Asteroids - Extreme Events: A Physical Reconstruction and Risk Assessment - page 243

from hot spot volcanoes as gaseous fluoride compounds, and it can be released as
PGE carbonyls within volcanic aerosols. High Ir concentrations have been found
in ashes from the Mt Kilauea volcano, Hawaii. These iridium-rich particles are
likely to be derived from deep magma sources and it is possible that they can be
adsorbed onto organic and inorganic particles. Sawlowics (1993)suggests that
deposition of these particles, or dissolution in seawater, can possibly explain
some enhanced Ir and PGE levels. Also, because there are few PGE data sets for
mantle-derived rocks, it is difficult to differentiate between anomalies derived
from terrestrial and extraterrestrial sources (Evanc et al. , 1993).
Biological processes also appear to be able to concentrate Ir from seawater.
Some bacteria and fungi can dissolve Ir from both igneous rocks and from
meteoritic materials. The mobilised elements can then be concentrated in the
sedimentary environment by living or dead cyanobacterial mats, other bacteria
and fungi. Micro-organisms can also dissolve Ir during diagenesis and redeposit it
in adjacent sediments (Sawlowics, 1993). Soils, marine and freshwater organisms,
therefore, appear able to dissolve and/or concentrate Ir and other PGE. Dyer et al.
(1989)demonstrated that some micro-organisms can enhance, disperse or erase
catastrophic iridium anomalies. They can also create anomalies from terrestrial
sources of iridium. Dyer et al. (1989)showed that through the redistribution of
iridium over short distances in sediments, micro-organisms may be responsible
for the presence or absence of iridium anomalies as well as for the apparent
time interval represented by an anomaly. Common micro-organisms worldwide
appear able to affect the chemistry of iridium over a wide pH range by concen-
tration or dissolution. It is important, therefore, that the microbial chemistry
is taken into account when examining Ir anomalies associated with extraterres-
trial impacts as conclusions about the nature of an event, the size of the impact
body and the scale of an event may be misinterpreted (Dyer et al. , 1989). Despite
therole that biological processes may have in producing Ir anomalies, there is
little doubt that such anomalies, when all other factors are taken into account,
are a signature marker for past bolide impacts with Earth.
Along with the K--T boundary, Ir anomalies occur during the Cambrian, Devo-
nian and Tertiary, some of which also correspond to major extinction events.
Wallace et al. (1991)focused on analysing the sedimentary Ir anomalies found
near the Frasnian--Famennian (F--F) boundary (Upper Devonian) in the Canning
Basin, Western Australia. This is one of the major extinction event boundaries.
The mass extinction event appears to have occurred slightly after the Ir anomaly
in the stratigraphic record at this time. Although meteorite impact has been sug-
gested to explain the Ir anomaly and the mass extinction events, it has also been
suggested that the filamentous microbe Frutexites may have collected Ir and also
Pt directly from seawater. The Ir anomaly is found together with this microbe