GeoLog

sediment

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

Seawater, fish larvae and sediments – a snapshot of an ecosystem off South Africa

Earlier this month Jens Weiser set out from southern Africa to find out more about the region’s biology and geology. Back aboard FS Meteor, he’s searching for layers of lagoonal muds to see what the climate was like here in the late Quaternary…

After quite a lot of transit, we arrived at our first big station off Durban on Wednesday afternoon. On our way here we used the time to introduce ourselves and our work to the rest of the group. Every participant had prepared a short talk about a recently finished project or his/her home institution. Since we all come from different parts of the world (South Africa, Germany, Kenya, Madagascar, Mauritius and Spain) and are involved in very different projects, these sessions were a nice opportunity to think outside the box for a while. I had for example never heard a talk on the ecology of fish larvae before, so meant that I could look at the environment from another perspective, rather than my usual geological approach.

We started our fieldwork with a CTD (Conductivity, Temperature, Density)-cast, as we will on every major stop from now on. The device is lowered down to a specific depth, constantly measuring the physical properties of the water. These will give a good insight into the architecture of the water mass and should correlate with the biological data too.

The biological work included plankton nets, multi nets and a ring trawl. The nets are used to retrieve plankton in various sizes and from varying depths, since the opening of the multi net can be opened and closed independently. As it is towed vertically back to the ship, different levels of the water column can be sampled. The ring trawl is towed while the vessel is moving and recovers plankton from the surface. During analysis, the biologist on board will focus on the fish larvae and copepods. As we are following the Agulhas Current to the South, changes in their age and abundance will give insight into the ecology of these organisms in this hydrologically dynamic region.

The multi net is lowered to a desired depth. On its way back to the surface it recovers plankton samples from varying intervals of the water column. (Credit: Jens Weiser)

The multi net is lowered to a desired depth. On its way back to the surface it recovers plankton samples from varying intervals of the water column. (Credit: Jens Weiser)

The weather was far from perfect the entire night, but when we wanted to start the geological work just before sunrise the strong rain and winds of up to 8 on the Beaufort Scale made things really uncomfortable. Since the selected sites looked promising on the PARASOUND (sub-seafloor, high-resolution imaging) instrument, we wanted to try it anyway. Promising, in this case, means: a thin layer of coarse, sandy sediments overlying lagoonal muds – what we were searching for. The first device we tried was the gravity corer, which uses only a heavy weight to force the drill pipe into the sediment. This did not return a satisfying core though, even when we changed the position. So we switched to the Vibrocorer, which uses a vibrating engine and is more suitable for coarse-grained sediments. This worked much better and throughout the day we were able to recover three cores of that were 5 metres long. Unfortunately none of them reached the lagoonal sediments, which are the more useful paleoenvironmental archive. A more detailed investigation in the next few days will reveal whether we can use the sand and shell debris we have recovered anyway. Some of the biologist are interested in them as well – it’s a good thing we are on such a multidisciplinary cruise.

The Vibrocorer is prepared on deck. It is the lowered to the seafloor, where it can recover cores from coarse-grained sediments. (Credit: Jens Weiser)

The Vibrocorer is prepared on deck. It is the lowered to the seafloor, where it can recover cores from coarse-grained sediments. (Credit: Jens Weiser)

By Jens Weiser, University of Bremen

Catch up on findings from the research cruise here.