VOLCANISM AND DIVERSIFICATION 101
Fig. 1. Summary of ecological responses of plankton and benthos to volcanic ash deposition in Ordovician fine
sediments of Llandrindod, central Wales. Vertical scale (with samples 2 cm thick) is dependent on
sedimentation rates, and should be treated as an example rather than absolute. Originally anoxic (dashed) and
dysaerobic (solid) situations are distinguished; benthic populations of Opsiconidion nudum and bryozoans
only occur under relatively high oxygenation and food supply.
Ecological effects of local ash deposition
To establish the effects of volcanic eruptions on
regional or global diversity, a detailed under-
standing of the ecological results of normal vol-
canic activity is required. Eruptions sufficiently
large to affect climate significantly are believed
to have dominantly destructive effects on biota
(e.g. Javoy & Courtillot 1989), but Huff et al.
(1992) found no evidence for this. Although
massive eruptions undoubtedly occurred during
the Ordovician (e.g. Huff et al 1992), they rep-
resent a rare influence beyond the normal range
of variability. The great majority of sedimentary
ash layers are less than 10 cm thick outside the
immediate vicinity of the source, suggesting
eruptions of similar size to the present day.
However, in many Ordovician successions, ash
beds are extremely abundant, implying a gener-
ally higher eruption frequency resulting from
widespread continental convergence (Stillman
1984).
Detailed, small-scale logs showing the eco-
logical effects of discrete ash beds in Ordovician
sediments of the Welsh Basin (Botting 2000)
have allowed the reconstruction of local popu-
lation dynamics. Dysaerobic Llanvirn and basal
Caradoc siltstones of the Builth-Llandrindod
Inlier, Central Wales, have yielded consistent
patterns of faunal abundance. Palaeoenviron-
mental analyses are dominated by the local
development of the Builth volcanic cone in the
upper part of the Llanvirn, with the spectacular
but controversial shoreline unconformity of
Jones & Pugh (1949) underlying teretiusculus
Biozone siltstones near Builth. Water depth
during the late Llanvirn at Llandrindod is
estimated at equivalent to mid to outer shelf
(50-150 m), based on the dominance of the
Raphiophorid Community of Fortey & Owens
(1978), proximity to the Builth cone and palaeo-
continental margin (from Cope et al. 1992), and
sedimentology (Botting 2000); Cope (1999)
inferred intertidal or shallow sublittoral con-
ditions for beds underlying the volcanic succes-
sion. Many of the sections are organic-rich, with
faunas indicative of oxygen limitation, including
persistent benthic anoxia in parts of the
murchisoni and teretiusculus Biozones. The
faunal logs are mostly dominated by the partly
pseudoplanktic inarticulate brachiopod Apato-
bolus micula (Botting & Thomas 1999), with
abundant graptolites, ostracodes and chitino-
zoans in some sections. The occurrence of all
