Soils, relict landscapes and paleoclimate in the Atacama Desert, Chile part of Vignettes:Vignette Collection
The exceptionally dry Atacama Desert, adjacent to the Central Andes in northern Chile, contains many relict landscapes (landscapes formed in the past, but preserved on the present surface; Figure 1). One remarkable aspect of the relict geomorphic surfaces in the Atacama is their age. Whereas most relict landscapes on Earth are typically hundreds of thousands to a few million years old, relict surfaces in the Atacama can be as old as 10 to 15 million years. These preserved landscapes are important because they can be used to understand the geomorphic evolution of a region (Mortimer, 1973), infer rates of tectonic uplift (Hoke and Garzione, 2008), and to reconstruct past environmental and climatic conditions (Brock and Buck, 2009). The preservation of relict landscape surfaces in the Atacama Desert is the result of the uplift of the Central Andes and the hyperarid climate in the Atacama. The uplift of the Central Andes caused the deep incision of streams and the abandonment of former stream landforms. Once removed from fluvial modification by streams draining the Andes, these landscapes were preserved as arid environments are prone to considerably less erosion and weathering than wet, humid environments (Figures 2 & 3). Therefore, the extreme aridity of the Atacama Desert has left many of these landscapes unaltered since their formation. Relict landscape surfaces, which have not experienced considerable erosion or geomorphic modification, are generally capped by thick soils (e.g., Brock and Buck, 2009). These soils reflect the main soil forming factors (climate, organisms, relief, parent material, time) during their development, and therefore can be used to infer past climatic and environmental conditions (e.g., Brock & Buck, 2009). This is especially true when it is possible to compare soils of various ages in a region. Our research on relict landscapes in the Atacama Desert focused on the Pampa de Tana region in northern Chile, between the deeply incised rivers of the Quebrada de Camarones (~19°15'S) and Quebrada de Tana (19° 30'S). This landscape surface has been dated to between ~12 and 9 million years old (Mya) based on radiometric ages (40K/40Ar) of volcanic ashes interbedded in the underlying strata, as well as lavas atop the landscape surface. However, many parts of the relict landscape have been altered by geomorphic activity since their formation. Our research objectives were to map the ~10 Mya unaltered relict landscape surface, and by examining soils on geomorphic surfaces of various ages, develop an understanding of the climatic and environmental changes since the formation of this landscape (i.e. the last 10 million years). The ~10 Mya relict landscape surface on the Pampa de Tana is capped by a thick (~4m) gypsum-cemented soil (Figure 4). We mapped the relict landscape surface based on the presence of this distinct soil at the surface. As gypsum is a very soluble salt, we can infer that hyperarid conditions have persisted in this region since the formation of this landscape surface. Otherwise, we might see features in the soil that formed under wetter conditions, such as the accumulation of clay minerals and calcium carbonate. We see no evidence of these features or other indicators of wetter climatic conditions preserved in this soil. The gypsum-cemented soil is also buried in some places by landslide deposits and fluvial deposits that are between ~8 to 5 Mya (Figure 5). Therefore, in these places the soil is a paleosol (fossil soil), which has not been influenced by soil forming factors over the last 5 to 8 Ma. Remarkably, the cemented gypsic soil looks almost exactly the same in places where it has been buried for 8 to 5 million years as it does in places where it is still exposed! This tells us that the majority of soil formation occurred during the early history of this soil, and that it is essentially a relict soil. In other places, the relict landscape surface has been eroded or buried by younger Quaternary stream deposits and younger soils (Figure 5). Chemical and mineralogical analysis of these soils shows that they contain gypsum, but also contain higher concentrations of halite and other saline minerals that are more soluble than gypsum. This may suggest that this region of the Atacama has become more arid over the last few million years. Our analysis of the soils on the relict landscape surfaces of the Pampa de Tana indicate that this landscape has been extremely dry and unvegetated since its formation ~10 million years ago. Soils of various ages, from >8 Ma to modern, show no evidence of accumulation of clay minerals or accumulation of calcium carbonate, clear indicators of semi-arid conditions in a vegetated landscape (Rech et al., 2006). Instead, we see soils cemented almost entirely with gypsum. In the modern Atacama Desert, gypsum-cemented soils are restricted to areas that receive <3 cm of precipitation per year! We also see that the abundance of more soluble minerals such as halite increasing in the younger soils, suggesting that climate might be getting drier!
Stream response to Climate Change, Atacama Desert, Chile part of Vignettes:Vignette Collection
Climate on earth is constantly changing. Earth's climate can change gradually over millions of years (tectonic-scale) due to changes in greenhouse gases or the slow movement of tectonic plates, or climate can change periodically over tens of thousands of years as a result of slight changes in earth's orbit (orbital-scale climate change). Clear evidence for dramatic orbital-scale climate change includes the large ice sheets that covered the northern portions of North America and Europe just 20,000 years ago. Climate, however, can also change on much shorter time scales, over decades, centuries, or millennia, for a wide variety of reasons. As you would expect, the rate and nature of geomorphic processes also respond to climate change. For example, if an arid region becomes wetter, the rates of chemical weathering may increase or flash floods may occur more frequently. Although most geomorphic systems will respond to changes in climate, it can be difficult sometimes to predict how a specific system, or parts of a system, will respond. This is important to better understand and predict how geomorphic processes and associated systems will respond to future climate change. In this study, we examined how perennial streams in the Atacama Desert have responded to short term climate changes (centuries to millennia) during the last 10,000 years (the Holocene Epoch). The Atacama Desert, situated between the Andes Mountains and Pacific Ocean in northern Chile, is probably the driest and oldest desert on earth (Figure 1). Large areas devoid of any vegetation recieve <3 cm of rainfall per year, and many areas only receive a rainfall event one or two times a decade! Some streams, however, have perennial stream flow as a result of high levels of ground water that is sourced from snow melt and precipitation in the Andes. These streams, however, are different than most in that although perennial, they only experience a few stream discharge events per year from precipitation falling on the western flank of the Andes. As a result, the beds of these streams are covered by dense wetland vegetation. These are called in-stream wetlands (Figure 2). In particular, we were interested in whether these streams cut down into their stream bed (incised), or accumulated sediment (aggrade) during periods of wetter climate. Therefore, we needed an independent measure of precipitation in this region over the last 10,000 years. We decided to use fossil vegetation preserved in rodent middens to reconstruct past precipitation (Latorre et al., 2002). In arid lands, small rodents build nests using the surrounding local vegetation. These nests become cemented with the rodent's urine over time. These urine-encased nests become time capsules of past plant communities, the age of which can be determined by radiocarbon dating (Figure 3). When collected along the lower elevation limit of vegetation in the Atacama (which is controlled primarily by precipitation with temperature only having a minor influence), these vegetation assemblages can be used to reconstruct past precipitation amounts (Latorre et al. 2002). We also needed to identify when streams in the Atacama incised into their stream beds and when they aggraded. We can determine when streams incised into their streambeds and when they aggraded by mapping and dating the fluvial terraces within these stream valleys (Figure 4). Fluvial terraces represent old, inactive floodplains of a stream. However, these fluvial landforms are notoriously difficult to date. We are fortunate that the terrace deposits of in-stream wetlands contain an abundance of terrestrial plant fossils (Figure 4 inset). By radiocarbon dating these plant fossils, and mapping the stratigraphy of the fluvial terrace deposits, we can reconstruct the history of incision and aggradation of several stream systems in the Atacama over the last 10,000 years (Rech et al. 2002; 2003). If climate change (paleoprecipitation) is really the cause of aggradation and incision in these stream systems, then we should be able to reproduce a similar history of stream incision in several different stream systems. When we combined our records of precipitation and stream incision, we see that streams generally accumulate sediments during wetter periods, and cut into their beds during dry periods. Is this what you expected? Why? Our current understanding of the climatic influence on these streams is that the dominant control on aggradation/incision is the resistance of the stream bed to erosion and not stream power (flow). When the water table is high perennial flow is present in the stream, the stream bed is armored by dense vegetation. However, during periods of drier climate, the water table drops due to reduced groundwater recharge in the Andes. Consequently, plants die and the in-stream wetland sediments, which are mostly silt, become extremely vulnerable to erosion and incision. After the stream has adjusted to the new (lower) water table, it will slowly accumulate sediments and aggrade until the next major drought and lowering of the water table. We now believe that we can use these stream deposits to reconstruct the history of major droughts in the area, and possibly identify the possible changes in ocean and atmospheric circulation that are responsible for causing major droughts in the Atacama. It is important to note that not all streams will respond directly to changes in climate, or that they may respond very differently to climate than perennial streams with in-stream wetlands in the Atacama. In other regions, the response of a stream may be influenced primarily by land use changes, or tectonics. Or, streams may continuously be aggrading and then incising to maintain equilibrium and adjust to fluctuations in sediment load.