Tag Archives: Sea level fall

The Great Barrier Reef and the Last Glacial Maximum (LGM)

The 2,300 km stretch of coral reefs and islands in the Coral Sea off the coast of Queensland, Australia is the largest single structure on Earth built by living organisms. The dominant reef builders are four hundred species of coral, most of which are a symbiosis that conjoins marine invertebrates in the class Anthozoa – part of the phylum Cnidaria – and photosynthesising single-celled eukaryotes known as dinoflagellates. These algae are mainly free-living marine plankton, some species of which evolved to be co-opted by corals. Their role in the symbiosis is complex; on the one hand providing energy in the form of sugars, glycerol and amino acids; on the other consuming the coral polyps’ carbon dioxide output. The latter is fixed, in the case of hard corals, by the secretion of calcium carbonate: the key to reef formation.

Marine photosynthesisers demand clear water in the upper few tens of metres of the sea, together with sunlight least affected by the atmosphere, as in the tropics where the sun rises to the zenith year round. The coral animal-algae connection limits reef growth to shallow seas, the top of the reef being close to mean sea level, sometimes rising above it at low tide. Hence the formation of fringing and barrier reefs. In the case of atoll reefs, a connection with sea-floor volcanoes that rose from hotspots on the oceanic abyssal plains to form active volcanic islands that began to sink once they became extinct. The pace at which reefs can grow is generally able to match that of crustal subsidence so that atolls remain throughout the Western Pacific. Reef growth is also capable of coping with global sea-level changes, so that the present top level of the Great Barrier Reef has been in balance with the generally static sea level of the Holocene since the ice caps of the last glaciation melted back to roughly their present extent about seven thousand years ago.

There are many cases of different reef levels on and around islands that match the sea-level fluctuations during the last Ice Age.  High-resolution bathymetry produced by multi-beam sonar across the eastern edge of parts of the Great Barrier Reef reveals a series of submerged terraces down to almost 120 m below modern sea-level (Yokoyama, Y. and 17 others 2018. Rapid glaciation and a two-step sea level plunge in the Last Glacial Maximum. Nature, v. 559, p. 603-607; doi:10.1038/s41586-018-0335-4). Globally, the LGM began at around 31 ka when sea level fell by about 40 metres, thanks to massive accumulation of glacial ice at high latitudes. Previous studies to chart the changes in global mean sea level during the LGM suggested a steady fall until about 20 ka, followed by rapid rise as ice caps melted back. The multinational team led by Yusuke Yokoyama of the University of Tokyo, obtained precise ages of coral samples from different depths in drill cores through the coral terraces. These data revealed a more complex pattern of sea-level change, in particular a hitherto unsuspected plunge between 21.9 and 20.5 ka of 20 m to reach -118 m. This immediately preceded the warming-related rise that continued to Holocene levels.

GBR Bathymetry

High-resolution sonar images of the sea floor at two sites on the eastern edge of Australia’s Great Barrier Reef. They show terraces associated with, the lowest of which corresponds to the Last Glacial Maximum. (Credit: Yokoyama et al. 2018, Figure 1)

Curiously, this massive phenomenon is not shown by sea-level estimates derived from the records of changing oxygen isotopes in ocean-floor sediments and ice cores. The team’s complex modelling incorporated global changes in land and sea-bed levels, and thus changes in the volume of the ocean basins, due to the changing isostatic effects of both ice-cap and ocean masses. From these it is possible to reach an interesting conclusion (Whitehouse, P. 2018. Ancient ice sheet had a growth spurt. Nature, v. 603, p. 487-488; doi:10.1038/d41586-018-05760-3). Rather than an increase in snowfall onto ice-caps, their retreat may have been hindered by thickening of marginal floating ice shelves that created buttresses around Antarctica and the northern ice sheets. Slowed glacial flow to the oceans could have promoted ice sheet growth for a time as melting of calved icebergs was hindered, especially in the case of the ice sheet over northern North America. Certainly, this crucial climatic turning point was a lot more complex than previously believed.

Volcanism and sea level fall

Most volcanic activity stems from the rise of hot, deep rock, usually within the mantle. Pressure suppresses partial melting, so as hot rock rises the greater the chance that it will begin to melt without any rise in its temperature. That is the reason why mantle plumes are associated with many volcanic centres within plates. Extension at oceanic ridges allows upper mantle to rise in linear belts below rift systems giving rise to shallow partial melting, mid-ocean ridge basalts and sea-floor spreading. These aren’t the only processes that can reduce pressure to induce such decompression melting; any means of uplift will do, provided the rate of uplift exceeds the rate of cooling at depth. As well as tectonic uplift and erosion, melting of thick ice sheets and major falls in sea level may result in unloading of the lithosphere.

During Messinian Stage of the late Miocene up to 3 km of evaporitic salt was deposited in the deepest parts of the Mediterranean Basin. One mechanism might have been faster evaporation of seawater than its resupply from the Atlantic through the Straits of Gibraltar, similar to the way in which salts is deposited below the Dead Sea. But the salt layer beneath the modern Mediterranean Sea bed has interleaved riverine sediments containing fossils of land plants. The Straits had closed and the Mediterranean Sea evaporated away. From about 6 to 5.3 Ma ago sea level fell by 3 to 5 km, only returning to normal when the Straits reopened to launch the huge Zanclean flood, with which the Pliocene of southern Europe and North Africa commenced. A team from the Universities of Geneva, Orleans and Paris and the Instituto de Ciencias de la Tierra Jaume Almera in Barcelona has tested the hypothesis that the Messinian Crisis affected volcanic activity in the area (Sternai, P. et al. 2017. Magmatic pulse driven by sea-level changes associated with the Messinian salinity crisis. Nature Geoscience, v. 10 online; doi:10.1038/ngeo3032).

From the record of salt precipitation, Pietro Sternai and colleagues, reckon that the main phase of unloading of the Mediterranean Basin began at around 5.6 Ma. Allowing for loading by the thick evaporites they calculated that the effect of the loss of water mass was equivalent to an unloading of 15 MPa in the deeper Eastern Mediterranean and 10 MPa in the west. Using standard pressure-temperature melting curves for the upper mantle, they then estimated that any magma chambers affected by the decrease in pressure could yield up to 17% more melt. Radiometrically dated lavas and igneous dykes within the Mediterranean region became more frequent and the number of events more than doubled during the time of main salt deposition.

https://upload.wikimedia.org/wikipedia/commons/thumb/6/64/DenglerSW-Stromboli-20040928-1230x800.jpg/1200px-DenglerSW-Stromboli-20040928-1230x800.jpg

Stromboli, one of the most active volcanoes in the Mediterranean Basin (credit: Wikipedia)

In May 2017 a study of subglacial volcanoes in West Antarctica based on radar mapping of the solid surface identified 138, 91 of them previously unknown (van Wyk de Vries et al. 2017. A new volcanic province: an inventory of subglacial volcanoes in West Antarctica.  Geological Society, London, Special Publication 461) They lie within a buried rift system and are covered by thick ice. Only one volcano in Antarctica is known to be active, Erebus, which is part of the cluster. Most of the news items stemming from the publication mentioned the possibility that the buried volcanic tract could be adding to the instability of the West Antarctic Ice Sheet through heating up its base. The WAIS is the ice sheet most feared to collapse seawards leading to a rise of about 3 m in global sea level. If the 2 km thick WAIS did slide off its underlying crust it might possibly trigger reactivation of the volcanic cluster.