Use of carbon isotope and C/N geochemistry in reconstructing vegetation communities: a mid- to late- Holocene palaeoenvironmental investigation from Romney Marsh, Kent, UK

Abstract

The use of stable carbon isotope geochemistry and C/N ratios for coastal palaeoenvironmental reconstructions is a relatively new and under-researched field. This research seeks to understand the processes which govern the carbon isotope and C/N ratios of past and present vegetation communities, particularly alder carr and saltmarsh. Romney Marsh is a large, extensively studied reclaimed coastal wetland in south east England containing a 2-metre thick peat layer that formed between c. 5500-1500 cal. yrs. BP. The palynological data from two mid- to late- Holocene cores, Hope Farm and Little Cheyne Court allows comparison to the ς¹³C and C/N data collected in this study. Ninety three contemporary plant samples, 25 contemporary soil samples and 234 fossil peat samples were collected and analysed for their carbon isotope and C/N ratios. The results show that contemporary alder carr woodland plant material have more negative ς¹³C (-26%₀ to -32%₀) than saltmarsh plant material (-22.5%₀ to -26%₀). The C/N of the plant material from the sampled vegetation communities varied dependent on the type material collected. In general leaf material has lower C/N (around 20) than non leaf material (btween 15 and 80). In the fossil core, former raised bog, sedge fen, alder carr and saltmarsh communities, identified by pollen and microfossil remains, have been tested for ς¹³C and C/N. There is eveidence for post-depositional changes in the alder carr communities, with a negative shift of between 0.7%₀ and 2%₀ from the contemporary soil to fossil peat. Three periods of increased higher water availability have been identified within the Little Cheyne Court core, using ς¹³C, at 4600 - 4000; 3200 to 2700,; to 2200 to 1500 cal. yrs. BP. Spectral analysis has found evidence for cycles, with periodicities of 1005 and 1675 years. The research has also provided evidence for past environmental stress on the vegetation communities , including changes in precipitation and changes in local coastal conditions, principally relative sea level