Several factors can mitigate against recovery of Mesolithic finds. Commercial excavation and fieldwork for example often fails to appreciate the extremely small size of many Mesolithic finds. Mesolithic sites can also be threatened by development. As well as typical threats due to development deep ploughing can damage or even destroy existing subsurface sites (as illustrated by Cockrell for the magnesium limestone zone). Upland wind turbines can also threaten sites under peat, not merely through the turbines themselves but through access roads built across the moors. Probably the most serious threat to Mesolithic sites however comes from the effects of climate change on Mesolithic sites preserved under upland peat.
Upland blanket bogs are fragile ecosystems and uniquely sensitive to changes in temperature and rainfall. Significant steps were taken in south Yorkshire to remedy what can be seen as the ‘first phase’ of human impact on upland peat landscapes in which substantial areas were severely degraded through the effects of industrial pollution and overgrazing in the early and mid 20th century. Projects as part of ‘Moors for the Future’ have had an important in the regeneration of peat uplands for example through the reduction in grazing and changes to upland drainage as well as active planting and over interventions. As a result there has been substantial regeneration of the upland peat in South Yorkshire. There will however be a ‘second phase’ of human impact on upland peat due to global changes in climate, and whilst land use practices have a substantial impact in retaining upland peat, global climate change may be a tide that cannot be turned in the same way.
Mesolithic sites under upland peat can be some of the most high integrity sites known. This is because if material is deposited at a time of incipient peat formation the normal agents of movement (burrowing animals, root action) are not active and finds stay very near to their original locations. At March Hill site A for example most finds lay within about 1cm of the original land surface, and re-fitting demonstrated coherent ‘snapshots’ of activity around four hearths (figures 8 and 9). Thus these sites can potentially provide highly significant evidence about the period, as long as they are recorded with the exceptionally high resolution methods demanded (Spikins, Ayestaran, and Conneller 1995; P. Spikins et al. 2002).
Figures 8 and 9: distribution of finds at March Hill (Marsden Moor) in plan, and in relation to palaeosol (P. Spikins et al. 2002)
The upland peat in South Yorkshire is one of the regions most vulnerable to climate change because it lies at the limits of ecological conditions which promote peat formation (Gallego-Sala and Colin Prentice 2012). Current temperatures are on average already 0.5 – 1 degree C higher than in the 1970s, and some degree of continued climate change is inevitable due to previous emissions. In fact even with strong international action global temperature has a fifty percent chance of rising by 2 degrees C by the end of the century (Committee on Climate Change 2017).
Firstly, climate change models predict increased sediment loss through erosion in the regime, under the influence of more variable climate regimes (Li, Holden, and Irvine 2016). Such erosion may not be gradual but as a result of ‘freak’ events such as unusually dry summers which dissicate peat, followed by sudden high rainfall events in autumn and winter. Land management practices mitigate against the effects of such events, however remaining marginal face peat deposits potentially overlying high integrity sites may be particularly vulnerable to this type of erosion. Plans for future management and research cannot assume that erosion will be a gradual process as sudden ‘freak’ erosional events are more likely.
Secondly, climate change will influence peat uplands through changing ecological zones. Blanket bog becomes unviable in all of the current climate models by 2050 (Committee on Climate Change 2013, see figure 10). The timescale of subsequent changes is unclear, Natural England for example note that ‘Peat will persist for long periods even when new peat is not forming’ (Natural England 2010, NE 257 Englands Peatlands, p34). However the nature of the vegetation will ultimately change, and we may see the rise of more resistant sphagnum, an increase in scrub and heathers or more tussocky peat. Changing vegetation may seriously impact sites with high integrity at a depth where roots affect the deposits for example.
We may currently occupy a time frame between the first phase of human impact on upland peat (due to local air pollution and overgrazing) and a second phase (under the influence of global climate changes). This is therefore the point to consider the threats to Mesolithic sites under peat posed by climate change (over the short, medium and long term), whether preservation in situ is viable and whether a pro-active approach to generating a well documented record of the upland Mesolithic is now necessary. Public engagement and a co-ordinated approach between the regions affected (Derbyshire, South Yorkshire, Greater Manchester and West Yorkshire) will be important.