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Imaggeo on Mondays: Exploring the East African Rift

10 Mar

This week’s Imaggeo on Mondays is brought to you by Alexis Merlaud, an atmospheric scientist from the Belgian Institute for Space Aeronomy. While the wonders of the African atmosphere feature in his photography, the East African Rift has a much bigger tale to tell. Drawing from all aspects of geoscience Alexis shares its story…

Kilimanjaro from Mount Meru. (Credit: Alexis Merlaud, distributed via imaggeo.egu.eu)

Kilimanjaro from Mount Meru. (Credit: Alexis Merlaud, distributed via imaggeo.egu.eu)

This picture shows Kilimanjaro, Africa’s highest mountain, at sunrise. It was taken from Socialist Peak, which marks the top of Mount Meru, some 70 km to the southwest. Both mountains are located in Tanzania and are among the largest stratovolcanoes of the East African Rift Zone. Unlike Kilimanjaro, Meru is active and its most recent eruption occurred in 1910.

Stratovolcanoes, also called composite cones, are built-up by alternating layers of lava flows, pyroclastic rocks, and volcanic ash. During a large eruption, huge quantities of ash and sulphur dioxide can reach the stratosphere, where they can affect the climate for several years, as did the eruptions of Krakatau in 1883 and Pinatubo in 1991. Sulphur dioxide is converted to sulphuric acid droplets, which spread with the ashes throughout the stratosphere. These aerosols screen some of the sunlight, decreasing the average surface temperature by about one degree. The temperature in the stratosphere simultaneously rises by a few degrees, due to the enhanced absorption of sunlight by aerosols.

There is a difference in the tectonic processes associated with these South East Asian volcanoes and the East African Rift: the former are located above a subduction zone while the rift is a divergent boundary.  An example of large volcanic eruption in a divergent zone is the Laki (Iceland) eruption in 1783, which yielded severe meteorological conditions and reduced harvests for several years in Europe. This eruption may have also helped trigger the French Revolution in 1789.

Plate tectonics in East Africa created Kilimajaro and have also played a role in early human evolution, by shaping the local landscape and the long-term climate, thus modifying the environment of our ancestors. East Africa is the area in the world where most of the hominid fossils have been discovered, including Homo sapiens – the oldest fossil record is 200,000 years old and started to move out from Africa 100,000 years ago!

A final thanks: thanks Cristina Brailescu for help climbing Meru and Emmanuel Dekemper for support on editing the picture. 

By Alexis Merlaud, Belgian Institute for Space Aeronomy

Imaggeo is the EGU’s open access geosciences image repository. Photos uploaded to Imaggeo can be used by scientists, the press and the public provided the original author is credited. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. You can submit your photos here.

Imaggeo on Mondays: Iceland’s highlands

3 Mar

This week’s Imaggeo on Mondays provides a little insight into what you might find beneath your feet as you explore the Icelandic highlands… 

Autumn mountain vegetation, Central Highland, Iceland. (Credit: Ragnar Sigurdsson/arctic-images.com via imaggeo.egu.eu)

Autumn mountain vegetation, Central Highland, Iceland. (Credit: Ragnar Sigurdsson/arctic-images.com, distributed via imaggeo.egu.eu)

You can stumble upon wild blueberries, better known to botanists as vaccinium uliginosum, in cool temperate regions of the Arctic, as well as other mountainous areas including the Pyrenees, Alps, and Rockies. They thrive in wet acidic soils – the sort you might find in heathlands, moorlands and stretches of Arctic tundra, and can carpet the ground beneath coniferous forests too!

Here’s a close-up! (Credit: Kim Hansen via Wikimedia Commons)

Here’s a close-up! (Credit: Kim Hansen via Wikimedia Commons)

Imaggeo is the EGU’s open access geosciences image repository. Photos uploaded to Imaggeo can be used by scientists, the press and the public provided the original author is credited. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. You can submit your photos here.

Geosciences column: Shifting the O in H2O

28 Feb

Wherever you are in the world’s oceans, you can identify particular bodies of water (provided you have the right equipment) by how salty they are. You can get a feel for how productive that part of the ocean is by measuring a few chemical components in the water column. And, year on year, you will see a recurring pattern in how things like temperature, salinity and oxygen content vary with depth. This last property – the oxygen content – is vital for life in the oceans, but recent decades have seen shifts in the amount available.

There is always more oxygen at the surface than there is at depth. When waves break they mix an abundance of tiny air bubbles into the water, providing oceans with their oxygen supply, which is mixed into the deep through large-scale ocean circulation and storms over winter. At the surface, algae make the most of the abundant light to photosynthesise, beginning the base of the marine food web and adding a little more oxygen to the water in the process. These microscopic plants are eaten by animal plankton (zooplankton), which are, in turn, eaten by other plankton, crustaceans, fish, and a plethora of other predators – none of which contribute to the ocean’s oxygen. Instead, they, and a multitude of microbes, slowly use up more and more of the supply as they respire and there comes a point in the water column where there is no longer enough oxygen for these aerobic animals to survive – the oxygen minimum zone (OMZ).

The surface ocean, where oxygen begins its journey to the deep. (Credit: Anna Lourantou, distributed via imaggeo.egu.eu)

The surface ocean, where oxygen begins its journey to the deep. (Credit: Anna Lourantou, distributed via imaggeo.egu.eu)

What marks the boundary of this zone is dependant not on the properties of the water, but the life that lives there – it is the point when marine organisms experience hypoxic stress, usually an oxygen concentration in the range of 60–120 μmol kg−1. Below this, life in the marine environment is very different indeed. Anaerobic microbes thrive below the OMZ, making the most of life in an environment where there is very little oxygen in each litre of seawater.

The boundary between oxygen-rich water and the OMZ is known as the oxygen limiting zone (OLZ), and during the day many small swimming species take refuge here to avoid their predators. In the Eastern Pacific, you reach the OLZ when there’s 60 μmol kg−1 oxygen in the water, and the OMZ when there’s a mere 20 μmol kg−1.

Waves are key to mixing oxygen into the ocean. When they break at the surface they mix air bubbles into the water, taking oxygen from the atmosphere into the sea. (Credit: NOAA Okeanos Explorer Program)

Waves are key to mixing oxygen into the ocean. When they break at the surface they mix air bubbles into the water, taking oxygen from the atmosphere into the sea. (Credit: NOAA Okeanos Explorer Program)

The depth of the OMZ depends on temperature. Because warmer water is capable of containing less dissolved gas than cold, the OMZ is found at shallower depths in the tropics, and occurs at shallower depths in the summer than it does over winter. Winter weather allows more oxygen to be mixed into the deep ocean as storms break down the sea’s stratification, bringing nutrients to the surface and replenishing supplies closer to the sea floor. However, when there’s a lot of production at the surface (which draws down the oxygen) and the replenishment at depth is slow, large oxygen minimum zones persist from year to year.

Recently though, the upper boundaries of these zones have been shifting to shallower depths, resulting larger hypoxic regions in the ocean. Since the 1960s, the OLZ in the Gulf of Alaska, for example, has shifted some 100 metres shallower. Why?

The oceans are absorbing more heat in response to climate change. Because high temperatures reduce oxygen solubility, they reduce the amount of dissolved oxygen at the surface. The increase in surface heat also creates stronger stratification in the ocean, making it harder for oxygen to be mixed deep into the water column, and reducing dissolved oxygen at depth. Ocean circulation systems are also in a state of change, with systems like the Atlantic meridional overturning circulation in decline. Such changes in ocean circulation will also affect the amount of oxygen that’s mixed into the deep sea.

Atlantic meridional overturning circulation, better known as AMOC. Red arrows show warm water circulation in the upper 1100 m and blue arrows show the southward flow of cold, deep water. (Credit: Smeed et al., 2014)

Atlantic meridional overturning circulation, better known as AMOC. Red arrows show warm water circulation in the upper 1100 m and blue arrows show the southward flow of cold, deep water. (Credit: Smeed et al., 2014)

Working out whether this is part of a long-term trend is a difficult task, as records of deep ocean oxygen only stretch back to 70 years ago. Only a longer record of observations will help determine the trend, but for now we can be sure that shoaling oxygen minimum zones will change the amount of habitat available to species either side of the line between oxygen-rich and oxygen-poor.

By Sara Mynott, EGU Communications Officer

References:

Gilly, W. F., Beman, J. M., Litvin, S. Y., & Robison, B. H.: Oceanographic and biological effects of shoaling of the oxygen minimum zone. Annual Review of Marine Science, 5, 393-420, 2013

Smeed, D. A., McCarthy, G. D., Cunningham, S. A., Frajka-Williams, E., Rayner, D., Johns, W. E., Meinen, C. S., Baringer, M. O., Moat, B. I., Duchez, A., and Bryden, H. L.: Observed decline of the Atlantic meridional overturning circulation 2004–2012, Ocean Sci., 10, 29-38, 2014

Imaggeo on Mondays: Friends in the field

24 Feb

Out in the field you encounter all sorts of wildlife and while mosquitos are the most frequent (and most unwelcome), they generally don’t interfere with your equipment or your data. The same can’t be said for all animals though, and many scientists have to strap their equipment out of reach, barricade it with barbed fences or place it in a relatively indestructible black box. It’s a particular problem when you need to head back to the lab or lecture theatre, and leave your equipment alone to collect precious scientific data remotely.

Animals can also cause a ruckus when you’re on site – after all, what’s more exciting than a geoscientist and their portable laboratory? This is surely the question that played on the minds of these bovine beasties before interfering with a geoelectrical survey, a method used to monitor CO2 storage and map groundwater.

Does it work? (Credit: Robert Supper, distributed via imaggeo.egu.eu)

Does it work? (Credit: Robert Supper, distributed via imaggeo.egu.eu)

While surveying groundwater in Salzburg, Austria, Robert Supper caught a crowd of curious cows taking a closer look at his equipment. “During the measurements on a meadow, we were inspected by a drove of cows, which immediately started to taste electrodes and cables,” he explains.

“On geoelectrical surveys in rural areas, we often encounter an interesting phenomenon: cows or sheep completely ignore us until we finish the installation of cables and electrodes. As soon as we are ready and want to start the measurements, they start to inspect everything, sniff on the equipment, nibble on the cables, stumble over the profile or (worst case) shit on it. If everything was tested correctly by them, they disappear,” Supper adds. Take care when you’re working in a rural area, you might just get some company.

By Sara Mynott, EGU Communications Officer

If you are pre-registered for the 2014 General Assembly (Vienna, 27 April – 2 May), you can take part in our annual photo competition! Up until 1 March, every participant pre-registered for the General Assembly can submit up three original photos and one moving image related to the Earth, planetary, and space sciences in competition for free registration to next year’s General Assembly!  These can include fantastic field photos, a stunning shot of your favourite thin section, what you’ve captured out on holiday or under the electron microscope – if it’s geoscientific, it fits the bill. Find out more about how to take part at http://imaggeo.egu.eu/photo-contest/information/.

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