Greenstone belts of the Canadian Superior province
and the Baltic Shield (3.0–2.7 Ga) consist of many lava
formations that have been interpreted to be remnants of
accreted oceanic plateaus. The evidence for an oceanic
plateau origin is based on the occurrence of pillow basalts
and komatiites with no terrestrial sedimentary intercalations or sheeted dike swarms, possessing broadly chondritic La/Nb ratios and low positive e Nd (see reviews in
Ernst and Buchan, 2001; Kerr, 2003).
Proterozoic oceanic plateau terrains have also been
identified in the Birimian Province of Western Africa,
the Arabian–Nubian Shield, the Flin Flon Belt in Canada,
and the Iron King Volcanics in Arizona. More detailed
summaries of these accreted terrains and other Precambrian oceanic plateaus are given in Ernst and Buchan
(2001) and Kerr (2014).
Environmental impact of oceanic plateau
formation
Many periods of oceanic environmental crisis in Earth’s
history are marked by significant black shale deposits,
which indicate oxygen-poor conditions in the deep ocean,
i.e., oceanic anoxic events (OAEs). In the Cretaceous,
these events are associated with periods of mass extinction
and oceanic plateau formation particularly around the
Cenomanian–Turonian boundary (93.5 Ma) and in the
Aptian (124–112 Ma).
Cenomanian–Turonian Boundary (CTB) event
As reviewed in detail by Kerr (1998, 2005), the CTB event
is characterized by the worldwide deposition of organicrich black shales, indicating widespread anoxia in the
oceans at this time. The CTB is also marked by a rise in
sea-level and significantly higher average global surface
temperatures than the present day. These higher temperatures are most likely due to an increase in global atmospheric CO 2 to 3–5 times higher than preindustrial levels
(Fletcher et al., 2008). These phenomena were accompanied by an extinction event that resulted in the demise of
26 % of all known genera (Raup and Sepkoski, 1986).
The CTB event has been linked to the formation of oceanic plateaus (Kerr, 1998, 2005; Kuroda et al., 2007). An
oceanic plateau trace element signature in black shales
was first noted by Kerr (1998) and was subsequently
reported by Snow et al. (2005). More recent isotopic studies (Kuroda et al., 2007) on CTB black shales from Italy
showed that their Pb isotope signatures are significantly
shifted toward the geochemical composition of the Caribbean plateau, thus implying a substantial input from the
plateau.
Links between CTB oceanic plateau volcanism and
environmental perturbation
Most plume-related volcanism around the CTB occurred
in the oceans, with the formation of significant portions
of the Caribbean, Ontong Java, and Kerguelen plateaus.
The probable worldwide environmental effects of such
volcanism are reviewed below and summarized in
Figure 3.
The net result of the emplacement of millions of cubic
kilometers of magma both into and onto the preexisting
ocean crust in combination with the uplift associated with
buoyant plume head impingement on the base of the oceanic lithosphere will be to raise the sea-level, due to the
displacement of seawater. The progressive rise in global
sea-level throughout the mid-Cretaceous (Haq et al.,
1987) may signal the gradual impingement of the Caribbean, Ontong Java, and Kerguelen plume heads below
the oceanic lithosphere. This uplift of oceanic lithosphere
and subsequent magma emplacement are also likely to
have resulted in the disruption of oceanic circulation systems such that cool, polar (oxygenated) water was not circulated to lower latitudes, leading to increased anoxia in
the oceans (Kerr, 1998). In addition to the CO 2 -induced,
elevated, global oceanic temperatures, hydrothermal
fluids from oceanic plateau volcanism may also have contributed to warmer oceans and, thus, to anoxia, since oxygen is significantly less soluble in warmer water (see
review in Kerr, 2005).
Calculations suggest that increased volcanic activity
around the CTB is likely to have added ~10
17 kg of CO 2
to the atmosphere (Kerr, 1998). This volcanic activity
probably also released substantial amounts of SO 2 , chlorine, fluorine, and H 2 S into the seawater, resulting in the
CTB oceans being more acidic. This could have resulted
in the dissolution of oceanic carbonate and may help to
explain the lack of carbonate at the CTB (Kerr, 1998,
2005), which would also release more CO 2 to the atmosphere. Higher atmospheric CO 2 levels and nutrient
upwelling from the deep ocean is likely to have resulted
in increased productivity in ocean surface waters
(Figure 3), leading to the widespread deposition of black
shales and, so, ultimately a reduction in CO 2 levels
(Jarvis et al., 2011).
Other links between oceanic plateau formation
and black shale formation
Black shales are relatively common in the geological
record and are generally associated with, sometimes
severe, environmental disruption (Wignall, 1994; Bond
and Wignall, 2014). It is highly significant that these black
shale events also correlate with the formation of oceanic
plateaus or plume-related volcanic rifted margins (Kerr,
2014).
During the Aptian–Albian (125–100 Ma), three distinct oceanic anoxia events (with associated black shales)
occurred, indicating a prolonged period of global environmental disturbance (Wignall, 1994). The Aptian–Albian
was also a major period of oceanic plateau formation in
the Pacific and Indian Oceans (Coffin and Eldholm,
2005). Although many of the geochemical features of
the Aptian–Albian OAE are very similar to those of the
CTB, one of the black shale horizons in the mid-Aptian
shows sharp decrease in d
13
C. This decrease is consistent
564
OCEANIC PLATEAUS
and the Baltic Shield (3.0–2.7 Ga) consist of many lava
formations that have been interpreted to be remnants of
accreted oceanic plateaus. The evidence for an oceanic
plateau origin is based on the occurrence of pillow basalts
and komatiites with no terrestrial sedimentary intercalations or sheeted dike swarms, possessing broadly chondritic La/Nb ratios and low positive e Nd (see reviews in
Ernst and Buchan, 2001; Kerr, 2003).
Proterozoic oceanic plateau terrains have also been
identified in the Birimian Province of Western Africa,
the Arabian–Nubian Shield, the Flin Flon Belt in Canada,
and the Iron King Volcanics in Arizona. More detailed
summaries of these accreted terrains and other Precambrian oceanic plateaus are given in Ernst and Buchan
(2001) and Kerr (2014).
Environmental impact of oceanic plateau
formation
Many periods of oceanic environmental crisis in Earth’s
history are marked by significant black shale deposits,
which indicate oxygen-poor conditions in the deep ocean,
i.e., oceanic anoxic events (OAEs). In the Cretaceous,
these events are associated with periods of mass extinction
and oceanic plateau formation particularly around the
Cenomanian–Turonian boundary (93.5 Ma) and in the
Aptian (124–112 Ma).
Cenomanian–Turonian Boundary (CTB) event
As reviewed in detail by Kerr (1998, 2005), the CTB event
is characterized by the worldwide deposition of organicrich black shales, indicating widespread anoxia in the
oceans at this time. The CTB is also marked by a rise in
sea-level and significantly higher average global surface
temperatures than the present day. These higher temperatures are most likely due to an increase in global atmospheric CO 2 to 3–5 times higher than preindustrial levels
(Fletcher et al., 2008). These phenomena were accompanied by an extinction event that resulted in the demise of
26 % of all known genera (Raup and Sepkoski, 1986).
The CTB event has been linked to the formation of oceanic plateaus (Kerr, 1998, 2005; Kuroda et al., 2007). An
oceanic plateau trace element signature in black shales
was first noted by Kerr (1998) and was subsequently
reported by Snow et al. (2005). More recent isotopic studies (Kuroda et al., 2007) on CTB black shales from Italy
showed that their Pb isotope signatures are significantly
shifted toward the geochemical composition of the Caribbean plateau, thus implying a substantial input from the
plateau.
Links between CTB oceanic plateau volcanism and
environmental perturbation
Most plume-related volcanism around the CTB occurred
in the oceans, with the formation of significant portions
of the Caribbean, Ontong Java, and Kerguelen plateaus.
The probable worldwide environmental effects of such
volcanism are reviewed below and summarized in
Figure 3.
The net result of the emplacement of millions of cubic
kilometers of magma both into and onto the preexisting
ocean crust in combination with the uplift associated with
buoyant plume head impingement on the base of the oceanic lithosphere will be to raise the sea-level, due to the
displacement of seawater. The progressive rise in global
sea-level throughout the mid-Cretaceous (Haq et al.,
1987) may signal the gradual impingement of the Caribbean, Ontong Java, and Kerguelen plume heads below
the oceanic lithosphere. This uplift of oceanic lithosphere
and subsequent magma emplacement are also likely to
have resulted in the disruption of oceanic circulation systems such that cool, polar (oxygenated) water was not circulated to lower latitudes, leading to increased anoxia in
the oceans (Kerr, 1998). In addition to the CO 2 -induced,
elevated, global oceanic temperatures, hydrothermal
fluids from oceanic plateau volcanism may also have contributed to warmer oceans and, thus, to anoxia, since oxygen is significantly less soluble in warmer water (see
review in Kerr, 2005).
Calculations suggest that increased volcanic activity
around the CTB is likely to have added ~10
17 kg of CO 2
to the atmosphere (Kerr, 1998). This volcanic activity
probably also released substantial amounts of SO 2 , chlorine, fluorine, and H 2 S into the seawater, resulting in the
CTB oceans being more acidic. This could have resulted
in the dissolution of oceanic carbonate and may help to
explain the lack of carbonate at the CTB (Kerr, 1998,
2005), which would also release more CO 2 to the atmosphere. Higher atmospheric CO 2 levels and nutrient
upwelling from the deep ocean is likely to have resulted
in increased productivity in ocean surface waters
(Figure 3), leading to the widespread deposition of black
shales and, so, ultimately a reduction in CO 2 levels
(Jarvis et al., 2011).
Other links between oceanic plateau formation
and black shale formation
Black shales are relatively common in the geological
record and are generally associated with, sometimes
severe, environmental disruption (Wignall, 1994; Bond
and Wignall, 2014). It is highly significant that these black
shale events also correlate with the formation of oceanic
plateaus or plume-related volcanic rifted margins (Kerr,
2014).
During the Aptian–Albian (125–100 Ma), three distinct oceanic anoxia events (with associated black shales)
occurred, indicating a prolonged period of global environmental disturbance (Wignall, 1994). The Aptian–Albian
was also a major period of oceanic plateau formation in
the Pacific and Indian Oceans (Coffin and Eldholm,
2005). Although many of the geochemical features of
the Aptian–Albian OAE are very similar to those of the
CTB, one of the black shale horizons in the mid-Aptian
shows sharp decrease in d
13
C. This decrease is consistent
564
OCEANIC PLATEAUS