EDO8078 - Decoy pond, Decoy Heath, Morden Bog National Nature Reserve (NNR); geoarchaeological borehole survey and palaeoenvironmental assessment 2023

OASIS: ‘A targeted geoarchaeological borehole survey was undertaken in and around the Decoy Pond at Morden Bog National Nature Reserve (NNR), Decoy Heath, Wareham. The geoarchaeological borehole survey and palaeoenvironmental assessment were undertaken to provide further information on the archaeological and palaeoenvironmental potential of the deposits in the area of the pond that may be impacted by the proposed restoration, and to provide recommendations for further work, where appropriate. A total of 28 hand auger boreholes were undertaken using a Russian corer. Borehole WA-25a was selected as the master sequence for palaeoenvironmental assessment. A second borehole, WA-10a, was also radiocarbon dated for chronological comparison against the deposits in WA-25a. The sequence of deposits at the Site comprised well preserved plant remains in a peaty matrix, overlying humified peat, in turn overlying coarse Sands. These Sands are considered likely to represent fluvial deposits of either Upper Pleistocene or early Holocene date, and form the topographic template upon which the organic deposits of Holocene age accumulated. The organic deposits are widespread both within and outside of the pond, and form part of the sequence that has accumulated during the Holocene in a stream valley draining south towards the River Piddle. No basal (e.g. clay) lining to the pond was identified during the borehole survey, and on the basis of the expected age of the underlying Sands, it is considered unlikely that a lining was used in the construction of the pond. There is a general decline in the surface elevation of the Sands from a level of c. 8.3 m OD in the west to c. 7.4 m OD in the east; in the area of the pond, a depression in the surface of these Sands is evident to a level of c. 7.8 m OD. How the surface of these Sands relates to the pond is currently unclear; it may relate to a natural feature, perhaps associated with a broadly north-south aligned early Holocene palaeochannel, the surviving topography of which may have been exploited during the construction of the pond; alternatively, construction of the pond may have resulted in a cut in to these Sands to a level of c. 7.8 m OD, the overlying material having accumulated within this feature. The combined results of the palaeoenvironmental assessment and scientific dating, including radiocarbon dating and the analysis of Spheroidal Carbonaceous Particles (SCPs) and provisional age modelling for the deposits, have helped to test these hypotheses. SCPs were observed in every analysed sample (at levels between 0.40 and 0.68 m bgl) suggesting that this part of the sequence post-dates the mid-19th century. A marked increase in SCP concentration observed at 0.48 m represents a date at, or close to, 1950 AD. However, on the basis of the radiocarbon dating, there is significant uncertainty as to the age of the deposits in the lower part of the sequence. The basal samples, derived from the lower peat unit and overlying silty peat, are typified by the presence of pine and birch, in addition to heathland shrubs and aquatic taxa, likely forming in wet heath or valley bog, adjacent to dry heath. Borehole WA-25a recorded a stratigraphic boundary at 0.98m, where a silty peat is overlain by a peat rich in organic remains. The palynological assessment has also identified a shift in the pollen signal at this boundary. Those samples derived from above 0.98 m bgl contain a more diverse arboreal signal, along with an abundance of charcoal within the upper sample from the silty peat, to infer the potential for allochthonous input (e.g. fires) prior to the onset of peat development above this unit. Isolated grains of beech are also recorded above 0.59m bgl, and the presence of likely cereal pollen above 0.52m bgl. The presence of beech provides a maximum age of c. 3000 BP for the uppermost samples. Diatoms were absent from the bottom three samples of core WA-25a (below c. 0.98 m bgl). Above this level, the diatom assemblage is indicative of acid, oligotrophic, shallow water habitats; the diversity of acidophilous, acidobiontic and other non-planktonic taxa in the top samples suggest that the sediments are derived from pond sediments rather than diatoms from the mire surface preserved in the peat. Morden Bog NNR Decoy Pond Palaeoenvironmental Assessment Report iv Document ref. 273310.04 Issue 1, Aug 2023 On the basis of these results, the pollen and diatom assessments are indicative of a transition in the sediments at 0.98 m bgl, at the interface between the humified peats and the poorly humified plant remains that may relate to a transition from peat associated with the naturally forming valley bog/wet heath to deposits forming within the decoy pond. Further refinement to the age model for WA-25a, in the form of additional SCP analysis below 0.70 m bgl and additional radiocarbon dating in the lower part of the sequence (below 0.98 m bgl) may help to refine our understanding of the chronology of the sequence and to test these biostratigraphic boundaries. Recommendations for any further palaeoenvironmental analysis should be made on the basis of these results.’