| Abstract: |
Wessex Archaeology (WA) was commissioned by Boskalis Westminster Limited (BWL), the dredging contractor, on behalf of the Defence Infrastructure Organisation (DIO), to undertake a Stage 3 palaeoenvironmental assessment of deposit recovered in vibrocores (DT-010, DT-026 and DT-029), as part of the Queen Elizabeth Class (QEC) Capital Dredge Project at Her Majesty’s Naval Base (HMNB) Portsmouth.
In addition, three borehole logs have been made available from the 2017 Ground Investigation program (ESG 2017) and are used to update the existing Site deposit model (WA 2016). The additional three boreholes contain a mixture of sand, silt, clay and gravel and contain no deposits of geoarchaeological potential.
Targeted palaeoenvironmental assessment (plant macrofossils, pollen, diatoms, foraminifera and ostracods, supported by radiocarbon dating) was only recommended on those vibrocores containing organic horizons and the immediately overlying and underlying alluvial sediments. These sediments have the highest geoarchaeological potential in terms of suitable deposits for dating and likelihood of containing the widest range of well-preserved palaeoenvironmental indicators. Radiocarbon dating has demonstrated that the peat deposits in vibrocores DT-010, DT-026 and DT- 029 are all late Mesolithic in date. These are amongst the first dated peat deposits from Portsmouth Harbour and add to our understanding of coastal development along the south coast under the influence of rising sea-levels.
The peat deposit in vibrocore DT-010 (6470–7040 cal. BC) is broadly comparable in date to the peat located c. 500m to the south in vibrocore DT-026 (6390–7030 cal. BC). The single date from the top of the peat in vibrocore DT-029 (6590-6900 cal. BC) suggests the peat at this location may have ceased forming slightly earlier than to the north around DT-026 Pollen preservation and concentrations were variable across vibrocores DT-010, DT-026 and DT-029, with full assessment counts achieved on 3 out of 6 samples from DT-010, all three samples from DT-026 and 5 out of 6 samples from DT-029. Diatoms, foraminifera and ostracods were largely absent from samples below the peats with the best results achieved from the two samples overlying the peat in DT-010.
Pollen in the peat includes a tall herb swamp and willow carr that gave way to a wetland woodland dominated by oak and hazel. There are indications of remnant local pine populations within the wider landscape, a picture indicated in earlier pollen work in the 1940s by Godwin, and in pollen studies from nearby Langstone Harbour. There is little evidence for human activity in the peat.
Sea-level rise eventually resulted in the cessation of peat formation and the development of intertidal mudflats and saltmarsh accumulating under rising sea-levels.
No further work is recommended on boreholes DT-010, DT-026, DT-029 due to the poor preservation/absence of diatoms, foraminifera and ostracods, the variable preservation of pollen, and the absence of evidence for human activity in the palaeoenvironmental record. |
