Peat and diatomite deposits to the north and east of Lough Neagh, the largest lake in the British Isles, contain a history of palaeohydrological fluctuations during the Holocene. Previous investigations of the diatomite suggested that it formed during a period of higher water levels mainly between ca. 8000-6000 cal. yr BP (Jessen 1949; Mitchell 1955; Smith & Collins 1971; Smith, 1981). In a more recent study, pollen and coleopteran records revealed that the peats formed in a eu- to mesotrophic floodplain environment. The study identified what appeared to be a widespread flooding event at ca. 4300 cal. yr BP, represented by the formation of diatomaceous lenses within the peat associated at three sites with Hekla 4-type tephra (Plunkett et al., 2004). New evidence obtained through AMS 14C dating at two sites reveals chronological reversals that signify sediment reworking around the time of diatomite deposition. Because it now seems likely that the diatomaceous lenses are the result of redeposited diatomite, the identity of the Hekla 4-type tephra in the floodplain deposits is called into question. One possibility is that a flood event around the time of Hekla 4 eroded in situ peats and redeposited sediment containing older diatomite. Alternatively, the tephra may derive from reworked Hekla 5/Lairg A tephra that was erupted c. 6900 cal. BP, and the precise age of the flood event is unknown. Disturbance of the sequences is not clearly evident in any of the pollen records, but the coleopteran fauna at one site contains significant variations in levels of preservation between different families (e.g. signs of erosion and mottling of the chitin in some groups and excellent levels of preservation in others), suggesting allochthonous and autochthonous faunal elements. It now seems likely that the fauna, at least at this site, was at least partially subjected to poorly understood taphonomic processes, presumably related to flooding, erosion and deposition on the floodplain. These new findings highlight that erosion and redeposition of organic floodplain sediments as a result of palaeohydrological events can potentially escape notice in the absence of a high-resolution chronological framework. The value of tephrochronology in the study of floodplain deposits must also be viewed with caution.
|Publication status||Published - 2006|
|Event||HOLIVAR Open Science Meeting 'Natural Climate Variability and Global Warming' - London, United Kingdom|
Duration: 12 Jun 2006 → …
|Conference||HOLIVAR Open Science Meeting 'Natural Climate Variability and Global Warming'|
|Period||12/06/2006 → …|