Hydrogeochemical evaluation of Caleta de Los Loros Patagonian salt marsh in response to the geomorphological evolution of the coast during the Holocene

Abstract

Sea level oscillations occurred during the Quaternary conditioned the marsh landscape by limiting the factors that regulate water salinity in this environment. The aim of this work was to evaluate geochemical processes that condition groundwater salinity in Caleta de Los Loros marsh considering geomorphological – climatic factors associated with the Holocene evolution of this Argentinean Patagonia coastal area. In this sense, a mapping and a geomorphological characterization was performed based on satellite images, digital elevation models and field surveys. A monitoring network including both surface and groundwater was made in order to measure in situ pH and electrical conductivity and to take samples to determine major ions and stable isotopes in the laboratory. The set of results obtained highlights that variations in salinity, major ions composition and isotopic signal recorded in the groundwater of Caleta de Los Loros marsh vary in the different sectors of the intertidal plains resulting from geomorphological evolution from the middle Holocene to the present day. The older intertidal deposits (2100 years B.P.) currently located in the more continental sector host high salinity environments with high marshes vegetated with Salicornia sp. The hydrochemical and isotopic signal indicates that in them salinity responds primarily to evaporitic salt dissolution – precipitation cycles. Meanwhile, more coastal intertidal deposits have less than 500 years and they host low marsh environments vegetated with Spartina sp. Hydrochemistry and isotopic composition in them reflect that tidal water flooding is the main water contribution to the system, being marsh groundwater hydrogeochemichally similar to surface water. Locally, contributions from adjacent beach ridges and hydrochemical variations associated with CO_2(g) dissolution were also recognized. Understanding environmental changes that occur in marshes as a product of their geomorphological quaternary evolution allows to understand the distribution of salinity patterns that develop in them and to give an idea of the time scale on which the salinization of these environments occurs.