by Anthony Lucas
An alternative perception of sea level rise and climate change.
The Earth’s geological record documents variation in sea level as a standard process in the evolution of our planet. Submerged cities of once flourishing civilisations are indicators of sea level change within the short period of human evolution. The recently discovered remains of the ancient Egyptian cities of Canopus and Herakleion, some eight metres below the Mediterranean off the coast of the Nile delta, provide an insight into the scale of sea level rise over the last few thousand years. They also provide an insight into the future of the American city of New Orleans.
Canopus, Herakleion, New Orleans and Continental Rifts are examples of geological subsidence. Uplift is the opposite process to subsidence. The way these processes are perceived is determined by an observer’s frame of reference which then determines the interpretation of the geological setting.
Rift zones are the product of subsidence generated by continental crustal extension. They are places in the crust which have been extended beyond its capacity to stretch and thin. When the crust can stretch no further, continued crustal extension results in structural failure in the form of faulting, subsidence then occurs along the faults. This is the description of a rift from the frame of reference of an observer viewing the rift from the continental crust, from outside the rift. If a disinterested observer had their frame of reference set within the rift zone, they would report that over geological time the continental crust on either side of the rift was uplifted. The point here being that an observer’s perspective of subsidence or uplift is entirely dependent on a subjective selection of their frame of reference.
Crustal extension in North East Egypt is expressed as the Suez – Cairo faulting system. The Northern extension of the faulting system trends directly towards the Nile Delta. ‘Seismic data, Fig 1, indicates that a large N- to NW – trending extensional basin of Oligocene to late Miocene age lies buried beneath the present day Nile delta fan’ William Bosworth and Ken McClay.2001. This basin is known as the Manzala Rift.
Fig 1:- Seismic activity of North Eastern Egypt from the Red Sea at Suez, through the Cairo -Suez faulting system and the Manzala Rift, to the Nile Delta and on into the Mediterranean via Abu Qir Bay. Dahy, SA. 2012.
Subsidence effects of the Manzala Rift have been observed along the coast of the Nile Delta in the lost Egyptian cities of Canopus & Herakleion, Fig 2. The gradual subsidence of these cities is clearly described by DB Stealy et al., 2004. This description is from the continental crustal frame of reference where the decreased elevation of the lost cities is attributed to, conceived as, a combination of subsidence and sea level rise. DB Stealy provides a practical graphic to illustrate the relative movements, Fig 3,
Fig 2:- Location of DB Stealy et al study area in the Abu Qir Bay, on the North Western margin of Nile Delta in Egypt. CT – traces of the relict Canopic branch, EC – Eastern Canopus, H- Herakleion.
The observation of events at Canopus and Herakleion, Fig 3, from the frame of reference within the rift, differs from the crustal frame of reference in three respects. First, the arrow indicating ‘Land subsidence’ is reversed to indicate uplift of the continental Limestone crust.
Secondly, the relative sea level rise becomes absolute sea level rise, although it will be perceived from the crustal frame of reference as virtually no relative sea level rise.
Third, the Holocene strata are represented as gradually lowering, whereas from the rift frame of reference they remain at the site and elevation of deposition. The Holocene strata effectively become the datum against which absolute movement of sea level rise and continental uplift are measured.
Fig 3: – Graphic from DB Stealy, et al., illustrating relative movement at Canopus. ‘An interpretation of processes that submerged Herakleion and Eastern Canopus to depths of 5 – 7 metres below sea level from ca. 500BC to present’.
Stealy, et al., assess subsidence at Canopus of 8 metres over 2500 years. This equates to an average of 3.2 mm/year, while NASA recently reported satellite measurement of sea level rise as 75mm for 3.4 +/-0.4mm/year. This is a remarkable agreement. Satellites use a dynamic geodetic elliptical frame of reference described at this NOAA, Frequently Ask Questions. ‘Why does the reference frame change for GPS orbits’. This frame of reference makes adjustments to elevation datum points when solid earth processes result in changes to the elevation of datum points.
The conventional explanation of subsidence within a rift is the view from the crustal frame of reference. As the rift opens up the material within the rift sink down into the resulting void under the force of gravity. However when a rift is viewed from the frame of reference within the rift, looking out towards the crust, the origin of the forces generating the rift invokes an alternate tectonic model. The rift is generated by the same extensional forces, but it is the rift itself that is static, while the surrounding crust is uplifted. Instead of gravity causing the rift to subside another force is uplifting the crust. The solid earth process generating the uplifting force is described in this video, as the process of Super Elevation, Fig 4.
Fig 4:- Super Elevation illustration after Maxlow J, 2012, Expanding Earth video No 5 of 16, illustrating the scale of potential uplift due to Super Elevation on an expanding earth from Permian to the present.
The South African Rift Valley (more accurately described as the East Africa Rift), is a well-known active rift that is easily observable at the earth’s surface. Rifting systems exhibiting similar features to the East Africa Rift are 1), the Europe Cenozoic Rift of the Rhine River, which passes through the Netherlands, as the Lower Rhine Embayment, then extending into the North Sea and 2), the North American Realfoot Rift of the Mississippi River which flows through Mississippi and Louisiana, as the Mississippi Embayment, then onto New Orleans on the Mississippi Delta.
The Mississippi Delta is located on the Southern edge of the North American continental crust and is not subject to uplift from the process of super elevation. It is perceived to subside as the continental crust uplifts and absolute sea level increases. When considered from the frame of reference within the rift, the sediments of the Mississippi Delta become an analogue of the Nile Delta sediments of Canopus Fig 3, and can be thought of as at their location of deposition while the crust is uplifted and sea level rises. Mississippi Delta subsidence was defined by Dokka, et al., 2006 as ‘The average subsidence rate for GPS sites located on the delta is 5.2 +/- 0.9 mm/year relative to the Earth’s center of mass, or 7mm/year relative to mean sea level.’
Fig 5:- Location of New Orleans on the sediments of the Mississippi Delta, South of the Michoud fault and the continental North American Crust. Roy K. Dokka et al., 2006 (adapted from Peel, et al., 1995)
The Nile Delta cities of Canopus and Herakleion and the Mississippi Delta city of New Orleans are both built on delta sediments over an actively subsiding rift, at the intersection of the rift and the coastal edge of the continental crust. As the geological setting of the lost cities is almost identical to New Orleans, it is possible to propose Canopus and Herakleion as a model for the future subsidence of New Orleans. To this extent it can be expected that New Orleans will continue subsiding at an average rate of about 3.4 mm/year, so that New Orleans should reasonably be expected to be 8 metres under the Gulf of Mexico in another 2500 years. That is one third of a metre per 100 years.
Delta based relative subsidence can then be used for long term policy planning, infrastructure investment and risk assessment in New Orleans, where the continental crust is uplifting and sea level is rising, encroaching on and inundating the delta. The wider issue is then the climate change debate, which can be framed in a paradigm where sea level rise is natural and ongoing, while the earth’s continental crusts are uplifted by Super Elevation.
References
JC note: As with all guest posts, please keep your comments civil and relevant. This post was submitted to me via email. While I find the topic interesting, I claim no personal expertise on this topic.
Filed under: Oceans
