The rectangular and oriented lakes in the Bolivian Amazon are not tectonic, and now what?

Our latest paper has been published a few days ago in Geomorphology. The title is: "The origin of oriented lakes: Evidence from the Bolivian Amazon". Here goes a very short version of it.
The presence of hundreds of rectangular and oriented lakes is one of the most striking characteristics of the Llanos de Moxos landscape (Fig. 1). Many different mechanisms have been proposed for their formation, including subsidence resulting from the propagation of bedrock faults through the foreland sediments, scouring caused by large-scale flooding, paleo deflation combined with wind/wave action and human agency. Nevertheless, amid this diversity of hypothesis, the most commonly accepted cause of lake formation to date has been tectonics.

Figure 1. Landsat image of oriented and rectangular lakes in the Llanos de Moxos

Plafker’s tectonic model (Fig. 2) has never been tested. If faulting is involved, the displacement should be visible and measurable through sediment profiling. The only element needed is a stratigraphic marker that allows the measurement of the vertical displacement.

Figure 2. Tectonic model for lake formation (Plafker, 1967). According to Plafker, the lakes' rectangular shape results from the propagation of bedrock fractures through unconsolidated sediments.

Thanks to our recent discovery of a paleosol below mid-Holocene fluvial sediments in the south-eastern LM (Lombardo et al., 2012), where several lakes are found, it is now possible to test the tectonic hypothesis. If lakes were formed by local subsidence induced by bedrock faults, we should find the paleosol at a greater depth below the lake than in the area surrounding it. 

This is how we cored the lakes

Stratigraphic profiles from transects that cut across the borders of three lakes show otherwise (Fig. 3): the depth of the paleosol is the same. Hence, tectonics, as the mechanism behind the formation of the lakes, can be ruled out. The origin of the Moxos rectangular and oriented lakes is still very much unresolved. A more detailed discussion about the possible mechanisms behind the lakes' formation can be found in Lombardo & Veit (In Press). 

Figure 3. Stratigraphic transects from the outside to the inside of the lakes. Dotted white lines define the lakes’ basins. The early to mid-Holocene paleosol acts as a stratigraphic marker (see Fig. 2). Cores 52, 63, 81, 170, 205 and 210 provide the reference depth of the paleosol outside the lakes; cores 77 and 204 have been performed in areas of the original lakes’ basins that have been infilled; cores 78, 169, 171 and 209_b come from inside the lakes. Continuous black lines reconstruct the original lake bottom (previous to lacustrine infilling); dashed black lines connect the paleosol. Source of digital images: Google earth.

References:

Lombardo, U., May, J.-H., & Veit, H. (2012). Mid- to late-Holocene fluvial activity behind pre-Columbian social complexity in the southwestern Amazon basin The Holocene DOI: 10.1177/0959683612437872
Lombardo, U., & Veit, H. (2013). The origin of oriented lakes: Evidence from the Bolivian Amazon Geomorphology DOI: 10.1016/j.geomorph.2013.08.029
Plafker, G. (1964). Oriented Lakes and Lineaments of Northeastern Bolivia Geological Society of America Bulletin DOI: 10.1130/0016-7606(1964)75[503:OLALON]2.0.CO;2

Effects of the expanding agriculture frontier in the Bolivian Amazon

Last week, a paper published in Nature spurred a lot of debates on the internet about the future of agriculture and our ability to feed the 9 billion people that the world will have in 2050. One important aspect related to this debate is the availability of agricultural land. However, people do not always have a clear idea of what expanding the agriculture land means. Here an example of what is happening in one of the most biodiversily rich places of the world: the Bolivan Amazon. Let’s just have a look at a few Google earth’s images of the Lake Peroto in the Llanos de Moxos (dep. of Beni in the Bolivian Amazon).

Image taken the 17 of May 2003. Land use is extensive cattle ranching. Typical floating vegetation (locally called “yomomo”) is growing along the shores of Lake Peroto.

Image taken the 20th of Nov 2004. Land use is extensive cattle ranching. It is the end of the dry season, and the water level is extremely low. In the greenish areas inside the lakes water is almost completely evaporated. This is normal in the Llanos de Moxos, where seasonality can be extreme

Image taken the 2nd of July 2009. Land use is extensive cattle ranching. We are at the beginning of the dry season, water level is quite high, the water is clearly visible.

Image taken the 8th of August 2011. Land use is extensive cattle ranching plus industrial agriculture. 70 hectares of land in the surroundings of the lake have been used to produce rice.

What happened to the lake in 2011? Now the lake is almost completely covered with vegetation... Is it dry? Or is it the first effect of land use change: eutrophication? Based on my personal experience of having been working in the Llanos de Moxos for more than 10 years now and based on the fact that the nearby Lake Suarez is full of water (photo below) on the 15 of August 2011, I would conclude that Lake Peroto is in process of eutrophication.

Lake Suarez. Image taken the 15th of August 2011. The lake is full.

Industrial agriculture in the Beni is already causing deforestation and threatening archaeological sites (see this). Now it is also affecting lake ecology. Is industrial agriculture what people in the Beni really need? Does this kind of agriculture make any sense in a place with a density population of 1 person per square kilometre? Here, a very important part of the population lives in indigenous communities whose subsistence depends on local natural resources. What will happen to them in ten years’ time if these resources are depleted?

Ref:

Seufert V, Ramankutty N, & Foley JA (2012). Comparing the yields of organic and conventional agriculture. Nature PMID: 22535250

No limits

We solved the problem! there is a very easy way to work in the lakes: learn to walk on the water:-))

We thanks our Anna for the photos, she is the official reporter of the blog:-)