Driving mechanisms of coastal change: Peat compaction and the destruction of late Holocene coastal wetlands

This paper examines the role of peat compaction as a driving mechanism behind the widespread inundation of a late Holocene coastal wetland in southeast England, UK. Detailed stratigraphic and dating evidence (lithology, grain size, foraminifera, pollen and radiocarbon dates) from a sample site in Romney Marsh documents the gradual inundation of a coastal wetland after 1263–1085 cal. yr BP (c. 700–850 AD) and the establishment of a saltmarsh. Shortly thereafter there was a rapid increase in water depth that was associated with the deposition of nearly 4 m of laminated intertidal mudflat and tidal channel sediments, prior to site reclamation from the sea by AD 1460. Grain-size data and statistical analysis of sand and mud laminae thicknesses suggest the laminated sediments accumulated rapidly (c. 0.2 m per year) as heterolithic tidal rhythmites. Rapid compaction of the thick peat bed that underlies the study site provided the accommodation space for their deposition. This process began with the gradual tidal inundation of the site, but accelerated following the widening of a breach in a coastal barrier in the 13th century. Compaction lowered the peat surface by at least 3 m and was associated with widespread landscape change. The study demonstrates the powerful influence that compaction had on the evolution of the late Holocene landscape at this site and, we believe, at many other coastal lowlands in northwest Europe. This process is likely to have been a key driving mechanism behind rapid late Holocene coastal change, far exceeding the longer-term effects of either eustatic change or crustal uplift/subsidence.