Palynological analysis of a 2,000 year-old bat-guano deposit in Wales (UK)

Suzanne A. G. Leroy (1) and Michael J. Simms (2)
    (1) Department of Geography and Earth Sciences, Brunel University, Uxbridge UB8 3PH, UK
    (2) Department of Geology, Ulster Museum, Botanic Gardens, Belfast BT9 5AB, N. Ireland, UK

The karst system is very well developed in Wales (UK) and its extension has only been recently explored and mapped. Ogof Draenen, near Blaenavon in south-east Wales, is the most recent major cave discovery (1994) with already > 70 km of passages across a vertical range of 148 m.

With the exception of a small chamber, Siambre Ddu, which lies directly above the main Ogof Draenen system and which is an important roost for lesser Horseshoe (Rhinolophus hipposideros) and a few Greater Horseshoe (Rhinolophus ferruequinum) bats, very few bats have been seen in the main system. Hence, the guano accumulations found in parts of the Ogof Draenen system are unexpectedly large. The heaps, several sqm and > 0.5 m thick, represent volumes unmatched by any other cave system in the British Isles.

A guano sample from Ogof Draenen was radiocarbon dated at 2000 yr ago, placing the sample in the Romano-British period.

Pollen analysis was applied to it as well as nine other samples distributed over the cave at various distances from the presumed entrance and also through the heaps themselves. Comparisons were done to a moss pollster and a mud sample from the area and to two modern guanos from Agen Allwedd cave (5 km to the north-west) which currently is one of the largest active roosts for Lesser Horseshoe bats in Britain and lies close to their present northern limit in Europe.

The main results are
  • that most of the Ogof Draenen guano is probably of the same age;
  • that the modern samples are clearly characterised by the presence of planted trees, invasive plants, and grassland and heathland (vs the fossil samples which reflect a closed forest);
  • that insect-pollinated plants such as Ilex, Acer, Hedera and Impatiens glandulifera are over-represented in the guano samples.
  • that various ways of incorporation of the pollen grains to the guano act jointly. Insect eating and grooming by the bat as well as direct air transport by draughts created by temperature difference between the cave and outside take place.