Asteroids - Extreme Events: A Physical Reconstruction and Risk Assessment - page 239

event resulting in worldwide simultaneous volcanic eruptions of unprecedented
magnitude.
Spherule layers have been found at more distant locations from the proposed
Gulf of Mexico K--T impact site. Norris et al. (1999)examined cores from the
northwestern Atlantic region and described the biostratigraphy and lithology
for the K--T boundary. This time zone was marked by a 10 cm thick spherule
layer assumed to have been derived from an extraterrestrial impact event. The
composition of clasts within the spherule bed suggests that the grains were
derived from either a metamorphic basement or a continental source area asso-
ciated with a carbonate platform. This is consistent with the subsurface geology
in the area around the Chicxulub crater. The size of the spherules also correlates
well with spherules identified in the Gulf of Mexico. An Ir anomaly in the upper
part of the spherule bed is interpreted as further evidence of deposition of an
ejecta layer following an extraterrestrial impact (Norris et al. , 1999).
Spherule beds are not always attributed to impact events. Koeberl et al. (1993)
examined several such beds in the Archean Barberton Greenstone Belt, South
Africa (Reimold et al. , 1997). While previous studies had suggested extraterres-
trial impacts as the cause of the spherule beds (Lowe and Byerly, 1986), Koeberl
et al. (1993)suggest that the mineralogy of the sequence indicates that extensive
hydrothermal activity of volcanic rocks is a more likely cause. They also note that
thespherule layers consist of several distinct sublayers, which if the impact event
hypothesis were proposed, would mean that a number of separate events would
have needed to have occurred over a 30 Ma interval at the one site. Deposition of
thespherule layers at Barberton are dated to about 3.2 Ga, and while others have
suggested that the multiple event hypothesis could have been due to the period
of the Late Heavy Bombardment, Koeberl et al. (1993) note that this ended about
3.8 Ga, hence before deposition of the Barberton spherules. It is also unlikely
that three distinct impacts could occur in the same region especially when this
number of spherule layers from this time interval has not been recognised else-
where. Koeberl et al. (1993) further noted that the microtektites found here occur
in the form of solid beds, which is unusual for well documented impact sites.
Such microtektites are also much smaller than the Barberton spherules, and are
dispersed over a relatively large area as well as having petrological and chemical
characteristics that differ from those of the Barberton spherules. Koeberl et al.
(1993)suggest that a better explanation is that the spherule layers have resulted
from widespread volcanic activity, followed by several phases of hydrothermal
and tectonothermal activity leading to siderophile element mobilisation and
redistribution in the course of sulphide mineralisation.
Simonson and Harnik (2000)found that there appear to be differences
between Precambrian and Phanerozoic spherule layers. On average, early