Review of erosion dynamics along the major N-S climatic gradient in Chile and perspectives

Chile is an elongated country, running in a north-south direction for more than 30° along a subduction zone. Its climate is progressively wetter and colder from north to south. This particular geography has been used positively by a growing number of studies to better understand the relationships between erosion processes and climate, land use, slope, tectonics, volcanism, etc. Here we review the erosion rates, factors, and dynamics over millennial to daily periods reported in the literature. In addition, 21 new catchment mean erosion rates (suspended sediment and 10Be) are provided, and previous suspended sediment-derived erosion rates are updated. A total of 485 local and catchment mean erosion rates are reported. Erosion rates vary between some of the smallest values on earth (10 −5 mm/a) to moderate values ≤0.5 mm/a compared to other active ranges. This review highlights strong limitations concerning the quantification of local erosion factors because of uncertainties in sampling point location, slope and rainfall data. For the mean erosion rates E for the millennial and decennial catchments, a model of the form E ∝ S/ [1 − (S/0.6)2] Rα with α = [0.3,0.8] accounts for 40 to 70% of the erosion variance, confirming a primary role of slope S compared to precipitation rate R over this time scale. Over the long-term, this review points to the long (5 to >10 Ma) response time of rivers to surface uplift in north-central arid Chile. Over millennia, data provide evidence for the progressive contribution of extreme erosion events to millennial averages for drier climates, as well as the link between glacier erosion and glacier sliding velocity. In this period of time, a discrepancy exists between the long-term offshore sedimentological record and continental decennial or millennial erosion data, for which no single explanation appears. Still, little information is available concerning the magnitude of variation of millennial erosion rates. Over centuries, data show the variable role of groundwater in the dynamics of suspended load and document a decrease in erosion over hundreds of years, probably associated with historical harvesting

Geochemistry of Chilean rivers within the central zone : distinguishing the impact of mining, lithology and physical weathering

Several rivers of Chile from the latitude 30 degrees-38 degrees have been sampled during a stable anticyclonic period (October 2008). Firstly, our aim was to evaluate the dissolved chemical composition (major and trace elements) of poorly known central Chilean rivers. Secondly, we used a co-inertia analysis (see Doledec and Chessel in Freshw Biol 31:277-294, 1994) to explore the possible relationships between the concentrations of elements and the environmental parameters [surface of the basin (km(2))/mining activity (%)/average height (m)/watershed mean slope (%)/% of the surface covered by vegetation, sedimentary rocks, volcano-sedimentary rocks, volcanic rocks, granitoid rocks/erosion rate (mm/year)]. Globally, the major elements concentration could be explained by a strong control of mixed silicate and carbonate and evaporate lithology. The statistical treatment reveals that the highest metal and metalloids loads of Tinguiririca, Cachapoal, Aconcagua, Choapa, Illapel and Limari could be explained by the contribution of the mining activities in the uppermost part of these watersheds and/or by the higher geochemical background. Indeed, it remains difficult to decipher between a real mining impact and a higher geochemical background. Even if these rivers could be impacted by AMD process, the size of these watersheds is capable of diluting AMD waters by the alkaline character of tributaries that induce acid neutralization and decrease the level of metals and metalloids

Sources, enrichment, and redistribution of As, Cd, Cu, Li, Mo, and Sb in the Northern Atacama Region, Chile: Implications for arid watersheds affected by mining

Long-established and widespread mining activities in the Northern Atacama Region of Chile have historically impacted the environment. Most notably, the Potrerillos and El Salvador mines, until 1976, were responsible for dumping over 150 ∙ 106 tons of tailings into the El Salado River, discharging directly into the bay of Chañaral on the coast. Water resources in the Northern Atacama Region are scarce; the few include the El Salado River and the Pedernales, Maricunga, and Laguna Verde basins. This region also contains two highly sensitive national parks: the Pan de Azúcar on the coast and the Nevado de Tres Cruces in the Andes. Protecting available water resources in this inherently dry region is critical and environmental degradation that has occurred has not been reported in terms of the most important superficial pollutants. In order to specifically evaluate the metals and metalloids polluting superficial water and fluvial sediments, a 3 year-long survey was carried out in the basins of the Northern Atacama Region. Additionally, impacts of the El Salado River flood in March 2015 were evaluated. When compared to the average concentrations of dissolved elements in river water worldwide, the most enriched elements of the Northern Atacama Region are, in decreasing order: Li, As, Mo, ± Cd, Sb, and Cu. In the case of fluvial sediments, compared to the composition of the upper continental crust, samples are enriched in the following elements (in decreasing order): As, Cu, Mo, Li, ± Cd and Sb. In surface waters, dissolved As, Li, Mo and Cd are naturally enriched, concentrations of Cu and Sb are inferred to be related to mining activities. In fluvial sediments, concentrations of As, Li and Cd are of natural origin while Cu, Mo and Sb are related to the exploitation and mineral treatment of porphyry copper deposits. During the intense March 2015 flood event, contaminant elements were remobilized in the Andes Mountains and El Salado Alto Basin, and concentrations increased in the El Salado Bajo Basin predominantly due to the creation of a hydrologic connection between adjacent basins. Despite the presence of world-class porphyry Cu-Mo and iron oxide copper‑gold deposits in the region, some of which have been mined since the end of the 19th century, concentrations of dissolved Cu are lower than previously reported. This is likely related to circumneutral pH and the complexation of Cu as a cation in contrast to As and Mo which might be stable as HAsO42 − and MoO42 −, respectively, in solution over long distances

Geodynamic processes in the Andes of Central Chile and Argentina

The effect of mean precipitation rate on erosion is debated. Three hypotheses may explain why the current erosion rate and runoff may be spatially uncorrelated: (1) the topography has reached a steady state for which the erosion rate pattern is determined by the uplift rate pattern; (2) the erosion rate only depends weakly on runoff; or (3) the studied catchments are experiencing different transient adjustments to uplift or to climate variations. In the Chilean Andes, between 27°S and 39°S, the mean annual runoff rates increase southwards from 0.01 to 2.6 m a-1 but the catchment averaged rates of decadal erosion (suspended sediment) and millennial erosion (10Be in river sand) peak at c. 0.25 mm a-1 for runoff c. 0.5 m a-1 and then decrease while runoff keeps increasing. Erosion rates increase non-linearly with the slope and weakly with the square root of the runoff. However, sediments trapped in the subduction trench suggest a correlation between the current runoff pattern and erosion over millions of years. The third hypothesis above may explain these different erosion rate patterns; the patterns seem consistent with, although not limited to, a model where the relief and erosion rate have first increased and then decreased in response to a period of uplift, at rates controlled by the mean precipitation rate