War and soil!

There was an unusual little paper in Geoderma’s last issue (Hupy & Schaetzl, 2008: Soil development on the WWI battlefield of Verdun, France. Geoderma 145: 37-49. DOI: 10.1016/j.geoderma.2008.01.024). While soil science is pretty far from what I do on a daily basis now, I had great interest in the subject when I was an undergrad and still finds it fascinating. Furthermore, I visited Verdun in 1994 and saw the remains of the battlefield and was amazed how little it had recovered - craters were absolutely everywhere and vast areas were cordoned off with warnings about un-exploded ammunition.  A study combining the two topics would be just up my alley!

Anyway, H&S studied soil development in and adjacent to artillery craters at three sites, corresponding to the three major soil types in the battlefield.  These in turn are related to the underlying geology and geomorphology of the area.  The area is structurally defined by a series of NE-SW running cuestas creating a sets of valleys and ridges.  In the valleys low-lying poorly drained areas have soils suffering from waterlogging (Pseudogleys), while the ridges have soils influenced by the shallow limestone bedrock (Brunified Rendzina and Calcareous Brown).  

The data were collected from the disturbed soil profiles (in the craters) compared to the ‘undisturbed’ profiles (adjacent to the craters). In 3 study sites, one for each major soil type  in the area, 3 soil profiles were dug in each of 3 craters for a total of 27 soil profiles.

At the end of the battle, no soil horizons were likely to exist in the area due to the intensive ‘bomturbation’ of the area. H&S found that a measurable amount of soil development have occurred since the disturbance in 1916: The crater bottoms have accumulated a thick layer of organic matter, the freshly exposed (in 1916) limestone bedrock have been weathered and its byproducts leached down-profile.  Weathering at the crater bottoms have occurred at a faster rate than elsewhere, most likely due to run-off accumulating in the crater bottoms.  Also earthworm activity was found to have moved humus-rich material into cracks and fractures in the limestone, thereby increasing the area exposed to leaching by humic acids.  

This means that soil formation has now been enhanced in crater bottoms located at the ridge crests due to 1) water being focused in the craters and 2) organic matter being collected at the crater bottoms.  In the valleys between the cuesta ridges soil formation has slowed as the crater bottoms are more or less permanently below the water table.  Soil formation has also increased on the slopes of the ridges, as the craters delay the run-off and increase the infiltration of surface water into the soils, thereby enhancing soil formation.

The vegetation in the area has generally not been restored to its pre-war state of beech, hornbeam and oak, due to the scale of the devastation.  Pine has been planted in heavily visited areas due to the soft lighting created by pine trees.  H&S try to make the claim that the soil development in the 88 years since the battle can be used as indicators of landscape recovery/resilience, in the absence of re-colonizing vegetation.  

However, it’s not clear to me exactly how.  Their data seems to show to me that while the same soil-forming processes are acting now as before the battle, the location of the resulting soils are different, for example as evidenced by the  development of soils in the crater bottoms on ridge crests.  Consequently reovery hasn’t been particularly successful (as I see it), but this is never really spelled out by H&S.  However, I still like the paper simply for being curious about soil development in a heavily bombarded area!

Anthropogenic erosion

Many people probably realise that a wide range of geological processes acting on the surface of our planet cause sediment erosion and movement: Rain water draining in a gully in a field, or waves slowly eating away coastal cliffs, for example. Landslides, aeolian (wind) activity and glaciers also erode sediment. 

Humans also move sediment (for example construction and mining activities). But how much? More or less than the 'normal' geological processes acting on the surface of the earth? These questions formed the basis of a couple of papers by Hooke (On the history of humans as geomorphic agents. Geology 28 (2000): 843-846) and Wilkinson (Humans as geologic agents: A deep-time perspective. Geology 33 (2005): 161-164. doi:10.1130/G21108.1), and they just blew my mind. 

Wilkinson looked at how much the deep-time sediment flux was, using the volume of surviving continental and oceanic sedimentary rock through the geological epochs from the Lower Cambrium to the Pliocene. As we go back in time less and less sedimentary rock remains, because there has been more time to erode it, and by fitting a curve to the remaining volume of sedimentary rock it is possible to estimate the cycling rate of the sedimentary rocks. At present 0.14% +- 0.06% of all sedimentary rock is being eroded every million year - equivalent to a lowering of the surface of the earth of 24+-11 m every million year.  This number is the rate we would expect without humans also acting on the surface of the planet.  So now on to the humans.

Human sediment-moving activities can be divided into direct (for example mining and construction) and indirect (for example agriculture and forestry) activities. In his paper, Hooke showed that movement of rock and soil during construction accounted for ~30% of all sediment transported by humans, the balance being made up by erosion from agricultural activities. The US Department of Agriculture estimates that development of pastureland results in a soil loss of 400 m every million year, while cropland tillage results in a soil loss of up to 1400 m every million year. By estimating the global area being tilled and the global area used as pastureland, Wilkinson estimated that the erosion resulting from these practices was equivalent to a lowering of the earth's surface at a rate of 360 m every million year - more than 10 times faster than the deep-time erosion rate obtained from natural processes. Humans are, therefore, much, *much* more efficient in eroding sediment than mother Nature herself! 

As amazing as that may seem, an even bigger surprise (to me) arose when Wilkinson then computed the historical rates of erosion caused by humans, and compared that to the deep-time erosion rates (the 24 m per million year above).  Hooke provided data on the per capita annual amounts of soil and rock movements from construction and agriculture for the last 5000 years.  By multiplying with the population estimate he could then estimate the total erosion due to human activity.  Wilkinson's comparison with the deep-time erosion rates showed that the two curves crossed each other around the end of the viking age (approximately year 1000).  This means that for the last 1000 years humans have on an annual basis eroded more sediment than all other natural processes acting on the surface of the planet combined.

That's wild! I first read about this 3 years or so ago and I'm still amazed by it.  Who would have thought that our ancestors just by digging around with primitive shovels, dragging wodden ploughs through the earth and planting some crops could erode as much sediment as all other natural processes combined? And this was in year 1000! Today it's 10 times as much! Wow!