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#Post#: 84--------------------------------------------------
NCGT SEA LEVELS
By: Admin Date: January 29, 2017, 8:31 pm
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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
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).
...
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.
...
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.
...
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.
...
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.
...
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.
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