also braved the wet bog in order to take a pollen core.

Thanks to everyone’s efforts we were able to come away with a 7.5m peat from the bog at Red Loch, which gave us an additional 2m from the previous year. We took the samples using a Russian corer, which traps 0.5m of peat in a central chamber allowing us to push down and then bring up this amount of peat at a time.

Coring

Corer

The peat samples are then transferred into plastic guttering (cut to 1m in length) ready to be transported back to the lab for sub-sampling.

Peat in the core chamber

Wrapping the cores

Wrapped core

Background

This project has been commissioned by the Discover Bute Landscape Partnership Scheme to provide the Isle of Bute with its first full Holocene (the last 10,000 years) pollen diagram. The island is one of the few Scottish islands not to have such a study undertaken on it and as thus the history of vegetational change on Bute is currently not well understood. The island does have a rich archaeological history, which can be seen to stretch back to the Mesolithic period (c.8800-4000 years ago). This has been recently publicized in the excellent Archaeological Landscape of Bute by Geddes and Hale (2010). 

In order to complete this study a site had to be located and then chosen on the basis of whether it would offer the opportunity to reconstruct vegetational change and communities over this (large) timeframe. This was undertaken previously as part of a palaeoenvironmental audit of the island and the results of this can be downloaded for free from the Discover Bute Landscape Partnership Scheme website: http://www.discoverbute.com/Documents.aspx

The site of Red Loch in the north of the island has been chosen for study due to its potential to contain deep sediments that are likely to date back to the Early Holocene and thus allow reconstruction for the whole Holocene period. Initial results showed that peat here extended >5.5m in depth with radiocarbon dates showing peat accumulation from 7300-7040 cal BC to cal AD 1420-1620; showing peat accumulation from the Mesolithic through to the late medieval period.

This part of the project will now be looking to extend this work and build a Master Chronology for vegetation change on Bute.