Download Oceanic LIPs: The Kiss of Death

Oceanic LIPs:
The Kiss of Death
Oceanic plateaus have
been drilled from the
JOIDES Resolution drill
ship. PHOTO ODP
Andrew C. Kerr1
ceanic plateaus represent large areas (~1 × 106 km2) of thickened
oceanic crust formed from rapidly erupted lava (<3 Myr). These
plateaus have formed throughout most of geological time. They
generally correlate with periods of environmental catastrophe characterised
by oceanic anoxia, leading to black shale formation and mass extinction
events. Such correlations are particularly evident in the Cretaceous and can
be partly attributed to the release of CO2 during oceanic plateau formation,
which ultimately resulted in a runaway greenhouse effect. Additionally, sea
level rise and disruption of oceanic circulation patterns by displacement of
seawater during plateau formation contributed to increased environmental
stress and biotic extinction.
O
(93.5 Ma) and during the Aptian
(124–112 Ma) (Sliter 1989;
Bralower et al. 1993; Jahren 2002).
This link appears to extend back to
the Precambrian: Condie et al.
(2001) have noted that significant
black shale events occurred at ~1.9
and 2.7 Ga and that these correlate
with the formation of mantle
plume-derived large igneous provinces (LIPs) and warmer palaeoclimates. Thus, there seems to be a
temporal association, throughout
a significant proportion of geologKEYWORDS: mass extinction, oceanic plateau, ical time, between periods of
black shale, anoxia, mantle plume global oceanic environmental
crises and oceanic plateau formaINTRODUCTION
tion. In the remainder of this contribution the possible
Although the potentially devastating environmental causal links between oceanic plateaus and oceanic environimpact of continental flood basalts has been extensively mental change will be reviewed, with particular reference to
discussed by many authors (Wignall 2001; articles by Self et Cretaceous events.
al. and Wignall this issue), the global environmental effects
of oceanic plateaus – the marine equivalent of continental
OCEANIC PLATEAU VOLCANISM AND
flood basalts – have received comparatively little attention.
Oceanic plateaus represent over-thickened areas of oceanic
crust (>10 km) in which the bulk of the >1 × 106 km3 lava
volume appears to have erupted in less than 2–3 Myr. These
plateaus generally cover an area in excess of 1 × 106 km2
and result from anomalously high rates of melt production
in the mantle. These high melt-production rates are most
likely due to the excess heat from a deep-rooted mantle
plume (Campbell this issue).
Oceanic plateaus are more buoyant than oceanic crust of
normal thickness generated at a mid-ocean ridge. This
buoyancy means that oceanic plateaus are more resistant to
subduction, a feature which results in partial accretion onto
continental margins. In this way oceanic plateaus can be
preserved in the geological record and can be recognised
back to the earliest Archaean.
Periods of oceanic environmental crisis can be identified in
the geological record by the occurrence of black shales,
which are indicative of low-oxygen or oxygen-absent conditions in the deep ocean. Vogt (1989), Sinton and Duncan
(1997), and Kerr (1998) have noted the coincidence of
global oceanic anoxia, black shale deposition, mass extinction and oceanic plateau formation in the Cretaceous, particularly around the Cenomanian–Turonian boundary
1 School of Earth, Ocean and Planetary Sciences
Cardiff University, Main Building
Park Place, Cardiff, Wales
CF10 3YE, UK
E-mail: [email protected]
ELEMENTS, VOL. 1,
PP.
289–292
GLOBAL OCEANIC ANOXIA AT THE
CENOMANIAN–TURONIAN BOUNDARY
Oceanic Volcanism
Arguably, the clearest link between oceanic plateau volcanism and environmental perturbation can be seen around
the Cenomanian–Turonian (C–T) boundary. This time
period is marked by the formation of the Caribbean–
Colombian oceanic plateau (eastern Pacific) and parts of
both the Kerguelen Plateau (Indian Ocean) and possibly the
Ontong Java Plateau (western Pacific) (FIG. 1). Also around
this time, India and Madagascar were beginning to rift
apart, and this event was associated with volcanism at the
Marion hotspot, which resulted in flood basalt eruptions on
Madagascar and basaltic lavas offshore. Due to the continued breakup of Gondwana, the length of the global ridge
system increased in the mid–late Cretaceous, resulting in a
significantly greater volume of lava erupted globally at midocean ridges. Kerr (1998) has calculated that the peak production of oceanic crust (both intrusive and extrusive)
around the C–T boundary was of the order of 45 × 106 km3,
with ~10 × 106 km3 of this erupted on the seafloor (FIG. 2).
Stratigraphic Characteristics
The stratigraphic succession around the C–T boundary is
characterised by black organic-rich shales, signifying anoxic
oceanic conditions. This black shale event was associated
with a second-order mass extinction event marked by the
demise of 26% of genera (Sepkoski 1986). The C–T boundary is also characterised by a sharp increase in δ13C from 1.5
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Cenomanian–Turonian plate tectonic reconstruction
showing the location of ~90–93 Ma and 123–110 Ma
large igneous provinces. Cenomanian–Turonian boundary black shale
deposits are also shown. The black shale localities are taken from Arthur
et al. (1987), Herbin et al. (1987), Summerhayes (1987), and Yurtsever et
al. (2003).
FIGURE 1
to 4‰ in pelagic limestones, a decline in 87Sr/86Sr of seawater and evidence for a significant transgression (FIG. 2).
Oxygen isotope evidence reveals that globally averaged surface temperatures at the C–T boundary were 6 to 14°C
warmer than now (Kaiho 1994). This temperature rise was
caused by elevated CO2 levels in the atmosphere. Modelling
by Berner (1994) suggests atmospheric CO2 levels at this
time were up to six times greater than pre-industrial levels
and reached a peak around the C–T boundary.
Links between Oceanic Plateau Volcanism
and Environmental Catastrophe
The coincidence of extensive oceanic plateau volcanism
and the physical and chemical phenomena outlined above
demand that we look for the causal links between oceanic
plateau volcanism, global oceanic anoxia and warming, and
mass extinction events.
The formation of LIPs on both the continents and the
oceans is often accompanied by lithospheric uplift and
doming (Larson 1991; Nadin et al. 1997). Under the oceans,
this elevation, in combination with the displacement of
water due to the eruption of ~10 × 106 km3 of lava onto the
ocean floor, results in a significant rise in sea level (FIG. 3).
This mechanism may provide an explanation for at least
part of the estimated ~100 m sea level rise which reached a
maximum at the C–T boundary. The elevation of the sea
floor, from both plume uplift and voluminous lava extrusion, during oceanic plateau formation could also have disrupted important oceanic circulation systems around the
C–T boundary. The Caribbean–Colombian oceanic plateau
formed close to the proto-Caribbean seaway between North
and South America (FIG. 1). At this time, the only major
source of deep, cold, oxygenated water for the juvenile
Atlantic was the Pacific, and the water had to pass through
this proto-Caribbean seaway. The formation of such a
major volcanic edifice and the associated shallowing of seawater so close to this oceanic gateway would have restricted
the flow of deep oxygenated water from the Pacific to the
Atlantic and thus increased the extent of oceanic anoxia in
the Atlantic (de Boer 1986).
Although volcanism undoubtedly contributed to the elevated CO2 contents in the C–T atmosphere, it is doubtful if
volcanism alone could have released enough CO2 to cause
the higher temperatures calculated to exist at this time (Self
et al. this issue). However, it is likely that a complex positive
ELEMENTS
Diagrams showing changes in key environmental indicators, sea level, and oceanic crust production between 110
and 80 Ma. The dotted horizontal line represents the Cenomanian–
Turonian boundary (CTB). Diagram updated from Kerr (1998).
FIGURE 2
feedback mechanism triggered by the volcanically derived
CO2 led to increased CO2 and elevated temperatures (FIG. 3).
The initial emission of CO2 from oceanic plateau volcanism
was probably accompanied by the release of a considerable
amount of SO2 and halogens (Self et al. this issue), which
would have made the oceans locally more acidic (FIG. 3).
This increased acidity would have led to the dissolution of
shallow-water carbonates, thus releasing more CO2 to the
atmosphere. [Significantly, Arthur et al. (1987) have noted
that the C–T boundary is characterised by a lack of carbonates.] Thus, the addition of carbonate-derived CO2 plus volcanic CO2 to the atmosphere at the C–T boundary would
have caused global warming of both the atmosphere and
the oceans. Since the solubility of CO2 in seawater decreases
by 4% for every 1°C rise in temperature, warming would
have resulted in the release to the atmosphere of yet more
CO2 which was previously dissolved in the oceans. Thus, a
positive CO2 feedback mechanism would have been established (FIG. 3). Kerr (1998) has proposed that such a scenario
would relatively rapidly result in the establishment of a
runaway greenhouse effect.
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Increased atmospheric CO2 levels, in combination with disrupted oceanic circulation patterns and associated
upwelling of nutrients from the deep ocean, would have
resulted in increased biogenic productivity in ocean surface
waters (FIG. 3). This increased biological activity led to
removal of CO2 from the atmosphere and provided a mechanism for reducing the amount of atmospheric CO2. The
δ13C peak in shallow ocean sediments at the C–T boundary
reflects increased burial of marine organic carbon in the
oceans and is due to the preference of organic matter for
isotopically light carbon.
The decrease in 87Sr/86Sr in the stratigraphic record (from
0.70753 to 0.70735), which started in the late-Cenomanian
and continued until the mid-Turonian (FIG. 2), may be a
reflection of the addition to seawater of hydrothermal fluids with a low 87Sr/86Sr from oceanic plateau volcanism.
Conversely, the rise in 87Sr/86Sr from the mid-Turonian
onwards may signify increased continental weathering
resulting from global warming and its associated climatic
disturbance. Continental weathering is another mechanism
which can reduce the amount of atmospheric CO2.
Higher oceanic temperatures would also have contributed
to oceanic anoxia since the solubility of O2 in seawater
decreases by 2% for every 1°C temperature rise (de Boer
1986). However, given that globally averaged ocean temperatures appear to have increased by at most 6°C (FIG. 2),
the consequent ~10% reduction in the solubility of O2 in
seawater is not enough to explain widespread oceanic
anoxia. Several additional mechanisms by which oceanic
plateaus can contribute to the depletion of dissolved O2
have been discussed by Sinton and Duncan (1997). The first
of these is the reduction of dissolved O2 in seawater by the
reaction of trace metals and sulphides in hydrothermal
fluids with the O2. Although basaltic lava flows can be oxidised by hydrothermal fluids, both during and after eruption, this process is volumetrically insignificant when compared to the much greater effect of the oxidation of metals
in hydrothermal fluids in lowering the amount of dissolved
oxygen in seawater (FIG. 3). The eruption of a 1 × 104 km3
oceanic plateau basalt lava flow would release a similar volume of hydrothermal fluids at 350°C into the ocean (Cathles, cited in Sinton and Duncan 1997). Sinton and Duncan
(1997) have calculated that the complete oxidation of the
ELEMENTS
Flow diagram of the likely physical and chemical environmental effects of oceanic plateau formation (see text for
a detailed description).
FIGURE 3
Fe2+, Mn2+, H2S, and CH4 in 1 x 104 km3 of hydrothermal
fluid would use up ~6% of the total dissolved oxygen in the
present-day ocean. However, as we have seen, the C–T
ocean was significantly warmer and contained less dissolved oxygen than the present-day ocean. Therefore, the
proportion of dissolved oxygen removed by 1 × 104 km3 of
hydrothermal fluid at the C–T boundary would have been
significantly greater than 6%.
Another mechanism for removing dissolved oxygen from
seawater, as discussed by Sinton and Duncan (1997), is the
stimulation of the growth of living organisms (organic productivity) in the oceans by the injection of hydrothermal
iron into surface waters. Vogt (1989) has suggested that
hydrothermal plumes, even those several orders of magnitude smaller than 1 × 104 km3, would have been capable of
rising into oceanic surface waters. Thus, as noted by Sinton
and Duncan (1997), a large (1 × 104 km3) hydrothermal
plume (with a low brine content) could easily rise through
the water column and spread laterally over a significant
proportion of the ocean surface. The trace metal–rich
waters of such massive hydrothermal plumes may well have
stimulated increased levels of organic productivity in nutrient-poor surface waters (Sinton and Duncan 1997). Coale et
al. (1996) have shown that the addition of Fe into ocean
surface waters can result in a rapid increase in the amount
of phytoplankton. If a similar hydrothermal fluid–induced
phytoplankton bloom occurred around the C–T boundary,
the net effect would have been a further reduction in the
amount of dissolved O2 as organic material decayed and
sank through the seawater column (FIG. 3).
FURTHER LINKS BETWEEN OCEANIC
PLATEAU VOLCANISM AND
ENVIRONMENTAL DISTURBANCE
The link between oceanic plateau volcanism and global
oceanic anoxia is given further credence by the occurrence
of Aptian (124–112 Ma) black shales, which probably represent one of the most extensive concentrations of organicrich black shales in the geological record (Jenkyns 1980;
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Hallam 1987; Bralower et al. 1993). It is no coincidence that
one of the most extensive periods of plume-related oceanic
plateau formation occurred in the Pacific and Indian oceans
during this period, including the Ontong Java Plateau, Hess
Rise, Manihiki Plateau, the East Mariana and Nauru basins
and a significant proportion of the Kerguelen Plateau
(Eldholm and Coffin 2000). Although many of the chemical and physical characteristics of the Aptian oceanic
anoxic event are very similar to those of the C–T boundary,
one of the black shale horizons in the early Aptian exhibits
a sharp decrease in δ13C, unlike the C–T boundary, which
shows a sharp increase in δ13C (Jahren 2002).
Present-day methane hydrates buried in ocean floor sediments possess very low δ13C (around –60‰). Thus the dissociation and catastrophic release of hydrates may have
caused the sharp decrease in δ13C and contributed to global
warming and oceanic anoxia in the early–mid Aptian
(Jahren 2002). It has been proposed that methane release
was triggered by tectonic events related to mantle plume
uplift (Jahren 2002). However, while tectonic processes
undoubtedly played a role, I contend that LIP-induced
global warming was of greater importance and was probably of sufficient magnitude to cause dissociation of
methane hydrates and consequent release of methane. This
release would have accelerated global warming and anoxia
since methane is a much more potent greenhouse gas than
carbon dioxide. Furthermore, atmospheric oxidation of
methane would consume a significant amount of free oxygen. Methane release may also have occurred around the
C–T boundary, but its distinctive? δ13C signal may have
been diluted by higher δ13C resulting from volcanism
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ELEMENTS
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Increased oceanic volcanism as a cause of oceanic anoxia
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This is obviously an interesting model, but how applicable
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