GeoLog

sediment

Could beavers be responsible for long-debated deposits?

Could beavers be responsible for long-debated deposits?

Following her presentation at the European Geosciences Union General Assembly in Vienna, I caught up with geomorphologist and environmental detective Annegret Larsen from the University of Lausanne, Switzerland, about beavers, baffling sediments and a case she’s been solving for the past seven years.

Back in 2012 the German geomorphology community was seriously debating the source of buried black soils, a stark black layer of sediment found in floodplain deposits all over Europe. Such dark sediments are usually associated with organic, carbon-rich materials, like peat. But unlike the other dark deposits, these soils are low in organic carbon, leading to a wide spectrum of ideas about their origin.

“They’re almost everywhere, and many people have had big fights about them and where they come from. Fire might have played a role, or human impact, or a rising water table associated with changes in climate,” explains Larsen.

The soils themselves are quite variable. Some deposits are quite muddy, while some trap fragments of long-dead plants. “They look a little like the relic of a swamp, containing grassy vegetation, sticks, leaves and little nuts, and they’re mainly black,” said Larsen. At the University of Lausanne, Switzerland and the University of Manchester, UK, she and her colleagues have been studying the composition and chemistry of black soils in an effort to understand how they formed.

Recently, Larsen has uncovered a possible connection between the black soil deposits and European beaver habitats. She presented her findings at the annual EGU meeting earlier this month.

The accused: a European beaver. Credit: Per Harald Olson via Wikimedia Commons

The idea began to take shape while Larsen was driving within the Spessart region of Switzerland. During her travels, she had found the soil situated in environments where beaver populations had been dwelling for some 25 years.

“There are huge swamps, what we call beaver meadows. And the vegetation communities are just like the ones found in those deposits,” said Larsen.

This discovery led her to develop a field experiment with the aim to determine whether beavers could be responsible for these puzzling black deposits.

“It’s like a big mystery for me. To find out if the black floodplain soil really come from when there was a widespread beaver population, before humans eradicated the beaver, I need to understand what the beaver does nowadays, and that’s how I started the project.”

Larsen thinks the beaver-created landscapes change with age, and she has been keeping a close watch on four sites across Switzerland and Germany, where beaver communities have been established for up to 25 years.

The long-toothed mammals have striking impacts on the landscape, which differ depending on where they build their dam. Upstream architecture results in beaver cascades, a series of closely packed ponds, each separated by a beaver dam. Down river, efforts go into one ‘megadam’ that stretches across a slow, meandering section of the stream and cause it to spill out into a large swampy floodplain.

The cascades, Larsen describes, are pretty dynamic. “Sediment gets trapped behind each dam, then they get strained, breach and break, causing sediment to flush downstream. It’s collected by the next dam and that then overtops and then that breaks” and the process starts all over again.

One of Larsen’s field sites: the Distelbach beaver reach. Credit: Annegret Larsen

Beaver meadows begin as large expanses of water, ponds teeming with semi-aquatic vegetation. Over time, fine sediment gathers in the ponds. As the sediment builds up, the area becomes a swamp – a patchwork of shrubs, trees, running water and tough, grassy plants. “You definitely get an explosion in diversity, but it’s a complete change, the area becomes a wetland,” adds Larsen.

And the wetland contains plants that resemble those found in the buried floodplain soils.

“For me, it’s fascinating to think about how all our streams would have looked with a beaver in there: before humans impacted those streams, before humans eradicated the beaver, and before [humans] settled there. There must have been beavers everywhere. Every stream would have been a beaver stream. And a beaver stream looks totally different [to what we see today].”

With the deposits all over Europe, it isn’t hard to imagine that, in years past, beavers shaped the streams, swamps and landscapes of the continent. It’s feasible that these regions might have been swampy landscapes at one point in history.

So, are the beavers behind the black soils? “I think we’re on a good path to contribute to this discussion. It’s at least as reasonable as fire and climate,” she replies.

Larsen makes a strong case, but the jury, it seems, is still out.

By Sara Mynott, EGU Press Assistant

Imaggeo on Mondays: Small scale processes, large scale landforms

Imaggeo on Mondays: Small scale processes, large scale landforms

This picture was taken in a sea cliff gully landscape at the Portuguese coast. It shows the microrelief which small scale wash and erosional processes produce in these poorly consolidated sediments. These small scale landforms could be interpreted as initial stages of larger scale gully landforms, which can be seen in the back. This highlights the importance of regarding scales and scale linkages in the geosciences.

Description by Jana Eichel, as it first appeared on imaggeo.egu.eu.

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: Sediments make the colour

Imaggeo on Mondays: Sediments make the colour

Earth is spectacularly beautiful, especially when seen from a bird’s eye view. This image, of a sweeping pattern made by a river in Iceland is testimony to it.

The picture shows river Leirá which drains sediment-loaded glacial water from the Myrdalsjökull glacier in Iceland. Myrdalsjökull glacier covers Katla, one of Iceland’s most active and ice-covered volcanoes.

A high sediment load (the suspended particles which are transported in river water) is typical for these glacial rivers and is visible as the fast-flowing glacial river (on the right of this image) appears light brown in colour. The sediment is gradually lost in the labyrinth of small lakes and narrow, crooked connections between lakes as can be seen as a gradual change in colour to dark blue.

The sediment load, height of the water  and chemistry of this and other glacial rivers are measured partly in real-time by the Icelandic Meteorological Office. This is done for research purposes and in order to detect floods from subglacial lakes that travel up to several tens of kilometers beneath the glacier before they reach a glacial river.

These glacial outburst floods do not only threaten people, livestock and property, but also infrastructure such as Route 1, a circular, national road which runs around the island. They occur regularly due to volcanic activity or localized geothermal melting on the volcano, creating a need for an effective early-warning system.

Advances in the last years include the usage of GPS instruments on top of a subglacial lake and the flood path in order to increase the early-warning for these floods. In 2015, the GPS network, gave scientists on duty at the Icelandic Meteorological Office 3.5 days of warning before one of the largest floods from western Vatnajökull emerged from beneath the ice.

The peak discharge exceeded 2000 m3/s,  which is comparable to an increase in discharge from that of the Thames to that of the Rhine.  This flood was also pioneeringly monitored with clusters of seismometers, so called arrays (from University College Dublin & Dublin Institute for Advanced Studies, Ireland), that enabled an early-warning of at least 20 hours and allowed to track the flood front merely using the ground vibrations it excited. The flood propagated under the glacier at a speed of around 2 km/h; so assuming you can keep up the speed over nearly a day you can escape the flood by walking while it is moving beneath the glacier.

Related publications about the tracking of these subglacial floods will emerge in the published literature soon (real time update available at www.evapseibl.wordpress.com).

By Eva Eibl, researcher at the Dublin Institute for Advanced Studies.

Thanks go to www.volcanoheli.is who organised this trip.

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/.

 

The known unknowns – the outstanding 49 questions in Earth Sciences (Part IV)

We are coming to the end of the known unknowns series and so far we have explored issues which mainly affect the inner workings of our planet. Today we’ll take a look at the surface expression of the geological processes which shape the Earth. Topography significantly affects our daily life and is formed via an interplay between primarily tectonics and climate, but it also affected by biological, mechanical and chemical processes at the Earth’s surface. We’ve  highlighted how advances in technology mean detailed study of previously inaccessible areas has now become possible, but that doesn’t mean there aren’t still plenty of questions left unanswered!

Earth’s landscape history and present environment

Drainage patterns in Yarlung Tsangpo River, China (Credit: NASA/GSFC/LaRC/JPL, MISR Team)

Drainage patterns in Yarlung Tsangpo River, China (Credit: NASA/GSFC/LaRC/JPL, MISR Team)

  • Can we use the increasing resolution of topographic and sedimentary data to derive past tectonic and climatic conditions? Will we ever know enough about the erosion and transport processes? Was also the stocasticity of meteorological and tectonic events relevant in the resulting landscape? And how much has life contributed to shape the Earth’s surface?
  • Can classical geomorphological concepts such as ‘peneplanation’ or ‘retrogressive erosion’ be understood quantitatively? Old mountain ranges such as the Appalachian or the Urals seem to retain relief for > 10^8 years, while fluvial valleys under the Antarctica are preserved under moving ice of kilometric thickness since the Neogene. What controls the time-scale of topographic decay? (Egholm, Nature, 2013)
  • What are the erosion and transport laws governing the evolution of the Earth’s Surface? (Willenbring et al., Geology, 2013) Rivers transport sediment particles that are at the same time the tools for erosion but also the shield protecting the bedrock. How important is this double role of sediment for the evolution of landscapes? (Sklar & Dietrich, Geology, 2011, tools and cover effect); (Cowie et al., Geology, 2008, a field example).
  • Can we predict sediment production and transport for hazard assessment and scientific purposes? (NAS SP report, 2010)
  • What do preserved 4D patterns of sediment flow tell us from the past of the Earth? Is it possible to quantitatively link past climatic and tectonic records to the present landforms? Is it possible to separate the signals of both processes? (e.g. Armitage et al., Nature Geosc, 2011).

    Smaller-scale patterns at the limit between river channels and hillslopes (Credit: Perron Group, MIT)

    Smaller-scale patterns at the limit
    between river channels and hillslopes (Credit: Perron Group, MIT)

  • Can we differentiate changes in the tectonic and climate regimes as recorded in sediment stratigraphy? Some think both signals are indeed distinguishable(Armitage et al., Nature Geosc, 2011). Others, (Jerolmack &Paola, GRL, 2010), argue that the dynamics intrinsic to the sediment transport system can be ‘noisy’ enough to drown out any signal of an external forcing.
  • Does surface erosion draw hot rock towards the Earth’s surface? Do tectonic folds grow preferentially where rivers cut down through them, causing them to look like up-turned boats with a deep transverse incision? (Simpson, Geology, 2004).
  • How resilient is the ocean to chemical perturbations? What caused the huge salt deposition in the Mediterranean known as the Messinian Salinity Crisis? Was the Mediterranean truly desiccated? What were the effects on climate and biology, and what can we learn from extreme salt giants like this? (e.g. Hsu, 1983; Clauzon et al., Geology, 1996; Krijgsman et al., Nature, 1999; Garcia-Castellanos & Villaseñor, Nature, 2011). Were the normal marine conditions truly reestablished by the largest flood documented on Earth, 5.3 million years ago? (Garcia-Castellanos et al., Nature, 2009).

The next post will be our final post in the series and we will list open questions on how climate has contributed to shape the surface of planet Earth, from its surface to the emergence of life and beyond.

Have you been enjoying the series so far? Let us know what you think in the comments section below, particularly if you think we’ve missed any fundamental questions.

By Laura Roberts Artal, EGU Communications Officer, based on the article previously posted on RetosTerricolas by Daniel Garcia-Castellanos, researcher at ICTJACSIC, Barcelona