Magnetic properties of lake sediments from Lake Chalco,central Mexico,and their palaeoenvironmental implications

Lake Chalco (99.0°W, 19.5°N) in the Basin of Mexico, was formed during the Pleistocene after the emplacement of the Chichinautzin volcanic field that closed the former drainage system. The lake sediment record has been influenced by a number of factors, including glacial–interglacial cycles, local volcanism, erosion of soils and anthropogenic disturbances. The magnetic properties of the lake sediments and the associated tephra layers of the last 16500 yr have been studied. It is found that the magnetic properties of the Lake Chalco sediments are very distinctive. Magnetic concentration varies by a factor of 1000 and magnetic stability also varies over an extremely wide range. The predominant magnetic mineral is titanomagnetite in addition to an imperfect antiferromagnetic phase, possibly goethite. An unusually large range of coercivities is found in certain of the tephras. Down‐core variations in magnetic properties closely follow climatic/environmental changes previously established by other proxy methods. The late Pleistocene and late Holocene lake sediments display a higher concentration of magnetic minerals than the early–middle Holocene sediments. In the non‐volcanic sediments, fluctuations in the magnetic concentration reflect changes in both the intensity of erosion, as represented by ferrimagnetic and paramagnetic minerals, and by the maturing of soils, as represented by geothite. Copyright © 2000 John Wiley & Sons, Ltd.     

Does wetland watershed land use influence amphibian larval development? A relevant effect of agriculture on biota

Agriculture practices have been widely proposed as the major cause of amphibian population decline. However, the majority of these results have been based on laboratory experiments. The present study was conducted to test the repercussion of wetland sediment provenance on amphibian larval development. Bufo bufo larvae were used in two different treatments in an outdoor experiment, the first one using sediment from one wetland surrounded by intensive agricultural practices, and the second using sediment with no record of agrochemical uses. A negative effect was observed in the agricultural treatment, from subcellular to individual level. The results showed that the sediment from agricultural practices watershed generates a reduction in survival. Furthermore, individuals that developed under this treatment showed a lower total length and a delay in the time necessary to complete metamorphosis; these features are connected with recruitment success. In the same way, biochemical analysis showed high values of lipid peroxidation in metamorphs developing in sediments from an agricultural area. Finally, the results obtained highlight the importance of considering the sediments, not only the water, as origin treatment, allowing us to understand the consequences on amphibian populations that inhabit areas affected by intensive agriculture.

     

Ocean Geography vis-à-vis Global Change and Sustainable Development

The associated development of global change research programs and the adoption of a global approach to the ocean by Agenda 21, Chapter 17 have generated an increasing need for investigations into coastal, deep ocean, and island management. To contribute to the development of ocean sciences vis-à-vis this need, in 1996 the International Geographical Union (IGU) launched the Oceans program. In 1998, the International Year of the Ocean, this program was implemented and converted into the Oceans 21 program, convened by the IGU and the Intergovernmental Oceanographic Commission of UNESCO. The epistemological and conceptual backgrounds of this program framework, jointly with its key contents, are presented and discussed, as well as its consistency with the program areas and sectoral issues of Agenda 21.     

Striped weakfish (Cynoscion guatucupa) population structure in waters adjacent to Rio de la Plata,environmental influence on its inter-annual variability

The springtime temporal variations of striped weakfish (Cynoscion guatucupa) population structure available to the commercial fishery and its relationship to environmental factors were evaluated in the Uruguayan coastal zone (35° −33.4°S), from 86 stations sampled in 1994, 98, 99 and 2003. We examined the inter-annual variability of age-class structure over four years under different oceanographic conditions: (1) 1998 (El Niño year) was characterized by elevated water temperature; (2) 1999 (La Niña year) was characterized by decreased water temperature; and (3) 1994 and 2003 were ‘typical years’ with intermediate values in those parameters. To determine whether or not major shifts in population structure occurred between years we used ANOSIM and SIMPER analysis to determine which age-class typified and discriminated between years. A canonical correspondence analysis (CCA) was used to define the temporal pattern of age-class structure of C. guatucupa and to estimate its associations with environmental factors (depth, temperature, salinity, vertical stratification, and zonal and meridional component of the wind). The C. guatucupa population structure showed significant difference between 1998 and 1999 and 1994–2003. During 1998 and 1999, the population structure was dominated by adults (between 4 and >7 age-class), while that in 1994 and 2003 was dominated by juveniles (between 0 and 3 age-class). CCA results indicated that zonal wind and salinity has a major influence on the temporal pattern of C. guatucupa population structure. Juvenile population structure was associated with low salinities and occurred when the wind field forced an inflow of freshwater into the marine coastal area from the Rio de la Plata estuary, while the adult structure, associated with high salinities, occurred when the area was dominated by a wind driven inflow of higher salinity shelf water. These results support the hypothesis that the short-term environmental synoptic condition has a greater influence on the distribution and population structure of C. guatucupa than long-term environmental variability.     

A cautionary note on automated statistical downscaling methods for climate change

The urge for higher resolution climate change scenarios has been widely recognized, particularly for conducting impact assessment studies. Statistical downscaling methods have shown to be very convenient for this task, mainly because of their lower computational requirements in comparison with nested limited-area regional models or very high resolution Atmosphere–ocean General Circulation Models. Nevertheless, although some of the limitations of statistical downscaling methods are widely known and have been discussed in the literature, in this paper it is argued that the current approach for statistical downscaling does not guard against misspecified statistical models and that the occurrence of spurious results is likely if the assumptions of the underlying probabilistic model are not satisfied. In this case, the physics included in climate change scenarios obtained by general circulation models, could be replaced by spatial patterns and magnitudes produced by statistically inadequate models. Illustrative examples are provided for monthly temperature for a region encompassing Mexico and part of the United States. It is found that the assumptions of the probabilistic models do not hold for about 70 % of the gridpoints, parameter instability and temporal dependence being the most common problems. As our examples reveal, automated statistical downscaling “black-box” models are to be considered as highly prone to produce misleading results. It is shown that the Probabilistic Reduction approach can be incorporated as a complete and internally consistent framework for securing the statistical adequacy of the downscaling models and for guiding the respecification process, in a way that prevents the lack of empirical validity that affects current methods.     

Environmental factors controlling the spatiotemporal distribution of microbial communities in a coastal, sandy aquifer system (Doñana, southwest Spain)

Currently, aquifers are considered to be ecosystems that interchange materials and energy with other systems located in their surroundings. The aquifer system of Doñana (southwest Spain) has been studied over recent decades from a hydrogeological point of view, although nothing is known about its biological or ecological aspects. In order to describe the general characteristics of its microbial communities, bacterial abundance, cell biomass, bacterial biomass and microbial activities of functional groups were investigated by sampling, over a 2-year period, 13 wells located in the vicinity of four very productive shallow lakes in the most superficial part of this coastal, sandy aquifer system. Multivariate analysis of variance (MANOVA) indicated differences in abundance and biomass variables among sampling locations, seasons and sampling locations × seasons. Principal component analysis showed that temperature and dissolved oxygen appeared to be the most important factors controlling the temporal variability of microbial communities. Hydrological connectivity between surface water and groundwater was important in the control of the spatiotemporal distribution of microbial communities. Due to this hydrological connection, the aquifer system and the wetlands constitute a unique entity, a unique ecosystem, called the “hydroecosystem”, where microbial communities could play a central ecological role.     

Landslide-dam paleolakes in the Central Pyrenees,Upper Gállego River Valley,NE Spain: timing and relationship with deglaciation

The production of a cartographic landslide inventory in the glaciated headwaters of the upper Gállego River, Axial Zone of the Spanish Pyrenees, revealed several paleolakes. These relict lake basins are related to the blockage of drainages by postglacial earthflows and slides, mainly developed on Paleozoic slates. The paleolakes are recorded either by non-dissected infilled basins or by hanging and incised lacustrine terraces that persisted for thousands of years. The development of lakes upstream of landslides, was favored, despite their erodibility, by high-displacement rates and the occurrence of paired landslides on both sides of the valley. Radiocarbon and single-grain optically stimulated luminescence (OSL) ages obtained on the lake sediments range between 41.5?±?3.9 ka and 15.1?±?0.3 ka. These chronologies indicate prolonged landslide activity long after the early deglaciation in the upper Gállego River Valley. The landslide-lake dating results support the interpretation that the regional maximum ice extent (MIE) in the Pyrenees occurred during the marine isotope stage 4 (MIS 4), long before the global Last Glacial Maximum (LGM).     

The Ocean after christopher columbus — thoughts about society and the marine environment

The objective of this paper is to consider the Columbus’ enterprise from the point of view of the evolution of the involvement of society in the marine environment. Recently Columbus has been regarded as a protagonist of the Renaissance because he was able to produce interaction between the willingness to discover new geographical frontiers, the use of knowledge for developing political strategies, the search for riches, the conquest of extra-Mediterranean and extra-European areas. This approach sustained both the palaeo- and neo-industrial stages in which the ocean environment as a whole was involved in conquest-oriented and resource-use strategies. Both Enlightenment and Positivism were the natural prolongation of Columbus’ approaches and those of the Renaissance. Changes occurred in the seventies: (i) the general system-based approach, jointly with the growing attention to the ocean ecosystem, have led to approaches in which sea uses and the natural context are regarded as interacting systems; (ii) in particular, the ocean ecosystem has been increasingly regarded in a no deterministic way; (iii) United Nations’ policy has encouraged policies based on the duoenvironment anddevelopment. As a final result, in terms of cultural values, the post-Columbus era has initiated just as the quincentenary of his enterprise is being celebrated.