Humans, Lakes, and Plate Tectonics in the Amazon Basin

Tectonics and surface processes have historically interfered with landscape and therefore with human societies. Earthquakes and floods are the most dramatic phenomenons in this respect, but there might be more subtile ways for the Solid Earth to shape our life style.
According to genetical and linguistic studies, the first humans arrived to the Americas in several migrations, starting ~15000 yr ago, around the end of the last glaciation. This age seems confirmed by datations of human coprolites from caves in Oregon (see Paisley Caves, at Lake Chewacan, original Science paper by Gilbert et al., 2008, here).

But when did these first migrants reach South America is as poorly constrained as controversial. In fact, it is now questioned that South America was populated from the north. Direct migrations from Asia across the Pacific may have occurred repeatedly during the Holocene. Equally debated are the causal relationship between three ubiquitous processes in the continent: the onset of a warmer climate since 10-15 kyr BP (Fig. 1), the massive extinction of most of the largest fauna of the continent, and the expansion of humans.

Fig. 1. Three proxies for surface temperature of the Earth, as a function of time (years BP). Young Dryas cold period is visible around 11kyr BP. Source: William M. Connolley in Commons

The puzzle becomes even more complicated because the Amazon basin is one of the most active sedimentary basins of the Cenozoic (last 65 million years), presently accumulating tens of centimeters of sediment every year, fed from the erosion of the second largest tectonic building in the world: the Andes. Needless to say, this long-term tectonic drive is also very sensitive to climate changes, in ways that are no yet well understood.

A candidate region to study the interactions between tectonics, climate and environment during the arrival of the hominids to South America is the Beni Basin. The mechanics of paleoenvironment must be well understood first, though. The Llanos de Moxos (LM) is the seasonally flooded savannah at the south-western Amazonia. This region is very sensitive to changes in hydrology and climate because of its extremely low topographic gradients and small changes in precipitation patterns can result in large shifts of the forest-savannah ecotone (Hamilton et al., 2004; Mayle et al., 2007).
The Llanos de Moxos (LM) is likely the region in Amazonia that was most intensely modified by pre-Columbians during the late Holocene (Erickson, 2008), but recently discovered earlier archaic archaeological sites suggests that hunter-gatherers inhabited the LM already since the early Holocene (Lombardo, 2012). From preliminary data, it seems that these early societies disappeared and the area was re-occupied by the so-called "earth movers" (Mann, 2000, 2008) only after a 2,000 year hiatus in the archaeological record. Some of the early Holocene archaeological sites were buried by sediments deposited when rivers entered a phase of high rate of avulsions (Lombardo et al., 2012). These shifts suggest that one or more major environmental changes happened in the LM during the Holocene causing the abandonment and burial of the early sites.

Fig. 2. Topographic map from Aalto et al., 2003. The Beni and Mamore river flood plains within the Llanos, northern Bolivia. The two rivers are the principal sediment and water sources for the Madeira River, the

largest sediment source for the Amazon

Little is known about the causes and the timing of these Mid-Holocene environmental changes of the LM landscape. Climate change is the usual suspect, as suggested by the coeval deposition of a sedimentary lobe of the Río Grande that buried some of the early sites (Lombardo et al., 2012) and the shift toward wetter conditions inferred from paleo-ecological archives (Baker et al., 2001; Mayle et al., 2000). However, current models of the long-term evolution of foreland basins consider the topography and drainage of these systems as mainly controlled by vertical surface motions related to 1) the rates of sediment supply and accumulation; 2) the long-term, long-wavelength isostatic subsidence of the Amazon foreland basin in response to the progradation of the Andean thrust belt and the increasing sediment weight (DeCelles & Giles, 1996; Garcia-Castellanos & Cloetingh, 2012); and 3) the tectonic uplift of a flexural forebulge in the external parts of the basin (Figs. 3 to 5), related to the same processes.

Fig. 3. Cartoon showing the standard partition of foreland basins and the main

processes involved in their evolution. From a hydrological point of view, the Amazon foreland

basin is an overflowed basin where sediment has overfilled the foredeep and the drainage is

carrying sediment beyond the forebulge uplift. After Garcia-Castellanos & Cloetingh (2012).

If tectonics have been the drive for the geological evolution of the Amazon foreland basin, it is reasonable to surmise that the neotectonic motions are still shaping the recent drainage history of the LM (see refs. by Dumont). But what are these motions? The green cover of the area difficults a direct assessment of what's going on in terms of neotectonics in the basin. What should we expect from the accumulated knowledge about the long-term history of foreland basin systems? (e.g. Garcia-Castellanos & Cloetingh, 2011; Fig. 3).

Fig. 4. After Roddaz, Baby, Herail et al., 2005, EPSL. Proposed model for the evolution of the Iquitos forebulge and the backbulge based on sediment provenance.

Fig. 5. After Roddaz, Baby, Herail et al., 2005 - Tentative Map for the Amazonian forebulge and backbulge associated to the weight of the Andes. 

T he geological history of Amazonia has been shaped by the uplift of the Andes during the Tertiary. Most of the area consists of a Tertiary sedimentary infill that accumulated in the trough resulting from the weight of the Andes, as its tectonic napes staked on top of the southamerican plate. When the Andes reached a significant elevation, they caused the onset of orographic precipitation, concentrating most rainfall along the eastern flank of the Andes. This resulted in higher erosion rates that changed the sedimentation regime in the Andean foreland and in Amazonia. The thickness of the deposited sediments reached more than 1000 meters. It is not clear if this happened between 23 and 10 Ma, due to the mechanism described above (Hoorn et al. 2010); or between 9 and 4.5 Ma, due to a reduction in the subduction angle of the Nazca plate in the Central Andes that shifted the sedimentation area eastward (Latrubesse et al. 2010). Nowadays these sediments constitute what is known as the Pebas  formation, which outcrops in several areas of Peru and Brazil. What did Amazonia look like during this period? The most accepted hypothesis suggests that a huge lake and wetland system formed in the foreland basin: the so-called Lake Pebas, presumably connected with the Caribbean Sea. The instability of this connection would contribute to explain the high diversity of fresh water fish with marine ancestors that now live in the Amazon river system (Hubert & Renno, 2006). For more info on this see How lake-like was Lake Pebas?

In this tectonic context, let's move back to the human colonization of the region:
In the Llanos de Moxos savannah, between 400 and 1400 AD, pre-Columbians built hundreds of monumental earth mounds, known locally as “lomas”. These earth mounds are planned, complex buildings made by one or more pyramids built on top of elevated platforms (Fig. 6). Monumental mounds can be up to 20 meters high and can cover up to 30 hectares. There are more than 350 of these pre-Columbian buildings in this area (see A story of people and rivers in the Amazon 5000 years ago, original paper here):

Figure 6. From this The Holocene paper.

Earlier than that, during the early Holocene (between 11,000 and 5,000 years ago) this portion of Amazonia was relatively dryer than today, inundations were less frequent and rivers transported few sediments. During these stable climatic conditions there was no deposition of fluvial sediments in the savannahs and soils were forming all over the Llanos de Moxos. But things changed during the Mid-Holocene, between 5 and 4 ky BP. The Rio Grande entered in a period of frequent avulsions and high sedimentation, probably triggered by wetter conditions. As a result, in the South-eastern LM, a ­fluvial distributary system formed. Suddenly, the landscape was transformed into a large swamp, dominated by something similar to an interior delta or a sedimentary lobe. The former soils were buried and the landscape became a mosaic of patches of savannahs closely interwoven and sometimes enclosed by forested paleo-levees.

Fig. 7. Google map of the Llanos de Moxos area.

Only by then, in the Late Holocene, did the pre-Columbians begun transforming the landscape. The lobe deposition favored the development of a complex pre-Columbian society by increasing the region’s agricultural potential. Firstly, it created a convex-up topography, which greatly reduced its susceptibility to ­flooding; secondly, the construction of the elevated ­fluvial levees significantly improved drainage conditions at the local scale. Furthermore, the Río Grande also provided relatively younger sediments derived from its Andean catchment that are rich in nutrients. Thus, the Río Grande removed the two biggest obstacles faced by tropical agriculture in the rest of Amazonia: severe waterlogging and poor soils. But the Río Grande’s job was not perfect: fl­uvial levees enclosed patches of ­floodplain, resulting in ponding and pronounced waterlogging. Thus pre-Columbian people had to transform the landscape through the construction of a drainage system in order to further improve agricultural conditions (Fig. 6).

The network of canals had a significant impact on the local edaphology: it pushed the forest-savannah boundary towards the savannah, eventually increasing the area of well-drained, usable land. The new inhabitants were lucky because they had several lakes placed on the top of the sedimentary lobe. Building canals that transported the water from the lakes to the agricultural fields they were able to perform agriculture even during the dry season.

These interpretations disregard the tectonic origin of the basin and its long-term control on environment commented above (see also refs. by Dumont). The ­fluvial landscape created by Río Grande was probably an important factor behind the emergence of the monumental mounds culture in the South-eastern LM, as it provided favorable soils, nutrients and drainage characteristics. Pre-Columbians additionally domesticated that environment by building a network of drainage canals. But what has been the role of the tectonic deformation during these events is yet an open question calling for answers, yet another reto terrícola. 

References:

• Aalto, R., Maurice-Bourgoin, L., Dunne, T., Montgomery, D.R., Nittrouer, C.A., Guyot, J.-L., 2003. Episodic sediment accumulation on Amazonian flood plains influenced by El Niño/Southern Oscillation. Nature, 425(6957), 493-497.
• DeCelles, P.G., and Giles, K.A. (1996) Foreland basin systems. Basin Research, 8, 105–123.
• Dumont, 1996 - Tectonophysics.     Dumont, Fournier - 1994 - Quaternary International.
• Garcia-Castellanos, D. & S. Cloetingh, 2011. Modeling the interaction between lithospheric and surface processes in foreland basins. In: Tectonics of Sedimentary Basins: Recent Advances, C. Busby & A. Azor (eds.). Blackwell Pub. Ltd., 152-181, doi:10.1002/9781444347166.ch8 [pdf]
• Lombardo, U., & Prümers, H. (2010). Pre-Columbian human occupation patterns in the eastern plains of the Llanos de Moxos, Bolivian Amazonia Journal of Archaeological Science, 37 (8), 1875-1885 DOI: 10.1016/j.jas.2010.02.011
• Lombardo, Jan-Hendrik May, & Heinz Veit (2012). Mid- to late-Holocene fluvial activity behind pre-Columbian social complexity in the southwestern Amazon basin The Holocene

Lombardo, U., May, J., & Veit, H. (2012). Mid- to late-Holocene fluvial activity behind pre-Columbian social complexity in the southwestern Amazon basin The Holocene, 22 (9), 1035-1045 DOI: 10.1177/0959683612437872