reworking after the middle Devonian Kaluga impact into
an epicontinental, about 300 m deep sea (Masaitis, 2002).
The Chicxulub impact event at the boundary between
Cretaceous and Paleogene (K/Pg) took place in shallow
water, near the shelf edge at the Yucatan peninsula
(Mexico) (Schulte et al., 2010). In that case, a 10 km bolide
hit, generated earthquakes, and large parts of the outer
shelf/shelf edge collapsed, triggering slumps and slides,
avalanches, gravity flows, turbidity currents, and tsunamis.
The 142-million years old Mjølnir structure was found
in the Barents Sea during geophysical exploration; later
structural, mineralogical, and geochemical analysis
proved its meteoritic origin (Dypvik et al., 2010). Mjølnir
looks different to subaerial craters with its subdued and
possibly gullied rim, chaotic marine succession, and
marine fossils in the black shale formations above and
below. On top of the folded, slumped, and fractured sediments forming the Mjølnir crater base, there is a sedimentary package about 20 m thick. It consists of breccias,
partly from autobrecciation and mass-flow deposits
including turbidites, forming the syn- and early
postimpact succession. It is all topped by black shales,
demonstrating a return of the black clay sedimentation in
the basin, which prevailed before impact.
So far, no deep sea impact craters have been found, in
spite of the large areas of the Earth which are and have
been covered by deep seas. Davison and Collins (2007)
claim there is still no definite relationship between impactor properties, water depth, and final crater diameter.
Artemieva and Shuvalov (2002) and Gisler et al. (2010)
presented similar relations between water, impactor size,
and cratering as given by the ratio between bolide size
and water depth and (H/D). For shallow craters with a ratio
below 3–4, decreasing crater diameters are modeled with
increasing water depth, but a ratio larger than 7–8 will
result in no crater at all, though possibly some disturbance
to the seafloor. Intermediate ratios may, according to
Davison and Collins, result in relatively broad, complex
craters. Artemieva and Shuvalov (2002) claim water
depth/bolide diameter ratios larger than 25 will have no
visual bottom effects.
The Eltanin impact (Bellingshausen Sea, SE Pacific)
represents the only known large meteorite impact into a
deep ocean. This Pliocene event (2.15 million years ago)
(Figure 1) involved an approximately 1 km diameter bolide
hitting the sea where it was about 4,000 m deep, and no crater was developed on the sea floor, but projectile ejecta and
debris (reworked seafloor sediments) were spread in a
diameter of at least 600 km across the ocean floor (Kyte,
1988; Kyte et al., 1988; Gersonde et al., 1997; Artemieva
and Shuvalov, 2002; Wünnemann and Lange, 2002). The
Eltanin-disturbed zone on the seafloor is characterized by
a 20–40 m thick unit of chaotically mixed sediments that
most probably originated from impact-induced turbulent
currents (Gersonde et al., 1997; Gersonde et al., 2002).
Detailed observations of spherule beds in Australia,
South Africa, and Greenland are possible remnants of Precambrian (Archean) oceanic impact activity, but without a
recognized crater. These possible Precambrian craters may
have been eroded or consumed during plate tectonic activity, and presently only the spherule layers have been found
(Simonson et al., 2002; Simonson and Glass, 2004). It is
speculated whether these spherules could reflect large oceanic impacts, maybe comparable or larger than Eltanin of
the SE Pacific.
Consequences of impact
In addition to the impact itself and associated earthquakes,
several other related processes can create hazardous situations in the oceans and along the coastlines (Figure 3);
here, atmospheric effects, tsunamis, and currents must be
mentioned.
Several authors (e.g., Ward and Asphaug, 2000;
Artemieva and Shuvalov, 2002; Shuvalov, 2002; Ward
and Asphaug, 2002; Gisler et al., 2010; Glimsdal et al.,
2010) have stated that tsunamis will be generated, but
according to Gisler et al. (2010), the real dangers from
wet impacts are mainly atmospheric: blast wave from the
explosion, winds, fallout, and high temperatures over small
distances. Possible contamination effects of impact should
also be mentioned as suggested partly causing the K/Pg
extinction tied to the Chicxulub impact (Schulte et al.,
2010). The K/Pg extinction was to a large extent controlled
by bolide and target area composition, water depth, and
impact velocity and direction (Schulte et al., 2010).
In the Chicxulub case, a bolide possibly 10 km in diameter hit a shallow to deep sea with significant lateral
bathyal variations from 100 m to probably 2,000 m
(Gulick et al., 2008; Schulte et al., 2010), creating an
about 180 km diameter crater. The K/Pg extinction was
triggered by the Chicxulub impact and its major pollution
consequences such as soot and dust distribution partly
shutting down photosynthesis, thermal radiation and different aerosols, acid rain, and possible change in the ocean
circulation (Pope et al., 1997; Schulte et al., 2010). It
should however be noted that several effects contributed
to the large biological extinction which has to be evaluated
from the background of ambient environmental conditions
and the existing ecosystem (Kring, 2003).
The potential hazard of meteorite impact and tsunami
generation in the ocean is controversial (Ward and
Asphaug, 2000; Ward and Asphaug, 2002; Korycansky
and Lynett, 2005; Korycansky and Lynett, 2007;
Wünnemann et al., 2007; Gisler et al., 2010) with respect
to the destructive power of the large waves (tsunamis) generated. It is important to mention that several types of tsunamis/waves are generated by impacts (Matsui et al.,
2002; Glimsdal et al., 2010). Recent studies demonstrate
that tsunamis generated by impacts are in general less
destructive than the more common tsunamis generated by,
e.g., plate tectonic movements. This reflects the nonlinear
shoaling of the impact tsunamis (Korycansky and Lynett,
2007; Wünnemann et al., 2007; Glimsdal et al., 2010;
Gisler et al., 2010). Impact tsunami may indeed undergo
strong diminution of amplitude on continental slopes and
MARINE IMPACTS AND THEIR CONSEQUENCES
465
an epicontinental, about 300 m deep sea (Masaitis, 2002).
The Chicxulub impact event at the boundary between
Cretaceous and Paleogene (K/Pg) took place in shallow
water, near the shelf edge at the Yucatan peninsula
(Mexico) (Schulte et al., 2010). In that case, a 10 km bolide
hit, generated earthquakes, and large parts of the outer
shelf/shelf edge collapsed, triggering slumps and slides,
avalanches, gravity flows, turbidity currents, and tsunamis.
The 142-million years old Mjølnir structure was found
in the Barents Sea during geophysical exploration; later
structural, mineralogical, and geochemical analysis
proved its meteoritic origin (Dypvik et al., 2010). Mjølnir
looks different to subaerial craters with its subdued and
possibly gullied rim, chaotic marine succession, and
marine fossils in the black shale formations above and
below. On top of the folded, slumped, and fractured sediments forming the Mjølnir crater base, there is a sedimentary package about 20 m thick. It consists of breccias,
partly from autobrecciation and mass-flow deposits
including turbidites, forming the syn- and early
postimpact succession. It is all topped by black shales,
demonstrating a return of the black clay sedimentation in
the basin, which prevailed before impact.
So far, no deep sea impact craters have been found, in
spite of the large areas of the Earth which are and have
been covered by deep seas. Davison and Collins (2007)
claim there is still no definite relationship between impactor properties, water depth, and final crater diameter.
Artemieva and Shuvalov (2002) and Gisler et al. (2010)
presented similar relations between water, impactor size,
and cratering as given by the ratio between bolide size
and water depth and (H/D). For shallow craters with a ratio
below 3–4, decreasing crater diameters are modeled with
increasing water depth, but a ratio larger than 7–8 will
result in no crater at all, though possibly some disturbance
to the seafloor. Intermediate ratios may, according to
Davison and Collins, result in relatively broad, complex
craters. Artemieva and Shuvalov (2002) claim water
depth/bolide diameter ratios larger than 25 will have no
visual bottom effects.
The Eltanin impact (Bellingshausen Sea, SE Pacific)
represents the only known large meteorite impact into a
deep ocean. This Pliocene event (2.15 million years ago)
(Figure 1) involved an approximately 1 km diameter bolide
hitting the sea where it was about 4,000 m deep, and no crater was developed on the sea floor, but projectile ejecta and
debris (reworked seafloor sediments) were spread in a
diameter of at least 600 km across the ocean floor (Kyte,
1988; Kyte et al., 1988; Gersonde et al., 1997; Artemieva
and Shuvalov, 2002; Wünnemann and Lange, 2002). The
Eltanin-disturbed zone on the seafloor is characterized by
a 20–40 m thick unit of chaotically mixed sediments that
most probably originated from impact-induced turbulent
currents (Gersonde et al., 1997; Gersonde et al., 2002).
Detailed observations of spherule beds in Australia,
South Africa, and Greenland are possible remnants of Precambrian (Archean) oceanic impact activity, but without a
recognized crater. These possible Precambrian craters may
have been eroded or consumed during plate tectonic activity, and presently only the spherule layers have been found
(Simonson et al., 2002; Simonson and Glass, 2004). It is
speculated whether these spherules could reflect large oceanic impacts, maybe comparable or larger than Eltanin of
the SE Pacific.
Consequences of impact
In addition to the impact itself and associated earthquakes,
several other related processes can create hazardous situations in the oceans and along the coastlines (Figure 3);
here, atmospheric effects, tsunamis, and currents must be
mentioned.
Several authors (e.g., Ward and Asphaug, 2000;
Artemieva and Shuvalov, 2002; Shuvalov, 2002; Ward
and Asphaug, 2002; Gisler et al., 2010; Glimsdal et al.,
2010) have stated that tsunamis will be generated, but
according to Gisler et al. (2010), the real dangers from
wet impacts are mainly atmospheric: blast wave from the
explosion, winds, fallout, and high temperatures over small
distances. Possible contamination effects of impact should
also be mentioned as suggested partly causing the K/Pg
extinction tied to the Chicxulub impact (Schulte et al.,
2010). The K/Pg extinction was to a large extent controlled
by bolide and target area composition, water depth, and
impact velocity and direction (Schulte et al., 2010).
In the Chicxulub case, a bolide possibly 10 km in diameter hit a shallow to deep sea with significant lateral
bathyal variations from 100 m to probably 2,000 m
(Gulick et al., 2008; Schulte et al., 2010), creating an
about 180 km diameter crater. The K/Pg extinction was
triggered by the Chicxulub impact and its major pollution
consequences such as soot and dust distribution partly
shutting down photosynthesis, thermal radiation and different aerosols, acid rain, and possible change in the ocean
circulation (Pope et al., 1997; Schulte et al., 2010). It
should however be noted that several effects contributed
to the large biological extinction which has to be evaluated
from the background of ambient environmental conditions
and the existing ecosystem (Kring, 2003).
The potential hazard of meteorite impact and tsunami
generation in the ocean is controversial (Ward and
Asphaug, 2000; Ward and Asphaug, 2002; Korycansky
and Lynett, 2005; Korycansky and Lynett, 2007;
Wünnemann et al., 2007; Gisler et al., 2010) with respect
to the destructive power of the large waves (tsunamis) generated. It is important to mention that several types of tsunamis/waves are generated by impacts (Matsui et al.,
2002; Glimsdal et al., 2010). Recent studies demonstrate
that tsunamis generated by impacts are in general less
destructive than the more common tsunamis generated by,
e.g., plate tectonic movements. This reflects the nonlinear
shoaling of the impact tsunamis (Korycansky and Lynett,
2007; Wünnemann et al., 2007; Glimsdal et al., 2010;
Gisler et al., 2010). Impact tsunami may indeed undergo
strong diminution of amplitude on continental slopes and
MARINE IMPACTS AND THEIR CONSEQUENCES
465