Author Topic: NCGT SEA LEVELS  (Read 23 times)

Admin

  • Administrator
  • Full Member
  • *****
  • Posts: 174
    • View Profile
NCGT SEA LEVELS
« on: January 29, 2017, 08:31:28 pm »
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.

« Last Edit: January 29, 2017, 08:33:15 pm by Admin »

Social Buttons