Soil erosion and atmospheric carbon

We have seen that intensive agriculture requires soils to be processed, enhanced and engineered to maximise productivity. The soils that surround the Mediterranean basin are poor; their structure is weak and there is little organic material. Clearing the land of natural flora to create fields removed the root structures which consolidated the soil. Ploughing destroyed what little soil integrity existed and intense rainfall washed the dusty soils into valley bottoms (García-Ruiz, 2010; Romano et al, 2013, pp.115-6).

The phenomenon of alluviation into Mediterranean valleys was originally observed by Vita-Finzi (1969) and expanded by Leopold and Vita-Finzi (1998) who identified two phases: the reddish  "Older Fill" dating between 30,000 and 10,000 BP and the grey-brown "Younger Fill" dating to between AD 400 and AD 1850. The very fact that a geological structure can be identified across a region and for it to be strongly linked with human activity is supportive of the argument for an Anthropocene. It must be acknowledged, however, that the evidence for a direct causal link is equivocal, for example see Ackerman et al. (2014, pp.239-40), who note that erosion may actually be caused by the abandonment of farming infrastructure, as does García-Ruiz (2010).


1. Abandoned farms, 2. Crops in hills, 3. Steep vineyards, 4. Steep olive and almond orchards, 5. Soils degraded by irrigation. (García-Ruiz, 2010, p.2)

Van Oost et al (2007) considered whether this type of soil erosion could cause organic carbon in soil to be released into the atmosphere. Having surveyed a wide range of soils samples from Europe and North America, they concluded that the eroded material is burying carbon in the places to which it is transported and that carbon is sequestered in the eroded locations. They concluded, controversially, that the effect of erosion on atmospheric carbon is neutral; agricultural erosion does not offset the fossil fuel contribution, neither does it contribute to atmospheric carbon. In response, Lal and Pimentel (2008) argued that soils exposed by erosion take up carbon slowly, that the transport of eroded soils releases carbon and that the deposition of eroded soils releases methane.

Kuhn et al. (2009) considered the relationship between soil erosion and the carbon cycle in more detail. They highlighted three issues which impact on the process:

1. Interrill erosion is the movement of soil across a surface caused by the impact of raindrops loosening the soil structure and the splash of raindrops transporting material. It affects organic material more than inorganic material and so has the effect of concentrating organic carbon in interrill sediments by up to 50%. This surface concentration has the effect of increasing the carbon exchange with the atmosphere.
2. Rill erosion is the transport of soils and sediments by the action of flowing water. This action moves material downslopes and sorts material by size (Goldberg and Macphail, 2006, pp.72-84). The sorting and transporting preferentially affects organic carbon material compared with mineral sediments, creating concentrations of organic carbon-rich soils and carbon-poor sediments at a rate not found in nature.
3. Erosion system stationarity concerns the assumptions that (i) the erosion rate is consistent and that (ii) there is a linear relationship between soil erosion and the release of organic carbon. In fact upper soil horizons are organic-rich and therefore carbon-rich. The release of carbon through erosion thus diminishes over time as the richer horizons are removed (though this ignores any effects of manuring and other agricultural soil enrichment activities).

These issues do not offer a conclusion as to whether Van Oost et al. were correct. They do, however, demonstrate that agriculture changes the dynamics of soil erosion, and that those dynamics affect the exchange of carbon between the biosphere and the atmosphere. Agriculture thus leaves more permanent markers in our environment.

This post reads more like a section from an academic essay than a blog. That is because I thought it was important to show that answering my titular question: "What Started the Anthropocene, Farming or Factories?" is not about showing that Ruddiman, or anyone else, is right or wrong. It is about understanding the range of independent and interdependent processes that are caused or impacted by human activity and quantifying their effect on all aspects of our environment.


References cited not available online:
Goldberg, P. and Macphail, R., 2006. Practical and Theoretical Geoarchaeology. Oxford. Blackwell Publishing Limited.
Vita-Finzi, C., 1969. The Mediterranean Valleys Geological Change in Historical Time. Cambridge. Cambridge University Press.