Marcela A. Espinosa

Paleoenvironmental reconstruction of Holocene coastal deposits from the Southeastern Buenos Aires Province, Argentina

An estuarine sequence outcropping at La Ballenera Creek (BuenosAires Province), dated between 6,800 and 4,100 14C years BP, wasinvestigated for diatoms and molluscs. The sea level history along the BuenosAires coastline has been reconstructed from discrete beach ridge sequencesalong low-lying plains. The La Ballenera profile is located on a cliffcoast where a sequence recorded environmental changes. Fifty-eightdiatom species were grouped based on their salinity and life formcharacteristics. Cluster analysis allowed the division of the sequence intothree diatom zones. Mollusc tolerances were also used to discern theenvironmental changes induced by sea-level fluctuations. The base of thesequence recorded the initiation of the marine influence about6,790–6,200 years ago. Diatom assemblages consist ofbrackish-freshwater tychoplankton accompanied by brackish benthicdiatoms. Among the molluscs, the freshwater-brackish species Heleobiaparchappii dominates. Between 6,200 and ca. 4,800 14C years BP, anestuarine lagoon environment is indicated by benthic and epiphyticmarine-brackish diatoms, as well as by the estuarine snail Heleobiaaustralis. After 4,800 14C years BP, the diatom assemblages and therelative abundance of freshwater molluscs indicate a marshy environment withlower salinity content. The La Ballenera records the salinity changes thataffect an estuary that infilled during the 2,700 years after the maximum sealevel reached in mid-latitudes of South America.

Fidelity Of Dead Diatom Assemblages In Estuarine Sediments: How Much Environmental Information Is Preserved?

Abstract The main objective of this contribution is to evaluate the environmental fidelity of dead diatom assemblages along two microtidal estuarine systems from southeastern Buenos Aires Province (Argentina). Living communities (inferred from counting protoplasm-containing cells) were compared to dead diatom assemblages through several fidelity metrics. Gradient analysis (by canonical correspondence analysis) was applied in order to assess the quantitative relationship between diatom assemblages and the environmental gradient. The presence of allochthonous components in diatom assemblages was assessed by analyzing the distribution of the main ecological groups. Results indicated a good agreement between living communities and total assemblages in surface sediments, as well as between total surface and subsurface assemblages from both estuaries. A high percentage of the variance in diatom assemblage composition was explained by the environmental gradient, particularly by sediment composition and salinity, indicating that taphonomic alterations play a minor role in structuring these assemblages. The good preservation of diatom thanatocoenoses in estuarine sediments makes them accurate indicators of the environmental conditions at the point of deposition, providing useful information for paleosalinity reconstructions in coastal settings.

Coastal environmental changes associated with Holocene sea-level fluctuation: Southeastern Buenos Aires, Argentina

Abstract Sea level was 2 m higher than present 6000 years ago at southeastern Buenos Aires Province, Argentina. This fluctuation has caused several environmental changes including: Drift reversal : Geomorphological features and radiocarbon dating suggest that the Mar Chiquita barrier grew from north to south during the regression from 4000 years BP. In contrast, mineral dispersals, coastal defenses, inlet migrations and wave statistics show that present beach drift is to the north, in the same direction as storm-induced shelf sediment transport. Coastal mollusk mass mortalities : Lagoonal deposits, that accumulated during the regression, contain abundant remains of Mactra isabelleana, Tagelus plebeius and Labiosa plicatella . These species are rare in present coastal lagoons. Many causes have been proposed for the absence of these species in present lagoons and include: sudden sea-level decline, outlet obstruction producing variation in salinity, and increased fluvial freshwater inputs. The regional character of these mortalities and growth curves indicates that taphonomic-feedback processes related to the scarce 2 m involved in the transgressive-regressive cycle are the reason for mollusk mortality. Sand consumption . During the latter stages of the Holocene transgression, sand ramps (up to 10 m height) at Mar del Plata cliffs, and sand barriers (up to 2 km width) north of Mar Chiquita village, developed as a result of high wave energy and a plentiful sand supply. The subsequent regressive phase produced lagoons, tidal flats, marshes and cheniers. The alongshore growth of barriers meaned their cannibalization (misfit foredunes) and narrowing driftwards. These processes occurred before the drift reversal mentioned above.

Dead diatom assemblages in surface sediments from a low impacted estuary: the Quequén Salado river, Argentina

In the context of a main project that aims to recover modern data on diatom distribution applicable to paleosalinity reconstructions in coastal areas of Southern South America, the composition and distribution of dead diatom assemblages in the littoral zone of the Quequén Salado estuary (Argentina) were studied. Diatom zones were defined along the estuarine gradient by cluster analysis and related to the salinity range and sediment composition by Canonical Correspondence Analysis. Four diatom zones were identified. A mixture of marine, brackish and freshwater diatoms, probably allochthonous, characterized the inlet (zone I). Marine/brackish taxa, represented mainly by Paralia sulcata dominated zone II, characterized by polyhaline conditions and sandy sediments. Zone III was characterized by mesohaline conditions, muddy sediments and the dominance of the estuarine diatom Amphora helenensis. Brackish/freshwater and freshwater diatoms dominated the headwaters (zone IV), where salinity was always below 5‰. The comparison of Quequén Salado diatom assemblages with previous results from the Quequén Grande estuary showed a similar taxonomic composition between both estuaries. However, differences in the salinity ranges of the estuaries (related to differences in the degree of human impact and tidal range) lead to a displacement in their spatial distribution along the longitudinal estuarine axis. This paper contributes to the knowledge of the ecological requirements of South American estuarine diatoms and provides useful data for paleosalinity reconstructions in the region.

Sediment and pollutant distribution along the Negro River: Patagonia, Argentina

The Negro River extends from the Andes to the Atlantic Ocean supplying water to several cities and to the most important fruit-bearing productive region of Patagonia. Floodplains from the Upper Valley are artificially irrigated with water delivered from the main channel. Pesticides are increasingly used without strict government control. Dams constructed at the tributary rivers retain sediment and therefore the clean water delivered becomes free of sediment and therefore is able to erode certain levees during high-discharge events. Bedload–transport fraction (sand dunes) increases at the Upper Valley and is fixed at the levees of the Lower Valley by riparian vegetation. This riparian vegetation is dominated by Salix humboldtiana and Schoenoplectus californicus while Spartina sp is abundant at the estuarine marsh area. Organochlorine compounds (OCCs) were measured by GC-ECD including dichloro-diphenyl-trichloroethane (DDT), dichlorophenyl-dichloroethylene (DDE) and dichloro-dyphenyl-dichloroethane (DDD) endosulfans, hexachlorocyclohexanes (HCH) and chlordanes. OCCs were preferentially accumulated at the Upper Valley in relation to the agricultural activities developed. The dominance of DDE and the parental endosulfan can be explained by their past and current use, respectively. PCBs (different congener groups) showed low levels as a consequence of chronic pollution related to the mud fraction. The high concentration in pollutants at the Upper Valley is significantly reduced to the Middle and Lower Valleys due to filtering effects, assumed to be induced by the riparian vegetation. At the estuary, very low OCCs levels were measured probably due to the dilution induced by mesotidal dynamics. The sedimentation rate of the lower floodplain is of 1 mm/year. The bottom of the main channel is dominated by 2D sand dunes. At the inlet, a sandy gravel bank is composed of megaripples 0.5 m high. Tidal currents guarantee the stability of the inlet, and particularly the asymmetric flood-tidal delta. Wave action induces a net longshore sand transport towards the north.

Diatom-inferred salinity changes in relation to Holocene sea-level fluctuations in estuarine environments of Argentina

Espinosa, M.A., Hassan, G.S. & Isla, F.I., September 2012. Diatom-inferred salinity changes in relation to Holocene sea-level fluctuations in estuarine environments of Argentina. Alcheringa 36, 377–391. ISSN 0311-5518. The analysis of diatoms from three sedimentary sequences in the Quequén Grande River basin (Buenos Aires province, Argentina) has allowed the reconstruction of local and regional palaeosalinity changes in relation to Holocene sea level fluctuations. An established diatom-based salinity transfer function is used in combination with autecological methods to reveal a maximum sea level between ca 7000 and 6000 14C years BP. This marine influence was recorded in deposits exposed 2 km and 12 km upstream, but not 32 km from the mouth. The fossil diatom assemblages are characteristic of an estuarine lagoon with maximum inferred salinities of 15‰. When the Holocene transgression began to flood the former valley (ca 7000 14C years BP), brackish lagoons evolved into estuarine settings in the lower valley, but shallow freshwater ponds with salinities <4‰ persisted at 32 km from the mouth.

MÚLTIPLES INDICADORES DE LOS CAMBIOS AMBIENTALES DESDE EL HOLOCENO MEDIO EN EL NORESTE DE PATAGONIA (BAJO DE LA QUINTA), ARGENTINA

Abstract. MULTIPROXY RECORDS OF MID-HOLOCENE ENVIRONMENTAL CHANGES IN NORTHEAST PATAGONIA (BAJO DE LA QUINTA), ARGENTINA. Paleoecology is a useful tool in the exploration of changes occurring in the environment over time. Particularly, multi-proxy studies provide a more comprehensive picture of climatic conditions in the past. This work is a new contribution to the currently available paleoenvironmental information on the northern Patagonian Atlantic coast. It aims to reconstruct the paleoenvironmental history of Bajo de la Quinta (northeastern Patagonia) using pollen, diatoms, organic matter, carbonates and geomorphological studies. The section suggests changes in climate and vegetation occurring since the middle Holocene, revealing an important change from arid to semiarid conditions during the late Holocene —at 3000 years cal. BP. This may be related to more frequent contributions of moisture from the Atlantic and a weakening of the western winds. These environmental conditions favored the development of the shrub communities known as Monte. Such a development was synchronous with geomorphological changes of the coast, where connection to the sea was interrupted to originate a lagoon. The diatom content suggests the deposit represents a shallow fresh-brackish environment possibly exposed to periodic desiccation. This multi-proxy study improves and complements the current paleoenvironmental offsetting accepted for the northern coast of Río Negro Province.

Reconstrucción Paleoambiental de la Planicie Costera de Mar Chiquita (Provincia de Buenos Aires, Argentina) Durante el Holoceno, Basada en Diatomeas

Abstract. HOLOCENE PALEOENVIRONMENTAL RECONSTRUCTION OF THE MAR CHIQUITA COASTAL PLAIN (BUENOS AIRES PROVINCE, ARGENTINA) BASED ON DIATOMS. Diatom assemblages from a core at the north of Mar Chiquita coastal plain were analyzed in order to identify environmental changes due to salinity and depth. The core contains sediments dated at the base in 5802±70 14C years BP. Four lithological units were defined: greenish-grey silt, brownish-grey sandy silt, heterolithic facies and brown sandy silt. Two hundred diatom species were identified and grouped based on their salinity tolerances and life form. Cluster analysis defined two diatom zones. The lower section of the sedimentary sequence (from the base to 90 cm depth, Zone I) was dominated by Paralia sulcata (Ehrenberg) Cleve and Psammococconeis cf. disculoides (Hustedt) García accompanied by marine-brackish plankton, epipsammon and epiphytes indicating coastal marine conditions. From 90 cm towards the top (Zone II), salinity and depth decrease: P. sulcata decline and appear Planothidium delicatulum (Kützing) Round and Bukht (brackish) and Hippodonta hungarica Grunow (brackish freshwater taxa), benthic and epiphytes diatoms increased. Multidimensional Scaling Analysis (MDS) showed that fossil diatom assemblages were similar to the modern assemblages from the inlet of Mar Chiquita coastal lagoon. These new diatom and sedimentological data support the regional model for the Holocene sea-level fluctuation and allow inferring the existence of a marginal coastal system during the regressive phase that evolved into a brackish-freshwater coastal lagoon and marsh.

Modern diatoms from a temperate river in South America: the Colorado River (North Patagonia, Argentina)

Diatom assemblage composition and distribution from surface sediment samples of the Colorado River, Patagonia were analysed in relation to environmental variables using multivariate techniques. The aim of this study was to determine the ecological preferences of the taxa and provide analogues for paleoenvironmental reconstructions in estuarine and fluvial environments of southern South America. Cluster analysis identified two main diatom assemblages, one grouping sites with a marine influence, and another group influenced by typical riverine conditions. Canonical correspondence analysis indicated that major ion concentrations contribute significantly to explain the variation in the composition of diatom assemblages. A total of 208 taxa belonging to 56 genera were recorded. In estuarine sites, the assemblages were characterized by marine-brackish tychoplankton, such as Cymatosira belgica, Rhaphoneis amphiceros, Delphineis minutissima, Paralia sulcata and Paralia sulcata var. coronota. Diatom communities in riverine sites were dominated by freshwater taxa such as Staurosira venter, Pseudostaurosira brevistriata, Punctastriata glubokoensis and Punctastriata lancettula. Two Punctastriata species that were difficult to distinguish with LM were examined in detail with SEM, and the value of detailed LM and SEM analysis for distinguishing Punctastriata spp. and other small fragilarioids is discussed. Based on their autoecological affinities, it is likely that the dominance of small fragilarioids is related to the increase in ionic concentration in the basin, as a consequence of both precipitation decrease, and an increase in urban, industrial and agricultural activities. The construction of a flood-control structure in the northern branch of the delta modified the diatom community so that marine taxa were replaced by brackish-freshwater taxa in response to changes in salinity, substrata and water depth.

Response of Diatoms to Late Quaternary Climate Changes

Diatoms are very useful proxy indicators to reconstruct past climate changes. Studies are based on qualitative and quantitative analyses that allow to infer variables related directly to climate as temperature, or indirectly as salinity, depth, productivity and pH. Reconstructions based on these methods rely on the general assumption that past environmental requirements of the fossil diatom taxa have remained similar to those of their closest living representatives. In this way, the environmental information obtained from living organisms can be used as analogs and extrapolated to the fossil record, particularly in Late Quaternary studies. Diatom records from lacustrine deposits from Argentina, ancient lakes from South America, and marine cores from Southeastern Atlantic Ocean and Eastern Equatorial Pacific were reinterpreted with the aim to correlate them to climatic changes during Marine Isotope Stage 3 (MIS 3) in the Southern Hemisphere. Marine records allowed paleo-reconstructions of productivity and upwelling conditions; at the same time continental records were used to interpret the lake-level histories. The high temporal resolution of diatom assemblages in both environments makes it possible to identify abrupt climate changes between ca. 60 and 30 cal. ka B.P. The future integration of diatom datasets constructed from different environments will solve the analogy problems between fossil and modern assemblages and increase the potential for reliable quantitative reconstructions of Late Quaternary climate in southern South America.