Maximum depositional age estimation revisited     

Pieter Vermeesch 《地学前缘(英文版)》2021,12(2):843-850

In a recent review published in this journal,Coutts et al.(2019)compared nine different ways to estimate the maximum depositional age(MDA)of siliclastic rocks by means of detrital geochronology.Their results show that among these methods three are positively and six negatively biased.This paper investigates the cause of these biases and proposes a solution to it.A simple toy example shows that it is theoretically impossible for the reviewed methods to find the correct depositional age in even a best case scenario:the MDA estimates drift to ever smaller values with increasing sample size.The issue can be solved using a maximum likelihood model that was originally developed for fission track thermochronology by Galbraith and Laslett(1993).This approach parameterises the MDA estimation problem with a binary mixture of discrete and continuous distributions.The‘Maximum Likelihood Age’(MLA)algorithm converges to a unique MDA value,unlike the ad hoc methods reviewed by Coutts et al.(2019).It successfully recovers the depositional age for the toy example,and produces sensible results for realistic distributions.This is illustrated with an application to a published dataset of 13 sandstone samples that were analysed by both LA-ICPMS and CA-TIMS U–Pb geochronology.The ad hoc algorithms produce unrealistic MDA estimates that are systematically younger for the LA-ICPMS data than for the CA-TIMS data.The MLA algorithm does not suffer from this negative bias.The MLA method is a purely statistical approach to MDA estimation.Like the ad hoc methods,it does not readily accommodate geological complications such as post-depositional Pb-loss,or analytical issues causing erroneously young outliers.The best approach in such complex cases is to re-analyse the youngest grains using more accurate dating techniques.The results of the MLA method are best visualised on radial plots.Both the model and the plots have applications outside detrital geochronology,for example to determine volcanic eruption ages.  相似文献   

Isoplot R: A free and open toolbox for geochronology   总被引：1，自引：0，他引：1  

Pieter Vermeesch 《地学前缘(英文版)》2018,9(5):1479-1493

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Physical controls on sand composition and relative durability of detrital minerals during ultra‐long distance littoral and aeolian transport (Namibia and southern Angola)          下载免费PDF全文

Eduardo Garzanti  Alberto Resentini  Sergio Andò  Giovanni Vezzoli  Alcides Pereira  Pieter Vermeesch 《Sedimentology》2015,62(4):971-996

Sediment in coastal Namibia to southern Angola is supplied dominantly from the Orange River with minor additional fluvial input and negligible modifications by chemical processes, which makes this a great test case for investigating physical controls on sand texture and composition. This study monitored textural, mineralogical and geochemical variability in beach and aeolian‐dune sands along a ca 1750 km stretch of the Atlantic coast of southern Africa by using an integrated set of techniques, including image analysis, laser granulometry, optical microscopy, Raman spectroscopy and bulk‐sediment geochemistry. These results contrast with previous reports that feldspars and volcanic detritus break down during transport, that sand grains are rounded rapidly in shallow‐marine environments, and that quartzose sands may be produced by physical processes. Mechanical wear is unable to modify the relative abundance of detrital components, including pyroxene and mafic volcanic rock fragments traditionally believed to be destroyed rapidly. The sole exceptions are poorly lithified or cleaved sedimentary/metasedimentary rock fragments, readily lost at the transition to the marine environment, and slow‐settling flaky micas, winnowed and deposited offshore. Coastal sediments tend to be depleted in relatively mobile amphibole, preferentially entrained offshore or re‐deposited in sheltered beaches, while less mobile garnet is retained onshore. No detrital mineral displays a significant increase in grain roundness after 300 to 350 km of longshore transport in high‐energy littoral environments from the Orange mouth to south of the Namib Erg, but all minerals get rapidly rounded after passing into the dunefield. Pyroxene and opaques get rounded faster than harder quartz and garnet, but sand mineralogy remains unchanged. Excepting strong transient selective‐entrainment effects, physical processes are unable to modify sand composition significantly. Selective mechanical breakdown can be largely neglected in quantitative provenance analysis of sand and sandstone even in the case of ultra‐long‐distance transport in high‐energy environments dominated by strong persistent winds and waves.  相似文献   

Separating baseline conditions from anthropogenic impacts: example of the Damour coastal aquifer (Lebanon)     

Wisam M. Khadra  Pieter J. Stuyfzand 《水文科学杂志》2013,58(10):1872-1893

Abstract

A new structured approach is presented to derive groundwater baseline conditions, in this case for a dolomitic limestone aquifer suffering from salinization and other anthropogenic impacts. It builds on the HydroChemical System Analysis (HCSA) to map different groundwater bodies (hydrosomes) and hydrochemical zones within them, each of which show significant differences in baseline conditions. It also comprises a rigorous elimination scheme for samples affected by bias or pollution. The method is applied to the Damour coastal aquifer system, south of Beirut (Lebanon). Concentrations of Cl, Cl/Br, ²H, ¹⁸O and Ca/Sr were used to discern five hydrosomes and to determine mixing ratios. The dominant hydrochemical facies was (sub)oxic, calcareous and salinized, indicating a very low reduction capacity of the aquifer system, strong dissolution of dolomitic limestone and clear traces of seawater encroachment. The method proposed was capable of filtering out baseline conditions for 16 main constituents, 64 trace elements and two isotopes.  相似文献   

The dichotomous Markov process with nonparametric test application; a decision support method in long-term river behavioural analysis: the Zayandeh Rud River; a case study from central Iran   总被引：1，自引：1，他引：0  

Mehdi F. Harandi  Shantia Yarahmadian  Mohammad Sepehrifar  Pieter H. A. J. M. van Gelder 《Stochastic Environmental Research and Risk Assessment (SERRA)》2014,28(7):1889-1896

We use the Dichotomous Markov Noise model with constant transition rates to describe the dynamics of fluctuations in the water level as a stochastic process, which is imposed on river discharge changes. By applying this model, two different regimes are determined for the long-term behaviour of the river. Based on these regimes, we define two nonparametric classes of the overall increasing/decreasing nature of the water level in the long-term behaviour, which are separated by an exponential steady state regime. In this paper, we develop a nonparametric testing procedure to test exponentially (steady state regime) against an alternative overall decreasing level distribution. The proposed test predicts the long-term regime behaviour of the river. The mathematical tools introduced to handle the problem should be of general use and the testing procedure can be considered as a new mathematical tool in the study of water level dynamics. Under conditions of data austerity and as a case of study, we examine the stochastic characteristics of the Zayandeh Rud (Zāyandé-Rūd or Zāyanderūd, also spelled as Zayandeh-Rood or Zayanderood) River (Isfahan, Iran) water level.  相似文献   

Resonance properties of tidal channels with multiple retention basins: role of adjacent sea   总被引：1，自引：1，他引：0  

Pieter C. Roos  Henk M. Schuttelaars 《Ocean Dynamics》2015,65(3):311-324

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Patterns in groundwater chemistry resulting from groundwater flow   总被引：18，自引：7，他引：11  

Pieter J. Stuyfzand 《Hydrogeology Journal》1999,7(1):15-27

 Groundwater flow influences hydrochemical patterns because flow reduces mixing by diffusion, carries the chemical imprints of biological and anthropogenic changes in the recharge area, and leaches the aquifer system. Global patterns are mainly dictated by differences in the flux of meteoric water passing through the subsoil. Within individual hydrosomes (water bodies with a specific origin), the following prograde evolution lines (facies sequence) normally develop in the direction of groundwater flow: from strong to no fluctuations in water quality, from polluted to unpolluted, from acidic to basic, from oxic to anoxic–methanogenic, from no to significant base exchange, and from fresh to brackish. This is demonstrated for fresh coastal-dune groundwater in the Netherlands. In this hydrosome, the leaching of calcium carbonate as much as 15 m and of adsorbed marine cations (Na⁺, K⁺, and Mg²⁺) as much as 2500 m in the flow direction is shown to correspond with about 5000 yr of flushing since the beach barrier with dunes developed. Recharge focus areas in the dunes are evidenced by groundwater displaying a lower prograde quality evolution than the surrounding dune groundwater. Artificially recharged Rhine River water in the dunes provides distinct hydrochemical patterns, which display groundwater flow, mixing, and groundwater ages. Received, May 1998 · Revised, August 1998 · Accepted, October 1998  相似文献   

Spectroscopic properties of \hbox {Cr}^{3+} in the spinel solid solution \hbox {ZnAl}_{2-x}\hbox {Cr}_{x}\hbox {O}_4     

Louisiane Verger  Olivier Dargaud  Gwenaelle Rousse  Emese Rozsályi  Amélie Juhin  Delphine Cabaret  Marine Cotte  Pieter Glatzel  Laurent Cormier 《Physics and Chemistry of Minerals》2016,43(1):33-42

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Chronology and palaeoenvironmental implications of the ice‐wedge pseudomorphs and composite‐wedge casts on the Magdalen Islands (eastern Canada)          下载免费PDF全文

Audrey M. Rémillard  Bernard Hétu  Pascal Bernatchez  Jan‐Pieter Buylaert  Andrew S. Murray  Guillaume St‐Onge  Martin Geach 《Boreas: An International Journal of Quaternary Research》2015,44(4):658-675

The Magdalen Islands are a valuable terrestrial record, evidencing the complex glacial and periglacial history of the Gulf of St. Lawrence. Thirteen structures interpreted as ice‐wedge pseudomorphs or composite‐wedge casts were observed at four sites on the southern Magdalen Islands and testify to the former presence of permafrost under periglacial conditions. These features truncate Carboniferous sandstone or Last Glacial Maximum (LGM) glacial and glaciomarine diamicts, both overlain by subtidal or coastal units. Six optically stimulated luminescence (OSL) and four radiocarbon ages were obtained from both host and infilled sedimentary units. These ages provide the first absolute chronological data on these structures, shedding new light on the relationships between glacial and periglacial phases. Our chronostratigraphic data suggest that, after the deglaciation and the emersion of the archipelago, thermal contraction cracks grew during the cold period of the Younger Dryas (11–10 ka; 12.9–11.5 cal. ka BP). The Younger Dryas, which is well documented in the Maritime Provinces of Canada, occurred after a pedogenesis phase associated with the Allerød warm period evidenced by the well‐developed palaeopodzol ubiquitous on the Magdalen Islands.  相似文献   

Modelling of membranes in the material point method with applications          下载免费PDF全文

Fursan Hamad  Dieter Stolle  Pieter Vermeer 《国际地质力学数值与分析法杂志》2015,39(8):833-853

The objective of this paper is to formulate and validate an accurate MPM approach for the numerical simulation of the large displacement of membranes containing soil. In the proposed approach, the membrane is discretised by a surface mesh that allows accurate simulation of membrane stresses. The membrane is free to move through a three‐dimensional grid for a continuum consisting of tetrahedral elements. The approach is applied to model a geocontainer being released from a split barge, taking into account the frictional contact between the geotextile and the barge. No‐slip contact is assumed between the geotextile and the soil inside. The effect of geocontainer interaction is investigated by dropping a second container. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献