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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
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