Evidence of an Ice Age at the start of the Middle Jurassic.

The Jurassic lasted approximately 65.3 million years, from about 201.3 million years ago till about 145 million years ago, and is generally considered to have been a period of exceptional greenhouse warming. It is divided into the Early, Middle and Late Jurassic, each with its own distinctive climates and faunas (as well as shorter subdivisions). The Early Jurassic ended 174.1 million years ago with the Toarcian Extinction Event, a period of exceptional warming triggered by volcanic activity in South Africa that sharply increased the amount of carbon dioxide in the atmosphere, which in turn triggered an anoxic ocean event (warmer water is less able to hold dissolved oxygen), causing many groups of marine organisms to die out. Recent evidence has suggested that this may have been followed by a period of severe cooling at the beginning of the Middle Jurassic; this has included signs of ice rafted debris in Jurassic Arctic basins and the formation of the mineral glendonite, a form of calcite which only forms in cold water, though it has not been suggested that there was significant glaciation on land (neither pole was covered by a continental land-mass in the Jurassic).

In a paper published in the journal Nature Communications on 11 December 2015, Christoph Korte of the Department of Geosciences and Natural Resource Management at the University of Copenhagen, Stephen Hesselbo of the Camborne School of Mines and Environment and Sustainability Institute at the University of Exeter, Clemens Ullmann of the Department of Geosciences and Natural Resource Management at the University of Copenhagen and the Camborne School of Mines and Environment and Sustainability Institute at the University of Exeter, Gerd Dietl of the Staatliches Museum für Naturkunde Stuttgart, Micha Ruhl of the Department of Earth Sciences at the University of Oxford, Günter Schweigert, also of the Staatliches Museum für Naturkunde Stuttgart and Nicolas Thibault, again of the Department of Geosciences and Natural Resource Management at the University of Copenhagen examine evidence for a cooling event at the start of the Middle Jurassic from oxygen isotope levels from Belemnite, Bivalves and Brachiopod shells from the Laurasian Seaway.

Oxygen has three stable isotopes, Oxygen-16, Oxygen-17 and Oxygen-18, with the heaviest, Oxygen-18 being the most abundant. As Oxygen-18 is slightly heavier at than the other isotopes water molecules containing this isotope require slightly more energy  to evaporate at the water surface, so under cooler conditions the proportion of Oxygen-18 in the water rises and under warmer conditions it falls. Many marine organisms with calcite shells store oxygen in their shells in a proportion that matches the surrounding water, enabling palaeontologists and geochemists to use this proportionality as a reliable proxy for the temperature in ancient marine deposits.

During the Jurassic the continents were joined together into a single supercontinent, Pangea. This was roughly 'C' shaped, with the interior part of the 'C' forming an equatorial ocean, the Tethys. However the Jurassic had a very warm climate, with subsequent high sea levels, and many areas of the continents were flooded. In particular an epicontinental sea (sea which lies on submerged continental crust), the Laurasian Seaway covered much of what is now Europe, connecting the tropical Tethys Ocean to the Boreal Sea around the North Pole, and allowing water to circulate between the two (in the modern world the Bering Straight is another such epicontinrntal seaway, connecting the Pacific to the Arctic across an area that is part of the North American Continental Plate, an area which was exposed during the Pleistocene glaciations).

By examining shells from the early Middle Jurassic of the Laurasian Seaway that showed little sign of alteration (recrystalization of replacement of the original calcite crystals), Korte et al. were able to detect a rise in Oxygen-18 of 2.5 parts in a thousand, equivalent to a drop in temperature of 10 degrees centigrade, over a period of about half a million years.

During this period the area that is now the North Sea, and which was then part of the Laurasian Seaway, is known to have undergone considerable uplift, driven by a rising volcanic plume from deep within the Earth, similar to the Yellowstone Plume beneath modern Wyoming (such a plume would not need to erupt as the surface to cause significant uplift, emplacing new material on the underside of the overlying tectonic plate would have the same effect), while at the same time Ammonite distributions across the Laurasian Seaway became fragmented by latitude (i.e. instead of having the same group of species across the seaway as in the Early Jurassic, different populations began to appear in the north and south). Korte et al  suggest that the rising North Sea Dome may have blocked the Laurasian Seaway, preventing currents running between the Tethys Ocean and Boreal Sea, thereby causing cooling in the Boreal Sea and possibly the formation of pack ice.

Late Early Jurassic palaeogeography. (a) Map shows the connection between the equatorial Tethys Ocean and the Boreal Sea via the Laurasian Seaway; the latter including the Viking Corridor which was several hundred kilometres wide. Red arrows mark generalized palaeocurrents. (b) Detail of Laurasian Seaway palaeogeography with the region affected by North Sea Dome as determined by the generalized outer limit of the Toarcian subcrop. Brown arrows represent the siliciclastic sediment supply/transport in relation to domal uplift. Sample locations are numbered and identified by stars (Hebrides Basin (I; Scotland), Cleveland Basin (II; England), Swabo-Franconian Basin (III; Germany) and Lusitanian (IV; Portugal)/Basque Cantabrian basins (V; Spain)). SNS, Southern North Sea Basin, W, Wessex Basin. Korte et al. (2015).

See also...

The reaction of marine invertebrates to global warming during the Early Jurassic Toarcian Extinction Event.                                                   About 183 million years ago a major eruptive episode in the Karoo-Ferrar Large Igneous Province of South Africa lead to an abrupt rise in global atmospheric...

A Late Jurassic lagerstätte from central Poland                                                          The term lagerstätte is used by Palaeontologists to describe a particularly rich fossil source; a site where fossils are either exceptionally numerous...

Opportunistic Bivalves during the Early Jurassic Toarcian Oceanic Anoxic Event. The Toarcian Oceanic Anoxic Event is an extinction event that took place in the Early Jurassic, about 183 million years ago. It took place in four phases, thought to have been related to Milankovitch Cycles. During each phase the temperature of the global ocean is thought...

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Eruptions on Mount Nabro, Eritrea.

Mount Nabro is a stratovolcano (a cone-shaped volcano of the kind seen in Hollywood movies, made up of successive layers of ash an lava) in the southeast Danakil Desert in Eritrea, close to the Ethiopian border. It forms part of the Bidu Volcanic Complex, along with the Dubbi and Sork Ale volcanoes in Eritrea and Mallahle and Bara Ale volcanoes in Ethiopia. Until this year there have been no recorded eruptions at Nabro, and it had been assumed to be extinct, though due to its remote location it had not been studied extensively.

Satellite image of Mount Nabro.

On 12 June 2011 the area around Mount Nabro was shaken be a series of earthquakes, which rose in intensity throughout the day. At approximately 9 pm local time on 13 June a large volcanic eruption took place in the region, creating a column of ash that rose 13.5 km into the air. At first this was thought to have been caused by an explosion on Mount Dubbi, the only volcano in the area to have erupted in recorded history (in 1861), causing concerns for the settlement of Afambo, which is close to Dubbi, but Nabro was later identified as the source of the eruption from satellite photographs. Within hours the ash column had started to cause disruption to flights in the region.

Satellite image of the eruption on Mount Nabro.

On 16 June the Eritrean Government announced an evacuation of the people of Afambo, Nebro and Sireru, the three settlements closest to the eruption.

By the end of June the ash cloud had cleared sufficiently for NASA to obtain pictures of the situation on the ground, revealing that a large amount of lava had issued from the caldera of the volcano, producing a flow of lava that extended westwards for over 12 km before producing a lava lake. It was also becoming apparent by this point that the evacuation by the Eritrean Government had not been entirely successful and that seven people in Eritrea had lost their lives as a result of the eruption, and a further three had been injured.

Satellite image of the lava flows from Mount Nabro.

A week later the neighboring Ethiopian state of Afar reveled that at least 31 people had died their, and that the local government was having extreme difficulty coping with deteriorating environmental conditions associated with the eruption, particularly water pollution and damage to crops and livestock. The Afar Government estimated that 48 000 people were in immediate danger from the ash-fall and that over 167 000 more were potentially at risk. Clouds of poisonous sulphur dioxide gas were also reported to be blowing across the border. It launched an appeal for aid, but this was overruled by the Federal Ethiopian Government in Addis Ababa.

On 9 September 2011 the area began to experience seismic activity again with small earthquakes persisting for the next three days. On 28 September NASA again reported sighting fresh lava flows on Mount Nabro.

The Bidu Volcanic Complex is located on the Afar Triple Junction, a complex area tectonically, where three spreading plate margins meet. To the northwest the Red Sea Rift runs between Africa and the Arabian Peninsula, a new ocean ridge where oceanic crust is being formed, with the potential to turn the Red Sea into an ocean over the next few million years. To the northeast the Aden Ridge forms a similar divergent margin between the Arabian Peninsula and the Indian Ocean, although in this case both plates are moving northward, the Arabian Peninsula is simply moving faster than the Indian Ocean for the time being. It is likely that as Arabia collides with Eurasia it will slow down and the Indian Ocean will overtake it, closing the Aden Rift. To the south the Great Rift Valley is slowly splitting Africa in two, creating a long chain of volcanoes heading south to Mozambique. Some geologists believe that a mantle plume beneath the Afar Triangle is fueling this, pushing the plates apart rather than their being drawn apart by seismic activity elsewhere.

The hypothetical Afar Plume.

See also Tourists evacuated due to volcanic activity on El Hierro, The Dangers of a modern Laki-style eruption in Iceland and Volcanoes on Sciency Thoughts YouTube.