Sea level fluctuations and large igneous provinces

On a global scale, shifts in sea level recorded by stratigraphers and on seismic profiles stem from one of two main processes: changes in land-ice volume and the volume of the ocean basins.  The latter most often results from changing rates of sea-floor spreading, so that when it is rapid a greater volume of the lithosphere near spreading centres retains sufficient buoyancy to displace the oceans onto continental margins.  During slow spreading, cooling of the lithosphere and an increase in its density enlarges the deep abyssal plains, so that the oceans withdraw to low levels.  The mid-Cretaceous saw vast outpourings of plume-related lavas onto the floor of the West Pacific.  So large, that they reduced the volume of the Pacific basin enough to result in continental flooding that was unprecedented in the Phanerozoic Eon.

On a local scale, changes in sea level recorded by the stratigraphic record include those due to local processes, generally ascribed to tectonic events at continental margins, which involved rising continental lithosphere.  However, one of the greatest forces for local change in the continental freeboard is changing density of the lithosphere due to thermal effects.  Anywhere once affected by major igneous events should record relative falls in sea level during the acme of magmatism, and rises when activity waned.  The British Tertiary Igneous Province, a precursor to the eventual rifting of the North Atlantic under the influence of the Iceland plume is a good candidate for charting magma-sea level connections.  The central volcanic complexes of the Hebrides, and their enveloping flood basalt piles formed at the start of the Palaeocene (~60 Ma).  Around that time, much of the British Isles underwent several kilometres of vertical uplift and exhumation, whose effects remain today.  In the surrounding marine basins, this event is recorded by Palaeogene sandstone bodies, presumable derived by erosion of the uplifted crust.  Yet local Palaeogene sediments also record episodes of rising sea level.  John Maclennan and Brian Lovell of the French Institut de Physique du Globe and Cambridge University have modelled the likely effect on sea levels around the British Isles by crustal underplating of magmas formed during the BTIP magmatism (Maclennan, J. & Lovell, B. 2002.  Control of regional sea level by surface uplift and subsidence caused by magmatic underplating of the Earth’s crust.  Geology, v. 30, p. 675-678).

Up to 8 km of mafic igneous rocks seem to have ponded at the base of the British Isles’ crust while the BTIP was active.  This estimate stems from the fact that the lavas of the province evidence high-pressure fractional crystallization.  Calculations of the percentage of cumulates needed to generate the bulk chemistry of the BTIP lavas suggest that their volume far outweighs that of the volcanic part of the province.  Given estimates of the volume of underplated cumulates, modelling boils down to examining the consequences for lithospheric density of initial heating and its subsequent relaxation.  The Palaeogene sedimentary record provides good support for the model, with massive uplift from 60-56 Ma (the period when the BTIP was forming).  Sudden sea-level rise at the end of this period never reached the level prior to magmatism; in fact it amounts to one half the estimated uplift.  That is precisely in line with the underplating model.


One response to “Sea level fluctuations and large igneous provinces

  1. Excellent work, excellent article. Let me write a couple of words on social & legal context

    So if an island nation is submerged beneath the ocean, does it maintain its membership in the United Nations? Who is responsible for the citizens? Do they travel on its passport? Who claims and enforces offshore mineral and fishing rights in waters around a submerged nation? International law currently has no answers to such questions.

    United Nations Ambassador Phillip Muller of the Marshall Islands said there is no sense of urgency to find not only those answers, but also to address the causes of climate change, which many believe to be responsible for rising ocean levels.

    “Even if we reach a legal agreement sometime soon, which I don’t think we will, the major players are not in the process,” Muller said.

    Those players, the participants said, include industrial nations such as the United States and China that emit the most carbon dioxide and other so-called greenhouse gases. Many climate scientists say those gases are responsible for global warming. Mary-Elena Carr of Columbia University’s Earth Institute said what is now an annual sea level rise of a few millimeters will increase dramatically by the year 2100. “The biggest challenge is to preserve their nationality without a territory,” said Bogumil Terminski from Geneva. International legal experts are discovering climate change law, and the Pacific island nation of Tuvalu is a case in point: The Polynesian archipelago is doomed to disappear beneath the ocean. Now lawyers are asking what sort of rights citizens have when their homeland no longer exists.
    t present, however, there appear to be at least three possibilities that could advance the international debate about ‘climate refugee’ protections and fill existing gaps in international law.

    The first option is to revise the 1951 Convention on the Status of Refugees to include climate (or environmental) refugees and to offer legal protections similar to those for refugees fleeing political persecution. A second, more ambitious option is to negotiate a completely new convention, one that would try to guarantee specific rights and protections to climate or environmental ‘refugees`.

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