the amazon natural laboratory
The Amazon Region is as large as it is important for the Earth hydroclimate system and biodiversity. It is also not well understood because of this vastness. As landscapes are the dynamic tapestry that link the underlying Earth's processes with the biosphere, hydrosphere, and atmosphere, understanding how the Amazon landscapes evolved through time and what processes govern its evolution has implications ranging from fundamental scientific advances to drawing conservation strategies.
My group uses cosmogenic nuclides, topographic analysis, structural analysis, and sediment provenance to decipher the style, timing, and governing mechanisms of evolution of the Amazon system from decadal to million-year timescales.
landscape change and
Figure 1: Conceptual model of a river capture in the Amazon region driven by intraplate faulting (i.e. neotectonics). Source: Val et al (2014).
Figure 2: Fault datasets showing changes in orientations. (A) Faults cutting bedrock only (not the soil horizons). (B) Faults cutting through bedrock and soil. Source: Val et al (2014).
Figure 3: Conceptual model of a river capture in the eastern Amazon region driven by differences in bedrock lithology. Source: Fadul et al (2022).
The Amazon is largely a low-slope region of the Earth that has not undergone fast tectonic perturbation for hundreds of millions of years. Typically this kind of history would lead to homogenous landscapes and with little sediment yield, indicating equilibrium. For this reason, many geoscientists might mistake it for a "boring" place to study landscape changes. Yet, the Amazon contains widespread evidence of landscape change and disequilibrium. Why? Why is there so much geomorphic activity in tectonically inactive settings?
Neotectonics: Despite being in the middle of the South American continent, there are different internal forces that can reactivate old faults and weakness zones in the crust and cause displacement of the underlying "floor" of the Amazon region. These displacements sometimes affect the surface and cause river network reorganization, erosion of interfluves, plateaus, and floodplains, ultimately reshaping the overall structure of the landscape.
Near Manaus, in the eastern Amazon, we've documented a river capture which was potentially triggered by intraplate NW-SE faulting in the area (Figures 1 and 2). We suspect that the orientation of the lower Negro River valley is at least partly influenced by faults.
• Erosion of an active fault scarp leads to drainage capture in the Amazon region, Brazil
Val, P.; Silva, CL; Harbor, DJ; Morales, N; Maia, TFA; Amaral, FR. 2014, Earth Surface Processes and Landforms, Vol. 39, p. 1062-1074, doi: 10.1002/esp.3507
Lithologic (autogenic) influences: Tectonically dead landscapes can be very much geomorphically alive! Camila Fadul, a former MSc student in my group investigated the transient landscapes of the eastern Guiana Shield with contributions from then undergraduate student Pedro Oliveira (now PhD student at CUNY). Fadul et al (2022) demonstrated how variable bedrock substrates near the outlet of neighboring river basins could explain most of the transient topography observed in the area upstream of where the hard rocks outcropped (Figures 3 and 4). This is likely the primary mechanisms of changes in river network shapes (i.e. river captures), generation of paleosurfaces, and overall geomorphic disequilibrium in the eastern Guiana Shield. Because these features are related to hard rocks exhuming to the surface and the observed features are seen in other settings, what Fadul et al. (2022) documented is possibly an important mechanism controlling Earth’s intraplate long-term topographic evolution, the lithologic control of base level.
• Ongoing landscape transience in the Eastern Amazon Craton consistent with lithologic control of base level.
Fadul, C. M.; Oliveira, P.; Val, P, 2022. Earth Surface Processes and Landforms, doi: 10.1002/esp.5447
- Ongoing work - Based on Fadul's findings, we are now diving into the role of hard rocks as triggers of landscape transience in continent interiors in the absence of external triggers such as tectonics or climate. We are working with Dr. Daniel Peifer (University of Tübingen) to investigate wind-gap formation and evolution in the Guiana Shield. My group is investigating the geomorphic response to the exhumation of hard rock lithologies in continent interiors. We are using empirical observations of type-localities mostly in Brazil (i.e. Amazon and Paraná basins) and using numerical models of landscape evolution (LandLab).
Figure 4: Photograph of the sandstone ridges (dark green) forming higher and steeper topography in the eastern Amazon. Photo credit: Self
Timing of landscape change
Figure 5: Review of the river network history of northern South America. Source: Albert et al (2018).
The Amazon river system underwent drastic changes in the geologic past (Figure 5). Instead of a west-to-east river system across the northern South American continent, the Amazon basin was divided in at least two separate drainage basins: the eastern Amazon basin which had headwaters near the city of Manaus and drained through its current lower half into the Atlantic ocean; and the western Amazon basin which drained westward, through a large wetland region before diverting northward through today's Orinoco river system.
The timing of integration of this paleo-river system into today's transcontinental basin is highly debated. Most of the temporal evidence showing the onset of a transcontinental system comes from offshore sedimentology and stratigraphy and suggest that pervasive changes to landscapes across northern South America ensued in the last 10 Ma, but the exact timing (late Miocene or Plio-Pleistocene, or both?) is widely debated. Importantly, there is little onshore evidence of this timing, mainly because sedimentary deposits are not well preserved in this low-slope river system. Also, what processes impacted the pre- and post- transcontinentalization landscapes? How did the landscape respond after the west-to-east integration?
Through cross-disciplinary collaborations with palynologist Carina Hoorn (U of Amsterdam) and biologist James Albert (LSU) and others, we reviewed the geologic history of the Amazon basin and its relationship with the evolution of the Andes mountains, global climate, and its importance for the evolution of the biodiversity in this region (Figure 6). We've also identified some sedimentologic and paleontologic evidence of potential cyclical marine incursions in the Miocene megawetland system in western Amazonia.
Figure 6: Compiled data showing the temporal evolution of the Andes mountains, stratigraphy, provenance, and global climate records in Amazonia. Source: Hoorn et al (2022).
• Cyclic sediment deposition by orbital forcing in the Miocene wetland of western Amazonia? New insights from a multidisciplinary approach.
Hoorn, C; Kukla, T; Bogotá-Angel, G.; van Soelen, E; González-Arango, C; Wesselingh, FP; Vonhof, H; Val, P.; Morcote-Rios, G; Roddaz, M; Dantas, EL; Santos, RV; Damsté, JSS; Kim J-H; Morley, RJ, 2022 Global and Planetary Change, 210, doi: 10.1016/j.gloplacha.2021.103717
• The Miocene wetland of western Amazonia and its role in biogeography.
Hoorn, C; Boschman, LM; Kukla, T; Sciumbata, M; Val, P, 2022 Perspective Article, Botanical Journal of the Linnean Society, doi: 10.1093/botlinnean/boab098
- Ongoing work - We are currently investigating how we may be able to use the topography of the Amazon region to constrain the timing of past changes in the Amazon river system base level. This is the topic of Pedro Oliveira's PhD at Queens College, CUNY.
The current environmental pressures that the Amazon region is facing - from climate change to forest degradation and deforestation - are met with an urgent need to integrate the scientific data across disciplines. However, the mechanisms interlinking the geosphere with the hydro-, bio-, and atmosphere in the Amazon system are largely underconstrained. In 2022, we released a comprehensive review of the Geology and Geodiversity of the Amazon region (Figure 7). See below for a quick summary.
Figure 7: Geodiversity of the Amazon region. Source: Val et al. (2022), in The Amazon Assessment Report.
The Amazon Assessment Report: In 2020 the United Nations' Sustainable Development Solutions Network (SDSN) assembled the Science Panel for the Amazon to write the first Scientific report about the current understanding of the Amazon system, from geologic history to human-induced environmental impacts, societal consequences, and risks to the local and original peoples.
I co-led Chapter 1 of the report with Carina Hoorn (U of Amsterdam) on the Geology and Geodiversity of the Amazon: Three billion years of history.
A few key messages from the report:
• Modern Amazonian landscapes can only be understood in the context of geological and climatic processes operating over hundreds of thousands to billions of years.
• Amazonian geodiversity arises from the heterogeneous distribution of lithologies in the geological substrate and edaphic (soil) conditions at many spatial scales, under the perennial influence of var-ied hydrological and biological process, at the surface and subsurface.
● It took hundreds of millions of years for the Amazon to develop the rich tapestry of landforms, soils, and ecosystems we see today, but humans degrade these unique ecosystems at a much faster rate. Decisions should be made to avoid further degradation and consider the time necessary for the Amazon to recover, which, if at all, will not be on a human-relevant timescale.
Albert et al (2023) featured in the cover of Science Magazine on Jan, 27 2023.
The Amazon Report highlighted how fast and efficient humans are at altering the Amazon system. In a follow-up paper led by James Albert (LSU), we've gathered our learnings from the Amazon Report and compared the rates of human activities to those of Amazon ecosystems. We found that humans are clearing forests and altering the ecosystem and landscapes 100s to 1,000s of times faster than the natural rates of forest recovery, biodiversity accumulation, and other ecosystem rates (Figure 8).
• Human impacts outpace natural processes in the Amazon
Albert, J, Carnaval, AC, Flantua, SGA, Lohmann, LG, Ribas, CC, Riff, D, Carillo, JD, Fan, Y, Figueiredo, JJP, Guayasamin, JM, Hoorn, C, Melo, GH, Nascimento, N, Quesada, CA, Ulloa Ulloa, C, Val, P, Arieira, J, Encalada, AC, Nobre, CA, 2023. Science, doi: 10.1126/science.abo5003
• Chapter 1: Geology and geodiversity of the Amazon: Three billion years of history
Val, P.; Figueiredo, J; Melo, G; Flantua, SGA; Quesada CA; Fan, Y; Albert, JA; Guayasamin, JM; Hoorn, C, 2021. In: Nobre C, Encalada A,… (Eds). Amazon Assessment Report 2021. United Nations Sustainable Development Solutions Network, New York, USA. Available from theamazonwewant.org/spa-reports/. doi: 10.55161/POFE6241 - INVITED
Figure 8: Deforestation history in the Amazon region (left) and the comparison between natural processes and anthropogenic activities (bottom). Source: Albert et al (2023).