PE1 (Figure 73)
A column of 64.5cm depth was taken from the lowest peat exposure at Port Eynon using a combination of a monolith tin to 25cm depth and a Russian Auger for the lower 39.5cm. A total of 25 deposition events are identified with a series of peats interspersed by minerogenic sediments. The earliest deposit is a dark friable peat of which the top c.7cm has been obtained, followed by c.32cm of organic clay. Above this lies a c.4cm layer of very friable dark peat, followed by c.6cm of mixed peaty clay. A c.18cm layer of dark organic clay develops above this, followed by a very thin (c.2mm) peat layer, 6cm of dark organic clay, another very thin peat layer and a further 2cm of dark organic clay. Above this a lighter coloured clay, still with organic components has developed (c.4cm), followed by a 1cm thick peat layer and then c.8cm of the lighter organic clay. This is followed by seven thin layers of between 2-4mm in thickness in the following sequence: sand, grey clay, sand, grey clay, sand, grey clay, sand. After this an organic clay develops for c. 5 cm, followed by 1cm of sand and another c.6cm of organic clay. Above this c.6cm of black fibrous peat is followed by 2cm of sandy peat suggesting a mixing of material.
The uppermost layer within the column is a further 10cm of dark fibrous peat.
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Figure 73: Sediment column sample PE1
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PE2 (Figure 74)
The 49cm column was taken from one of two exposures identified in the upper intertidal zone at Port Eynon using two monolith tins. The lowest material sampled was a coarse, grey coloured sand. Above in order of deposition is c. 15cm of brown fibrous peat with frequent wood inclusions overlain by 1cm of slightly finer grained sand, 3cm of a dark fibrous peat, 2cm of sand and c.17cm of black fibrous peat. The top 6cm of the column is a continuation of the preceding peat, but showing clear signs of bioturbation.
Figure 74: Sediment column sample PE2
171 PE3 (Figure 75)
This column sample of 46cm in depth obtained using two monolith tins is from the second upper beach peat exposure located towards the middle of the bay. It consists of a homogenous black fibrous peat with potential reed inclusions throughout. The upper 8cm of the column shows evidence for bioturbation, including in situ worm action.
Figure 75: Sediment column sample PE3
PE4
A deep mixed peaty clay deposit overlain by over 50cm of sand was a revealed during auger prospection. Though the deposit appeared to be directly overlain by sand, it was positioned much deeper than the other recorded exposures. A spot sample was collected for palaeoenvironmental analysis to investigate whether the deposit represented the same environment as the surface exposures on the beach or whether it could be evidence of an earlier landscape.
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Radiocarbon Dating
A series of samples from the top and bottom organic peat deposits at PE1 and PE2 on Port Eynon beach and the top of the lowest sampled peat exposure at Broughton Bay were submitted for radiocarbon dating in order to create a chronological framework for the archaeological and palaeoenvironmental evidence collated in this investigation. The radiocarbon dates are presented in Table 9 in stratigraphic order, from lowest to highest, with Port Eynon dates listed first. They are stated first as conventional radiocarbon ages in accordance with recommendations made by Stuiver and Polach (1977, 363) using the Trondheim convention (Stuiver and Kra 1986) and then as calibrated ranges at 68% and 95% probabilities.
Sample Sample
Table 9: Calibrated radiocarbon ages at 68.2% and 95.4% probabilities.
Figure 76: Calibrated radiocarbon dates plotted using OxCal v4.3 (Bronk Ramsey 2009). The red box highlights the anomalous date that will be excluded from further modelling at this stage.
Figure 76 plots the calibrated dates in stratigraphic order at Port Eynon. Broughton Bay is included for comparison. Sample PE1-2, stratigraphically situated between PE1-1 and PE1-3 presents a date earlier than PE1-3, despite being stratigraphically younger. To investigate this issue the column was re-examined visually where it became clear that the area sampled was
173 more mixed with sand inclusions than initially identified and is suggestive of movement and mixing of materials and potential contamination with older carbon sources. This could explain the anomalous result, but without further sampling and dating it is not currently possible to present a full explanation. For the purposes of this study the PE1-2 date will be removed from further modelling attempts, but it acts as an indication that peat accumulation was not necessarily linear and that numerous additional factors must be taken into account when investigating these deposits.
Figure 77: Calibrated radiocarbon dates displayed alongside the corresponding section of the radiocarbon calibration curve: a-c = PE1 (please note the discarded PE1-2 is included here for full disclosure), d-e = PE2, f= Br1.
Graphs are created in OxCal c.4.3 (Bronk Ramsey 2009) using calibration datasets IntCall13 and MARINE13 (Reimer et al. 2013).
The radiocarbon calibration curve has been inspected at each of the date ranges measured (Figure 77). The curve has a general downward trend during the total period covered and the upper samples from columns PE1 and PE2 have dates that fall on slight plateaus of the curve (PE1-1 and PE2-1) (Figure 77: a and d). As a result, the probable date range is wider than it would be at other points on the curve. This may be an issue if dating was focussed on the identification
a)
b)
c)
d)
e)
f)
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of narrow date ranges, however these dates remain appropriate for this range finding exercise and allow an overall chronology to be developed for the sites.
Figure 78: P_Sequence model log_10 (k/k_0) where k_0=1, allowing k to take any value from 0.01-100
The age-depth model output (Figure 79) indicates that the lower sequence (PE1) formed over a time period of up to 460 years between 5480 and 5020 cal BC, with 65cm of varied material deposited. The upper sequence (PE2) formed at a slower rate, depositing 45cm of more homogenous material over a maximum of 530 years between 4490 and 3790 cal BC. Between the formation of these two sequences, there is a hiatus of up to 700 years. Without further radiocarbon dates, more in depth analysis of accumulation rates is not possible, as the layers within the sequence would have formed at different rates dependent on environmental conditions. For example, freshwater peats will take longer to accumulate as they are a very low energy environment, reliant on the introduction of vegetation matter over a long period.
Minerogenic sediments related to sea level rise are more likely to accumulate quickly, particularly once they are part of the daily tidal cycle, and more so within the lower saltmarsh zones (Pratolongo et al. 2009, 93). The effects of erosion must also be taken into account. These are not visible within the sequence, but can account for missing periods of time within the accumulation record. Without direct dating evidence, however, these occurrences cannot be identified and although a slowing or speeding up of accumulation can be inferred from the lithological and palynological records, they cannot be proven.