Deglacial sequences typically include backstepping grounding zone wedges and prevailing glaciomarine depositional facies. However, in coastal domains, deglacial sequences are dominated by depositional systems ranging from turbiditic to fluvial facies. Such deglacial sequences are strongly impacted by glacio‐isostatic rebound, the rate and amplitude of which commonly outpaces those of post‐glacial eustatic sea‐level rise. This results in a sustained relative sea‐level fall covering the entire depositional time interval. This paper examines a Late Quaternary, forced regressive, deglacial sequence located on the North Shore of the St. Lawrence Estuary (Portneuf Peninsula, Québec, Canada) and aims to decipher the main controls that governed its stratigraphic architecture. The forced regressive deglacial sequence forms a thick (>100 m) and extensive (>100 km2) multiphased deltaic complex emplaced after the retreat of the Laurentide Ice Sheet margin from the study area ca 12 500 years ago. The sedimentary succession is composed of ice‐contact, glaciomarine, turbiditic, deltaic, fluvial and coastal depositional units. A four‐stage development is recognized: (i) an early ice‐contact stage (esker, glaciomarine mud and outwash fan); (ii) an in‐valley progradational stage (fjord head or moraine‐dammed lacustrine deltas) fed by glacigenics; (iii) an open‐coast deltaic progradation, when proglacial depositional systems expanded beyond the valley outlets and merged together; and (iv) a final stage of river entrenchment and shallow marine reworking that affected the previously emplaced deltaic complex. Most of the sedimentary volume (10 to 15 km3) was emplaced during the three‐first stages over a ca 2 kyr interval. In spite of sustained high rates of relative sea‐level fall (50 to 30 mm·year?1), delta plain accretion occurred up to the end of the proglacial open‐coast progradational stage. River entrenchment only occurred later, after a significant decrease in the relative sea‐level fall rates (<30 mm·year?1), and was concurrent with the formation and preservation of extensive coastal deposits (raised beaches, spit platform and barrier sands). The turnaround from delta plain accretion to river entrenchment and coastal erosion is interpreted to be a consequence of the retreat of the ice margin from the river drainage basins that led to the drastic drop of sediment supply and the abrupt decrease in progradation rates. The main internal stratigraphic discontinuity within the forced regressive deglacial sequence does not reflect changes in relative sea‐level variations. 相似文献
Using an Atmospheric Global Circulation Model, we assess the relevance of selected atmospheric mechanisms for climate evolution of Saharan and sub-Saharan regions since the Miocene. First, we test the influence of the East-African Rift System uplift on atmospheric dynamics. Although the uplift played an important role in triggering East-African rainfall, no significant impact over central and western Africa has been detected. We also analyse the feedbacks of a giant lake on the climate of Chad basin. First results infer a negative feedback of the giant lake on the water balance, as convection is weakened by the cold water surface and as water evaporated from the lake does not feed the basin hydrological cycle. Lastly, we suggest that colder than present sea surface temperatures over the Gulf of Guinea reinforce the West-African monsoon, by enhancing the moisture advection engine via stronger thermal contrast between the ocean and the continent. 相似文献
Ice-core samples from Upper Fremont Glacier (UFG), Wyoming, were used as proxy records for the chemical composition of atmospheric
deposition. Results of analysis of the ice-core samples for stable isotopes of nitrogen (δ15N, ) and sulfur (δ34S, ), as well as and deposition rates from the late-1940s thru the early-1990s, were used to enhance and extend existing National Atmospheric
Deposition Program/National Trends Network (NADP/NTN) data in western Wyoming. The most enriched δ34S value in the UFG ice-core samples coincided with snow deposited during the 1980 eruption of Mt. St. Helens, Washington.
The remaining δ34S values were similar to the isotopic composition of coal from southern Wyoming. The δ15N values in ice-core samples representing a similar period of snow deposition were negative, ranging from -5.9 to -3.2 ‰ and
all fall within the δ15N values expected from vehicle emissions. Ice-core nitrate and sulfate deposition data reflect the sharply increasing U.S.
emissions data from 1950 to the mid-1970s. 相似文献
Libyan Desert Glass (LDG) is a SiO2-rich natural glass whose origin, formation mechanism, and target material are highly debated. We here report on the finding of a lens-shaped whitish inclusion within LDG. The object is dominantly composed of siliceous glass and separated from the surrounding LDG by numerous cristobalite grains. Within cristobalite, several regions rich in mullite often associated with ilmenite were detected. Mineral assemblage, chemical composition, and grain morphologies suggest that mullite was formed by thermal decomposition of kaolinitic clay at atmospheric pressure and T ≥ 1600 °C and also attested to high cooling rates under nonequilibrium conditions. Cristobalite contains concentric and irregular internal cracks and is intensely twinned, indicating that first crystallized β-cristobalite inverted to α-cristobalite during cooling of the SiO2-rich melt. The accompanied volume reduction of 4% induced the high density of defects. The whitish inclusion also contains several partly molten rutile grains evidencing that at least locally the LDG melt was at T ≥ 1800 °C. Based on these observations, it is concluded that LDG was formed by high-temperature melting of kaolinitic clay-, rutile-, and ilmenite-bearing Cenozoic sandstone or sand very likely during an asteroid or comet impact onto Earth. While melting and ejection occurred at high pressures, the melt solidified quickly at atmospheric pressure. 相似文献
Polymetallic/ferro-manganese nodules (Mn-nodules) reach sizes of up to 10 cm in diameter and are abundantly found on the seabed.
To date, the origin of Mn-nodules remains unclear, and both abiogenic and biogenic origins have been proposed. In search of
evidence for a contribution of microbial processes to the formation of Mn-nodules, we analyzed those spherical nodules which
contain a concentrically banded texture in their interior. The Mn-nodules were collected at a depth of 5,152 m from the Clarion-Clipperton
Zone. Our high-resolution scanning electron microscopy analyses reveal first published evidence that endolithic microorganisms
exist and are arranged in a highly organized manner on plane mineral surfaces within the nodules. These microorganisms are
adorned on their surfaces with S-layers, which are indicative for bacteria. Moreover, the data suggest that these S-layers
are the crystallization seeds for the mineralization process. We conclude that the mineral material of the Mn-nodule has a
biogenic origin, and hope that these data will contribute to the development of biotechnological approaches to concentrate
metals from seawater using bacteria in bioreactors. 相似文献
Prediction of true classes of surficial and deep earth materials using multivariate spatial data is a common challenge for geoscience modelers. Most geological processes leave a footprint that can be explored by geochemical data analysis. These footprints are normally complex statistical and spatial patterns buried deep in the high-dimensional compositional space. This paper proposes a spatial predictive model for classification of surficial and deep earth materials derived from the geochemical composition of surface regolith. The model is based on a combination of geostatistical simulation and machine learning approaches. A random forest predictive model is trained, and features are ranked based on their contribution to the predictive model. To generate potential and uncertainty maps, compositional data are simulated at unsampled locations via a chain of transformations (isometric log-ratio transformation followed by the flow anamorphosis) and geostatistical simulation. The simulated results are subsequently back-transformed to the original compositional space. The trained predictive model is used to estimate the probability of classes for simulated compositions. The proposed approach is illustrated through two case studies. In the first case study, the major crustal blocks of the Australian continent are predicted from the surface regolith geochemistry of the National Geochemical Survey of Australia project. The aim of the second case study is to discover the superficial deposits (peat) from the regional-scale soil geochemical data of the Tellus Project. The accuracy of the results in these two case studies confirms the usefulness of the proposed method for geological class prediction and geological process discovery.
The topic of this paper is the retrieval of hidden or secondary information on complex spatial variables from geophysical
data. Typical situations of obscured geological or geophysical information are the following: (1) Noise may disturb the signal
for a variable for which measurements have been collected. (2) The variable of interest may be obscured by other geophysical
processes. (3) The information of interest may formally be captured in a secondary variable, whereas data may have been collected
for a primary variable only, that is related to the geophysical process of interest. Examples discussed here include mapping
of marine-geologic provinces from bathymetric data, identification of sea-ice properties from snow-depth data, analysis of
snow surface data in an Alpine environment and association of deformation types in fast-moving glaciers from airborne video
material or satellite imagery. Data types include geophysical profile or trackline data, image data, grid or matrix-type data,
and more generally, any two-dimensional or three-dimensional discrete or discretizable data sets.
The framework for a solution is geostatistical characterization and classification, which typically involves the following
steps: (1) calculation of vario functions (which may be of higher order or residual type, or combinations of both), (2) derivation
of classification parameters from vario functions, and (3) characterization, classification or segmentation, depending on
the applied problem. In some situations, spatial surface roughness is utilized as an auxiliary variable, for instance, roughness
of the seafloor may be derived from bathymetric data and be indicative of geological provinces.
The objective of this paper is to present components of the geostatistical classification method in a summarizing and synoptical
manner, motivated by applied examples and integrating principal and generalized concepts, such as hyperparameters and parameters
that relate to the same physical processes and work for data in oversampled and undersampled situations, parameters that facilitate
comparison among different data types, data sets and across scales, variograms and vario functions of higher order, and deterministic
and connectionist classification algorithms. 相似文献
Garnet in metapelites from the Wölz Complex of the Austroalpine crystalline basement east of the Tauern Window characteristically consists of two growth phases, which preserve a comprehensive record of the geothermal history during polymetamorphism. From numerical modelling of garnet formation, detailed information on the pressure–temperature–time (P–T–t) evolution during prograde metamorphism is obtained. In that respect, the combined influences of chemical fractionation associated with garnet growth, modification of the original growth zoning through intragranular diffusion and the nucleation history on the chemical zoning of garnet as P and T change during growth are considered. The concentric chemical zoning observed in garnet and the homogenous rock matrix, which is devoid of chemical segregation, render the simulation of garnet growth through successive equilibrium states reliable. Whereas the first growth phase of garnet was formed at isobaric conditions of ~3.8 kbar at low heating/cooling rates, the second growth phase grew along a Barrovian P–T path marked with a thermal peak of ~625°C at ~10 kbar and a maximum in P of ~10.4 kbar at ~610°C. For the heating rate during the growth of the second phase of garnet, average rates faster than 50°C Ma?1 are obtained. From geochronological investigations the first growth phase of garnet from the Wölz Complex pertains to the Permian metamorphic event. The second growth phase grew in the course of Eo-Alpine metamorphism during the Cretaceous. 相似文献