GeoLog

Geomorphology

Imaggeo on Mondays: Isolated atoll

Imaggeo on Mondays: Isolated atoll

Covering a total area of 298 km², the idylic natural atolls and reefs of the Maldives stretch across the Indian Ocean. The tropical nation is famous for it’s crystal clear waters and picture perfect white sand beaches, but how did the 26 ring-shaped atolls and over 1000 coral islands form?

Coral reefs commonly form immediately around an island, creating a fringe which projects seawards from the shore. If the island is of volcaninc origin and slowly subsides below sea level, while the coral continues to grow growing outwards and upwards, an atoll is formed. They are usually roughly circular in shape and have a central lagoon. If the coral reef grows high enough, it will emerge from the sea waters and start to form a  tiny island.

“I took this photo while flying over the Maldives, south of Malè, from a small seaplane,” describes Favaro, who took this stunning aerial image of an atoll above the Indian Ocean.

Pictured, goes on to explain Favaro,

“[is] part of the ring-shaped coral reef bounding the atoll. On the right side of the image there is the lagoon and on the left side the open ocean. The coral reef is interrupted twice by ‘Kandu’ (water passages in Dhivehi [the language spoken in the Maldives]), which are the places where water flows in and out of the atoll when the tides changes”.

Two small harbours and antennas suggest the two small islands are occupied by local people, not by a resort or hotels.

“What always strikes me is how they can live so isolated, in a place which doesn’t offer basic resources, such as drinkable water,” says Favaro.

Fresh water is scarce in this archipelago nation. Rainwater harvesting is unreliable; poor rainfall means depleted collection tanks and groundwater tables. The problem is being exacerbated by climate change which is altering the monsoon cycle and rainfall patters over the Indian Ocean. As a result, the country relies heavily on desalination plants (and imported bottled water) to sustain the nation and the 1 million tourists who visit annually.

This animation shows the dynamic process of how a coral atoll forms. Corals (represented in tan and purple) begin to settle and grow around an oceanic island forming a fringing reef. It can take as long as 10,000 years for a fringing reef to form. Over the next 100,000 years, if conditions are favorable, the reef will continue to expand. As the reef expands, the interior island usually begins to subside and the fringing reef turns into a barrier reef. When the island completely subsides beneath the water leaving a ring of growing coral with an open lagoon in its center, it is called an atoll. The process of atoll formation may take as long as 30,000,000 years to occur. Caption and figure credit: National Oceanographic and Atmospheric Administration (NOAA).

References and further reading

How Do Coral Reefs Form? An educational resource by NOAA

Amazing atolls of the Maldives – a feature on NASA’s Earth Observatory.

 

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

 

Imaggeo on Mondays: A prehistoric forest

Imaggeo on Mondays: A prehistoric forest

This stunning vista encompasses the south-western wilderness of Tasmania as seen from the Tahune air walk 60 m above the Huon river valley. In front lies the beginning of a huge UNESCO World Heritage Site, covering almost a fourth of the area of Tasmania. The site mostly consists of a pristine, temperate rainforest of Gondwanan origin that is home to the tallest flowering trees in the world; Eucalyptus spp. reach up to 100 m height in this region.

“I have never tasted the sense of a more remote place than this one. Give me more,” says Vytas Huth, who captured this stunning shot.

Gondwana was a supercontinent, consisting of present day Africa, South America, India, Madagascar, Australia and New Zealand. It formed when the even larger supercontinent of Pangaea broke up 250 million years ago.

Slowly, Gondwana started to break apart too. India tore away first, followed by Africa and then New Zealand. By the end of the Cretaceous, 65 million years ago, only South America, Australia and Antarctica remained joined.  It took a further 20 million years before Australia and Antarctica separated.

By the time Australia started being pulled northwards, the first glaciers were forming on Antarctica, as it began freezing over. Atop the old rocks which made up its bulk, animals and plants of ancient origin, travel northwards with the Land Down Under.

Because India and Africa broke away from the supercontinent so early on, few hallmarks of ancient Gondwana wildlife are left in their present biodiversity. In contrast, Australia and Tasmania remained connected to Antarctica and South America much longer and there are clear similarities in species across these continents.

“Fossil evidence suggests that temperate rainforest once extended across Australia, Antarctica, South America and New Zealand around 45 million years ago. Such fossils and the surviving species in Tasmania provide evidence of the ancient link to Gondwana”, reports the Tasmania Parks & Wildlife Service.

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

 

Imaggeo on Mondays: Viñales Valley

Imaggeo on Mondays: Viñales Valley

From last week’s unusual desert landscape to this week’s lush valley in Cuba…

The picture shows the Vinales Valley, a karstic depression with mogotes in western Cuba. Karst is the general term for landscapes formed when limestone is disolved by carbonic acid, in rain water. This leads, in particular, to the formation of an underground network of caves and rivers.

In the tropics, due to the heavy rains, the dissolution is fast. The ground collapses above the caves and the karst landscape may evolve to mogotes, which are isolated and steep-sided limestone hills; visible in this week’s featured image. Mogotes can also be found in Eastern Asia, for instance in Halong Bay (Vietnam) or Bohol Island (‘Chocolate Hills’) in the Philipines.

By Alexis Merlaud, Belgian Institute for Space Aeronomy

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

GeoSciences Column: When could humans last walk, on land, between Asia & America?

GeoSciences Column: When could humans last walk, on land, between Asia & America?

Though now submerged under 53 m of ocean waters, there once was a land bridge which connected North America with Asia, allowing the passage of species, including early humans, between the two continents. A new study, published in the EGU’s open access journal Climate of the Past, explores when the land bridge was last inundated, cutting off the link between the two landmasses.

The Bering Strait, a narrow passage of water, connects the Arctic Ocean with the Pacific Ocean. Located slightly south of the Arctic Circle, the shallow, navigable, 85 km wide waterway is all that separates the U.S.A and Russia. There is strong evidence to suggest that, not so long ago, it was possible to walk between the two*.

The Paleolithic people of the Americas. Evidence suggests big-animal hunters crossed the Bering Strait from Eurasia into North America over a land and ice bridge (Beringia). Image: The American Indian by Clark Wissler (1917). Distributed via Wikipedia.

In fact, though the subject of a heated, ongoing debate, this route is thought to be one of the ones taken by some of the very first human colonisers of the Americas, some 16, 500 years ago.

Finding out exactly when the Bering Strait last flooded is important, not only because it ends the last period when animals and humans could cross between North America and northeast Asia, but because an open strait affects the two oceans it connects. It plays a role in how waters move around in the Arctic Ocean, as well as how masses of water with different properties (oxygen and/or salt concentrations and temperatures, for example) arrange themselves. The implications are significant: currently, the heat transported to Arctic waters (from the Pacific) via the Bering Strait determines the extend of Arctic sea ice.

As a result, a closed strait has global climatic implications, which adds to the importance of knowing when the strait last flooded.

The new study uses geophysical data which allowed the team of authors to create a 3D image of the Herald Canyon (within the Bering Strait). They combined this map with data acquired from cylindrical sections of sediment drilled from the ocean floor to build a picture of how the environments in the region of the Bering Strait changed towards the end of the last glaciation (at the start of a time known as the Holocene, approximately 11,700 years ago, when the last ‘ice age’ ended).

At depths between 412 and 400 cm in the cores, the sediment experiences changes in physical and chemical properties which, the researchers argue, represent the time when Pacific water began to enter the Arctic Ocean via the Bearing strait. Radiocarbon dating puts the age of this transition at approximately 11, 000 years ago.

Above this transition in the core, the scientist identified high concentrations of biogenic silica (which comes from the skeletons of marine creatures such as diatoms – a type of algae – and sponges); a characteristic signature of Pacific waters. Elevated concentrations of a carbon isotope called delta carbon thirteen (δ 13Corg), are further evidence that marine waters were present at that time, as they indicate larger contributions from phytoplankton.

The sediments below the transition consist of sandy clayey silts, which the team interpret as deposited near to the shore with the input of terrestrial materials. Above the transition, the sediments become olive-grey in colour and are exclusively made up of silt. Combined with the evidence from the chemical data, the team argue, these sediments were deposited in an exclusively marine environment, likely influenced by Pacific waters.

Combining geophysical data with information gathered from sediment cores allowed the researchers to establish when the Bering Strait closed. This image is a 3-D view of the bathymetry of Herald Canyon and the chirp sonar profiles acquired along crossing transects. Locations of the coring sites are shown by black bars. Figure taken from M. Jakobsson et al. 2017.

The timing of the sudden flooding of the Bering Strait and the submergence of the land bridge which connected North America with northeast Asia, coincides with a period of time characterised by Meltwater pulse 1B, when sea levels were rising rapidly as a result of meltwater input to the oceans from the collapse of continental ice sheets at the end of the last glaciation.

The reestablishment of the Pacific-Arctic water connection, say the researchers, would have had a big impact on the circulation of water in the Arctic Ocean, sea ice, ecology and potentially the Earth’s climate during the early Holocene. Know that we are more certain about when the Bering Strait reflooded, scientist can work towards quantifying these impacts in more detail.

By Laura Roberts Artal, EGU Communications Officer

 

*Authors’s note: In fact, during the winter months, when sea ice covers the strait, it is still possible to cross from Russia to the U.S.A (and vice versa) on foot. Eight people have accomplished the feat throughout the 20th Century. Links to some recent attempts can be found at the end of this post.

References and resources:

Jakobsson, M., Pearce, C., Cronin, T. M., Backman, J., Anderson, L. G., Barrientos, N., Björk, G., Coxall, H., de Boer, A., Mayer, L. A., Mörth, C.-M., Nilsson, J., Rattray, J. E., Stranne, C., Semiletov, I., and O’Regan, M.: Post-glacial flooding of the Bering Land Bridge dated to 11 cal ka BP based on new geophysical and sediment records, Clim. Past, 13, 991-1005, https://doi.org/10.5194/cp-13-991-2017, 2017.

Barton, C. M., Clark, G. A., Yesner, D. R., and Pearson, G. A.: The Settlement of the American Continents: A Multidisciplinay Approach to Human Biogeography, The University of Arizona Press, Tuscon, 2004.

Goebel, T., Waters, M. R., and Rourke, D. H.: The Late Pleistocene Dispersal of Modern Humans in the Americas, Science, 319,1497–1502, https://doi.org/10.1126/science.1153569, 2008

Epic explorer crossed frozen sea (BBC): http://news.bbc.co.uk/2/hi/uk_news/england/humber/4872348.stm

Korean team crossed Bering Strait (The Korean Herald): http://www.koreaherald.com/view.php?ud=20120301000341

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