Sedimentary vs. diagenetic control on rhythmic calcareous successions (Pliensbachian of Asturias, Spain)

The diagenetic overprint in rhythmic hemipelagic successions can either enhance or change the original distribution of CaCO₃, but it is difficult to evaluate its effect because in most field examples, it is not possible to distinguish between sedimentary and diagenetic features. The rhythmic succession of the Pliensbachian of Asturias (Spain) shows alternation of bioclastic and laminated/burrowed intervals. The original content of carbonate brought by storms from shallow areas was larger in the bioclastic horizons. However, there is a widespread mismatch between facies alternations and the observed lithological rhythms. The diagenetic redistribution of CaCO₃ resulted in successive limestone–marl/clay couplets that do not match (either in number or in the location of the boundaries) the sedimentary cycles defined by facies alternations. We conclude that interpreting the limestone–marl rhythms as a direct response to primary changes is highly questionable, unless there is unequivocal proof of a sedimentary origin of the alternation of the two lithologies.     

Effect of the selective feeding ofGalaxias maculatus (Salmoniformes,Galaxiidae) on zooplankton of a South Andes lake

The effect of larval and juvenile predation ofGalaxias maculatus on a natural small-sized zooplankton was studied in a South Andes lake. Field data of zooplankton density and composition were combined with fish gut content analysis during a spring-summer season. Results showed thatGalaxias early stages ingested mostly planktonic preys. A shift was observed in prey electivity: larvae preferred copepod nauplii whereas juveniles preferred cladocerans. Larval predation provokes the failure of the summer cohort of the calanoid copepodBoeckella gracilipes. The effect of juveniles is more undefined, but seems to prevent the reappearance ofCeriodaphnia in the summer plankton.     

Unconformities as mineralogical breaks in the burial metamorphism of the Andes

Stratigraphical-structural units separated by regional unconformities in the Andes of Peru and Chile, display a pattern of low grade burial metamorphism. Each stratigraphical-structural unit shows a particular facies series covering part or all the range between the zeolite and the greenschist facies. These facies series were episodically generated as part of the geological evolution of each unit prior to its own folding. Mineralogical breaks are found to coincide with the regional unconformities and often cases of higher grade assemblages on top of lower grade ones occur. This pattern may be explained by a process of sealing of each unit after its particular metamorphic episode took place. Porosity and permeability conditioning P _f, as demonstrated for individual lava flows, are the significant controlling factors in the production of the metamorphic assemblages.     

Generation of overtides and compound tides in Amazon estuary

Tidal propagation in estuaries is affected by friction and fresh water discharge, besides changes in the depth and morphology of the channel. Main distortions imply variations in the mean water level and asymmetry. Tidal asymmetry can be important as a mechanism for sediment accumulation and turbidity maximum formation in estuaries, while mean water level changes can affect navigation depths. Data from several gauges stations from the Amazon estuary and the adjacent coast were analyzed and a 2DH hydrodynamic model was configured in a domain covering the continental shelf up to the last section of the river where the tidal signature is observed. Based on data, theoretical and numerical results, the various influences in the generation of estuarine harmonics are presented, including that of fresh water discharge. It is shown that the main overtide, M₄, derived from the most important astronomic component in the Amazon estuary, M₂, is responsible for the tidal wave asymmetry. This harmonic has its maximum amplitude at the mouth, where minimum depths are found, and then decreases while tide propagates inside the estuary. Also, the numerical results show that the discharge does not affect water level asymmetry; however, the Amazon river discharge plays an important role in the behavior of the horizontal tide. The main compound tide in Amazon estuary, Msf, generated from the combination of the M₂ and S₂, can be strong enough to provoke neap low waters lower than spring ones. The results show this component increasing while going upstream in the estuary, reaching a maximum and then slightly decaying.     

Rock magnetic and geochemical proxies for iron mineral diagenesis in a tropical lake: Lago Verde, Los Tuxtlas, East–Central Mexico

Magnetic and non-magnetic mineral analyses were conducted on a lacustrine sequence from Lago Verde in the tropical coast along the Gulf of Mexico that covers the last 2000 years. The site witnessed the transformation of the environment since the early Olmec societies until forest clearance in the last century. Through these analyses we investigated the processes that affected the magnetic mineralogy in order to construct a model of past environmental changes, and compare this model with the archeological record and inferred climatic changes in the northern hemisphere of tropical America. Volcanic activity has played a major influence on sediment magnetic properties, as a purveyor of Ti-magnetites/Ti-maghemites, and as a factor of instability in the environment. Anoxic reductive conditions are evident in most of Lago Verde's sedimentary record.

Direct observations of magnetic minerals and ratios of geochemical (Fe, Ti), and ferrimagnetic (χf) and paramagnetic (χp) susceptibility (χ) data, are used as parameters for magnetite dissolution (χp/χ, Fe/χf), and precipitation (χf/Ti) of magnetic minerals. Intense volcanic activity and anoxia are recorded before A.D. 20, leading to the formation of framboidal pyrite. Increased erosion, higher evaporation rates, lower lake levels, anoxia and reductive diagenesis in non-sulphidic conditions are inferred for laminated sediments between A.D. 20–850. This deposit matches the period of historical crisis and multiyear droughts that contributed to the collapse of the Maya civilization. Dissolution of magnetite, a high organic content and framboidal pyrite point to anoxic, sulphidic conditions and higher lake levels after A.D. 850. Higher lake levels in Lago Verde broadly coincide with the increased precipitation documented during the Medieval Warm Period (A.D. 950–1350) in the northern tropical and subtropical regions of the American continent. For the Little Ice Age (A.D. 1400–1800), the relatively moist conditions inferred are in concordance with the glacial advances recorded in central Mexico. Higher erosion rates reflect destruction of the rainforest over the last 40 years.     

Hydroclimatology of the Upper Madeira River basin: spatio-temporal variability and trends

Rising in the Andes, the Madeira River drains the southwestern part of the Amazon basin, which is characterized by high geographical, biological and climatic diversity. This study uses daily records to assess the spatio-temporal runoff variability in the Madeira sub-basins. Results show that inter-annual variability of both discharge and rainfall differs between Andean and lowland tributaries. High-flow discharge variability in the Andean tributaries and the Guaporé River is mostly related to sea surface temperature (SST) in the equatorial Pacific in austral summer, while tropical North Atlantic (TNA) SST modulates rainfall and discharge variability in the lowlands. There also is a downward trend in the low-flow discharge of the lowland tributaries which is not observed in the Andes. Because low-flow discharge values at most lowland stations are negatively related to the SST in the tropical North Atlantic, these trends could be explained by the warming of this ocean since the 1970s.

EDITOR A. Castellarin

ASSOCIATE EDITOR A. Viglione     

Estimating commercial property prices: an application of cokriging with housing prices as ancillary information

A vast majority of the recent literature on spatial hedonic analysis has been concerned with residential property values, with only very few examples of studies focused on commercial property prices. The dearth of studies can be attributed to some of the challenges faced in the analysis of commercial properties, in particular the scarcity of information compared to residential transactions. In order to address this issue, in this paper we propose the use of cokriging and housing prices as ancillary information to estimate commercial property prices. Cokriging takes into account the spatial autocorrelation structure of property prices, and the use of more abundant information on housing prices helps to improve the accuracy of property value estimates. A case study of Toledo in Spain, a city for which commercial activity stemming from tourism is one of the key elements of the economy in the city, demonstrates that substantial accuracy and precision gains can be obtained from the use of cokriging.     

From platform to basin to swell: orbital control on sedimentary sequences in the Oxfordian, Spain

Climatic, oceanographic and ecological changes that control the formation and deposition of sediment in shallow and deep depositional environments commonly occur with periodicities of a few 10 000 years. Consequently, in order to interpret sedimentary sequences in the geological past, high time resolution is required. This is best obtained by cyclostratigraphy. Three sections have been studied in the Oxfordian of north-eastern Spain: one represents a shallow, siliciclastic-carbonate platform with repetitive subaerial exposures, one an intraplatform basin with sponge bioherms, and one a swell where iron ooids and glauconite formed. The platform section displays a well-defined stacking pattern of depositional sequences; the deeper-water sections are well dated by ammonites. The correlation between the three sections is a best-fit solution integrating biostratigraphy, sequence stratigraphy and cyclostratigraphy. It is concluded that the small-scale depositional sequences formed in tune with the 100-ka orbital eccentricity cycle. An additional factor was differential subsidence that ruled basin morphology.