A geological record of multiple Pleistocene tsunami inundations in an oceanic island: The case of Maio,Cape Verde     

José Madeira  Ricardo S. Ramalho  Dirk L. Hoffmann  João Mata  Mário Moreira 《Sedimentology》2020,67(3):1529-1552

In the Central Atlantic archipelagos – the Canaries, Cape Verde, Madeira and the Azores – tsunami hazard is often regarded as low, when compared with other extreme wave events such as hurricanes and storms. The geological record of many of these islands, however, suggests that tsunami hazard may be underestimated, notwithstanding being lower than in areas adjacent to subduction zones, such as the margins of the Pacific and Indian oceans. Moreover, tsunamis in oceanic islands are generally triggered by local large-scale volcanic flank collapses, for which little is known about their frequency, making it difficult to estimate the probability of a new occurrence. Part of the problem lies in the fact that tsunami deposits are usually difficult to date, and few islands in the world exhibit evidence for repeated tsunami inundation on a protracted timescale. This study reports on the presence of abundant tsunami deposits (conglomerates and sandstones) on Maio Island (Cape Verde) and discusses their stratigraphy, sedimentological characteristics, probable age and tsunamigenic source. Observations indicate that four distinct inundation events of variable magnitude took place during the Pleistocene. One of the tsunami deposits yielded a high-confidence U/Th age of 78·8 ± 0·9 ka, which overlaps within error with the 73 ± 7 ka age proposed for Fogo volcano's flank collapse, an event known to have had a significant tsunami impact on nearby Santiago Island. This shows that the Fogo tsunami also impacted Maio, resulting in runups in excess of 60 m above coeval sea-level at ca 120 km from the source. Two older deposits, possibly linked to recurrent flank collapses of the Tope de Coroa volcano in Santo Antão Island, yielded lower-confidence ages of 479 to 390 ka and 360 to 304 ka. A younger deposit (<78 ka) remains undated. In summary, the geological record of Maio exhibits well-preserved evidence of repeated tsunami inundation, reinforcing the notion that tsunami hazard is not so low at volcanic archipelagos featuring prominent and highly-active volcanoes such as in Cape Verde.  相似文献   

What feeds shelf-edge clinoforms over margins deprived of adjacent land sources? An example from southeastern Brazil     

Uri Schattner  Francisco José Lobo  Adrián López-Quirós  Jorge Luiz dos Passos Nascimento  Michel Michaelovitch de Mahiques 《Basin Research》2020,32(2):293-301

In southeastern Brazil, the Serra do Mar coastal mountain range blocks the sediment influx from arriving at a ca. 1,500 km long continental margin comprising Santos and Pelotas basins. Despite this deprivation, the margin accumulated a ca. 1 km thick sedimentary succession since the Mid-Miocene. Examination of seismic reflection and oceanographic data indicates that shelf-margin clinoform formation exhibits a regional variability, with major sigmoidal clinoforms developed in the transitional area between both basins. Laterally, poorly developed oblique clinoforms constitute isolated depocenters along the shelf margin. The continuous clinoform development in the transitional area is attributed to the major influence on sediment transport patterns of several ocean bottom currents flowing along the margin, such as the Brazil Coastal Current, the Brazil Current and the Intermediate Water Brazil Current. These currents erode, transport and distribute sediments across the shelf break and upper slope from distant sediment sources located either north or south of the study area. The progressive southward strengthening of the Brazil Current could be responsible for a major southward sediment redistribution from the northern Campos Basin, and/or for sediment entrainment from northward-induced transport by the Brazil Coastal Current, originally derived from the De la Plata Estuary. In the transition between Santos and Pelotas basins, the Intermediate Water Brazil Current splits forming the Santos Bifurcation, allowing for a continuous depositional process and clinoform generation. We suggest that ocean bottom currents may shape other shelf-edge ‘contouritic clinoforms’ in continental margins mainly constructed by along-strike sediment transport largely driven by long-term geostrophic currents.  相似文献   

Revisiting an old profile: an updated geoarchaeological study at Nasera Rockshelter (Tanzania)     

David Manuel MartÍn-Perea  JosÉ-Manuel MaÍllo-FernÁndez  Alicia Medialdea  Juan MarÍn  Irene Solano-MegÍas  Agness Gidna  Audax Mabulla 《第四纪科学杂志》2020,35(7):951-960

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Using a shifted Grünwald-Letnikov scheme for the Caputo derivative to study anomalous solute transport in porous medium     

Pedro Victor Serra Mascarenhas  Ricardo Mendonça de Moraes  André Luís Brasil Cavalcante 《国际地质力学数值与分析法杂志》2019,43(11):1956-1977

We propose an extension of the shifted Grünwald-Letnikov method to solve fractional partial differential equations in the Caputo sense with arbitrary fractional order derivative α and with an advective term. The method uses the relation between Caputo and Riemann-Liouville definitions, the shifted Grünwald-Letnikov, and the traditional backward and forward finite difference method. The stability of the method is investigated for the implicit and explicit scheme with homogeneous boundary conditions, and a stability criterion is found for the advective-dispersive equation. An application of the method is used to solve contaminant diffusion and advective-dispersive problems. The numerical solution for the fractional diffusion and fractional advection-dispersion is compared with their respective analytical solutions for different time and space grid refinements. The diffusion simulation exhibited a good fit between the analytical and numerical solutions, with the explicit scheme going from stable to unstable as the time and space refinement changes. The fractional advection-dispersion application produced small deviations from the analytical solution. These deviations, however, are analogous to the numerical dispersions encountered in conventional finite difference solutions of the advection-dispersion equation. The new method is also compared with the traditional L2 method. Notably, an example that involves asymmetrical fractional conditions, a fractional diffusivity that depends on time, and a source term show how the methods compare. Overall, this study assesses the quality and easiness of use of the numerical method.  相似文献   

Drought regulates the burned forest areas in Mexico: the case of 2011, a record year     

José Manuel Zúñiga-Vásquez  Dario Cisneros-González 《国际地球制图》2019,34(5):560-573

During 2011, Mexico experienced record figures in terms of forest fires, with an affected area of 956,405 ha, and an association between this activity and the meteorological conditions is suspected. We addressed the question: Are the burned areas in Mexico associated with the duration and accumulation of drought? Using the G statistic, the cluster zones of burned areas in Mexico in 2011 were analyzed, and the value of accumulated drought that was most related to these cluster zones was determined. Global and local regression models were used to evaluate the drought-burned areas association. Two cluster zones were found (zones A and B). Accumulated drought of 24 and 12 months significantly explained the burned areas of zones A and B, respectively. The burned areas of Mexico in 2011 were non-randomly distributed, and accumulated drought significantly explained the magnitude of the areas affected by fire.  相似文献   

Long‐term effects of aquifer overdraft and recovery on groundwater quality in a Ramsar wetland: Las Tablas de Daimiel National Park,Spain          下载免费PDF全文

Silvino Castaño  Almudena de la Losa  Pedro Martínez‐Santos  Rosa Mediavilla  Juan Ignacio Santisteban 《水文研究》2018,32(18):2863-2873

Las Tablas de Daimiel National Park is one of Spain's most representative groundwater‐dependent ecosystems. Under natural conditions, water inflows combined brackish surface water from River Gigüela with freshwater inputs from River Guadiana and the underlying aquifer. Since the mid‐1970s, aquifer overexploitation caused the desiccation of the wetlands and neighbouring springs. The National Park remained in precarious hydrological conditions for three decades, with the only exception of rapid floods due to extreme rainfall events and sporadic water transfers from other basins. In the late 2000s, a decrease in groundwater abstraction and an extraordinarily wet period reversed the trend. The aquifer experienced an unexpected recovery of groundwater levels (over 20 m in some areas), thus restoring groundwater discharge to springs and wetlands. The complex historical evolution of the water balance in this site has resulted in substantial changes in surface and groundwater quality. This becomes evident when comparing the pre‐1980 groundwater quality and the hydrochemical status in the wetland in two different periods, under “dry” and “wet” conditions. Although the system is close to full recovery from the groundwater‐level viewpoint, bouncing back in the major hydrochemical constituents has not yet been obtained. These still appear to evolve in response to the previous overexploitation state. Moreover, in some sectors, there are groundwater‐dependent ecosystems that remain different to those found in preoverexploitation times. The experience of Las Tablas de Damiel provides an observatory of long‐term changes in wetland water quality, demonstrating that the effects of aquifer overexploitation on aquatic ecosystems are more than a mere alteration of the water balance and that groundwater quality is the key to aquifer and aquatic ecosystem sustainability.  相似文献   

Fate of persistent organic pollutants in the water column: does turbulent mixing matter?     

Jurado E  Zaldívar JM  Marinov D  Dachs J 《Marine pollution bulletin》2007,54(4):441-451

The effect of vertical turbulent mixing on the dynamics of persistent organic pollutants has long been overlooked and its role is still hardly understood. Here we present the first comprehensive analysis of the role of turbulent diffusion on the distribution of those contaminants and its interplay with sinking fluxes. To this end, a 1D dynamic coupled hydrodynamic-contaminant model has been developed and applied to a Mediterranean continental shelf environment. The hydrodynamic sub-model is adapted from COHERENS, the contaminant sub-model is an improvement from the BIODEP model and considers the contaminant in 3 phases: dissolved-colloidal-particulate. The simulation highlights the role of turbulence in determining the POP distribution and variability in the water column. In short, turbulent flux of contaminants strengthens the upward diffusion of sediment entrained contaminants and determines the extent to which inputs from the atmosphere mix into the water column. It acts in parallel with degradation and sinking fluxes, the combined effect yielding a surface enriched - depth depleted - benthic layer enriched region distribution, which presents similarities to reported experimental measures.  相似文献   

Phytoplankton community structure defined by key environmental variables in Tagus estuary, Portugal   总被引：4，自引：0，他引：4  

Brogueira MJ  Oliveira Mdo R  Cabeçadas G 《Marine environmental research》2007,64(5):616-628

In this work, we analyze environmental (physical and chemical) and biological (phytoplankton) data obtained along Tagus estuary during three surveys, carried out in productive period (May/June/July) at ebb tide. The main objective of this study was to identify the key environmental factors affecting phytoplankton structure in the estuary. BIOENV analysis revealed that, in study period, temperature, salinity, silicate and total phosphorus were the variables that best explained the phytoplankton spatial pattern in the estuary (Spearman correlation, rho=0.803). A generalized linear model (GLM) also identified salinity, silicate and phosphate as having a high explanatory power (63%) of phytoplankton abundance. These selected nutrients appear to be consistent with the requirements of the dominant phytoplankton group, Baccilariophyceae. Apparently, phytoplankton community is adapted to fluctuations in light intensity, as suspended particulate matter did not come out as a key factor in shaping phytoplankton structure along Tagus estuary.  相似文献   

Cosmological implications of massive gravitons     

Donald H. Eckhardt  José Luis G. Pestaña  Ephraim Fischbach 《New Astronomy》2010,15(1):175-179

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Monitoring of the groundwater chemical status in the Azores archipelago (Portugal) in the context of the EU water framework directive   总被引：1，自引：0，他引：1  

José Virgílio Cruz  Dina Pacheco  Raquel Cymbron  Sandra Mendes 《Environmental Earth Sciences》2010,61(1):173-186

A main challenge associated to EU Water Framework Directive corresponds to groundwater monitoring, both quantitative and chemical. The need for monitoring was also stressed by the Azores Water Plan. Monitoring of the chemical status of groundwater in Azores started in 2003 and has been progressively enlarged to all islands, totalizing 72 springs and 32 wells. A large number of parameters are analysed biannually, as major, minor and trace elements, pesticides and total hydrocarbons, as well as microbial indicators. Spring waters are mainly from HCO₃–Na type; instead water from wells is predominantly from the Cl–Na type, been differences attributed to their respective hydrogeologic framework. Springs discharge mainly from perched-water bodies, been influenced mainly from CO₂ in soils, silicate weathering, and seawater spraying and aerosols. Wells are in basal bodies, therefore subject to seawater intrusion influence, and 9% of Cl analyses made in wells exceed the standard value. Heavy metals, metalloids, hydrocarbons and pesticides all comply with standard values. Agriculture pollution also influences groundwater quality, as revealed by both NO₃ and PO₄ content, been the guide value exceeded respectively in 8 and 9% of the analyses. Despite the discontinuous variation of coliforms over time microbial indicators present an impact on water quality.  相似文献