Groundwater characteristics within loessic deposits: the coastal springs of Los Acantilados,Mar del Plata,Argentina

The urban growth of the southern neighborhoods of Mar del Plata City provoked significant changes in the groundwater balance of the loessic sequences. These regional loessic levels with a significant portion of volcanic ash layers were reported subject to fluoride and nitrate concentrations. Residential houses pump from sands located 70 m depth and withdraw the sewages to depths less than 5 m. These effects cause significant local and seasonal (summer) increments of the water table outcropping via springs at certain unconformities of the coastal cliffs. A mathematical model was applied to analyze the water level lowering at the productive levels, while there is a decrease in the quality of the upper levels subject to waste discharges. Much of this groundwater flow is concentrated in unconformities between different types of sediments. Human activities have affected the aquifer dynamics increasing the groundwater pumping rates and the return velocity of the sewages. This should be considered in the management strategies of coastal hydric resources.     

Holocene sea-level fluctuation in the southern hemisphere

If rising sea levels dominate in the northern hemisphere (NH), falling or fluctuating sea levels predominate in the southern hemisphere (SH). Endogenic processes (tectonics, isostasy or geoidal changes) could explain local or regional mean sea level (MSL) fluctuations but not an hemispherical one.Evidence from South America, Africa, Antarctica, Australia and the Pacific and Indian Oceans suggest that the Holocene transgression rose above the present MSL, in higher latitudes before the tropics. By plotting latitude against the age of MSL arrival at present coasts, good correlation is observed.Oceanic salinity mixing has been already proposed to explain this mid-Holocene sea-level fluctuation. Climate could be the only factor responsible for this hemisphere-wide behavior of MSL. It has been suggested previously that the climate of the SH precedes that of the NH by 3000 years. The climatic optimum, or maximum warmth, occurred predominantly about 6000 BP in the NH, but about 10–9000 BP in the SH.Short-term climatic effects on the sea level (monsoons, southern oscillation/El Niño phenomena) should have significant occurrences during the past in the windiest oceanic hemisphere.This latitudinal trend in former MSL should be considered when using shorelines as reference points for measuring vertical crustal movements.     

Anatomy and facies distribution of terminal lobes in ephemeral fluvial successions: Jurassic Tordillo Formation,Neuquén Basin,Argentina

In terminal fluvial-fan systems, characteristic proximal to distal variations in sedimentary architectures are recognized to arise from progressive downstream loss of water discharge related to both infiltration and evaporation. This work aims to elucidate downstream trends in facies and architecture across the medial and distal zones of terminal-fan systems, which record transitions from deposits of channel elements to lobe-like and sheet-like elements. This is achieved via a detailed characterization of ancient ephemeral fluvial deposits of the well-exposed Kimmeridgian Tordillo Formation (Neuquén Basin, Argentina). The fine sand-prone and silt-prone succession associated with the medial to distal sectors of the system has been studied to understand relationships between depositional processes and resulting architectures. Facies and architectural-element analyses, and quantification of resulting sedimentological data at multiple scales, have been undertaken to characterize sedimentary facies, facies transitions, bed types, architectural elements and larger-scale architectural styles. Eight bed types with distinct internal facies transitions are defined and interpreted in terms of different types of flood events. Channelized and non-channelized architectural elements are defined based on their constituent bed types and their external geometry. The most common elements are terminal lobes, which are composite bodies within which largely unconfined sandy deposits are stacked in a compensational manner; a hierarchical arrangement of internal components is recognized. Proximal feeder-channel avulsion events likely controlled the evolution of terminal-lobe elements and their spatiotemporal shifts. Stratigraphic relations between architectural elements record system-wide trends, whereby a proximal sector dominated by channel elements passes downstream via a gradational transition to a medial sector dominated by sandy terminal-lobe elements, which in turn passes further downstream to a distal sector dominated by silty terminal lobe-margin and fringing deposits. This work enhances current understanding of the stratigraphic record of terminal fluvial systems at multiple scales, and provides insight that can be applied to predict the facies and architectural complexity of terminal fluvial successions.     

Subantarctic macrotidal flats, cheniers and beaches in San Sebastian Bay, Tierra Del Fuego, Argentina

San Sebastian Bay is a large, semicircular coastal embayment situated on the Atlantic coast of Tierra del Fuego, Argentina. It is a high-energy, subantarctic environment with a tidal range of 10.4 m, influenced by large waves of Atlantic and local origin and swept by constant, strong westerly winds. A 17 km long gravel spit protects the Northern part of the Bay giving rise to a gradient in sedimentary processes. From south to north, are seven distinct sedimentary environments. Coastal sedimentation started at least 5200 years before present (BP) and a rapid progradation related to a relative sea-level drop has infilled about 6 kilometres of the Bay with a sedimentary sequence 10–11 m thick. 14-C dating of unabraded shells in the Chenier ridges of the relict part of the complex allows for a precise reconstruction of the stages of the progradation, that has slowed from 2.35 m/year at 5000 years BP to 0.6 m/year at present. The possible causes of the sea-level drop are discussed.     

Overpassing and armouring phenomena on gravel beaches

Armouring phenomena are common in river beds, gravel beaches and spits. The gravel segregation recognized at depth in three different beaches of Patagonia and Tierra del Fuego, Argentina, allows us to propose a mechanism for formation.

A mixed population of gravel set into motion by waves is deposited progressively. Granules and fine gravels with higher pivoting angles in the beach slope are easily trapped within the bed, while rounded pebbles continue rolling over (overpassing) it. Finally, decreasing flow velocity allows the deposition of the larger pebbles, thus armouring the beach.

A carpet of round clasts of uniform size is more stable than the same clasts in a mixed population armouring the bed and covering a uniform layer of rounded granules and fine gravels.     

Modern Diatom Assemblages in Surface Sediments from Estuarine Systemsin the Southeastern Buenos Aires Province, Argentina

Paleoecological reconstructions of Holocene sea-level changes in Argentinean coastal regions were based mainly on ecological data gathered from other regions, as there was a lack of information on modern estuarine diatom distributions. The aim of the present work was to assess the spatial variation of diatom assemblages in two representative estuaries of Argentina in order to gather ecological information for paleoecological reconstructions in the region. The two selected estuaries have different geomorphologic features and salinity regimes: Mar Chiquita Lagoon is shallow, which prevents the development of a stable salinity gradient as it occurs in the Quequén Grande River. Surface sediment samples were taken from selected stations representative of the environmental gradient from the inlet to the inner reaches of both estuaries. Cluster analysis defined three diatom zones at Mar Chiquita: marine/brackish assemblages dominate the inlet (zone I), where salinity, tidal range and current speed are higher. The brackish/freshwater tychoplankton Staurosira construens var. venter and Staurosirella pinnata dominate the inner lagoon (zone II), where environmental conditions are very variable and concentrations of suspended sediments are higher. Brackish/freshwater euryhaline diatoms dominate the headwaters (zone III). On the other hand, the Quequén Grande River was divided into three diatom zones: coastal taxa are distributed at the inlet (zone I), while the middle estuary (zone II) is dominated by brackish/freshwater euryhaline taxa. At the upper estuary region (zone III), freshwater diatoms dominate, and the halophobous Nitzschia denticula increased in abundance values. Diatom distributions were most closely related to the salinity gradient at Quequén Grande River than at Mar Chiquita Lagoon. Fossil data of a sequence from Mar Chiquita Lagoon (Las Gallinas Creek) were compared to the modern data set in order to search for analogies between fossil and modern diatom assemblages. DCA results showed that fossil diatom assemblages have modern counterparts. Most diatom assemblages of Las Gallinas Creek fall within Mar Chiquita zone III, representing a shallow brackish/freshwater environment, with low salinity fluctuations (~1–9‰) and no tidal influence. Therefore, our modern diatom data provide useful analogs to interpret paleoenvironments in the region.     

Short and medium-term coastal evolution of Necochea Municipality, Buenos Aires province, Argentina

The short and medium-term coastal evolution of Necochea Municipality, Buenos Aires province (Argentina) was studied. The medium-term evolution study was based on aerial photographs from 1967 and 1984, and satellite images from 2004, using the dune or cliff toe as coastline indicator. The short-term evolution was studied from February 2006 to December 2009 by means of seasonal beach profiles and sediment sampling. Results showed that in the medium-term at Necochea, the coastline has remained stable or has advanced. Whereas in the short-term, the analysis has evidenced incipient erosion processes. On the other hand, at Quequén the retreat of the coastline and the negative sedimentary balances (medium and short-term) are clearly indicating an accentuated erosion process. The different degrees of coastal erosion are related in part to beach vulnerability to wave storms but mainly to anthropogenic actions, which have affected the beach sediment supply. These results evidence the necessity to develop a coastal management programme.     

Holocene evolution of the Mar Chiquita coastal lagoon area (Argentina) indicated by pollen analysis

Palynologycal information was integrated with the aim to study vegetation history and environmental changes in Mar Chiquita coastal lagoon area (in the southeast of Buenos Aires province, Argentina) related to sea‐level fluctuations during the Holocene. Prior to 5000 yr BP a mosaic of psammophytic and halophytic communities developed on extensive sandy flats. Between 4000 and 2500 yr BP, during a period of sea‐level stabilisation, similar halophytic communities developed in the south and in the middle part of the present lagoon. Between 4500 and 3500 yr BP hydrophytic and halophytic communities developed in Laguna Hinojales. Beginning 2500 yr BP, when sea level was nearing its present position, modern plant communities became established. Pollen records suggest that littoral evolution of the area was much more complex than previous models have described, introducing new evidence of the geomorphological evolution of Mar Chiquita coastal lagoon. The estuarine environment was delimited by a system of barrier islands instead of a linear prograding barrier, as has been proposed. Copyright © 2005 John Wiley & Sons, Ltd.     

Downward particle fluxes in the Guadiaro submarine canyon depositional system (north-western Alboran Sea), a river flood dominated system

Three moorings equipped with sediment traps were deployed in the north-western Alboran continental margin to study downward particle fluxes in the Guadiaro submarine canyon depositional system. This area is located close to the Strait of Gibraltar and is influenced by the upwelling induced by the Atlantic Jet and by episodic flood events from the Guadiaro River. Sediment traps were installed in the Guadiaro Canyon, in the Guadiaro Channel and in the adjacent continental slope. The overall duration of the deployment was 12 months (from November 1997 to October 1998). Time-series of downward particle fluxes, major constituents (organic carbon, nitrogen, biogenic opal, calcium carbonate and lithogenics) and ²¹⁰Pb were determined near the surface at mid-depths and near the bottom. Total mass fluxes (TMF) in this area fluctuated more than two orders of magnitude and showed an important seasonal variability with higher fluxes in winter. Increases in TMF and lithogenics together with decreases in ²¹⁰Pb, organic carbon and opal were recorded in all traps coinciding with river floods, indicating a direct response of the system to these events and a rapid offshore transport of suspended matter affecting the entire water column. The channel site received similar particle fluxes to the western open slope site, indicating that this channel did not act as a preferential sediment conduit during the deployment period. In the Guadiaro Canyon, TMF were more than one order of magnitude higher, ²¹⁰Pb concentration was lower (one half) and organic matter was more degraded than at the channel site during spring and summer, as a consequence of receiving particles resuspended by internal waves and occasionally by trawling activities. These particles were mainly retained in the canyon, which works as a trap. Also, during spring and summer, the opal and organic carbon percentages increased in all traps both in magnitude and variability, and peaks seem to be associated with biological blooms.