102 JOSEPH P. BOTTING
Fig. 2. Idealized distribution of oxygenation effects around ash-fall deposits in a shelf region with immobile
water mass: central area of benthic oxygenation by strong overturning, and deoxygenation in lateral regions
due to partial overturning and plankton bloom. Anoxic regions begin to disperse following decline of plankton
bloom (one year), allowing immigration of benthos from exterior.
other faunal elements was noted, although data
were usually insufficient for meaningful abun-
dance comparisons. The complex overall
patterns are consistent throughout all studied
sections, minimizing the possibility of an arti-
ficial distribution. A generalization of the results
is shown in Figure 1 and explained below. Full
details of these data, including deeper-water
facies from which benthic blooms were absent,
will be published elsewhere, with full interpre-
tation, and are also available in Botting (2000).
The ash bed is usually barren, with only oc-
casional chitinozoans recorded. A brief, usually
minor bloom of small mobile benthos (ostra-
codes and simple burrows) immediately fol-
lowed ash deposition, overlapping with a
subsequent dramatic plankton bloom of Apato-
bolus micula and graptolites. The plankton
bloom showed a pseudo-logarithmic decline,
while A. micula also formed a partly overlapping
second bloom, of longer duration. Thereafter,
abundance gradually declined to, or below,
initial levels, unless benthic oxygenation became
established. The ratio of ostracode to graptolite
abundance varied according to initial con-
ditions; in originally anoxic facies, the plankton
(Apatobolus micula plus graptolites) bloom was
by far the greater, and vice versa for originally
dysaerobic substrates (Fig. 1). A horizontal
sequence, sampling the 2 cm immediately above
the ash upper surface, revealed steep faunal
abundance gradients (>300% over 10m), corre-
sponding with up to a 20% change in ash thick-
ness. A strong maximum in the immediately
post-ash plankton abundance overlay the thick-
est part of the bentonite.
Multiple lines of evidence were used to con-
strain the processes operating, and a coherent
interpretation developed, based on vertical
circulation of a stratified water column. Fine ash
deposited onto a standing body of water
descended initially as turbid flows rather than
discrete particles (Carey 1997). Provided a
critical water depth (strongly dependent on
several parameters) was not exceeded, these
flows replaced deep, dysaerobic water with
aerobic surface water, enabling a bloom of small
mobile benthos, restricted by low food supplies.
Corresponding upwelling induced a large-scale
plankton bloom, continuing until inertial circu-
lation ceased. This circulation could have been
maintained directly by ash deposition for several
weeks, through continuous fragmentation of
floating pumice and slow sinking of fine
particles. Following ash deposition, circulation
would be prolonged by the temperature inver-
sion of ash-free fluid, and perhaps enhanced by
large-scale particulate phytoplankton produc-
tion at the surface. Upwelling is expected to
have continued weakly for a few months, suf-
ficiently long to establish a substantial plankton
bloom involving several trophic levels. The
