Exploring fault rocks at the nanoscale

This paper aims to demonstrate the potential role of micro/nanoscale investigations in the study of fault rocks, given that a multiscale and multidisciplinary approach linking structural geology, petrology and mineralogy from the outcrop to the unit-cell scale is essential to the comprehensive characterization of geological materials. To explore fault rocks at the sub-micrometer scale, a fundamental contribution is provided by the Transmission Electron Microscope (TEM), which offers a real-time combination of high-resolution images, diffraction patterns, and chemical data for extremely small volumes, providing accurate mineralogical and nanostructural determinations.This paper focuses on a few selected examples, that are particularly appropriate for TEM investigation, in particular: 1) grain-size determinations on ultrafine-grained fault rocks; 2) detection and characterization of poorly crystalline minerals (e.g., clays) and amorphous phases within fault rocks; 3) identification of deformation-induced mineral reactions occurring within fault zones (e.g., dolomite decarbonation and serpentine dehydration); and 4) observations of recurrent deformation nanotextures in phyllosilicates. TEM investigation can provide an unexpected amount of data, much of which cannot be obtained by conventional techniques, and it can therefore significantly contribute to an understanding of the physico-chemical conditions for faulting and fault mechanics.     

Reinterpretation of the Ordovician rotations in NW Argentina and Northern Chile: a consequence of the Precordillera collision?

Early Paleozoic paleomagnetic data from NW Argentina and Northern Chile have shown large systematic rotations within two domains: one composed of the Western Puna that yields very large (up to 80°) counter-clockwise rotations, and the other formed by the Famatina Ranges and the Eastern Puna that shows (~40°) clockwise rotations around vertical axes. In several locations, lack of significant rotations in younger rocks constrains this kinematic pattern to have occurred during the Paleozoic. Previous tectonic models have explained these rotations as indicative of rigid-body rotations of large para-autochthonous crustal blocks or terranes. A different but simple tectonic model that accounts for this pattern is presented in which rotations are associated to crustal shortening and tectonic escape due to the collision of the allochthonous terrane of Precordillera in the Late Ordovician. This collision should have generated dextral shear zones in the back arc region of the convergent SW Gondwana margin, where systematic domino-like clockwise rotations of small crustal blocks accommodate crustal shortening. The Western Puna block, bordering the Precordillera terrane to the north, might have rotated counterclockwise as an independent microplate due to tectonic escape processes, in a fashion similar to the present-day relationship between the Anatolia block and the Arabian microplate.     

Macrobenthic surf zone communities of temperate sandy beaches: spatial and temporal patterns

The spatial and temporal patterns within the surf zone epibenthic assemblages were studied in a coastal fringe of Argentina to determine whether assemblage compositions, abundance, species richness and diversity vary spatially and temporarily. Sampling was conducted seasonally in two sandy beaches over 2 years with a benthic sledge used to collect the fauna in the upper centimeters of soft bottom sediments and the epifauna on the sediment surface. Physical variables were measured in the same coastal sites where biological sampling was conducted. A total of 58 morphospecies were collected. Peracarid crustaceans were the most abundant group. The mysid Pseudobranchiomysis arenae (new genus–new species) (29.73 ± 17.79 ind. per sample) and the isopod Leptoserolis bonaerensis (51.54 ± 22.35 ind. per sample) were the most abundant and common species and were present regularly throughout the sampling period. Differences in the surf zone community composition were found between the beaches; these differences could be related to variation in physical parameters such as sand grain size and wave climate, indicating the possible influence of the morphodynamic state of the beaches on the epibenthic assemblages. A seasonal abundance trend was detected, reflecting the changes in abundance of the two dominant species; the richness pattern was not easily detectable due to the sporadic appearance of non‐resident species in the surf zone, probably due to different causes, including dispersion by entry of water from surrounding areas, littoral currents and storms. The surf zone studied presents a complex and dynamic epibenthic community that appears to be influenced by the morphodynamic state of the beach and the dynamic of non‐resident species.     

Simulated carbon and nitrogen flows of the planktonic food web during an upwelling relaxation period in St Helena Bay (southern Benguela ecosystem)

A vertically resolved ecosystem model is developed to simulate the dynamics of the pelagic food web in St Helena Bay during a representative period of relaxation after an upwelling event. The proposed model aims at coupling three biogeochemical cycles (carbon, nitrogen and silicon), using several recently developed concepts of the stoichiometric approach. A consequence of this approach is that important qualitative aspects are introduced, such as indicators of phytoplankton physiological state or variable food C:N ratios. For instance, the sedimentation and exudation rates for phytoplankton vary according to physiological state. An attempt is made to parameterize and simulate the diel cycles for vertical migration and feeding rhythms of large zooplankton, two important mesoscale processes that are thought to influence the overall dynamics of the huge phytoplankton blooms in the region.Observations of the Anchor Station Experiment 1987 (ASE’87) are used to assess the quality of the model. There is overall agreement between observations and the corresponding simulated results. The timing, the magnitude, and the vertical structure of the phytoplankton bloom are well reproduced. The balances for carbon and nitrogen flows and stocks compare well to the numerous estimates found from the literature for the southern Benguela region.On the basis of the model results, the origin of the new nutrients, the fate of the carbon fixed by phytoplankton, and the importance of the microheterotrophic pathways are discussed. It is concluded that sediments of the St Helena Bay and surrounding areas may play a crucial role in increasing the level of phytoplankton production. The results also suggest that exudation is the main process by which the carbon fixed by phytoplankton would have been lost, and that microheterotrophic pathways would have been intense during the experiment.     

Vertical migrations of the mangrove snail Cerithidea decollata (L.) (Potamididae) through a synodic month

The vertical migration on mangrove trunks of the gastropod Cerithidea decollata was followed for 5 weeks, in a Kenyan mangrove. Most of the times, snails forage on the mud surface, during low tide, and climb back on trees well before the incoming tide. As soon as the sea retreats, the downward migration takes place and the snails spread again on the ground. The migratory behaviour of snails can vary widely, depending on the relative tide intensity, and different strategies can be exhibited. Individuals can spend several days on trees without migrating to the ground, around Spring Tides, or else, they might remain on the ground without bothering to migrate upwards, during Neap Tides, when the study area is not reached by the water. These irregular animal behaviours, relating to the complicated tide succession, can hardly be explained by the sole presence of an internal clock, and direct cues seem necessary to switch between different strategies, tuning the snails migratory behaviour to the actual local sea conditions.     

Reconstructing recent environmental changes using non-biting midges (Diptera: Chironomidae) in two high mountain lakes from northern Patagonia,Argentina

Remote lakes of northern Patagonia are ideal sites for examining climate- and non-climate-driven changes in aquatic ecosystems because there is little evidence of human influence and there is no detailed information on recent environmental trends in the region (i.e. the last 200 years). Subfossil chironomids (Diptera: Chironomidae) are useful paleoindicators due to their specific response to numerous environmental factors. Here, we analyze the chironomid subfossil assemblages from two remote lakes located in different environmental settings in Nahuel Huapi National Park of northern Patagonia, Argentina. Chironomids combined with sedimentary pigments (chlorophyll derivatives and total carotenoids) and organic matter provided information on the environmental history of the lakes for the last ca. 200 years. The ²¹⁰Pb chronology and tephra layers are used to establish the chronology of changes in the chironomid assemblages associated to different environmental factors that impacted the area during the period covered by the study. The deposition of volcanic ash affected the abundance and composition of chironomid assemblage throughout the record of both lakes. However, changing climate conditions and human activities are also responsible for chironomid changes in the last 50 years.     

Spatial variation in meristic and morphometric characteristics of sardine Sardinops sagax around the coast of southern Africa

Spatial variability in phenotypic characteristics within a fish population may be used to infer the existence of multiple stocks, and knowledge of the population structure of exploited species is important for their sustainable management. In this study we investigated geographic variability in meristic (vertebral count) and morphometric (body shape) characteristics of sardine Sardinops sagax from three southern African regions: Namibia, the South African west coast, and the South African south coast. There were significant regional differences in vertebral counts, with Namibian fish tending to have more vertebrae than South African fish, but with no difference between sardine off the South African west and south coasts. Body shape was characterised using morphometric landmarks, and shape changes were investigated using geometric morphometrics, with influences on fish shape assessed using multivariate regressions. To remove the effects of allometry, the 22 shape variables (represented by Procrustes coordinates) were regressed against fish size (represented by centroid size). There was a significant allometric effect, which was removed by using the residuals of these regressions in further analyses. A multivariate multiple regression was applied to the 22 size-corrected shape variables and to three covariates: condition factor, stomach fullness and fat stage. The significant influence of these covariates was removed by using the residuals from this regression in a canonical variate analysis, where corrected shape variables were classified in relation to region, year and sex. The Mahalanobis distances from the canonical variate analysis differed significantly for most combinations of classifier variables, with clear separations among the three regions. Sardinops sagax off Namibia had thicker bodies and more contracted tail regions than South African fish, whereas S. sagax off the South African south coast had thinner bodies and smaller heads than those off the west coast. These results support the hypothesis of multiple sardine stocks around southern Africa.