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664 New Concepts in Global Tectonics Journal, V. 4, No. 4,
December 2016. www.ncgt.org
A History of the Earth’s Seawater: Transgressions and
Regressions
Karsten M. Storetvedt
Institute of Geophysics, University of Bergen, Bergen, Norway
karsten.storetvedt@uib.no
“We do not have a simple event A causally connected with a
simple event B, but the whole background of the system in which
the events occur is included in the concept, and is a vital part
of it”
P.W. Bridgman, in: The Logic of Modern Physics, p. 83
Synopsis
The origin of sea water is viewed in the context of the model of
a slowly degassing Earth – an Earth which is more than likely
being far from having acquired thermo-chemical equilibrium. The
internal reorganization of the Earth’s planetary mass has led to
changes of its moment of inertia and thereby its rotation
characteristics – giving rise to variations in spin rate as well
as spatial redistribution of its mass (producing the dynamic
phenomenon of true polar wander). These dynamical pulsations are
seen as the engine behind the spasmodic behaviour of the
principal global geological phenomena. In the continuing
restructuring processes of the Earth’s interior, water has been
added incessantly (but episodic) to the surface, while hydrous
fluids have also played a central role in breaking down the
original thick pan-global crust – progressively forming ever
deeper ocean basins. Vertical uplifts and subsidence of the
evolving deep sea crust, in association with continental
transgression-regression cyclicity, are natural consequences of
a slowly degassing Earth. Thus, the unceasing transformation of
the felsic crust is intimately tied to the history of seawater –
along with a multitude of first-order geological, geophysical,
environmental and biological occurrences.
Keywords: Origin of seawater, sea-level changes, dynamic
drivers, geological history
(Received on 6 December 2016. Accepted on 31 December 2016)
Problem outline
o a large extent, the history of the Earth’s dynamo-tectonic
development is related to the origin of the oceanic water masses
and their surface oscillations – characterized by the advances
and retreats of epicontinental oceans. During major parts of
post-Precambrian time, the present land surface was extensively
covered by shallow seas, while today the continents are dryer
than at any time during the last 570 million years
(Phanerozoic). During the late Mesozoic, the continental
flooding was nearly as widespread as that of the Lower-Middle
Palaeozoic, though the highest sea-level may not have been
higher than 200-400 metres above the present shore line (cf.
Miller et al., 2005). “Today, a similar rise would inundate less
than half the area that was flooded in the Cretaceous, because
our continents stand high above the sea, whereas the Mesozoic
lands were low and flat” (van Andel, 1985). The same low and
flat continents were apparently the norm during the Palaeozoic
as well as in Precambrian time; the elevation of our continents
and continental mountain chains, as well as the mid-ocean
ridges, seems to have a quite recent origin – having basically
occurred during the last 5 million years of Earth history (cf.
Storetvedt, 2015 for references and a compilation of evidence).
Fluctuations in global sea-level result either from changes of
the volume of sea water on the planet’s surface or of deep sea
basins. Today, the growth and decay of major continental ice
fields are thought to be the most likely causal mechanism for
sea-level changes, but vertical oscillations of the deep sea
crust, caused by variable rates of hypothesized seafloor
spreading, has also been proposed – but without real success.
More recently, attempts have been made to relate sea-level
changes to climatic control (Miller et al., 2005; Zhang, 2005),
but none of these ideas seem to account satisfactorily for the
large number of recorded sea-level changes – with time scales
varying from long-term super-cycles over hundreds of millions of
years to rapid changes in the order of tens of thousands of
years or less. Modern compilations of Phanerozoic sea-level
trends have been given by a number of authors (e.g., Haq et al.,
1987; Hardenbol et al., 1998; Haq and Shutter, 2008), but the
description the overall sea-level pattern has not changed
significantly since the early work of Sloss (1963) and Vail et
al. (1977).
It is natural to think that seawater is intimately associated
with the Earth’s internal chemical reconstitution and degassing,
but when did the bulk of surface water accumulate? During
Precambrian times, there is no factual evidence for the
existence of deep sea basins, and the volume of surface water
was apparently modest – but there is ample evidence of
deposition in shallow marine waters within greenstone belts
(Figure 1) along with indications of fluvial activity (cf.
Windley, 1977). Perhaps an appropriate description of the
surface conditions in the late Precambrian can be unveiled by
the Grand Canyon sedimentary system – as described by Dunbar
(1949, p. 93-94):
“The Grand Canyon system is essentially unmetamorphosed, thus
contrasting in the most striking manner with the underlying
schists, which must be vastly older. The system begins with a
basal conglomerate resting on a peneplaned surface of the Vishnu
schists. Following this come limestone and then limy shale and
sandy shale and quartzites. The limestones, and probably a
larger part of the shales and quartzites, were deposited in
shallow marine water, but parts of the sandy shale and sandstone
are bright red and are so commonly mud **** as to suggest
deposition on a broad floodplain. The region was probably part
of a great delta plain in which submarine and subaerial
deposition alternated. And since these strata were formed near
sea-level, the region obviously subsided slowly [...] while
deposition was in progress.”
The Archaean aeon, which was characterized by features such as
the relative abundance of komatiite extrusions and a relative
scarcity of redbeds and carbonates, was succeeded by the much
more diversified geological record of the Proterozoic –
progressively distinguished by large sedimentary basins with
primitive living forms more abundantly recorded in surface
carbonates. Contrasting strongly with the Proterozoic situation,
the Cambrian experienced a general “transgression onto cratons,
with a classic orthoquartzite-to-shale sequence resting
unconformably on Precambrian and overlain by carbonates” (Hallam
1992). And suddenly, a diversity of complex life forms,
dominated by trilobites and brachiopods, appear in abundance at
the base of the Cambrian; this remarkable biological explosion
was probably a direct consequence of the rapidly increasing
volume of surface water. Thus began a major Lower-Middle
Palaeozoic submergence of the apparently flat and low-lying
continental masses that was accompanied by a rapid development
of sea-living creatures. According to Dunbar (1949, p. 155),
“the Early Cambrian oceans seem to have been somewhat openly
connected, so that intermigration was easy and the leading types
of life are much alike in various parts of the world”. Thus, the
Cambrian eustatic transgression probably represents the first
major supply of water to the Earth’s surface – degassed from the
interior of the Earth; the explosive prevalence of marine fauna
at that time is likely to have been a consequence.
Figure 1. Depiction of a transect across a block of late
Archaean greenstone belts – subsiding rift basins developing
along one of the pre-existing orthogonal fracture systems, with
associated volcanism and shallow water sedimentation. Diagram is
based on Cloos (1939).
For post-Precambrian time – ranging from 570 My to the Present,
the stratigraphic record is generally well exposed due to the
fact that epicontinental seas repeatedly covered substantial
parts of the present land surface. Based on geological maps,
depicting the distribution of shallow marine deposits, it is
possible to
evaluate the fluctuations of sea level with time. However,
compared with younger geological periods, Cambrian stratigraphy
is poorly known so, for these early times, eustatic correlations
of sea level are rarely possible (Hallam, 1992). Nevertheless,
since the late 19th century, a cogent picture has emerged that
suggests that during post-Precambrian time, significant regions
of the present continents have repeatedly been engulfed by
longer term shallow seas, despite the fact that the present
volume of marine surface water seemingly is larger now than ever
before (cf. Storetvedt, 2003).
Traditionally, it has been accepted that the major proportion of
the Earth’s seawater is the product of an early stage internal
differentiation and degassing while the planet was in a much
hotter state than now – giving rise to the hypothesis that the
Precambrian Earth was blanketed by a more or less pan-global
ocean concealing a rather featureless granitic crust (Süss,
1893). However, Rubey (1951) argued that the greater part of the
surface water is unlikely to have been exhaled through processes
of primordial segregation, it being more likely to have
accumulated through slow but progressive degassing via volcanic
action. He argued that the widespread epicontinental seas of the
past should not be confused with the aspect of seawater volume,
because “if the ocean basins have been sinking relative to the
continental blocks, then one must look largely to the ocean
floor, rather than to the continents, for evidence of a growing
volume of seawater”. In other words, Rubey was apparently
adhering to the crustal oceanization model of Barrell (1927).
Following Rubey’s reasoning, the formation of the world’s ocean
basins by internal mechanisms would be associated with a major
release of juvenile water. The fact that to some extent the
Earth continues to be volcanically active leads to the
conclusion that, unless there is a mechanism by which water can
recirculate back into the mantle – for which there is hardly any
evidence, the volume of surface water is larger now than ever
before despite the fact that the present continental landmasses
are significantly dryer now than during earlier Phanerozoic
history.
However, this long-term drying-up of the continents has been
discontinuous. Thus, major eustatic inundations during the
Lower-Middle Palaeozoic were followed by a sharp drop of the sea
level in the late Permian, and then another major inundation
resumed during the Mesozoic – culminating in the late
Cretaceous. On top of this mega-scale sea-level trend, it was
recognized early on that many shorter fluctuations of the shore
line had been superimposed on the longer trends. This Pulse of
the Earth – through its intimate association with the history of
surface water and the record of oscillating sea-level changes –
is the theme of this paper.
The level of the sea will be lowered when a substantial volume
of surface water is being stored in major ice fields, as during
the Quaternary ice age of the Northern Hemisphere. But during
post-Precambrian time, such global cold spells seems to have
been rare and of too short duration and the ice fields of too
limited an extent, to have had an appreciable effect on the
longer term trend of the global sea level. In this context, van
Andel (1985, p. 155) wrote: “The rate of sea-level change for
glaciations and deglaciations is measured in metres per 1,000
years, much too fast [for explaining Phanerozoic sea-level
variations], and we are quite certain that there were no ice
ages during the Mesozoic. Clearly, non-glacial eustatic changes
cannot be explained by changing the volume of water”. The
recorded inundations (transgressions) of the sea could be
equated with either an increasing volume of seawater or sinking
of the land, and sea-level retreats (regressions) with the
reverse. However, the history of seawater and the pulsating
eustatic sea-level stand seems to be closely associated with the
rest of the Earth’s evolutionary pattern – including the
episodicity of principal global tectonic events along with the
associated diversity of geological and environmental phenomena.
In other words, any sensible explanation of seawater and
sea-level oscillations demands a realistic system understanding
of Earth’s geological history.
Historical snapshots
Around the middle of the 19th century, it was commonly thought
that the Earth had cooled and chemically differentiated from an
original fluid magmatic state, and that seawater was a natural
product of the planet’s primeval de-volatilization. In the view
of the Austrian geologist Eduard Süss [1831-1914], the expelled
water had originally spread across a fairly featureless crustal
surface, but progressive cooling and planetary contraction had
produced crustal warping and fracturing leading to
redistribution of the surface water. Consistent with this view,
Süss (1893) conceived of an early Precambrian Earth covered by a
shallow pan-global ocean – an idea which Alfred Wegener later
took for granted (Wegener, 1912 and 1924). In Süss’ opinion,
continental and oceanic crust were compositionally similar and
interchangeable; he opined that during the presumed planetary
contraction, large areas of the surface had collapsed to become
deep sea depressions into which water masses previously residing
within the continental crust had drained. The volume of surface
water was considered to be constant, but it was well known that
the sea level relative to the continents had displayed rhythmic
variations: the sea advancing over low-lying lands –
transgressions, had alternated with sea-level retreats –
regressions, forming a pulsating global shore line succession
for which Eduard Süss coined the term eustatic. His dynamic
driving force was the Earth’s cooling and contraction.
In the Süssian tectonic system, the oceans had been growing for
a long time (i.e. an increasing part of the global crust had
been down-warped by the forces of thermal contraction) at the
expense of upstanding continents, thereby accounting for the
geological fact that the land masses – with their extensive
cover of ancient marine sediments – had been subjected to an
overall progressive drying-up during post-Precambrian time.
However, the common transgressive pulses were explained by a net
reduction in storage capacity of the developing oceanic basins –
attributed to the accumulation of transported terrigenous
material from surrounding regions of the crust. Regressions, on
the other hand, were ascribed to the increasing volume of the
deep sea basins formed as a consequence of contraction. Thus,
transgression and regressions were ascribed to different and
seemingly independent causes – which therefore smacked of an ad
hoc escape. Furthermore, the generally slow build-up of the
transgressive phases, as compared to the much more rapid
regressions, was an even greater problem for the Süssian theory.
The distribution of land and sea in the Upper Palaeozoic,
according to Eduard Süss, is shown in Figure 2. Süss postulated
the Gondwana palaeo-continent – an ancient mega-continent that
in the late Palaeozoic had united all southern land masses. When
a major part of Gondwana subsequently subsided (by the presumed
forces arising from planetary contraction), previous biological
migration routes had been broken. In this way, the Süssian
theory – representing vertical contraction-enforced oscillations
of the crust – could explain biological similarities between
continents now widely separated by oceanic barriers, for which
the competing American contraction model of James Dana (1873 and
1881) offered no solution. Dana’s contraction hypothesis was
strongly discrepant with that of Eduard Süss. According to Dana
the overall physical state of the Earth had not changed
significantly during most of its history: its internal state and
surface structures were assumed to be static, including the
configuration of continents and deep sea basins.
The ultimate product of the American version of the contraction
theory was a very slow- to-moderate episodic growth of the
continents through accretion along their margins. A central
theme in Dana’s model was the formation and episodic deformation
of fold belts; in his view, incessant crustal contraction had
produced recurrent down-warping, sedimentary accumulation,
compression, and then uplift. But why were the Rocky Mountains
located so far inland, and how had the intracontinental
Alpine-Himalaya tectonic belt formed? With regard to the
long-standing problems of the widespread marine deposits
blanketing the continents, Dana suggested that the primeval
oceans had been too shallow to accommodate the expelled
primordial water masses, implying that the present lands, in
their early history, had been submerged by epeiric seas which
had then drained into the subsequently-formed deep sea basins.
But this proposition did not readily fit such geological facts
as that the Lower-Middle Palaeozoic marine deposits blanketed
significant parts of North America, the extensive and
long-lasting Tethyan Sea had been a characteristic feature
across southern Eurasia for most of post-Precambrian time, and
the late Cretaceous (‘Cenomanian’) transgression had apparently
covered considerable parts of the continents (discussed later).
It seemed, therefore, that the North American contraction-based
evolutionary scheme was unable to account even for most
prominent surface geological features.
GONDWANALAND
Figure 2. A sketch of the suggested distribution of land (white)
and sea (light blue) in the Upper Palaeozoic – according to the
palaeogeographic model of Eduard Süss; in this synthesis,
Gondwana was an Upper Palaeozoic southern land mass which,
during subsequent contraction and vertical crustal down-warping,
had been turned into the present ocean-continent arrangement.
Note also the extensive intracontinental seaway, running E-W
across southern Eurasia and northern Africa, which Süss (1893)
named Tethys.
It became evident that the contraction theories did not provide
satisfactory explanations for the uneven distribution and global
tectonic interrelationships of the various tectonic belts. Thus,
even more than a century ago, the time was ripe for re-thinking
the basic concepts. Essential aspects like the origin of the
oceanic water masses, eustatic sea-level fluctuations, along
with their natural link-ups with other prominent geological
phenomena, such as tectonic belt formation – a necessity for a
functional global geological theory, had no ready explanation.
In many ways, geology was, and still basically is, a
fact-gathering enterprise without a realistic and functional
global mechanism. With respect to the state-of-the-art at the
end of the 19th century, the following short-list of principal
problems may suffice to explain the lack of a satisfactory
explanation for the geological facts as seen at that time
(Storetvedt, 2003):
# The extensive periodic flooding and subsequent long-term
draining of the land masses in post-Precambrian time, that left
behind a blanket of shallow marine sediments, had no
satisfactory explanation.
# Recurrent, but variable, sea-level fluctuations were well
established, but the origin of the internal processes that
produced these transgression-regression cycles, and how these
sea-level pulses tied to the overall long-term draining of the
present continents, remained unknown.
# Periods of transgression were much longer than the relatively
short and distinct periods of regression. What was the cause of
this discrepancy?
# A genetic relationship between oceanic depressions and
high-standing continents was likely, but how was this connection
to be understood and explained?
# The Earth’s hydrosphere had either formed during its early
history or accumulated progressively, through internal degassing
and volcanic action, since the birth of the planet. But if the
Earth began as a red-hot molten body, as was commonly taken for
granted, would it not be reasonable to think that degassed light
hydrogen and hot water vapour would largely have escaped into
space?
# In general, palaeo-biological problems were inexplicable
within the context of the present-day continental configuration.
Any functional global theory had to account for faunal and
floral similarities between continents now separated by deep
oceanic barriers, in addition to cases of endemism.
# It was gradually realized that major mountain chains had
formed in very recent geological time, regardless of the ages of
underlying tectonic disturbances. So were the deformation of
pre-existing sedimentary troughs (geosynclines) and their fairly
recent topographic uplift really closely connected phenomena –
as had been commonly assumed?
# During post-Precambrian times, the climatic zones had had
quite different orientations from those of today. In extreme
cases, the present polar regions had been tropical and vice
versa. What dynamic mechanism could have caused this profound
shift of the global climate system?
# If the shifts of climate belts were of global extent, the old
speculation of changes of the Earth’s body relative to the Sun
(a notion now called True Polar Wandering), originally discussed
by the famous German philosopher Johann Gottfried von Herder (in
1785: see Schwarzbach 1963), would gain strong evidence in its
favour. Furthermore, if spatial changes of the Earth’s mass were
a reality, how would its ellipsoidal shape affect sea-level
oscillations across the globe?
# The Caledonian, Hercynian and Alpine tectonic belts running
across Eurasia form a southward progression with decreasing age
– probably defining globe-encircling great-circle structures.
What was the cause of this dynamo-tectonic shift, and how was it
connected with the rest of Earth history – including aspects
such as (1) the relatively short-lived geological cataclysms
characterizing the principal geological time boundaries and (2)
the predominant sea-level super-cycles, with their superimposed
shorter period transgression-regression cyclicity?
As a result of the multitude of unsolved problems, central
European geophysicists began to explore new directions in global
tectonics. By integrating palaeo-climatology and geophysics, the
old notion of True Polar Wandering was substantiated – notably
by Kreichgauer (1902), while arguments in favour of continental
mobility were expounded. Thus, Damian Kreichgauer argued for a
westward rotation of the whole crust without altering the
relative continental positions, and Wettstein (1880) followed
Eduard Süss by suggesting that deep sea basins were sunken parts
of former land masses. Kreichgauer was apparently the first to
suggest a close dynamic link between tectono-magmatic belts and
the Earth’s rotation; later, Wegener (1912, 1915 and 1929) gave
Kreichgauer the credit for having discovered the pole-fleeing
force (later named the Eøtvøs force). Wegener followed
Kreichgauer by postulating the tectonic effect from the tidal
torques from the Sun and Moon – the pole-fleeing force
(Pohlflucht) and the Coriolis Effect as possible driving
mechanisms; these forces are indeed directed westward and
towards the time-equivalent equator. However, the vast
global-extent invasion of epicontinental seas during a major
part of the Palaeozoic and then again during the Upper Mesozoic
remained a puzzle for both Wegener and other central European
geophysicists.
In his discussion of polar wandering and its possible
consequences for sea-level changes, Wegener (1929, p. 159) wrote
that “Many authors […] have already discussed the fact that
internal axial shifts must be tied up with systematic
transgression cycles; this is because the earth is ellipsoidal
and because there is a time lag while it adjusts itself to the
new position of the axis, whereas the sea follows at once. Since
the ocean follows immediately any re-orientation of the
equatorial bulge, but the earth does not, then in the quadrant
in front of the wandering pole increasing regression or
formation of dry land prevails; in the quadrant behind,
increasing transgression or inundation [is the consequence]”.
Thus, Wegener interpreted sea-level changes as being intimately
tied to resettings of the equatorial bulge; however, in his
scheme, transgressions and regressions did not affect all
continents simultaneously – they were quadrant-dependent. This
view markedly contradicted the stratigraphic observations which
Eduard Süss and later workers regarded as evidence for eustatic
(global) sea-level changes.
During the following decades, a number of prominent geologists
paid special attention to the chronological distribution of
long-term changes of sea-level at variable scales (e.g.,
Barrell, 1917; Stille, 1924; Joly, 1925; Bucher, 1933; Umbgrove,
1939). Thus, Umbgrove (1942) wrote that “a great number of major
transgressions took place each being separated by periods of
widespread emersion [sic] of the continents. There was a
rhythmic advance and retreat of the sea. We can therefore only
conclude that the transgressions and regressions on the
continents must be ascribed solely to a world-embracing cause.
Stille expressed the synchronism of the great trans- and
regressions in his law of epeirogenic synchronism, which Bucher
formulated as follows: – “In a large way the major movements of
the strandline, positive and negative, have affected all
continents in the same sense at the same time”.
Though Umbgrove proposed that the eustatic movements were a
major rhythmic phenomenon throughout post-Precambrian time, the
cause of these oscillating motions were referred to unspecified
vertical pulsation processes in the mantle. In addition to the
global cyclicity and synchronicity, it had been known, since the
time of Eduard Süss that superimposed on the ‘first order’
post-Precambrian eustatic changes there were regional-scale
movements of the strandline. As we have seen above, Rubey (1951)
took an unconventional look at this problem suggesting that the
hydrosphere had been exhaled by episodic internal processes in
connection with sub-crustal thinning of continental crust thus
trending towards an oceanic
mode – an idea closely related to the oceanization model of
Barrell (1927). However, sedimentation on the ocean floor has
not been continuous; numerous Deep Sea Drilling Project (DSDP)
cores show that sedimentation and erosion are typically episodic
phenomena. Thus, Rona (1973) described hiatuses of up to tens of
millions of years in the late Mesozoic to Middle Tertiary
stratigraphic record of every principal ocean basin – expressed
by intervals of non-deposition and/or erosion, which he
tentatively associated with the transgression-regression
cyclicity on shallow continental crust. Nevertheless, the
ultimate question remains: which dynamo-tectonic mechanism
stands behind the eustatic sea-level changes and the associated
multitude of episodic surface geological phenomena?
An intermittently degassing Earth
Against the prevailing view of the 19th century – of an
initially hot and molten planet, there was indeed considerable
surface evidence for the presence of an assortment of discharged
internal gases – discussed by authors like Reyer (1877),
Guenther (1897) and Chamberlin (1897). The enormous gas blowouts
of the 19th century – Mt. Tambura in 1815 and Krakatoa in 1883 –
may have been reminders in this respect. Chamberlin (1897) –
struggling with the many unsolved problems in global geology –
took a completely new starting point by proposing that the
terrestrial planets had formed by aggregation of rocky dust
particles. He suggested that the early Earth probably began as a
very cold body (with temperatures near 0˚K) which
subsequently, as a consequence of entrapped radioactive
materials, gradually heated up. On this basis, the solid
material of an initial cold Earth could well have maintained at
least part of its primordial heterogeneity and, therefore, could
still be in a state of internal differentiation with associated
degassing. Adding to this untraditional view, Hixon (1920)
suggested that tectonic processes were diapiric phenomena caused
by the release of internal gases, and Ampferer (1944) discussed
the possibility of subsurface gas pressure powering vertical
tectonic processes. In addition, the closely related theory of
Earth Pulsation (e.g., Stille, 1924; Bucher, 1933), implying
distinct global and synchronous tectonic events alternating with
much longer periods of tranquility, was reiterated by Umbgrove
(1942 and 1947).
Cosmo-chemist and planetologist Harold Urey (Urey, 1952) – the
1934 Nobel laureate in chemistry for the discovery of deuterium
– restated the old view of Pierre-Simon Laplace and Immanuel
Kant (late 1700s) that the planetary system had formed by
aggregation of material from a flattened nebular disk
surrounding the Sun, comprising a cold mix of predominantly
hydrogen gas and particulate matter. On this basis, Urey argued
that differentiation of the Earth, into a metallic core and
silicate shells, could well be incomplete and therefore still in
progress. Consistent with this thinking, Turekian (1977) argued
that the present volume of surface water is considerable less
than what might be expected if all water had been driven off –
that is, if the Earth at an early state had been a hot molten
body. Karl Turekian pointed out that, if the chemical
composition of the original mantle was like that of average
carbonaceous chondrites (and the early Earth in a hot molten
state) – as is generally believed, the surface should contain at
least 20 times more water than is presently the case.
It can be envisaged that continuous planetary degassing and
related reorganization of the Earth’s interior mass has modified
both the internal and the outer regions of the Earth
progressively since early Archaean time – transforming an
initially thick proto-crust as well as progressively, and
episodically, increasing the volume of surface water (cf.
Storetvedt, 2003 and 2011). The gradual accumulation of fluids
and gases in the upper mantle and lower crust must have led to a
considerable increase in the confining pressure at these levels.
At each depth level, rocks and fluids would naturally be subject
to a common pressure – producing a kind of high pressure vessel
situation – with fractures being kept open just like those in
near-surface rocks at low pressures (Gold’s pore theory, see
Hoyle, 1955; Gold, 1999). This principle is well demonstrated in
the Kola and KTB (S. Germany) deep continental boreholes (which
reached maximum depths of 12 and 9 km, respectively) where open
fractures filled with hydrous fluids were found throughout the
entire sections drilled (e.g., Möller et al., 1997; Smithson et
al., 2000); brines were seen to coexist with crustal rocks and,
in the KTB site, the salinity of the formation water turned out
to be about twice that of present-day normal sea water (Möller
et al., 2005). In both drill sites, a variety of dissolved gases
and fluids was found; primitive helium was observed at different
depth levels indicating that the fluids were of deep interior
origin (Smithson et al., 2000). As there is no observational
evidence that deep oceanic depressions existed prior to the
middle-late Mesozoic (see below), the bulk of present-day
surface water must, in fact, have been exhaled from the deep
interior during later stages of the Earth’s history.
Nevertheless, there are reasons for believing that most of the
planet’s water is still residing in the deep interior.
In view of the extremely limited information on the physical
state of rocks even at shallow depths, modern studies of the
Earth’s internal constitution must rely on geophysical inversion
techniques, based primarily on seismological and geodetic
observations, supplemented by high-temperature, high-pressure
mineral physics and chemistry experiments. Nonetheless,
inversion techniques have no unique solutions so inferences
about the planet’s inner state and chemical constitution must
necessarily be strongly model-dependent – resting on
hypothetical scenarios of primordial accretion, temperature
development, and mass/energy transfer processes. Therefore, to a
large extent, the picture of the Earth’s interior has changed
according to the needs of whatever particular theories have
been/are invoked to explain surface geological phenomena.
Regrettably, purely speculative ideas from time to time have
become immaculate facts in all the sciences, and so it has been
with regard to the interior of the Earth. For example, in recent
decades deep continental drilling (Kola and KTB, S. Germany) has
demonstrated that the physico-chemical constitution and
structural state at even near-surface levels differ markedly
from long-held conventional views – albeit without having had
any noticeable effect on currently ingrained and popular views
(cf. Storetvedt, 2013). Or as expressed by Wilfred Trotter
(Trotter, 1941): “a little self-examination tells us pretty
easily how deeply rooted in the mind is the fear of the new”.
If we accept that the Earth formed by aggregation of cold gases
and rocky dust particles, the early planet must have been left
in a relatively undifferentiated state. It follows that chemical
elements must have experienced differentiation as the body
evolved toward some lower-energy state. Within the gas-filled
proto-planet, incremental coalescence of ferromagnetic
planetesimals can be expected to have led to heavier concretions
for which the gravitational influence outbalanced the
centrifugal effect. Thus, the heavier Fe-rich masses settled
inwards through the relatively less dense (presumably) gaseous
mass – gradually building up a high-density central core (see
Tunyi et al., 2001). As lighter elements like sulphur, carbon,
silicon, hydrogen and oxygen easily dissolve in high-pressure
metallic mixes (cf. Stephenson, 1981; Hunt et al., 1992; Okuchi,
1997), such lighter constituents can be expected to have
followed iron alloys into the core giving rise to the
well-established density deficit of the central body. According
to Gottfried (1990), the core must be the host of a significant
amount of hydride-metal compounds while the present
silicate-rich lower mantle must include an appreciable volume of
silicides – notably, silicon carbide. According to Stevenson
(1981) and many others, the core is not in equilibrium with the
mantle, and the presence of an irregular ‘topography’ of the
core-mantle boundary (CMB) region (cf. Morelli & Dziewonski,
1987) gives further evidence of a thermo-chemically active and
heterogeneous zone. It follows that the CMB region may represent
the fundamental trigger of endogenous energy – this eventually
leading to the observed range of geodynamic and surface
geological phenomena – including surface accumulation of water.
Lighter elements, originally entrapped in the relatively cold
(but slowly heating up) deep interior must have begun their
upward voyage at an early stage – necessarily taking part in a
number of phase changes en route. With the many lighter elements
now regarded as possible constituents of the deep Earth (cf.
Storetvedt, 2003 for references and discussion), it is of
paramount importance to consider the geodynamic and geological
consequences of buoyant volatiles – including a range of
hydrocarbon compounds which may provide the most important
mechanisms for internal mass transfer (Gold, 1979 and 1999). For
a planet undergoing irregularly-distributed degassing (both
temporally and spatially), one would expect lateral variations
of density arising from temperature differences, irregular
fracture distribution, and compositional heterogeneities. It is
significant that sub-oceanic and sub-continental mantle sections
display a relatively clear seismic difference – notably in the
outer few hundred kilometres (e.g., Dziewonski 1984; Dziewonski
and Woodhouse, 1987; Forte et al., 1995).
An important observation in this respect is that, when projected
onto the Earth’s surface, upstanding regions of the CMB
correspond to deep oceanic basins. Figure 3 demonstrates this
CMB-planetary surface relationship – suggesting that processes
at the outer core release energy and buoyant masses that on the
surface have led to the formation of deep sea basins (see
Morelli and Dziewonski, 1987) as well as, apparently, the whole
range of principal geodynamic and surface geological phenomena
(Storetvedt, 2003). Ruditch (1990), studying the distribution of
shallow water sediments in more than 400 deep sea drill holes in
the Atlantic, Indian and Pacific oceans, submitted that, since
the Jurassic, oceanic depressions have formed as a result of
large-scale chemical transformation and subsidence of an initial
thick continental crust; he argued that the world oceans had
evolved from separate and initially isolated basins – like those
currently observed on the continents.
Figure 3. The diagram illustrates the estimated topography of
the core-mantle boundary region obtained by PcP and PKP
residuals combined – simplified after Morelli & Dziewonski
(1987). Note that when projected onto the Earth’s surface, the
upstanding regions of the core-mantle interphase (cf. coloured
scale) correspond to deep oceanic depressions.
The fact that deep oceanic depressions apparently did not exist
prior to the late Mesozoic and that most seawater seems to have
accumulated during late Phanerozoic time suggests that both
planetary outgassing and the vertical transfer of internal mass
have been extremely slow – albeit markedly accelerating during
the Mesozoic. The irregular CMB topography, as outlined by
Morelli and Dziewonski, suggests that the core-mantle boundary
zone is a thermo-chemically active and heterogeneous region.
Whatever buoyant phases arise from the CMB region, the
implications of the broad regions of diapiric upwelling, aided
by hydrocarbons and hydrous fluids, are crustal thinning –
through eclogite formation and associated gravity-driven
delamination of the crust from its base upward. Hence, isostatic
subsidence and development of surface depressions would ensue.
Eclogitization commonly propagates along fractures and shear
zones, and the metasomatic front often defines bands of eclogite
trending along fractures – showing an abrupt transition from
granulite to eclogite facies. Granted the availability of
sufficient hydrous fluid, and with pressure conditions being
satisfied, the reaction to eclogite will predictably proceed
rapidly (Austrheim et al., 1996).
It has been demonstrated that natural occurrences of the
granulite-to-eclogite transition are strongly impeded when
hydrous fluids are absent (e.g., Austrheim, 1987 and 1990;
Walther, 1994; Leech, 2001; Austrheim et al., 1997). Thus,
Austrheim (1998) argues that hydrous fluids are much more
important than either temperature or pressure, and Leech (2001)
concluded that gravity-driven sub-crustal delamination (through
eclogite formation) is strongly controlled by the availability
of water. According to Austrheim et al. (1997), the
eclogitization process brings about material weakening which
make eclogites deform more easily than their protoliths – the
degree of deformability being further increased in the presence
of water. Thus, the large density increase consequent upon
eclogitization destabilizes the lower crust and makes it detach
from the relatively unaffected crust above (Leech, 2001). Figure
4 gives an illustration of this sub-crustal thinning process –
advancing upward and eventually forming deep sea basins.
Figure 4. Geological interpretation of a N-S seismic profile
across the North Pyrenean Fault Zone of the inner Bay of Biscay.
Gravity-driven eclogitized lower crust delaminates from the
lower crust and sinks into the upper mantle giving rise to the
Parentis Basin. Illustration is a simplified version after Pinet
et al. (1987). It is suggested that during Earth history a
presumed thick proto-crust has been progressively thinned and
chemically transformed – gradually implanting the present Moho
interface.
Throughout its history, the Earth must have lacked
thermochemical equilibrium, so in the process of reaching
internal stability, mass reorganization – aided by buoyant
volatiles – seems to have been at work to produce a
progressively evolutionary course of crustal thinning and
intermittent geological activity along with episodic
accumulation of the present volume of seawater (cf. Storetvedt,
2003). The discharge rate of juvenile water seems to have
accelerated greatly in Cretaceous and Tertiary times. Though
shallow seas may have existed in the Precambrian, Truswell and
Eriksson (1975) have argued that their tidal amplitudes were
only modest.
As a consequence of the Earth’s degassing and associated
internal mass reorganization, changes of its moment of inertia
would be a natural consequence – producing secular changes of
the globe’s rate of rotation as well as episodic, but generally
progressive, changes of its spatial orientation (true polar
wander). A method for studying the Earth’s spin rate (length of
day, L.O.D.) for the geological past was introduced by Wells
(1963 and 1970): by counting presumed growth increments in
recent and fossil corals, he estimated the number of days per
year back to the Lower Palaeozoic. A famous result from this
study was that Middle Devonian corals gave some 400 daily growth
lines per year – suggesting a pronounced slowing of the Earth’s
spin rate over the past 380 million years. Subsequent studies of
skeletal increments in marine fossils back to the Ordovician
were generally consistent with a higher rotation rate also in
the Lower Palaeozoic (Pannella et al., 1968). Creer (1975) and
Whyte (1977) summarized the palaeontological length of day data
available by the mid-1970. Figure 5 shows the graph of presumed
number of days during post-Precambrian time given by Creer. A
subsequent compilatory L.O.D. study by Williams (1989) gave
closely similar results – in addition to presenting fossil clock
data for the Mesozoic. More recently, a study by Rosenberg
(1997) concluded that at Grenville time (some 900 million years
ago) the year had 440 days.
From the zig-zag appearance of Figure 5, it is remarkable how
closely the established break-points of the L.O.D. curve
(numbered 1-4) – separating periods of deceleration from periods
of acceleration – correspond to times of global tectonic events.
These tectonic revolutions are: 1, the Alpine climax at around
the Cretaceous-Tertiary boundary; 2, the Appalachian-Palatinian
event near the Permian-Triassic boundary; 3, the late Devonian
Acadian disturbance; and 4, the late Ordovician Taconian event.
As will be outlined below, the inferred close relationship
between changes in the Earth’s rotation and global tectonics is
additionally associated intimately with prominent regressive
sea-level events and biotic mass extinctions.
Phanerozoic sea-level changes
The volume of sea water in the late Precambrian has remained
speculative, and relatively little is known about marine
stratigraphy and eustasy in the early Cambrian. Nevertheless, a
general Cambrian transgression onto progressively drowned
cratons (Matthews and Cowie, 1979) begins with a classic
orthoquartzite-to-shale succession followed by carbonites (cf.
Hallam, 1992 and references therein). From the modest sea water
incursion in the early Cambrian, the late Cambrian
epicontinental coverage of North America had increased by some
75 %, while in the late Ordovician to Middle Silurian the
shallow sea had enlarged to around 90 % or more (Dott and
Batten, 1976; Dott and Prothero, 1994). Thereafter, sea level
fell gradually to even below its present level at around the
Permian-Triassic boundary. Figure 6 shows the global
distribution of the Lower Silurian epicontinental seas.
Figure 5. Compilation of presumed days per month during the
Phanerozoic – based on growth rings in fossil shells –
simplified after Creer (1975). Numbers refer to break-points
which in turn represent prominent tectonic events corresponding
to the principal geological time boundaries.
Figure 6. A sketch map of the overall distribution of Lower
Silurian epicontinental seas (blue) superposed on the current
land masses. Note the relatively modest areal extent of dry land
(green). Due to the low and fairly flat global surface, the
overall shallow-water cosmopolitan faunas were widespread. The
diagram is simplified after Boucot and Johnson (1973). At that
time, the present oceanic domains are likely to have had thick
continental crust so these regions too are likely to have been
dominated by shallow epicontinental seas (cf. Storetvedt, 2003).
Cambrian stratigraphy is poorly known, and so are eustatic
sea-level variations during that era (cf. Hallam, 1992), though
a widely accepted transgression onto cratons is demonstrated by
the Exxon sea level curve (see below, and Figure 7).
Illuminating studies in North America (Bond et al., 1988) show
consistent sea-level changes for certain specific regions: in
North America, an overall eustatic rise in the Cambrian-early
Ordovician is followed by a marked sea-level fall in
Ordovician-Silurian time. The progressive Cambrian flooding of
the cratons probably represents the first major influx of water
to the Earth’s surface (as a result of degassing from the
interior) – the principal factor behind the explosion of marine
life at that time. In addition, world maps of the maximum degree
of shallow marine inundation (Strakhov, 1948; Termier and
Termier, 1952) demonstrated a similar eustatic high sea-level
during the Lower-Middle Palaeozoic.
The more detailed sea-level curve of the Exxon group (Vail et
al., 1977), based on onshore North American data, gave five
asymmetric sea-level cycles – each representing a relatively
slow transgression followed by a sharp basin deepening and a
related regressive event. Figure 7, showing the Exxon sea-level
curve for the Palaeozoic based on North American sequence
stratigraphy, demonstrates an obvious oblique saw-tooth-shaped
sea-level variation from the Silurian onwards, and an overall
regression culminates in a marked Permian low-stand. In an
attempt to eliminate any regional tectonic effects, Hallam
(1992) proposed a generalized eustatic sea-level curve as
depicted in Figure 8. For the time range concerned, the two
curves are remarkably similar.
Figure 7. The Palaeozoic section of the Exxon sea-level curve –
after Vail et al. (1977). Note the sharp regressive events
compared with the preceding and slower transgressive periods,
and the overall progressive continental draining after Silurian
time.
Figure 8. Generalized eustatic sea-level variations for the
Phanerozoic – after Hallam (1992). Star symbols mark the six
principal events of marine extinctions; note that these biotic
catastrophes correspond to times of sea-level minima (distinct
regressive events).
On the basis of a progressively degassing Earth, the inferred
reorganization of the internal mass would have dynamic
implications – periodically altering the planet’s moment of
inertia producing events of polar wander and variations in spin
rate (Storetvedt, 1997, 2003 and 2011).
These intermittent changes of planetary dynamics would naturally
affect the inventory of gasses and volatiles accumulated at the
outer levels of the Earth and trigger a range of
tectono-magmatic and surface environmental processes – including
crustal transformation and variations in the mass distribution
of seawater. This interlinking of geological phenomena,
influencing the Earth’s progressive, variegated and episodic
history, is the cornerstone of my Global Wrench Tectonics
theory. By postulating the proto-Earth as a relatively cold and,
hence, rather undifferentiated planetary body (cf. Storetvedt,
2011), its early history could be expected to have encompassed
slow volatilization that progressively would have added gases
and fluids to the developing upper mantle and crust, as well as
the hydrosphere and atmosphere – besides continuously changing
the planet’s internal constitution. In this way, geological
evolution as well as the Earth’s seawater history became
intimately associated with intermittent changes in planetary
rotation which, in the surface record, is expressed by
stratigraphic upheavals seen between the major geological time
boundaries.
Volatiles have a high vapour pressure so, if they are
incorporated into solid or liquid material during their
transport outwards, they will have a tendency to escape, atom by
atom, from their host compounds thereby increasing the local gas
pressure: at near-surface levels, the gases contributing to the
enhanced pressure may include methane and other alkanes, carbon
dioxide, carbon monoxide, hydrogen sulphide, hydrogen, nitrogen,
helium, and water – as vapour (see summaries by Gold, 1987 and
1999). Thus, the continuing build-up of pressure from volatiles
in the outer levels of the Earth can be predicted to have
triggered eclogitization and associated gravity-driven
sub-crustal attenuation, giving rise to isostatic subsidence and
basin formation. This process naturally began as continental
depocentres, but progressive delamination of the lower crust
(accelerating during the Phanerozoic), along with
degassing-related magmatic processes, eventually led to a thin
and basaltic deep sea crust as well as accumulating surface
water (cf. Storetvedt, 1997 and 2003). Thus, the slow build-up
of hydrostatic pressure beneath the evolving deep sea basins
would naturally provide a lifting power for the attenuated and
mechanically-weakened oceanic crust; this, in turn, would lead
to accumulated seawater that would gradually transgress
low-lying continental regions. Subsequently, associated
sub-crustal eclogitization and delamination would lead to basin
subsidence and eustatic regression – in addition to new supply
of pristine water from the interior. As demonstrated by Figures
7 & 8, the long-term eustatic sea-level changes, caused by
vertical motions of the evolving and progressively thinned
oceanic crust, has been an ongoing process notably since
Cambrian time. The important question is what dynamic mechanism
led to the relatively rapid influx of surface water during the
Palaeozoic?
The long-term build-up of fluids and gases in the upper mantle
and lower crust can be inferred to have led to a considerable
increase in the confining pressure at these levels setting off a
chain of related dynamo-tectonic and environmental processes.
Those parts of the upper mantle that received the greater amount
of degassing volatiles – the oceanic regions to be – underwent
long-term uplift, whereby the remaining continental blocks were
affected by transgressive super-cycles along with superimposed
events of higher frequency sea-level changes. In response,
sub-crustal eclogitization and associated delamination caused
broad regions to undergo overall progressive subsidence, while
corresponding regressive events affected less attenuated (higher
standing) crustal blocks. Dynamically, the episodic widespread
inward loss of heavier eclogitized sub-crustal sections led to
periodic planetary acceleration which, in turn, gave rise to
events of inertia-driven torsion of the increasingly fragmented
brittle shell. Hence, wrench tectonics processes were set in
action.
According to present geological and palaeomagnetic evidence, the
late Proterozoic-early Cambrian equator is only exposed in two
continental regions: (1) the Adelaide Geosyncline and
Warburton-Georgina-Bonaparte basins of Central Australia (Brown
et al., 1969) – with the continent in its pre-late
Cretaceous/early Tertiary orientation (see Storetvedt &
Longhinos, 2014a& b; Storetvedt 2015b) and (2) the Arctic
Canada-Baffin Bay-Davis Strait-Labrador Sea sector. The
remaining part of the topmost Precambrian palaeoequator cuts
across present-day oceanic regions (see Storetvedt, 2003 for
discussion). Consistent with this palaeo-equatorial orientation,
the Lower Cambrian Bradore Sandstone of northern Newfoundland
and Labrador shows near-horizontal remanence inclinations –
suggesting a palaeo-equatorial location (Rao & Deutsch, 1976).
From a more extensive palaeomagnetic and geological database, it
has been inferred that the North American craton resided at low
palaeolatitudes throughout the Upper Proterozoic (e.g., Link et
al., 1992; Storetvedt, 2003). Furthermore, palaeomagnetic data
indicate a palaeo-equatorial setting for the late Precambrian of
Australia (Embleton & Williams, 1986). The occurrence of redbeds
at various horizons of the Adelaide Geosyncline and the
widespread accumulation of carbonates, including stromatolitic
reef sequences, provide further evidence that Australia, during
the greater part of late Precambrian and Lower Palaeozoic times,
experienced tropical to sub-tropical conditions.
Palaeomagnetic data show that the Northern Appalachian foldbelt
– of late early Lower-Middle Palaeozoic age, strikes across
Newfoundland in a NE-SW direction and follows along the
corresponding palaeo-equatorial zone. Thus, in the Labrador Sea
region, the two palaeo-equatorial zones (late Precambrian and
Lower Palaeozoic, respectively) intersect each other at a fairly
steep angle, signifying an important spatial resetting of the
globe (an event of polar wander) in the early Palaeozoic. In the
wrench tectonic system, the equivalent anti-podal
palaeo-equatorial crossing corresponds to the Tasman-Adelaidean
junction in the Australia region; in the pre-late Cretaceous
setting of the continents, the Caledonian-Appalachian foldbelt
formed a great-circle girdling the globe along which the
Tasman-New England tectonic zone was located (see Storetvedt,
2003). Inferentially, the major event of polar wander in the
early-middle Cambrian – resetting the palaeo-equatorial bulge
and the corresponding polar flattening – must have caused a
significant hydrostatic pressure increase affecting the gas- and
fluid-rich upper mantle thereby triggering a number of
geological processes – such as sub-crustal eclogitization and
associated gravity-driven crustal loss to the upper mantle, as
well as ‘beginning’ isostatic basin subsidence, surface
volcanism driven by high-pressured volatiles, expulsion of a
significant volume of endogenous hydrous fluids to the surface –
along with gases including methane, hydrogen, helium, hydrogen
sulphide, hydrogen, etc. (cf. Gold, 1999; McLaughlin-West et
al., 1999; Lupton et al., 1999, and many others).
According to Figure 8, marked eustatic regressions characterize
principal geological time boundaries – which are thought to
correspond to times of sub-crustal attenuation and isostatic
basin subsidence, each event resulting in a distinct
tectono-magmatic upheaval caused by changes in the Earth’s
moment of inertia and thereby its rotation characteristics
(Storetvedt, 1997 and 2003). The late Cambrian
transgressive-regressive event was followed by subsequent
sea-level rises during the Palaeozoic – culminating in
regressive occurrences at the Ordovician-Silurian,
Silurian-Devonian, Devonian-Carboniferous and Permian-Triassic
boundaries. Thus, during the Palaeozoic, the rudimentary sea
basins of the late Cambrian were deepened and laterally
extended; although juvenile water from the interior was
periodically added to the surface, the overall global sea-level
fell ending in a marked low-stand at around the Permian-Triassic
boundary. Thus, during the Palaeozoic, due to dynamo-tectonic
processes, a substantial volume of seawater was added, but at
the same time the capacity of the developing oceanic basins had
grown so that the much less affected continental block was
significantly drained. In fact, the deep regression at the
Permian-Triassic boundary left more dry lands than existed prior
to the major influx of seawater during the Cambrian; a
rudimentary outline of the modern continents had thereby been
established.
A number of studies have demonstrated that during Phanerozoic
time, there was a strong correlation between distinct regressive
episodes and events of mass extinction – particularly of marine
faunas (e.g., Bayer & McGhee, 1985; Jablonski, 1986; Raup and
Sepkoski, 1982; Hallam, 1989; Hallam and Wignall, 1999). Thus,
Hallam and Wignall (1999) concluded that “Rapid high amplitude
regressive-transgressive couplets are the most frequently
observed eustatic changes at times of mass extinction, with the
majority of extinctions occurring during the transgressive pulse
when anoxic bottom waters often became extensive”.
The six main events of marine mass extinction, corresponding to
marked regressive events at principal geological time
boundaries, are shown in Figure 8. The sea-level high during
most of the Palaeozoic – reaching its maximum in late Ordovician
and Silurian times – was punctuated by a number of regressive
events. The most distinct sea-level falls occur at principal
geological time boundaries corresponding in turn to events of
crustal loss to the upper mantle, progressive isostatic
subsidence and cumulative development of oceanic basins, as well
as a range of environmental events. In this way, eustatic
sea-level variations are intimately tied to the range of
first-order events in the Earth’s history. By the end of the
Permian, the accumulated high volatile pressures in the upper
mantle had eventually been ‘exhausted’. During the Palaeozoic,
the flooded land masses had been subjected to a number of
distinct regressive events, each supposedly related to stages of
the progressively evolving deep sea basins, but the deep late
Permian regression exposed more dry land than since the
Precambrian. By now the evolving oceanic basins were in a rather
unfinished state, but the increasing eustatic transgression
during the Mesozoic, reaching its peak in the Upper Cretaceous
(Figure 9) and followed by a sharp regression at around the K/T
boundary, eventually gave rise to the modern deep sea basins.
During the predicted long-lasting crustal oceanization – that
gradually and episodically turned the once global-extent thick
continental crust into the present land-deep sea mosaic – the
volume of surface water must have increased exponentially, but
the capacity of the deep sea containers had clearly expanded
even more so that, today, we have more dry land than since the
early Cambrian.
At times of major volcano-tectonic upheavals, including mass
extinctions of marine fauna, the anoxic conditions discussed by
Hallam and Wignall (1999), may easily have entered the seawater
column. For example, some authors have suggested that the
combination of massive gas-driven volcanism, associated ocean
anoxic events and bursts of methane release may be responsible
for three major biological catastrophes – at 250, 200, and 65
million years respectively, while Max et al. (1999) considered
methane gas blow-outs as the actual source of fuel for the
global firestorm recorded by soot layers at the K/T boundary.
For the end of the Permian mass extinction – corresponding to a
deep regression and the loss of as much as 95 % of all species
on Earth, Erwin (1994) and Benton and Twitchett (2003)
considered widespread volcanic activity to be the most likely
cause. They concluded: “The extinction model involves global
warming by 6˚C and [a] huge input of light carbon into the
ocean-atmosphere system from the eruptions, but especially from
gas hydrates, leading to an ever-worsening positive-feedback
loop, the ‘runaway greenhouse’”. A global carbon isotope
excursion behind the catastrophic die-off of terrestrial
vegetation at the Permian-Triassic boundary was noted and
discussed by Ward et al. (2000), and Michaelsen (2002) -
studying the peat-forming plants across the northern Bowin
Basin, Australia - concluded that about 95% of the plants
disappeared rapidly at that time.
Figure 9. Part of the world map depicting the distribution of
shallow seas across the present-day continents in the Upper
Cretaceous. Diagram is based on Umbgrove (1942).
Hesselbo et al. (2000) presented evidence that, in the early
Jurassic, isotopically-light carbon dominated all the upper
oceanic, biospheric and atmospheric carbon reservoirs. They
suggested that the observed patterns were produced by voluminous
release of methane from marine deposits of gas hydrates, which
would be a natural consequence of the Earth’s internal degassing
(cf. Gold, 1999; Storetvedt, 2003). A similar dissociation of
oceanic methane hydrate has been suggested for the isotope
excursion at the Palaeocene-Eocene boundary (Dickins et al.,
1995; Katz et al., 1999). Thus, throughout the post-Precambrian
at least, the emission of major amounts of mantle-derived
methane is liable to have raised global atmospheric temperature,
notably at times of rapid eustatic excursions. The occurrence of
soot in and immediately above the K/T boundary and extinction
zone has been associated with a global firestorm (Wohlbach et
al., 1988), an observation that Gilmour and Guenther (1988)
referred to as “an incomplete combustion of methane” – a
conclusion with which Max et al. (1999) also concurred.
The Upper Cretaceous transgressive peak was interrupted by a
number of shorter-period sea level oscillations – presumably
interlinked with progressive sub-crustal attenuation, changes in
planetary rotation rate, and gas-driven volcanic activity in
many regions of the world. However, since then the seas have
gradually retreated from the continents. In oceanic regions,
this ‘multifarious’ global pulsation – often referred to as the
Alpine tectonic revolution – is well imprinted into the
geological record, either as horizons of erosion or
non-deposition (formed by stages of uplift of the developing
oceanic crust), and/or events of volcanic activity (cf.
Storetvedt, 1985). During the Upper Cretaceous, widespread
distribution of thinly-crusted deep oceans appeared for the
first time in Earth history. The deep sea basins that had
existed during the early-mid Mesozoic were only of limited
extent, consisting of circular to oval-shaped depressions
surrounded by a mosaic of sub-aerially exposed continental
masses less affected by sub-crustal attenuation. Within the deep
oceans, many fragments of former land can still be recognized by
a multitude of submerged aseismic ridges and plateaus with
anomalously thick crust. Thus, throughout most of the Mesozoic,
there existed land connections between the remaining continental
blocks, providing relatively free exchange of biota, though –
due to the accelerated loss of eclogitized crust (to the upper
mantle) by the end of the Cretaceous – the developing
‘asthenosphere’ had reached a more ‘mature’ stage: the irregular
brittle crust had become mechanically weakened as well as more
easily detachable from the underlying soft asthenosphere.
A dynamical consequence of heavier (eclogitized) crust sinking
into the deformable upper mantle was an increase in planetary
spin rate and/or events of polar wander – triggering
latitude-dependent wrench deformation of the inhomogeneous crust
(Storetvedt, 2003). Thus, for the first time, the modern
continental masses were separated by thin and deformable oceanic
crust and, due to an increasing planetary rotation, the land
masses became subjected to relative motions in situ. For the
larger continental blocks, these inertial rotations were only
minor. Figure 10 gives a sketch of the suggested overall Upper
Cretaceous palaeogeography – immediately before the onset of the
global wrench tectonic revolution at around the K/T boundary
which moderately changed the azimuthal orientations of the major
continents.
An overall regressive sea-level trend prevailed during the Lower
Tertiary, but by the beginning of the Miocene this tendency was
put in reverse. It may be argued that the second eustatic
sea-level super-cycle of the Phanerozoic, having been initiated
in the early Triassic, eventually came to a close in the late
Oligocene (cf. Figure 8); it had lasted for some 220 million
years and had included many minor eustatic rises and falls in
combination with tectono-magmatic pulses, some of them
accompanied by pronounced biological and environmental
consequences. Thus, a sharp event of polar wander took place at
around the Eocene-Oligocene boundary (ca. 35 million years ago),
amounting to an angular shift of 35˚ of the equatorial
bulge, bringing the Earth to approximately its present spatial
orientation. Thus, for the first time in Phanerozoic history,
the North Pole became positioned in the land-locked present-day
Arctic Basin, and the South Pole was displaced a corresponding
distance from its early Tertiary position in the South Atlantic,
onto the Antarctic continent. This polar wander event marks the
beginning of the well-established onset of the present
Antarctica ice cap; in Europe, the major latitudinal shift is
well demonstrated by palaeontological and palaeoclimatological
evidence (cf. Pomerol, 1982) – associated with a drastic cooling
(e.g. Buchardt, 1978).
Figure 10. Sketch map of the suggested palaeogeography of the
Earth by the end of the Cretaceous, prior to the subsequent
wrench tectonic continental rotations which disrupted former
trans-oceanic land ‘bridges’. In comparison with present-day
geography, it may be noted that the wrench rotations of the
Atlantic continents (their separation as well as their azimuthal
orientation) were only minor. Dark blue colour indicates deep
sea basins, while light blue represent ‘Cenomanian’
transgressive seas. Diagram is based on Storetvedt (2003).
In continental settings, the Eocene/Oligocene dynamic transition
triggered the eruption of the Ethiopian flood basalts (36.9 ±0.9
My), and a number of volcanic gas blow-outs took place at that
time – e. g., the Mistastin and Wanapitei Lake craters in
Canada, and the Popigai crater in Russia. In an Ar/40-Ar/39 age
study of the 100 km diameter Popigai structure, Bottomley et al.
(1997) noted the close match between the obtained age (36.9 ±0.2
My) and that of the North American tektites which had been
associated with the 85 km diameter Chesapeake Bay crater off the
eastern U.S. coast, with an age of 35.3 ±0.2 My (Poag et al.,
1994; Poag and Aubry, 1995). Adding to the diversity of global
geological phenomena occurring at this time, can be cited by the
volcanic ashes in the Massignano stratigraphic section of Italy,
dated at 35 ±0.4 My, which contain a distinct Ir peak – in
association with shocked quartz (Montanari et al., 1993).
The significant spatial shift of the Earth some 35 million years
ago must have led to considerable hydrostatic pressure increases
in regions of the volatile-rich and irregular asthenosphere. In
addition to events of continued crustal delamination, the
overpressure within the topmost mantle would create tectonically
fractured crustal ‘chimneys’ that served as a form of pressure
valves which on the surface would give rise to volcanism and
high-pressure blow-outs forming craters. In many ways, the major
shift of the equatorial bulge at around the Eocene/Oligocene
boundary may be seen as the terminal spasm of the Alpine
tectonic revolution which can be related to the widespread
tectono-magmatic activity at that time – notably in the oceans.
In the Exxon eustatic curve, a regressive event characterizes
the Eocene/Oligocene boundary, and the Lower Oligocene
transgression terminates in a deep regression in the Middle
Oligocene – serving as a marker horizon between the Rupelian and
the Chattian epochs (Haq et al., 1987).
In a study of the global distribution of late Lower Tertiary
stratigraphic hiatuses in the sea floor record, Keller et al.
(1987) found erosion events to have occurred at the
Eocene/Oligocene and Oligocene/Miocene boundaries; this is
consistent with the general observation of a close link between
tectonics and distinct regressive-transgressive couplets linked
with geological time boundaries. However, Keller et al. did not
find an erosional horizon corresponding to the relatively sharp
mid-Oligocene sea-level change in the Exxon curve which is well
demonstrated by a DSDP drilling transect of the South Atlantic
(see below). On the other hand, they found ‘corresponding’
erosional discordances in both the early and the late Oligocene.
In this context, it should be remembered that any major shift of
the equatorial bulge and polar flattening (such as that
occurring around 35 million years ago) would have been liable to
cause regional variations in asthenospheric volatile pressures
and related crustal effects, notably at low-to-intermediate
palaeolatitudes, thereby masking the true eustatic sea-level
variation in some regions.
Overall, the Oligocene showed a regressive tendency indicating
ongoing crustal loss to the mantle and related development of
deep sea basins. This late stage reconstitution of the crust
inevitably led to changes in the Earth’s moment of inertia,
increasing the confining pressure within the lithospheric lenses
as well as in the melt pockets at higher levels – paving the way
for a new round of more forceful tectono-magmatic events. Thus,
starting at around the Oligocene/Miocene boundary, ca. 22
million years ago and the sea encroached once more on the land,
culminating in an overall high stand in the Lower-Middle
Miocene. The Exxon proposal of the post-Oligocene (Neogene)
sea-level variations (Haq et al., 1987) is shown in Figure 11.
According to this scheme, for the Lower and Middle Miocene –
spanning a period of about 15 million years – the global
shore-line was raised by some 150 metres. This long-standing
transgression was punctured by two short-lived regressive
events, around 15 My ago, ending with a major sea-level drop
some 8 My years ago – the latter defining the Miocene sea-level
minimum. The Miocene Era was terminated by a distinct regressive
phase at ca. 5 My ago (end of the Messinian). These sea-level
low stands are most likely associated with events of planetary
acceleration – being a dynamic response to inward loss of
widespread eclogitized lower crustal segments.
In the Atlantic region, the oscillating mid-Miocene regressions,
with their related high-pressured volatile-rich asthenosphere,
is time-equivalent with the origin of the Columbia River basalts
(dated at 16.2 ±1 My) and with the Steinheim and Ries craters in
Germany (dated at ca. 15 My) – see Figure 11. Miocene and
younger elevations of the deep sea crust, giving rise to
continental transgression, affected broader crustal regions of
the world oceans. For example, in the islands of the Central
Atlantic (Cape Verde Islands, Ascension Island, Madeira and the
Azores), Lower-Middle Miocene and younger marine sedimentary
horizons are found at heights ranging between 400 and 500 metres
above present sea level (Mitchell-Thomé,1976), while Miocene and
younger volcanic activity shows widespread distribution in this
part of the Atlantic (see Storetvedt, 1985). The Neogene phases
of regression are inferred to be related intimately to the
youngest phases of oceanization – having transformed particular
regions of continental crust into oceanic-type structures. For
example, in the Mediterranean a number of isolated
circular-to-oval shaped depressions formed during the Messinian
– in association with a very thick succession of salt of
variable chemistry degassed from the mantle. Wezel (1985), for
example, argued that, in the late Miocene, the Tyrrhenian region
was the site of an upstanding intra-Alpine continental crust
that in Plio-Quaternary time underwent variable sub-crustal
thinning and vertical collapse activated by upper mantle
processes.
Figure 11. Diagram shows the Exxon eustatic sea-level curve for
post-Oligocene (Neogene) time – after Haq et al. (1987).
As we have argued above, periodic vertical motions of the sea
floor – reflecting build-up and subsequent release of upper
mantle volatile pressures – with related sedimentary
discordances and magmatic activity, are likely to have been a
persistent global feature and the ultimate cause of the
principal events of eustatic sea-level changes. Thus, Figure 12a
delineates the significant Miocene depositional break across the
South Atlantic, at latitude 30˚S, which inferentially
corresponds to the Lower-Middle Miocene transgressive phase
shown in Figure 11. The associated flooding of low-lying regions
of South America is outlined in Figure 12b. In an extended
sedimentary section at DSDP site 355 on the North Brazilian
margin, sedimentary hiatuses were recorded in the topmost
Cretaceous (Maastrichtian), at around the Eocene-Oligocene
boundary, and in the Middle Miocene – supporting the thesis of a
close connection between major phases of oceanic crustal uplift
and erosion with corresponding events of sea-level rise on
low-lying continental regions. Compilation of cored Mesozoic
sediments in sites of the western and eastern margins of the
Central Atlantic (Arthur, 1979; Storetvedt, 1985) again shows a
significant stratigraphic hiatus consistent with the major Upper
Mesozoic eustatic transgression.
Figure 12. Diagram (a) shows the ‘Middle’ Miocene sedimentary
break of the Deep Sea Drilling Project Leg 3 sites across the
South Atlantic at 30˚S (simplified after Maxwell et al.,
1970). This trans-oceanic depositional hiatus is regarded here
as a segment of a widespread deep sea crustal uplift having
produced the Lower-Middle Miocene eustatic sea-level rise.
Diagram (b) exemplifies the resulting mid-Miocene sea-level
(light blue) of South America (Webb, 1995).
Concluding summary
In this paper, the focus has been on the origin of Earth’s
surface water and the cause of sea-level changes for which the
crustal product is a continuing, albeit jerky, loss of
eclogitized gravity-driven continental material to the mantle –
eventually leading to formation of the present-day thin oceanic
crust and deep sea basins. As a result of the actual degassing
Earth model, today’s continents have, during the Phanerozoic,
been repeatedly flooded by slowly rising seas which after
sea-level high stands have subsequently retreated to form
distinct sea-level lows. It is an observation of paramount
importance, long noted by many authors, that the most marked
regressive events occur at times of principal geological time
boundaries – representing revolutionary episodes in Earth
history – in terms of tectonic, magmatic, biological and
environmental happenings. In this way, sea-level changes became
intimately linked to the rest of the planet’s first-order
geological manifestations.
Central in this discussion is that recurrent sea-level
low-stands eventually gave rise to ever-growing deep sea basins,
and the transgressive-regressive couplets continuously added
fresh surface water from the mantle. The first transgressive
super-cycle commenced in the early Palaeozoic – being closely
linked to the marine biological boom at that time, lasting till
the late Palaeozoic. However, a deep sea-level regression at the
Permian/Triassic boundary, adding a multitude of toxic gases and
fluids to the sea and the atmosphere, led to mass extinction and
the most severe crisis in the history of life (Raup, 1979). At
this time, the evolving deep sea basins had evolved into a
sizeable volume thus draining the continents – leaving more dry
land than ever before in post-Precambrian history. But internal
gases and fluids continued their upper mantle accumulation and
accompanying pressure increase – giving rise to a Mesozoic
uplift of the evolving oceanic basement, with an associated
overall major sea-level rise that culminated in the Upper
Cretaceous. The following regression and upper mantle gas
exhaustion led to another major biotic and environmental crisis
– at around the K/T boundary. By now the world oceans were
nearing their present state and extent, but continued to
demonstrate alternating cycles of sea-level changes, with
stratigraphic control, suggesting that the deep sea basins are
still under development. In addition, it is highly probable that
the volume of sea water has increased continuously to this day,
and the major part of the planet’s water may probably still be
residing in the interior.
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