Environmental controls on 10Be‐based catchment‐averaged denudation rates along the western margin of the Peruvian Andes          下载免费PDF全文

Regina Reber  Romain Delunel  Fritz Schlunegger  Camille Litty  Andrea Madella  Naki Akçar  Marcus Christl 《地学学报》2017,29(5):282-293

We present ¹⁰Be‐based basin‐averaged denudation rates for the entire western margin of the Peruvian Andes. Denudation rates range from c. 9 mm ka^?1 to 190 mm ka^?1 and are related neither to the subduction of the Nazca plate nor to the current seismicity along the Pacific coast and the occurrence of raised Quaternary marine terraces. Therefore, we exclude a tectonic control on denudation on a millennial time‐scale. Instead, we explain >60% of the observed denudation rates with a model where erosion rates increase either with mean basin slope angles or with mean annual water discharge. These relationships suggest a strong environmental control on denudation.  相似文献   

Variogram models must be positive-definite     

Margaret Armstrong  Romain Jabin 《Mathematical Geology》1981,13(5):455-459

The aim of this short article is to stress the importance of using only positive-definite functions as models for covariance functions and variograms.The two examples presented show that a negative variance can easily be obtained when a nonadmissible function is chosen for the variogram model.  相似文献   

Thermal structure and metamorphic evolution of the Piemont-Ligurian metasediments in the northern Western Alps     

François Negro  Romain Bousquet  Flurin Vils  Clara-Marine Pellet  Jeanette Hänggi-Schaub 《Swiss Journal of Geoscience》2013,106(1):63-78

In the Western Alps, the Piemont-Ligurian oceanic domain records blueschist to eclogite metamorphic conditions during the Alpine orogeny. This domain is classically divided into two “zones” (Combin and Zermatt-Saas), with contrasting metamorphic evolution, and separated tectonically by the Combin fault. This study presents new metamorphic and temperature (RSCM thermometry) data obtained in Piemont-Ligurian metasediments and proposes a reevaluation of the P–T evolution of this domain. In the upper unit (or “Combin zone”) temperatures are in the range of 420–530 °C, with an increase of temperature from upper to lower structural levels. Petrological evidences show that these temperatures are related to the retrograde path and to deformation at greenschist metamorphic conditions. This highlights heating during exhumation of HP metamorphic rocks. In the lower unit (or “Zermatt-Saas zone”), temperatures are very homogeneous in the range of 500–540 °C. This shows almost continuous downward temperature increase in the Piemont-Ligurian domain. The observed thermal structure is interpreted as the result of the upper and lower unit juxtaposition along shear zones at a temperature of ~500 °C during the Middle Eocene. This juxtaposition probably occurred at shallow crustal levels (~15–20 km) within a subduction channel. We finally propose that the Piemont-Ligurian Domain should not be viewed as two distinct “zones”, but rather as a stack of several tectonic slices.  相似文献   

Field evidence of hydraulic connections between bedrock aquifers and overlying granular aquifers: examples from the Grenville Province of the Canadian Shield   总被引：1，自引：0，他引：1  

Sandra K. Richard  Romain Chesnaux  Alain Rouleau  Roger Morin  Julien Walter  Silvain Rafini 《Hydrogeology Journal》2014,22(8):1889-1904

Field evidence of hydraulic connections between a bedrock aquifer and an overlying granular aquifer in the Canadian Shield (Grenville Province) is presented. This issue is rarely considered and investigated despite its important hydraulic and chemical consequences and its widespread occurrence worldwide. The methodology employed is based on complementary field tests conducted at specific experimental sites instrumented both in the rock and in the overlying deposits. One of the bedrock sites revealed a natural hydraulic connection with the overlying granular aquifer caused by the weathered surface of the uppermost bedrock. Another site revealed an artificial hydraulic connection between the bedrock and the granular aquifer created by an improperly sealed casing. A regional study showed that hydraulic connections yield an erroneous interpretation of the true hydraulic properties of the tested aquifer. The detection of hydraulic connections is therefore essential to properly define well-capture areas and contamination conditions. It is recommended to practitioners that pumping tests be performed as well as hydrochemical comparisons of each existing aquifer unit. Falling-head permeability tests are also helpful in verifying the quality of the seal at the bedrock–casing contact. More effective procedural controls and better well-construction practices are necessary to reduce the risks of cross-contamination induced by defective seals.  相似文献   

Oxygen isotope fractionation between apatite-bound carbonate and water determined from controlled experiments with synthetic apatites precipitated at 10-37 °C     

Christophe Lécuyer  Vincent Balter  François Fourel  Romain Amiot  Olga Otero  Gérard Panczer  Rossana Martini 《Geochimica et cosmochimica acta》2010,74(7):2072-584

The oxygen isotope fractionation between the structural carbonate of inorganically precipitated hydroxyapatite (HAP) and water was determined in the range 10-37 °C. Values of 1000 ln α() are linearly correlated with inverse temperature (K) according to the following equation: 1000 ln α() = 25.19 (±0.53)·T⁻¹ − 56.47 (±1.81) (R² = 0.998). This fractionation equation has a slightly steeper slope than those already established between calcite and water ( [O’Neil et al., 1969] and [Kim and O’Neil, 1997]) even though measured fractionations are of comparable amplitude in the temperature range of these experimental studies. It is consequently observed that the oxygen isotope fractionation between apatite carbonate and phosphate increases from about 7.5‰ up to 9.1‰ with decreasing temperature from 37 °C to 10 °C. A compilation of δ¹⁸O values of both phosphate and carbonate from modern mammal teeth and bones confirms that both variables are linearly correlated, despite a significant scattering up to 3.5‰, with a slope close to 1 and an intercept corresponding to a 1000 ln α() value of 8.1‰. This apparent fractionation factor is slightly higher or close to the fractionation factor expected to be in the range 7-8‰ at the body temperature of mammals.  相似文献   

Behaviour of lithium and its isotopes during weathering in the Mackenzie Basin, Canada     

Romain Millot  Nathalie Vigier 《Geochimica et cosmochimica acta》2010,74(14):3897-3468

We report Li isotopic compositions, for river waters and suspended sediments, of about 40 rivers sampled within the Mackenzie River Basin in northwestern Canada. The aim of this study is to characterize the behaviour of Li and its isotopes during weathering at the scale of a large mixed lithology basin. The Mackenzie River waters display systematically heavier Li isotopic compositions relative to source rocks and suspended sediments. The range in δ⁷Li is larger in dissolved load (from +9.3‰ to +29.0‰) compared to suspended sediments (from −1.7‰ to +3.2‰), which are not significantly different from δ⁷Li values in bedrocks. Our study shows that dissolved Li is essentially derived from the weathering of silicates and that its isotopic composition in the dissolved load is inversely correlated with its relative mobility when compared to Na. The highest enrichment of ⁷Li in the dissolved load is reported when Li is not or poorly incorporated in secondary phases after its release into solution by mineral dissolution. This counterintuitive observation is interpreted by the mixing of water types derived from two different weathering regimes producing different Li isotopic compositions within the Mackenzie River Basin. The incipient weathering regime characterizing the Rocky Mountains and the Shield areas produces ⁷Li enrichment in the fluid phase that is most simply explained by the precipitation of oxyhydroxide phases fractionating Li isotopes. The second weathering regime is found in the lowland area and produces the lower δ⁷Li waters (but still enriched in ⁷Li compared to bedrocks) and the most Li-depleted waters (compared to Na). Fractionation factors suggest that the incorporation of Li in clay minerals is the mechanism that explains the isotopic composition of the lowland rivers. The correlation of boron and lithium concentrations found in the dissolved load of the Mackenzie Rivers suggests that precipitation of clay minerals is favoured by the relatively high residence time of water in groundwater. In the Shield and Rocky Mountains, Li isotopes suggest that clay minerals are not forming and that secondary minerals with stronger affinity for ⁷Li appear.Although the weathering mechanisms operating in the Mackenzie Basin need to be characterized more precisely, the Li isotope data reported here clearly show the control of Li isotopes by the weathering intensity. The spatial diversity of weathering regimes, resulting from a complex combination of factors such as topography, geology, climate and hydrology explains, in fine, the spatial distribution of Li isotopic ratios in the large drainage basin of the Mackenzie River. There is no simple relationship between Li isotopic composition and chemical denudation fluxes in the Mackenzie River Basin.  相似文献